The influence of sewage effluent on coastal wetlands of Tin Can Bay: assessment of impacts and design for mitigation
Thesis submitted for the Master’s Degree in Environmental Management by Helen Hillier, 1996
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TABLE OF CONTENTS
Page No.
PREFACE 4
Acknowledgments 5
Statement of Originality 7
1.0 Introduction 9
1.1 Scope of the Study 11
1.11 Related Surveys
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1.12 Description of the Site 19
1.2 Legislative Issues 14
1.3 The Tin Can Bay Coastal Floral Community 17
1.31 Soils and Geology 17
2.0 WETLANDS 21
2.1 Introduction to the area 21
2.2 Wetlands - a definition 23
2.3 Wetland nutrient cycling & productivity 24
3.0 THE MANGROVE FOREST 28
3.1 General Description 28
3.2 Mangrove nutrient chemistry at TCB 32
3.3 Methodology
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3.4 Results and Discussion 51
3.7 Melaleuca Nitrogen results - Low flow 53
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3.8 Melaleuca Nitrogen results - Peak period 54
4.0 MANGROVE ANALYSIS
4.1 Mangrove - Low Flow results 54
4.2 Water Results and Discussion 56
4.3 Mangrove nutrient concentrations
in sediments and waters - peak period 58
4.4 Ion Uptake and Salinity 61
4.5 Redox, pH and Conductivity 64
4.6 Discussion 66
4.8 Heavy Metals 68
5.0 FRESHWATER INFLUENCE AND
ENERGY FLOWS 69
5.1 Effluent energy and its effect on
mangrove forest zonation and succession 73
5.2 Summary - Nutrient Cycling 77
6.0 EFFECT OF EFFLUENT ON SEAGRASS 80
6.1 General Discussion 80
6.2 Effect of point source pollution 82
7.0 CONSTRUCTED WETLANDS 82
7.1 Introduction 82
7.2 The existing TCB WPCW 84
7.3 Biological Nutrient Removal 89
7.4 Phosphorous Removal 90
7.5 Potential for constructed broadacre
wetlands 92
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7.6 Sub-surface flow wetlands 94
8.0 CONSTRUCTED WETLANDS VERSUS NATURAL
WETLANDS 97
8.1 System Components
8.2 Micro-organisms in wastewater treatment 100
8.3 Environmental Conditions 101
8.4 Microbiology and public health 101
8.5 Microorganisms and biological yields 103
8.6 Biological rates 104
9.0 POLLUTANT REMOVAL IN FREE WATER
SURFACE WETLANDS 105
9.1 Biochemical Oxygen Demand 105
9.2 Suspended Solids 109
9.3 Nitrogen Removal 110
9.4 Design for nitrification 111
9.5 Denitrification 113
9.6 Phosphorous 113
9.7 Pathogen Removal 116
9.8 Wetland Influent Characteristics 118
9.9 Effluent Quality 122
10.0 WETLANDS AND LANDSCAPE DESIGN 124
11.0 SITE SELECTION AND EVALUATION 126
11.1 Site Identification 126
11.2 Topography and soils investigation 127
11.3 Soils 129
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11.5 Hydraulic configuration 131
11.51 Flow path zonation 135
11.6 Subgrade construction 140
11.61 Placement of topsoil 142
11.62 Batter slopes and freeboard 143
11.7 Inlet and outlet structures 144
11.8 Aerators 147
11.9 Buffers and Corridors 149
12.0 VEGETATION
12.1 Planting and vegetation management 150
12.11 Soil Preparation 150
12.2 Planting densities and water control 152
12.21 Plant selection 153
12.3 Weed Control 158
12.4 Pest Control 159
12.5 Harvesting 160
12.6 Dewatering 162
12.7 Sedimentation management 163
13.0 MOSQUITO CONTROL 165
14.0 COMMUNITY INVOLVEMENT 169
15.0 SITE LAYOUT AND WETLANDS DESIGN 172
15.0 COSTS 173
16.0 RESTORATION AND MANAGEMENT OF MELALEUCA
WETLAND AT TCB WPCW 176
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16.1 Project Goals 177
16.2 Project objectives 178
APPENDIX ONE - FAUNA SURVEY
APPENDIX TWO - PLANTS CONSIDERED SUITABLE FOR ARTIFICIAL
WETLANDS FOR WASTEWATER TREATMENT IN QUEENSLAND (DEH)
BIBLIOGRAPHY
List of Tables
Table 1: Methods of Analysis
Table 2: Comparison of foliar nitrogen and
phosphorous concentrations in Avicennia marina
Table 3: Concentrations of total nitrogen in Melaleuca
Table 4: Concentrations of Ammonia nitrogen and
Nitrate Nitrogen in Melaleuca
Table 5: Sediment nutrient concentrations of nitrogen
and phosphorous in post-holiday period
Table 6: Concentrations of nutrients in waters
of Melaleucain the post-holiday period
Table 7: Mangrove sediment nutrient concentrations
in the post-holiday period
Table 8: Mangrove waters nutrient concentrations in
the post-holiday period
Table 9: Results of field testing of pH, redox and
conductivity
Table 10: Analytical results of grab sample from trickle
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filter during holiday period
Table 11: Predicted average effluent quality after
upgrade
Table 12: Comparison of 90 percentile values for
effluent nutrient concentrations
Table 13: Wetland zone characteristics
List of Figures
Figure 1: Plan of Site
Figure 2: Forest community zonation
Figure 3: Cross-section through transect
Figure 4: Cross-section showing height changes
Figure 5: Ion concentrations in Avicennia m. foliage
Figure 6: Energy flows in the system
Figure 7: Canopy height versus distance
Figure 8: Salinity versus distance
Figure 9: Vegetation and attached growth organisms
Figure 10: Natural wetland zones
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List of Photographs
Airphoto 1: Recent airphoto
Photograph 1: Increase in canopy height visible from
water tower
Photograph 2: Vegetation changes within the effluent
plume
Photograph 3: The meeting of coastal and terrestrial
vegetation within the effluent plume
Photograph 4: West Byron Treatment Plant
Photograph 5: West Byron Treatment Plant
Photograph 6: Inlet to SF Wetland at West Byron
Photograph 7: V-shape weir outlet
Photograph 8: Newly planted wetland cells
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EXECUTIVE SUMMARY
In 1983 Department of Local Government (DLG) recommended
augmentation of the Tin Can Bay Water Pollution Control
Works (WPCW) to 5000 equivalent population (EP). A report
was prepared by John Wilson and Partners in 1993 which
considered augmentation of the WPCW. This report included a
brief assessment by Fisheries Research Consultants of
discharge-related impacts on adjacent and downstream floral
communities. It appeared that continued discharge of
effluent into wetlands could have significant negative
impact on the coastal ecosystem adjacent to the site as a
result of nutrient enrichment.
With the introduction of the Environmental Protection
Act (1994) in March, 1995, a thorough assessment of the
potential for tertiary treatment of sewage effluent was
required. An assessment of the Tin Can Bay coastal floral
community and the potential 'carrying capacity' of the
effluent inundated wetlands was undertaken concurrently.
This assessment, undertaken from July to October, 1995,
found that the Melaleuca forest adjacent to the WPCW has
suffered some damage as a result of sheet flow of nutrient-
rich freshwater. Die-back of trees has occurred adjacent to
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the outflow pipes and local weeds and freshwater macrophytes
have invaded the understorey of the Melaleuca forest within
the effluent plume.
In the intertidal zone, normally devoid of vegetation,
freshwater macrophytes are thriving beside emergent
freshwater- tolerant mangroves.
The mangrove forest which is influenced by the effluent
exhibits significant differences in canopy height, life form
and tissue nutrient concentrations. The effluent however,
has not negatively impacted upon the mangrove forest
community. The mangroves have exhibited a high tolerance of
fluctuating edaphic conditions.
No seagrass beds have ever been documented adjacent to
the Tin Can Bay WPCW. Close to the site, on the opposite
side of Snapper Creek, seagrass beds have not been affected
adversely. No point source discharge within this estuary
could be pin-pointed as having an effect on the health of
seagrass beds. Changes in vegetation density and area of
beds are normal, and are influenced by many factors. The
ability of the wetlands to 'polish' effluent is well
documented and has been demonstrated at the receiving site
at Tin Can Bay. The ability of the wetlands to act as a
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'nutrient sink' has protected coastal marine flora from
degradation which can occur as a result of nutrient
enrichment of waters.
The edaphic conditions which exist at Tin Can Bay -
sandy clay loam underlain by metres of clay - is an
indication of the suitability of the Tin Can Bay Sanitary
Reserve as a site for construction of wetlands for tertiary
treatment of wastewater.
The existing Melaleuca quinquenervia trees and endemic
macrophyte species could be used as vegetation in the
wetlands, having already demonstrated an ability to polish
wastewater and a tolerance of inundation with effluent. The
site would also be suitable for the construction of broad-
acre forested wetlands such as those in existence at West
Byron WPCW.
A landscape management plan for the area will ensure
that ecological values can be restored at the existing
receiving site and habitat values can be maintained.
The Tin Can Bay Sanitary Reserve site represents a
unique opportunity for the development of an ecologically
sustainable and innovative wastewater treatment plant.
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PREFACE
Science never proves anything, it makes guesses and goes
by them as long as they work well.
Steinbeck
Our understanding of our environment improves every day,
yet every day human impacts reduce the availability of
natural resources for future generations. Research projects
are often initiated when a management system has had an
unforseen and undesirable effect upon the natural
environment, and the impacts of that system must be
mitigated. Common sense tells us that it is better that
environmental damage is avoided through prior scientific
investigation.
The Queensland Government passed the Environmental
Protection Act in 1994. The Bill came into effect in March,
1995. Although one could not expect an overnight
rehabilitation of our environment, it is to be hoped that
the implementation of well-researched and carefully
monitored environmental management systems will assist in
preserving the environment for future generations.
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It was with this objective in mind that this project
began. The author hopes that the management systems
described in this report will contribute to the protection
of this small yet significant part of the Queensland
coastline.
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ACKNOWLEDGMENTS
This project was commissioned by the Cooloola Shire
Council, whose financial assistance made the completion of
this work possible. The Senior Planner Brian Stockwell, and
Senior Engineer Geoff Newton, were instrumental in securing
this commission.
The author would like to thank Dr. Jim Davie for his
support, guidance and facilitation in writing the report and
for assisting me with field work. Dr. Cynthia Mitchell was
my internal supervisor and also provided guidance,
encouragement and assistance with the project.
Ms. Fleur Kingham, Director of the Environmental
Management course, was also supportive and facilitated my
writing this thesis.
The technical staff at Gatton Campus of University of
Queensland (John Foster, Lynn Hall and Kathy Raymont)
provided patient and valuable assistance in devising
implements for field work and setting up laboratory tests.
I would also like to thank Dr. Mike Olsen (Griffith
University) for support and advice; Tim Stevens and Nicole
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Buchanski for providing whatever information they could from
Department of Environment and Heritage; and the staff at Tin
Can Bay Works Depot for their assistance.
Department of Defence provided access to all
environmental assessments undertaken in the Wide Bay area
relevant to this study.
Last, but not least, I would like to thank my partner
Robert who ably assisted in fieldwork and kept our household
in order.
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STATEMENT OF ORIGINALITY
To the best of my knowledge and belief the research
reported here is original work for which I am entirely
responsible. Formal acknowledgment is made in the text to
previously published work or to professional discussions.
The material of this thesis has not been submitted
either in whole, or in part, for a degree at this or any
other University.
Helen Elizabeth Hillier.
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1.00 INTRODUCTION
In 1992, John Wilson and Partners (JWP) were
commissioned to prepare a planning report for the Tin Can
Bay Sewerage Scheme. Planning reports for augmentation of
both Tin Can Bay and Rainbow Beach Water Pollution Control
Works (WPCW) were prepared by the Department of Local
Government, (DLG) (1983), recommending augmentation of the
Tin Can Bay WPCW to 5000 equivalent population (EP). The
Tin Can Bay WPCW is overloaded and in need of augmentation
(JWP,1993).
A report was prepared by Fisheries Research Consultants
that considered the level of discharge-related impacts from
Tin Can Bay on adjacent and downstream floral communities.
The consultants concluded that "these floral communities
currently show no sign of detrimental impacts relating to
the effluent discharges or overflows". However it was
indicated that continued discharge of effluent into the
wetlands adjacent to the WPCW could have "significant direct
and indirect effects on adjoining floral communities, as a
result of nutrient enrichment"(JWP,1993).
Fisheries Research Consultants (FRC) concluded that the
augmentation of Tin Can Bay and Rainbow Beach sewerage
treatment works should include an "environmentally
sustainable effluent disposal system". Dr. John Thorogood,
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in his report for FRC (1993) advised that a monitoring
program that would consider the effect of effluent on the
adjacent coastal floral community be conducted. This
program would attempt to ascertain the "carrying capacity"
of the Melaleuca forest and mangrove community, assess their
present condition, and determine whether continued discharge
of a tertiary-treated effluent into this area would be
sustainable. The significance of the mangrove and seagrass
community as a resource for fisheries has not been
overlooked by the local community. This significance is
highlighted by the establishment of Fisheries Reserves in a
number of areas close to the Tin Can Bay WPCW.
This thesis considers the impact of effluent from the
Tin Can Bay treatment plant on the coastal vegetation, and
recommends a restoration strategy for the Melaleuca forest
at present suffering significant degradation. It also
considers the feasibility of a constructed wetland at the
WPCW for tertiary treatment of effluent prior to its release
into the waters of Snapper Creek estuary.
The Great Sandy Region Management Plan (GSRMP) states
that "specific standards may need to be reassessed for
specific sites or circumstances....Sewage treatment plants
operate at Tin Can Bay, Cooloola Village and Rainbow Beach.
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The capacity of these plants to handle anticipated increase
in visitor numbers is doubtful...Systems will be upgraded to
tertiary treatment as soon as possible." (GSRMP,1993)
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1.1 SCOPE OF THE STUDY
This study was undertaken over a four month period
coinciding with the University Spring Semester. Some data
related to water quality was obtained from consultants for
periods prior to this study, however most of the data
collection was limited to the period July-October,1995.
For this reason the collection program was limited to
analysis of foliage, soil and water samples. Airphoto
interpretation provided more information about vegetation
changes over the last forty years.
The point-source discharges from the effluent outflow
pipes located along the length of Pond 9 (see Figure 1)
create a darkened area visible from the air which correlates
with vegetation changes visible on the ground. (See Airphoto
1).
The effluent outflow can be clearly seen covering the
Melaleuca swamp, flowing through the mangroves and towards
the inlet point on Snapper Creek marked 'O' on Figure 1.
Transects were set up through the area covered by the
effluent plume.
Data required for the preliminary design phase of a
wastewater treatment wetland was collected concurrently.
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1.11 Related Surveys
A number of studies have been conducted by Fisheries
Research Consultants in the Great Sandy Region examining the
extent of seagrass beds. A review of this survey is
included.
The Department of Defence also commissioned a study of
vertebrate fauna in the Wide Bay area, and that report
underlines the importance of managing the wetlands and
wallum heath areas sustainably in order to protect
significant species found in this area. The findings of the
fauna survey are summarised in Appendix 1.
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1.12 Description of the Site
The floral community which is the focus of discharge-
related impacts described in this report is located on
Vacant Crown Land east of the Tin Can Bay Sanitary Reserve
(R1102,Por 58, Mch 3705).
The treatment plant itself is located on an elevated
earth platform, built in the early 1970's. The plant is
located to the south-west of Tin Can Bay township and
borders on Snapper Creek, a major tributary of the Tin Can
Bay estuary, which marks the southern entrance to the Great
Sandy Strait.
Open Melaleuca forest and wallum heathland adjoin the
plant to the north-west. To the south-east, a Melaleuca
forest with a dense understorey or reeds (in particular
Typha orientalis) receives all of the treated effluent from
the disinfection ponds of the WPCW platform. A narrow but
dense zone of Casuarina glauca, Casuarina equisetifolia and
Melaleuca quinquenervia on a low beach ridge separates the
freshwater wetland from a band of Juncus species, Typha,
Cyperus spp.species with emergent Avicennia marina and
Aegicerus corniculatum forming a low closed forest on the
normally sparsely vegetated mudflat.
The marsh has developed below the level of Mean High
Water Neap (MHWN). This grades into the mangrove community
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dominated by tall closed forest of Avicennia marina. The
seaward edge of this community is low closed forest of
Rhizophora stylosa. (See Figure 2)
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1.2 LEGISLATIVE ISSUES RELEVANT TO THE ENVIRONMENTAL
PROTECTION ACT (1994)
The Environmental Protection Act (1994)(EPA) was
proclaimed on March 1st, 1995. The object of this Act is:
"to protect Queensland's environment while allowing for
development that improves the total quality of life, both
now and in the future, in a way that maintains the
ecological processes on which life depends ("ecologically
sustainable development").
"The protection of Queensland's environment is to be
achieved by an integrated management program that is
consistent with ecologically sustainable development".
A sewerage treatment works of the size which exists at
Tin Can Bay is classed as a "Schedule 1, Level One,
environmentally relevant activity" in the Environmental
Protection Regulations,1995, and therefore the Local
Authority requires a licence to carry out that activity.
Wilful contravention of licence provisions could attract
a maximum penalty of 1665 penalty units or two years
imprisonment.
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The implementation of an environmental management
program (EMP) would allow the Council to improve waste water
treatment processes over time until discharge quality
complies with new licensed limits. However, EMP's for
periods in excess of three years require public notice.
Failure to comply with an EMP, once approved, constitutes an
offence (Freehill, Hollingdale and Page,1994).
Under the Environmental Protection (Water) Policy(EPP)
(1995), natural wetlands may not be used for the building of
artificial wetlands. However, the chief executive may give
approval for the construction of artificial wetlands for
wastewater treatment in degraded wetlands if the existing
ecological qualities of the degraded wetlands are not
significant (EPP,1995).
The administering authority would consider the following
aspects addressed in an environmental management plan in
making its decision:
(a) existing water quality and water quality
objectives;
(b) topography
(c) technology, management and nature of processes
being, or to be, used in carrying out the
activity;
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(d) water buffers;
(e) type of vegetation within water buffers; and
(f) sensitivity of aquatic ecosystems (EPWP,1995).
Each of these aspects will be addressed in this report.
From the information gathered for this report, it would
appear that the Tin Can Bay WPCW is a suitable site of the
development of constructed wetlands for the tertiary
treatment of sewerage.
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1.3 The Tin Can Bay Coastal Floral Community
1.31 Soils and Geology
The present ecosystem is a phase of a dynamic system in
which tectonic geological history has played an important
part in determining the present environment (Ridley,1957).
The evolution of the Maryborough Basin, the tectonic unit
containing the Tin Can Bay area, produced the basement
complex for the ecosystem of the coastal lowlands. The
vegetation forms part of the northward extension of the
Pleistocene and Holocene dune landscapes of the Cooloola-
Noosa region described by Coaldrake (1961,1962), Blake
(1938,1947), and Ridley (1957). Other factors which
determine vegetation patterns include topography and
climate. Coaldrake (1961) mapped out "suites" within the
coastal lowlands of South-East Queensland, based on these
evolving patterns of vegetation, soils, topography, geology
(parent material) and climate. The site studied here lies
in the Tin Can Bay suite.
The Tin Can Bay treatment plant site is located on a
coastal plain characterised by large areas of stabilised
sandy soils with a high clay content, often with impeded
drainage. The clay substrate, visible in the soil profile
at the municipal dump adjacent to the treatment plant, has
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been significant in the development of the Melaleuca
'swamp'.
The sandy soils support a complex vegetation mosaic of
open forest with the dominant trees being species of
Banksia, Eucalyptus, Melaleuca and Casuarina, with a grassy
or low shrubby understorey, and 'wallum' or 'ericoid' heaths
(Gillison,1985). These communities support a wide variety of
vegetation adapted to the nutrient poor conditions, and a
number of fauna species whose habitats and range have become
threatened by coastal developments
(FRC,1993;Driscoll,1991;WBM Oceanics,1995).
Heathland soils are typically acid, podzolised, and
deficient in available phosphorous and nitrogen
(Groves,1979). The vegetation has evolved in nutrient poor
conditions. The impact of increased nutrients on this type
of vegetation complex may be viewed at the Cooloola village
outflow. At that site, overland flow is used for tertiary
treatment of sewage. The ground is permanently inundated
and numerous heath plants have died, indicating the
unsuitability of the wallum heath for overland flow.
Occasional palms such as Livistonia australis and
Pandanus pedunculatus which are thriving despite recent
controlled burns in the area, indicate regular rainfall and
subtropical climate. The annual rainfall is 1532mm recorded
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at Double Island Point, with rain falling on 125 days per
year. The average temperature, recorded at Inskip Point,
ranges from a daily minimum of 18.5 to 23.9 degrees Celcius
(Australian Bureau of Meteorology,1995).
Casuarina glauca, Casuarina equisitefolia and Melaleuca
quinquenervia dominate the fringing littoral vegetation.
Occasional freshwater tolerant mangroves have been noted.
Freshwater reeds and sedges have colonised the area which
once supported only claypan and mudflat vegetation: Juncus
spp.,Typha spp. and Cyperus spp. dominate. Lantana camara
and groundsel (Baccharis halimifolia), epiphytic vines and
other local weeds were also noted in this area. Beyond the
effluent plume, typical claypan species such as Sporobolis
virginicus, Scirpus spp., Suaeda australis, Salicornia
quinqueflora exist, shaded by Avicennia marina shrubs and
seedlings.
The mangrove species, in a range of floristic and
structural categories, dominate the intertidal zone. As
described in the report by Fisheries Research Consultants
(1993), a number of mangrove species can be found including
Ceriops tagal ((Perr)C.B. Robinson,1908), Lumnitzera
racemosa (Willd,1803), Excoecaria agallocha (L.1759),
Avicennia marina ((Forsk.)Vierh.1907) and Rhizophora stylosa
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(Griff.1854). The seaward fringe is dominated by Rhizophora
stylosa.
Seagrass beds, predominantly Zostera capricorni, are
found on the opposite banks of Snapper Creek, and extensive
beds are found in the creeks lower reaches, however no
significant communities have been recorded in the area
adjacent to the treatment outflow for a number of years,
according to surveys conducted by Fisheries Research
Consultants (1992,1993).
The mangroves and saltmarsh adjacent to the Tin Can Bay
WPCW represent less than 1% of the 15,500 hectares of
mangrove and 2,800 hectares of saltmarsh in the Great Sandy
Region Management area (FRC,1993). There are many areas in
close proximity to the Tin Can Bay WPCW, such as Kauri
Creek, which have been declared Fisheries Habitat Reserves.
Failure to address downstream impacts of potential export of
significant levels of nutrients and pollutants offsite from
the WPCW could result in devaluation of these reserves.
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2.00 Wetlands
2.1 Introduction to the area
Snapper Creek meets the Tin Can Bay Inlet, which then
flows into the Great Sandy Strait. The small peninsula of
land which receives the effluent discharged from the
treatment plant lies in this estuary and the vegetation
within the plume has undergone significant changes as a
result of twenty years of inundation with nutrient-rich
freshwater.
Oral histories, airphoto interpretation, ground truthing
and records of vegetation
(Coaldrake,1961;Gillison,1985;Driscoll,1991) indicate that
the natural vegetation of this peninsula consisted of wallum
heath fringed by open forest of Melaleuca and Casuarina
trees with a grassey understorey. The trees marked the
boundary between terrestrial vegetation and fringing
littoral vegetation on the mudflat. The situation at
present is markedly different.
Airphotos are available from 1958, and these records
show that beyond the influence of the effluent plume, no
changes have occurred in the vegetation structure, biomass,
life form or species density.
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Adjacent to the point of discharge is a bowl-shaped
depression of approximately 1.5 hectares in size which
appears to have been exagerrated by earthworks involved in
the construction of the treatment plant. A combination of
factors, specifically topography and the clay substrate,
have resulted in the sheet flow of nutrient rich effluent
waterlogging the soils in this depression. Melaleuca
quinquenervia have opportunistically colonised this
environment, a situation mirrored at the constructed
wetlands at West Byron Sewage Treatmen Plant
(Andell,pers.comm.). A dense understorey of Typha orientalis
and other freshwater sedges and reeds has developed, and
various local weeds have colonised spaces in the canopy.
Airphoto interpretation and oral histories from plant
workers indicate that in the first few years of the WPCW's
operation, until approximately 1981, the Melaleuca forest
grew quickly and developed a tall dense canopy. Airphotos
from 1989 onwards indicate that die-back of trees occurred
adjacent to the source of the discharge, and the canopy
opened, allowing weeds to invade. An understorey of reeds
and sedges developed in the permanently waterlogged
conditions. At this point, just above the Mean High Water
Spring tide level, (and confirmed by field and laboratory
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testing of conductivity and nutrients) the nutrient-laden
freshwater provides optimum conditions for the growth of
macrophytes.
This "wetland" has developed as a result of artificial
anthropogenic changes rather than natural evolution. If the
effluent were no longer directed onto this 'swamp', in a
number of years the area would no doubt return to its
original state as a wallum heathland. The capacity of the
area in question to continue to cope with effluent with
nutrient concentrations at the present level appears
doubtful. This is confirmed by analysis discussed later. An
effluent with a lower nutrient concentration would mitigate
the significant negative impacts that wastewater has had on
the site to date. This change would have to be accompanied
by ongoing monitoring. In view of the degradation of the
site which has already occurred and the ready availability
of other more suitable sites adjacent to the existing plant,
it would be prudent to cease using this area as a receiving
site as soon as a new treatment works becomes operative.
2.2 Wetlands - a definition
A consideration of definitions of "wetlands"
demonstrates to the reader that there are many types of
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ecosystems, both natural and constructed, with varying
hydroperiods, which can be classed as "wetlands". For the
purpose of this study, the broadest and most widely used of
the internationally accepted definitions of wetlands will be
used. The Ramsar Convention on Wetlands of International
Importance, Especially as Waterfowl Habitat, was designed to
provide international protection of the widest possible
group of wetland ecosystems, and defines wetlands as "areas
of marsh, fen, peatland or water, whether natural or
artificial, permanent or temporary, with water that is
static or flowing, fresh, brackish or salt, including areas
of marine water the depth of which at low tide does not
exceed six metres"(Rees et al, 1993).
The hydrologic regime - permanent inundation - combined
with eutrophic conditions, would suggest that the Melaleuca
forest is acting as a nutrient source, exporting sediment,
organic matter and nutrient from the wetland area towards
the mangrove forest and the creek.
The nutrient-laden freshwater flow has resulted in
macrophytes growing on the coastal flat between the
terrestrial vegetation and the intertidal zone inhabited by
mangroves, a zone normally devoid of vegetation. Airphoto
interpretation indicates that this patch of vegetation has
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developed over the last three to five years, suggesting a
significant increase in the export of nutrients and
freshwater from the Melaleuca swamp. The change from
'nutrient sink' to 'nutrient source' indicates that changes
have occurred in the physical, chemical and biological
nutrient cycling process within the swamp.
2.3 Wetland nutrient cycling and productivity
Wetlands have been shown by many researchers to be
excellent nutrient sinks. Wetlands retain nutrients,
specifically nitrogen and phosphorous, in the substrate and
vegetation (Rees et al,1993). Heavy metals such as iron and
manganese are held in the soils, and wetlands have been
mined for bog iron in both the United States and Europe
(Hammer,1991).
Through adsorption and assimilation, the plants remove
nutrients for biomass production through filtration,
sedimentation, chemical reactions and biological
decomposition, with oxygen as a by-product. This natural
form of water pollution treatment results in increased
oxygen being made available for further aerobic bacterial
decomposition of nutrients and creates an environment which
can support a variety of aerobic aquatic organisms
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(Hammer,1991). Analysis of sediments, water and foliage from
the study site indicate that the capacity of the wetland to
act as a nutrient sink has been compromised, resulting in
export of nutrients across the intertidal zone.
Wetland nutrient cycling has important implications for
water quality, fish and prawn production, algal growth,
seagrass pasture growth and any downstream recreational
activities such as swimming. The benefits which occur in
coastal waters which are protected by wetlands are
eliminated if water quality is compromised by
eutrophication. Coastal fisheries rely upon wetlands as
breeding and nursery areas. The Kauri Creek Fisheries
Habitat Reserve, adjacent to the Tin Can Bay Inlet, is one
of a number of such reserves in the region managed under the
Great Sandy Region Management Plan (1993).
The ecological productivity of coastal ecosystems relies
upon detrital metabolic pathways that transform organic and
inorganic substances supporting other biogeochemical cycles
at the base of the food chain (Hammer,1991,p.71). More
advanced life forms such as sponges and jellyfish, orchids
and fungi, algae and insects, provide food for the larger
vertebrates and invertebrates higher in the food chain which
are of higher economic value.
37
37
Although ducks and wading birds often typify community
perception of wetlands, many birds, mammals and reptiles are
supported by wetland habitats.
Egg masses and tadpoles as well as adult frogs and toads
are important food for fish, snakes, birds, mammals and
other amphibians. Reptiles such as snakes lay their eggs in
terrestrial environments even though some may use wetlands
for food, cover and water. Many birds and a few mammals
rely upon emergent macrophytes and floating plants such as
duckweed (Lemna) as important food sources. Emergent
macrophytes also provide habitat and shelter for birds and
mammals found in wetlands. Wetland trees and shrubs provide
nesting sites and perches for birds. Fish and frogs also
take advantage of shady and covered areas in both shallow
and deep water (Hammer,1991; Rees et al,1993).
These are just some examples of the natural resource
value of wetlands and their importance as part of the
nutrient cycling process and as an energy source. Australia
will no doubt soon begin to recognise the potential of
wetlands to supply timber, honey, 'ti-tree' oil, nuts and
other harvestable products, following the example of
countries overseas.
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38
Wetlands supporting mangrove forests are important along
the eastern Australian coastline. In Moreton Bay alone it
has been estimated that the commercial and recreational
fisheries based on mangrove-dependant species are worth
about 280 million Australian dollars annually (Rees et
al,1993). The establishment of marine parks and habitat
reserves has helped to stem the tide of coastal development
which is the main cause of wetland destruction in Australia.
Wetlands are 'transitional' areas, strongly influenced
by external factors such as rainfall, solar radiation,
energy and nutrient inputs and surface and groundwater
flows, the percentage of wildlife to biomass is very high
(Hammer,1991). Wetlands are incredibly productive
environments, and community perception of their value is
changing rapidly in Australia.
At the study site, the WPCW is adjoined by a mangrove
forest. The mangroves are subject to daily tidal
inundation, flushing the load of freshwater and nutrients
through the mangrove 'sink'. The effect of this inundation
was the focus of most of the analysis carried out for this
report. Determining the capacity of the mangrove forest to
act as a nutrient sink in this location, and therefore its
39
39
capacity to protect the coastal waters from eutrophication,
became the focus of the author's brief.
3.00 The Mangrove Forest
3.1 General description
Mangrove ecosystems are characterised by their high
primary production and rapid nutrient turnover rate. This
process is assisted by periodic inundation from tides, and
sediments which are rich in organic matter. Mangroves have
extensive root systems which appear to be effective in
trapping nutrients, such as those from wastewater. Nutrient
enrichment may even have a beneficial effect in areas where
soil nutrient status is low (Boto,1992; Boto and
Wellington,1983;Clough et al,1983).
Mangroves have adapted to stressed environments
characterised by fluctuating salinities, fluctuating
anaerobic and aerobic substrates, fluctuating nutrient
levels, high winds and wave action (Wong et al,1995). Their
potential as a low-cost, low-maintenance method for
decreasing nutrient and heavy metal loads in sewage effluent
has been exploited for some time, although there is a dearth
of research into the effects of this form of wastewater
treatment on mangrove ecosystems (Clough et al,1983).
40
40
Clough (1983) wrote: "the common practice of discharging
treated sewage effluent directly into estuaries or tidal
channels fringed by mangroves would appear to be a
relatively ineffective scheme for improving the quality of
sewage effluent." He suggests that anaerobic mangrove soils
"seem likely to have the capacity to trap heavy metals and
pesticides without harm to mangroves themselves, although
these materials could have an adverse effect on sediment
fauna. The high organic carbon content of sewage effluent
may lead to further reduction of soil redox potential. This
may place additional stress on mangroves and their
fauna...In the long term this could be a major drawback to
the use of mangrove systems as sites for the discharge of
sewage effluent." This article was written in 1983, long
before the introduction of the Environmental Protection Act
(1994) which will ensure that any effluent discharged to
natural wetlands in future has been tertiary treated to
reduce nutrient and heavy metal loads.
The study site at Tin Can Bay has the advantage of being
relatively free of influences such as urban stormwater
runoff and agricultural pesticides. The treated effluent is
discharged at the terrestrial interface of the wetland
ecosystem, improving the chances of nutrient and heavy metal
41
41
immobilisation prior to mixing with seawater through tidal
inundation.
Historically, mangroves have been regarded as dumpsites
for disposal of sewage and wastewater, since many urban
centres are adjacent to rivers and coastlines, and wetlands
have been viewed as wastelands, particularly in Australia
where their productive and economic potential is only just
being realised (Clough et al,1983; Nedwell,1974).
If wetlands are perceived as potential wastewater
receiving sites three major management problems must be
considered:
(1) the wetland's ability to cope with the long term
influence of pollutants
(2) the downstream effects of increased nutrient
loadings in tidal waters and
(3) the effect of floods and cyclones on the efficiency
of the system to reduce pollutants if vegetation is severely
storm-damaged.
A thorough investigation of the effects of twenty-odd
years of effluent discharge to the Tin Can Bay wetlands
would take some years to complete. The following analyses of
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42
foliage, sediments and waters and changes in the wetland
vegetation complex are an abbreviated assessment of the
impact of secondary-treated effluent. The unique nature of
the site provides an excellent opportunity to assess the
long term impacts of effluent on the mangrove community.
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43
3.2 Mangrove nurient chemistry at the Tin Can Bay WPCW
The nutrient chemistry of the mangrove ecosystem is
determined by a suite of competing processes. Site specific
physical, chemical and biogeochemical processes determine
the chemistry of each constituent in waters, soils and
foliage. The particular values of interest in wetlands are
pH, substrate, porosity, cation exchange capacity, redox,
salinity, organic matter content, oxides and hydroxides of
iron, manganese and aluminium, carbonates. phosphates and
sulphides, microbial population sizes and activities
(DeBustamante,1990).
Odum (1984) suggested that ecosystem complexity
increases with increases in flows of water and nutrients.
The situation in the Tin Can estuary is representative of
this complex interaction of variables such as hydrology,
edaphic conditions, climate, nutrient cycling and
anthropogenic influences.
The freshwater influence affects salinity of water and
soil sediments and nutrient transport and consequently plant
and animal distributions and activity(Joshi et al,1975).
There is substantial evidence that natural wetlands can
still perform as natural habitats whilst enabling
44
44
improvements in water quality despite heavy nutrient loads
(Hammer,1989;Kusler et al,1990;Breen et al,1995;Greenway &
Bolton,1995). Most research has investigated the water
pollution control performance of emergent macrophytes rather
than forested wetlands. Less data is available for mangrove
forests however their ability to immobilise nutrients and
heavy metals appears to be excellent (Clough et al,1983;
Boto and Wellington,1984;Wonget al,1995). This ability is
influenced by tidal and drainage characteristics, and by the
redox state of the soil (Clough, Boto and Attiwill, 1983).
There must be limitations to the ability of a natural
wetland to accomplish water quality improvement. Hammer
(1991) warns that other important functional values of
wetlands are likely to be severely depressed before water
purification is significantly impaired.
The Tin Can Bay site may give some indication of the
limitations to load that a natural wetland can tolerate. The
situation at present with dieback of Melaleuca quinquenervia
occurring at the receiving site, and permanently inundated
areas colonised by freshwater macrophytes and local weeds,
indicates that some parts of the ecosystem are under stress.
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45
3.3 Methodology
A transect was set up through the mangrove forest in
order to screen for differences in sediment, water and
foliar nutrients which may occur as a result of the effluent
sheet flow. Airphotos gave a clear indication of the area
influenced by the effluent plume from its source to the
point at which it flowed into Snapper Creek. On the ground,
it appeared that the effluent influenced both the structure
of the forest, natural species diversity and the height of
the canopy. In the case of the Avicennia marina, for
example, at the terrestrial interface of the forest the
trees were approximately three to four metres taller than
adjacent trees which appeared to be beyond the influence of
the effluent plume. A compass was used to continue the
transect through the Rhizophora forest where the access was
made difficult by stilt roots. A boat facilitated sample
collection at the mouth of the creek.
Vorosmarty and Loder (1994) found that nutrient to
nutrient ratios varied from neap to spring tides as a result
of variations in inundation and suggested designing sampling
programs accordingly. For this reason, all samples were
collected as close as possible to Mean High Water Neap tides
at low tide.
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46
Samples of leaves were collected from the top of the
canopy of randomly chosen trees which appeared
representative of each structural or height change through
the transect. Leaves were sorted and washed with
demineralised water and the young leaves (the first leaf
pair) were dried at 80 degrees Celcius. The dried leaves
were ground to a powder before analysis.
Soil and water samples were taken from representative
sites with five replicates being taken in a 25 square metre
area. A modified 'Coile' sampler was driven into the mud and
a 'core' of 150 millimetres depth withdrawn. These samples
were analysed for trace elements, nutrients, pH and
conductivity, and loss on ignition (Davie,1983). Samples up
to one metre depth were taken for particle size analysis.
The following table details methods of analysis.
Parameter Methodology and Reference.
Sediments
Moisture Content Gravimetric determination @ 105C.R & H*
pH 1:5 soil/water extract.Conductivity
electrode R & H
Conductivity 1:5 soil/water extract.Conductivity
electrode R & H
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47
Total nitrogen Modified Kjeldahl digest and
distillation . Titrimetric .
determination. R & H.
Ammonium nitrogen Distilllation and titrimetric
determiination. R & H
Nitrate-nitrogen 1:5 soil/water extract. Colorimetric
determination
UV-Vis spectrophotometer
Total phosphorous Modified Kjeldahl digest. Colorimetric
determination -
ascorbic acid method (UV-Vis
spectrophotometer)
Sulphate 1:5/water extraction. Determinatio by
turbidimetric
method (APHA 4500-SO E)
Chloride 1:5 soil/water extract and titrimetric
determination.R &H
Trace elements,
potassium, magnesium
1:5 soil/water extract. Determination by
ICP-MS
(APHA 3120B)
Waters
pH Electrometric method. APHA 4500-H
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48
Conductivity Meter and conductivity electrode APHA
2510B
Total nitrogen Modified Kjeldahl digest and
diistillation. Titrimetric
determination. APHA 4500
Ammonium-nitrogen Distillation and titrimetric
determination. APHA 4500-
NH3
Nitrate-nitrogen Chromotropic acid method.APHA 418D
(1985)
Total phosphorous Modified Kjeldahl digest. Colorimetric
determination
ascorbic acid method (UV-Vis
spectrophotometer)
Table 1: Methods of Analysis
*Rayment,G.E. and Higginson,F.R. (1992) Australian
Laboratory Handbook of Soil and Water Chemical Methods.
Inkata Press, Melbourne.
Analysis was carried out by Envirotest Analytical
Services.
For foliage, total nitrogen was determined by a modified
Kjeldahl digest and titrimetric determination. Total
phosphorous was also determined by modified Kjeldahl digest
49
49
followed by colorimetric determination - ascorbic acid
method (UV-Vis spectrophotometer).
Sulphate, chloride, cations and trace elements were
analysed according to the methods specified in "Standard
Methods for the Examination of Water and Wastes" manual
published by the American Public Health Association
(APHA,1992). Nitric acid digest and Turbidimetric method was
used for the determination of sulphate (APHA 4500-SO4E).
Nitric acid digest and Argentometric method was used to
determine chloride levels (APHA 4500-CL B). Cations and
trace elements were determined using a nitric acid digest
and determination by ICP-MS (APHA 3120B).
50
50
3.4 Results and discussion
Mangroves demonstrate an ability to survive in
conditions of both nutrient enrichment and nutrient
limitations. No information has been collected on the
'optimum' requirements for growth of either Avicennia marina
or Melaleuca quinquenervia, although both species have been
found growing vigorously despite edaphic constraints and
varying hydrologic regimes.(Davie,1983;Greenway,1995). In
order to interpret the results they were compared to data
obtained by Henley (1978), Davie (1983), Clough, Boto and
Attiwill (1983), Boto and Wellington (1984), Davie (1992)
and Wong et al (1995). These researchers all presented data
collected from mangroves influenced by discharge of sewage
effluent, although the results presented by Wong et al(1995)
were the only results from a mangrove forest receiving
effluent from the landward rather than the seaward edge of
the forest.
Davie (1983) noted a trend towards increased levels of
total nitrogen, total phosphorous, sodium and chloride with
increased levels of water stress. The exception to this was
Luggage Point where the plants are subjected to nutrient-
enriched sewage effluent from the adjacent treatment plant
51
51
on Moreton Bay, Queensland. At this site, the values for all
elements were higher than those obtained elsewhere.
Wong et al(1995) found that the sewage discharge did not
cause any change in leaf nutrient content in either Kandelia
or Aegiceras species sampled. Clough et al (1983) suggested
that sewage discharge should produce a beneficial effect on
plant growth and productivity of the mangrove ecosystem due
to increased nutrient supply. With increased growth,
enrichment would also lead to elevated tissue nutrient
content. This was the case for Rhizophora species observed
by Boto and Wellington (1983), who noted significant
increases in foliar nitrogen and phosphorous for new leaves
at a site subject to ammonium and phosphate enrichment for
twelve months.
Henley(1978) found no alterations in age structure, tree
dimensions, leaf sizes, or community vigour in enriched
areas. However enriched sites appeared to have higher
foliage nutrient levels, reflecting the higher levels of
available nutrients in the environment.
Cooke et al(1990) suggested that nutrient additions from
wastewater should lead to "greater nutrient availability
...greater cycling, decomposition, and ultimately, to a
change in community composition to species more tolerant to
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52
nutrient loads." This appears to be the case at the Tin Can
Bay site.
Foliar nutrient levels of total nitrogen and total
phosphorous for Melaleuca quinquenerviaand Avicennia marina
trees were sampled through the transect set up within the
area covered by the effluent plume. Figures 2 shows the
transect through the forest. Figure 3 is a cross-section
diagram of the transect, showing the variation in elevation
with position and tide data for the area.
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53
Figure 2: Transect through the forest
Figure 3: Cross-section of the transect, showing the
variation in elevation with position and tide data for the
area
54
54
sedg
e
10m
100m
200m
foliagesoil0
20000
40000
Mangrove Foliage & Soils - Total N
foliage
soil
outf low
50
150
foliage
soil0
5000
10000
15000
20000
total nitrogen - melaleuca
foliage
soil
55
55
200m
210m
300m
400m
foliagesoil0
2000
4000
Mangrove Foliage & Soil - Total P
foliage
soil
outflow50m
150m
foliage
soil0
5000
Melaleuca Foliage & Soil Total P
foliage
soil
56
56
Concentrations of nitrogen and phosphorous are
signficantly higher than those recorded by Davie (1983) and
Henley (1978). As a comparison some of the data presented by
other researchers is listed with the concentrations found in
Avicennia marina foliage at the Tin Can Bay site in Table 1.
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57
Site Location Total N
(% dry
weight)
Total P
(% dry
weight)
Mud Island 1.8 .14
Luggage Point 2.4 .17
Kings Creek 1.5 .11
Sadroves
Creek(at outfall)
2.4 .25
Sadgroves
Creek (at outfall)
2.9 .22
Tin Can
Bay(landward)
2.6 .19
Tin Can Bay
(seaward)
1.6 .24
Table 2: Comparison of Foliar Concentrations in
Avicennia marinaof Total Nitrogen and Total Phosphorous (%
dry weight) .
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58
The important conclusion to be drawn from the data
collected at Tin Can Bay is that despite high nutrient
concentrations in the foliage, sediments and waters,
significant decreases in concentrations of nutrients in
waters were recorded during the 'low flow' period and 'peak
flow' period from the effluent outfall to the seaward fringe
of the forest. This would indicate that as a phase of the
treatment process the natural wetlands act as a nutrient
'sink'.
3.5 •Melaleuca P levels - Low Flow Period
Phosphorous levels did not drop dramatically in
Melaleuca foliage within the effluent plume - a 2% decrease
was noted. However the concentrations of total phosphorous
within the sediments of the Melaleuca forest decreased 92%.
Water concentrations dropped only 16%. These results
indicate that Melaleuca spp. accumulate significant excess
available P in their foliage. Some P is lost through sheet
flow from the system towards the mangrove forest. This
result concurrs with preliminary results from Bolton and
Greenway (1995) who have been examining the effect of
effluent applied to various species of Melaleuca.
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59
At this point in time, nutrients are exported through
sheet flow and tidal inundation from the Melaleuca forest,
through the intertidal zone of reeds and sedges and into the
mangrove forest. From airphoto interpretation, this export
of nutrients has increased over the last five years,
resulting in vegetation changes through the transect.
The band of emergent macrophytes which has colonised the
previously saline, bare intertidal mudflat since 1989 is
perhaps the most spectacular change which has occurred as a
result of the constant flow of effluent. This change has
coincided with a gradual die-back of Melaleuca trees
adjacent to the effluent outflow and throughout the 'swamp',
and an invasion of the forest understorey with weeds, Typha
spp. and other nutrient tolerant plants. It was reported
that at some stage the ponds received 'shock loads'. These
shock loads may have contributed to the forest die-back and
the reported death of native fauna in the area at that time.
Nitrogen levels in sediments decrease within the
Melaleuca forest from the outflow pipe towards the
Casuarina/Melaleuca fringe on the frontal dune. At this
point, however, the trees are tall with thick trunks, and
Foliage Projective Cover (FPC) is 100%, indicating that the
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60
energy supply from the nutrients has contributed to an
increase in biomass.
The substantial decrease in concentrations of total
nitrogen found in the foliage, waters and sediments of
Melaleuca indicate that this nutrient is being removed from
the system.
A small decrease in nutrient levels in sediments and
waters occurs in the macrophyte zone between the terrestrial
Melaleuca forest and the mangrove forest, although nutrient
levels remain significantly higher than natural sediments.
Of interest at this location in the transect are the results
of foliage analysis from emergent freshwater tolerant
mangroves growing in this macrophyte dominated zone. Total
nitrogen and phosphorous levels are higher than at any other
point on the transect, with nitrogen levels exceeding any
others recorded in Australia except the Sadgroves Creek
outfall near Darwin (Henley,1983). It appears that nutrients
are exported from the Melaleuca swamp, over the frontal dune
and into the intertidal zone, where the macrophytes and
emergent mangroves store nutrients in plant tissues and
sediments. This point on the transect marks the confluence
of two energy flows - the tidal energy and the sheet flow of
effluent. The marked increase in biomass, species density
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61
and diversity and canopy height is an indication of the
'energy' being cycled at this part of the ecosystem.(See
Section 'Energy Flows'). The following photographs show the
marked changes in canopy height which occur at this point
within the effluent plume.
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62
Photograph 1: The increase in canopy height is visible
from the water tower in the township of Tin Can Bay
Photograph 2: The vegetation changes within the effluent
plume can be viewed on the ground and on airphotos.
63
63
3.6 Melaleuca P Concentrations - Peak Period
Sediments and waters were analysed 20 days after the end
of the September holiday peak period. Assuming a detention
time in the lagoons of twenty days, sufficient time should
have elapsed for any significant nutrient enrichment in
sheet flow of effluent passing through the swamp to be
detected on analysis.
The following charts indicate the level of P in foliage,
sediments and waters within the Melaleuca wetland.
The significant increase in P concentration in
sediments, associated with a corresponding increase in total
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64
nitrogen concentrations suggests that effluent sheet flow,
rich in nutrients, is being stored and cycled within the
waterlogged soils of the frontal dune. Unfortunately, a
corresponding increase in nutrient concentrations occurs in
waters flowing through the Melaleuca forest, again
indicating that this climax community is unable to fixate in
the biomass all the energy available in the effluent, and
energy in the form of nutrient rich freshwater passes over
the frontal dune towards the mangrove forest.
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65
3.7 Melaleucanitrogen results - low flow period
The following table shows results of sampling of
foliage, sediments and waters within the Melaleucaforest.
Sample At outfall 50m 100m
Sediments 16100 1290 940
Foliage 12060 950 3010
Waters 83 6.53 6.13
Table 3: Concentrations of Total Nitrogen in Melaleuca
forest samples (mg/Kg)
Amm-Nit 0m 30 50 100 120
Water 2.9 4.6 5.1 11.5 2.2
Soil 100 97 30
Nit-Nit
Water 0.8 0.4 0.7 3.2 0.5
Soil 6.4 1.9 ,<0.1
Table 4: Comparison of concentrations of ammonium
nitrogen and nitrate nitrogen (mg/Kg) in Melaleuca forest.
As described earlier, a significant percentage of total
nitrogen is leaving the system.
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66
3.8 Melaleuca Nitrogen results - Peak period
Nutrients 0m 50m 100m
Total nitrogen 40 950 2240
Ammonium-
nitrogen
40 6 80
Nitrate-
nitrogen
0.9 4.8 3.0
Total
Phosphorous
160 1170 3400
Table 5: Sediment nutrient concentrations of nitrogen &
phosphorous in post-holiday period (mg/kg dry weight)
Nutrient 0m 50m 100m
Total nitrogen 6.7 100 180
Ammonium
nitrogen
3.7 2.2 1.7
Nitrate
nitrogen
0.2 0.8 0.2
Total
phosphorous
39 60 56
67
67
Table 6: Concentration of nutrients in waters of
Melaleuca swamp,post holiday period (mg/L).
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68
4.0 Mangrove analysis
4.1 Low flow results
The closed canopy and significant height increase
exhibited by the Avicennia marina forest at the terrestrial
interface within the effluent plume, along with very high
foliage nutrient concentrations, would suggest that a high
percentage of nutrients are taken up in the mangrove
tissues. This observation was proven on analysis. The
foliage and sediment nutrient concentrations then
significantly decrease from the landward to seaward edge of
the forest, as would be expected in an area subjected to
regular tidal inundation.
Total nitrogen in Melaleuca foliage decreases 75%, and
in Avicennia marina foliage there is a decrease of 57%. The
macrophyte understorey in the Melaleuca forest and the
edaphic conditions would contribute to the process of
nitrification and denitrification, removing nitrogen from
the system.
Sediment concentrations of total nitrogen decrease 94%
in the Melaleuca forest, however an increase was noted in
the Avicennia soil of 41.5%. Concentration of total nitrogen
in waters show a corresponding decrease of 96.8%. This
finding highlights the noted ability of mangrove sediments
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69
to immobilise nutrients, in particular phosphorous, whilst
nitrogen is lost from this ecosystem again through the
process of nitrification and denitrification. Oxygen is
replenished with tidal movement(Davie,1983; Clough, Boto &
Attiwill, 1983).
In the Avicennia forest, foliar phosphorous
concentrations decreased 36% from the landward edge to the
seaward edge where the waters flow into a tidal channel.
Soil concentrations decreased 46% and phosphorous
concentration in water decreased 86%. Overall water samples
collected from the effluent outflow pipe to the mouth of the
creek showed a decrease in total phosphorous of 82.5% and a
decrease in total nitrogen of 98.3%. Despite these
significant decreases certain areas showed elevations within
the tidal zone from MHWN to sea level.
This could be the result of pooling and poor drainage in
certain areas - interesting metabolic adaptations to
anaerobiosis and inundation which occur in mangrove forests
as noted by Boto and Wellington (1983).
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4.2 Water results and discussion
Overall a 98.3% reduction occurred in total nitrogen in
waters sampled from the effluent outfall to the receiving
waters at the seaward edge of the mangrove forest.
An 82.5% reduction occurred in total phosphorous
concentrations in waters from outfall to receiving waters.
Waters sampled showed dramatic reductions in nutrient
levels which in most cases corresponded with changes in
foliage and soil nutrient concentrations indicating that
each part of the forest was generally 'at equilibrium' with
the level of nutrients available to the plant. Odum (1984)
has indicated that at least twenty years of effluent
inundation is required before it is possible to assess the
full impact of nutrient enrichment and freshwater inundation
on a mangrove community. As this treatment plant has been
operational for about twenty years it should be possible to
draw definite conclusions from results obtained at this
point in time about the impact and sustainability of
discharging effluent into the wetland. All samples were
collected during neap tides. The first sampling episode was
71
71
during a normal flow period and the second was after a peak
holiday period.
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72
4.3 Mangrove nutrient concentration in sediments and
waters post holiday period
130m 200 250 300 400
TotalN 360 20 65 290 1170
Amm-N 16 5 20 8 49
Nit-N 2.4 5 0.9 12.1 0.3
Total P 1500 1350 97 75 200
Table 7: Mangrove sediment nutrient concentrations
(mg/Kg dry weight) post holiday period.
130 200 250 300 400
Total N 145 40 6.7 8.1 <0.4
Amm-N 0.9 0.2 <0.2 <0.2 <0.2
Nit-N 0.8 0.9 1.2 0.5 <0.1
Total P 37 52 23 2.1 <1.9
Table 8: Mangrove waters nutrient concentrations (mg/L)
post holiday period
Both nitrogen and phosphorous concentrations are very
high within the Melaleuca swamp. Concentrations in
interstitial waters show a steady decline as the effluent
flow passes through the mangrove forest. Nutrients held in
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73
sediments are high compared to those for natural sediments,
yet not as high as those in sediments sampled at the
Sadgroves Creek outfall mentioned earlier(Henley,1978).
Due to the clay content of sediments, losses due to
leaching are possibly reduced, underlining the role of
mangroves as nutrient and carbon traps protecting seagrasses
and coral reefs from degradation by uncontrolled nutrient
inputs (Morrell and Corredor,1993).
Changes in nutrient levels in a mangrove forest occur
more rapidly in interstitial waters than sediments
(Henley,1978). Total P levels in sediments and waters
decrease with increasing distance from the outfall.
Phosphorous is held in the mangrove sediments in a readily
available form for uptake by the mangroves rather than being
lost into tidal waters (Henley,1978).
Studies by Morell and Corredor (1993) and Henley (1978)
indicate that mangrove sediments are a sink, not a source,
for water column nitrates. The loss of nitrogen through
denitrification appears to be high in sediments exposed to
sewage effluent, although a peak in levels occurs in
sediments occurring at the outfall to Snapper Creek in this
situation. A corresponding low level in interstitial waters
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74
indicates that the nitrogen remains available to mangrove
production, and is not lost into tidal waters. (Clough et
al,1983).
Morell and Corredor (1992) document an inhibition of the
process of mineralisation of particulate organic matter and
nitrification, precluding mobilisation of nitrogen
accumulated in sediments. They concluded that:"Possible
environmental constraints for these processes include low
oxygen saturation levels in the water column due to high
temperature and the bacteriostatic effects of tannins
leaching from decomposing Rhizophora mangle." The low pH,
low oxygen penetration and high temperature environment
described by these authors is also found at the Tin Can Bay
site, although a high percentage of nitrogen is removed from
the system. Tidal flow must contribute to the oxygenation of
sediments allowing for nitrification to occur. The first
'wetland treatment phase' through the Melaleuca forest also
removes a significant percentage of nitrogen.
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4.4 Ion uptake and salinity
Data collected by Joshi et al(1975) indicates the
importance of the element potassium in the physiology of
mangroves. They also demonstrated an insufficiency of
calcium and magnesium in metabolic tissues under freshwater
conditions, a decrease in sodium and chloride uptake and
increase in calcium uptake. Walsh (1974) summarised the
views of other researchers who accept that mangroves appear
to grow well when salt is present in the soil water. Joshi
et al(1975) found that mangroves grew well under freshwater
conditions, however they were subject to the invasion of
fresh water marsh plants.
Overall, the mangroves adapt to fluctuating conditions
and find their own ecosystem equilibrium with the evolution
of a number of characteristics such as salt-exclusion, salt-
excretion, deposition of ions in senescent leaves, and ion
regulation by mangrove seedlings (Joshi et
al,1975;Davie,1983).
Figure 5 represents results of analysis of ion
concentrations in Avicennia marina foliage. Although this
represents the ions as comprising 100% of the total
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contained in the foliage, this is not the case. This table
is a representation to enable comparisons of relative
concentrations for the purposes of this discussion.
Control200m210m230m250m300m350m
0% 20% 40% 60% 80% 100%
Ion Concentrations in Avicennia marina foliage(% dry weight) Chloride
K
Na
Mg
Ca
Total N
Total P
Figure 5:Ion concentrations in Avicennia marina
foliage(% dry weight) at relative distances from effluent
pipe.
The results from Tin Can Bay support the findings of
Joshi et al (1975). At 200 metres from the outflow pipe -
the landward fringe of the mangrove forest - increased
levels of nitrogen correspond with decrease in sodium and
chloride uptake. At 250 metres from the outfall pipes, where
increased levels of foliage and soil total nitrogen and
phosphorous were previously noted, measures of conductivity
suggest that pooling of nutrient laden freshwater occurs.
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Small changes in levels caused by natural accretion and
tidal movement resulting in niche adaptations were noted
throughout the estuary during ground searches and boat
reconnaisance. The implication of this for ongoing
monitoring of the estuary is that sampling programs must be
designed to take account of these 'niches'.
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4.5 Redox, pH and Conductivity
Methodology
Redox potentials, conductivity and pH were measured
using a portable water quality monitor. Cores of
approximately 15cm depth were dug at sites along the
transect and allowed to fill with water and settle for 15
minutes. A calibrated monitor was lowered into each core and
read once the water had settled. In a strict sense, this
result would interpret a certain percentage of surface water
combined with interstitial water, although for the purposes
of this study, the results provided some interesting and
worthwhile data.
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Results and Discussion
Table 9 shows results of field testing of redox, pH and
conductivity.
Distance from
outflow
pH Redox Conductivity
At outlet 5.8 6 .6
50m 6.53 -132 .7
150m 6.1 -139 .69
200m 5.91 -165 1.1
230m 5.5 -66 15.3
300m 3.9 -8 18.9
400m 4.5 -152 46.0
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4.6 Discussion
Mangroves are mainly found growing on saline,
waterlogged, anaerobic soils (Clough,Boto & Attiwill,1983).
These writers have eloquently described the process of
oxidation-reduction reactions in mangrove soils:
"Initially, aerobic micro-organisms deplete the soil of
oxygen and the redox potential falls to around +350mV.
After the disappearance of oxygen, nitrate is reduced to
gaseous N2: this reaction is followed in turn by a sequence
of reduction reactions involving manganese, iron, sulphur,
and finally carbon dioxide. Sediment organic matter is the
source of energy for these microbiologically mediated
reduction reactions... The redox potentials of mangrove
sediments vary from one area to the next, depending on the
frequency and duration of tidal inundation, drainage,
sediment organic matter content, and the availability of
electron acceptors such as nitrate, iron, manganese and
sulphate....low redox potentials in mangrove and other
waterlogged soils lead to the release of phosphate,
particularly at low pH (<7)."
Boto and Wellington (1983) did further studies and found
a positive correlation between redox potential and mangrove
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biomass to enhanced oxygen transport to sediments via
mangrove roots. Despite low levels of dissolved oxygen
measure in interstitial waters at low tide, significant
nitrification of waters suggests that tidal inundation must
replenish available oxygen. In general cases, low redox
potentials in waterlogged mangrove soils lead to release of
phosphate, particularly at low pH. Phosphate becomes
available for plant growth. This appears to be the case at
Tin Can Bay as P levels drop in interstitial waters towards
the seaward fringe of the forest, whilst plant densities
have increased along the effluent flow path. Tidal waters
are protected from eutrophication which may result from
effluent flow by the presence of the mangrove forest
ecosystem.
The very low pH of the sediments in the Rhizophora
mangle are accompanied by very high sulphate levels: 2,300
mg/Kg during the low flow period and 10,280 mg/Kg during the
post-holiday period. Defining the meaning of this result
would require considerable time and resources, although
groundwater recharge of the forest appears to be the most
obvious answer(P.Matthews,pers.comm.) The presence of acid-
sulphate soils at the terrestrial interface of the forest
indicates that groundwater meets the soil surface at this
point within the mangrove forest, contributing to natural
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biogeochemical processes which predispose sediments to
reduced pH, decreased redox potential, anaerobiosis and free
hydrogen sulphide(Hutchings & Saenger,1987).
4.7 Loss on Ignition
Organic matter of the soil is determined as Loss-on-
ignition. Samples are taken from the top 15cm of soil using
a modified Coile sampler. Samples are air-dried, ground and
passed through a 2mm sieve prior to ignition. Five
replicates from each sample point along the transect were
made.
Determinations were obtained after ignition in a muffle
furnace for 10 hours at 370 degrees Celcius. Calculations
give net loss in weight of soils as a percentage of oven dry
weight (105 degrees Celcius).
Results indicated very high organic matter content,
however they varied according to the vegetation complex.
Mangrove trees have numerous adventitious roots near the
surface resulting in increased loss on ignition. The
Melaleucaforest had large amounts or organic matter above
the soil surface, with less solid organic matter below the
soil surface. The presence of benthic fauna contributes to
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the movement and processing of organic matter below the soil
surface in the mangrove forest as compared to the Melaleuca
forest.
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4.8 Heavy Metals
The potential effect of heavy metals on mangrove
sediments and water quality depends on the nature of the
material discharged, however some observations have been
made by University of Georgia(1981) and Dubinski et al
(1986) on sewage sludge dumping in mangrove forests. They
found only small concentrations in waters in the vicinity of
the dumpsite and concluded that wetlands (both tidal and
freshwater) have the potential for heavy metal retention.
Clough, Boto and Attiwill (1983) concluded from their work
that mangrove sediments have the capacity to remove and
immobilise sediments and heavy metals.
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4.9 Freshwater influence and energy flows
There seems little doubt that freshwater contributes to
mangrove development (Bunt, Williams & Duke,1982;Hutchings &
Saenger,1987). Freshwater is potentially an important source
of inorganic nutrients (Boto,1979). The high clay content in
the mangrove substrate in Tin Can Bay would indicate that
the tidal waters are less likely to leach these nutrients
and they remain available for plant growth.
Substrate development appears to have played an
important role in the focussing of energy flows within the
system being studied. Odum (1984) has described the "energy
signature" of a system as "the sum of all incoming energy
flows to an ecosystem and the pattern of their delivery".
In this system, different energies have a different
ability to do "work" within the system. "Zones" have
developed within the wetland in a response to frontal
energies. Sheet flows of freshwater arrive at the
terrestrial interface of the mangrove forest. Tidal energy
results in mixing of the nutrient rich freshwater with
seawater and the nutrients are cycled within the ecosystem.
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E.P. Odum noted in 1971:
"high rates of production, both in natural and cultured
ecosystems , occur when physical factors are favourable and
especially when there are energy subsidies from outside the
system that reduce the cost of maintenance. Such energy
subsidies may take the form of the work of wind and rain in
a rainforest, tidal energy in an estuary or the fossil fuel,
animal or human work energy used in the cultivation of a
crop."
The confluence of these energy flows appears to be the
zone of emergent macrophytes which has developed on the
claypan between the mangrove forest and the
Melaleuca/Casuarina community on the frontal dune. The dense
canopies of each zone of vegetation, accompanied by a
significant increase in canopy height give an indication of
the volume of nutrients being exported from the effluent
outfall. The nutrients are being cycled and stored within
the biomass at this point. Photographs on the following
pages and the Airphoto 1 vividly demonstrate the changes in
vegetation which have occurred.
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Photograph 3: The meeting of coastal and terrestrial
vegetation as a result of freshwater flows across the
intertidal zone.
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Figure 6 shows the impact of the two frontal energy
flows.
Freshwater and nutrients vs Wind waves tides sun
Energy flows within the system
Canopy Ht vs Distance Salinity Vs Distance
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5.91
Effluent energy and its effect on mangrove zonation and
succession
Horn (1974) defines succession as a pattern of changes
in specific composition of a community after a radical
disturbance such as the opening of the canopy after a
violent storm. Mangrove zonation has been interpreted as
succession however the characteristic colonisation of
mangroves along actively accreting shorelines means that
mangroves themselves are replaced by other species in a
process of succession and zonation only in the dynamic
littoral zone between the terrestrial and marine communities
(Pernetta,1993).
Zonation has been examined by many researchers and the
following four factors are generally agreed to be the
principle influence on zonation patterns (after
Chapman,1975):
(1) tidal inundation
(2) soil type
(3) salt content of the water and soil and
(4) light.
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Within each climatic-tidal environment, each mangrove
species possesses a particular physiological response to
habitat conditions. Habitats are influenced primarily by
geomorphic processes which in turn determine the nature of
the substrate. Substrate attributes such as moisture
content, texture salinity, redox potential and chemical
composition then determine the nature of mangrove
distribution within an area (Teas et al,1975).
The dynamics of the mangrove community at Tin Can Bay
have been examined by analysing substrate attributes, life
forms, canopy height and species composition. Overall
zonation and species density may be observed in airphotos
taken of the area since 1958.
The particular 'island' of vegetation being examined
within Snapper Creek has not changed at all since 1958
except for the area within the effluent plume. Indeed,
barring major climatic events and human influences,
significant changes would not be expected, although this is
not the case for all mangroves. In Tabasco, where average
rainfall is 1700mm per year, mangroves growing in a delta
have adapted to major changes in habitat conditions over the
last fifty years (Thom,1967). Rates of seaward progradation
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of up to 100 metres per year have been recorded in Indonesia
(Rees et al,1993).
Thom's work in Tabasco demonstrated that areas of active
mudflat progradation were first colonised by Avicennia,
prior to the colonisation of the wet sloppy surface by
Rhizophora. Rhizophora is usually assumed to have a pioneer
role. The presence of individual specimens of tall
(approximately 12m) mature Avicennia marina trees within the
dense seaward zone of the Rhizophora mangle at the study
site suggests that Avicennia may have initially colonised
the drier inactive mudflat of the promontory being studied,
followed by Rhizophora stylosa on the wetter, sloppy soils
of the accreting mudflat. Indeed, boat reconnaisance of the
narrow tidal channel receiving the effluent within the
mangrove forest showed that small 'islands' of accreting
sediments with slight increases in elevation above the tidal
margin are colonised by individual species of Avicennia
which are then surrounded by Rhizophora.
Other changes in zonation surrounding this tidal channel
can be observed from airphotos and ground truthing. The
zone of Rhizophora is denser and has prograded landward,
probably in response to the freshwater influence. Chapman
(1975) found that Rhizophora spp. are able to tolerate
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significant periods of inundation by freshwater and their
seedlings tolerate shade, giving them opportunity to
outcompete the Avicennia.
The Avicennia have moved slightly landward, and are up
to 4 metres taller than trees within the same tidal range at
the study site not influenced by the effluent. A few
emergent mangroves have grown in the 'halophytic zone'
directly abutting the zone of Melaleuca and Casuarina,
accompanied by other freshwater tolerant mangroves such as
Aegicerus corniculatum and Lumnitzera racemosa, with a dense
understorey of Juncus spp. to 1.5 metres.
As stated earlier, Odum (1984) suggested that some
twenty years of inundation would be required to accurately
assess the response of the mangrove community to effluent
flow. A detailed analysis would take some years, however
from this study site it can be concluded that changes in
zonation and succession will occur. Allogenic succession has
replaced autogenic succession within the area affected by
the effluent plume.
Most importantly, the effluent has not had a negative
impact on the mangrove forest itself. Other vegetation
communities on the frontal dune and in the littoral zone
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which do not possess the same ability to respond to
fluctuating environmental conditions, have not fared so
well.
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5.92 Summary - Nutrient Cycling in the mangrove forest
Like any natural system the nutrient cycling of matter
in a mangrove forest is a reflection of energy flows into,
through and out of the system (Odum,W.E.,1971)
Rivers (such as Snapper Creek), overland sheet flow of
freshwater, tides and rainfall transport nutrients and
energy into the mangrove forest. Chemical, physical and
biological reactions occur so that inorganic materials are
fixed by plants into organic compounds, which are then
returned to the forest floor as decomposing leaf litter.
Animals and benthic fauna redistribute mineral elements and
organic substances within and outside the system (Lugo &
Snedaker,1974). The final part of this equation is the
export of minerals and organic matter from the land and
intertidal zone into the sea.
One question posed in this thesis is whether the
nutrients exported from the mangrove forest into the tidal
waters are significantly greater in quantity than from a
forest not influenced by effluent? If so, is there a
resultant negative impact on the estuarine flora and fauna
of Snapper Creek and the Tin Can Inlet?
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The results of analysis of foliage, soils and water
collected from the receiving site of the Tin Can Bay WPCW
indicate that the Melaleuca quinquenervia swamp and the
mangrove forest are acting as "nutrient sinks", effecting
considerable improvement in the quality of waters flowing
into the Snapper Creek estuary.
Possible environmental constraints to the potential of
coastal lagoons to act as nutrient sinks were described by
Morell and Corredor (1992) and Kemp et al (1990). They
describe observing "summer depressions in nitrification..
and consequently denitrification" (as a result of low oxygen
saturation levels) and "the bacteriostatic effects of
tannins leaching from decomposing Rhizophora mangle",
suggesting that such depressions may be a permanent feature
of tropical coastal lagoons. Their observations supported
those of Wiebe(1986): that overall these lagoons act as
nutrient and carbon traps immobilising terrestrially-derived
nutrients and protecting seagrass beds from nutrient-rich
loads.
Tertiary treatment would contribute significantly to the
efficiency of the constructed and natural wetlands to
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immobilise nutrients from the sewage effluent (Clough et
al,1983;Wong et al,1995;Reed et al, 1995;)
Cloern et al (1983) found that following the breakdown
of a waste treatment plant near San Francisco Bay,
California, chemical and microbial changes occurred within
the tributary waters as decomposition and denitrification
depleted dissolved oxygen. Water quality improved rapidly
once normal tertiary treatment resumed. The implications
from these findings for Tin Can Bay are that following
overloading during peak periods, it can be expected that
water quality in Snapper Creek will return to normal, if any
nutrients are exported at all from the wetlands to the tidal
waters. At this point in time, nutrients are not exported
from the wetlands into the coastal waters. Upgrading the
WPCW would eliminate the potential for this to occur almost
entirely.
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6.0 Effect of effluent on seagrasses in the Tin Can Bay
area
6.1 General Discussion
Australia has large areas of seagrass beds and the
largest number of seagrass species. Seagrass flourishes in
the low-nutrient coastal waters, however in recent years
there have been wide-spread human-induced declines around
Australia. Seagrasses play an important role in coastal
ecosystems. These include (after Walker and McComb,1992):
• reduction in water movement
• trapping and binding of sediments
• organic detritus
• provision of a stable surface for colonisation
of epiphytes
• high rates of production
• contributions to detrital food chains
• contribution of calcium carbonate by epiphyte
deposition to sediments and
• essential roles in nutrient trapping and
recycling.
Six species have been recorded in the Great Sandy Strait
and Tin Can Bay with beds varying in density from sparse to
very dense. Zostera capricorni (Aschers) is the most
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prominent species (FRC,1992). Within the coastal waters
directly adjacent to the study area, no beds have been
recorded. A small area has been recorded further along the
mid to upper reaches of Snapper Creek
(Thorogood,pers.comm.).
Areas in the headwaters of the estuary were reported as
being covered with seagrass in 1992, however it is now
believed that the tannin stained waters were mistaken for
seagrass from the air, as no beds were found a short time
later during ground surveys.
A number of factors influence seagrass growth. Nutrient
enrichment not only enhances phytoplankton growth but also
the growth of macroscopic and microscopic algae on leaf
surfaces. Macroalgae dominate over seagrasses under
conditions of marked eutrophication, both as epiphytes and
as loose-lying species. This increased epiphytic growth
results in shading of seagrass leaves by up to 65%, thereby
reducing photosynthesis, and diffusion of gases and
nutrients to seagrass leaves (Borowitzka and Lethbridge,
1989).
Sediments can affect seagrasses if large volumes settle
for longer than a few months. Sediments may settle for a few
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months after a major flood event. Sediment imbalances may
also be caused by dredging and construction of harbours and
groynes which may change the wave energy regime. Increased
wave energy may increase erosion, leading to fragmentation
of seagrass beds and damage to the rhizome mat (Walker and
McComb,1992).
The growth of epiphytes and deposition of sediments can
interfere with carbon and nutrient uptake, and most
significantly the shading reduces photosynthesis and
productivity.
6.2 Effect of point source pollution
Several discharges are licensed by DEH to release waters
into the Great Sandy Strait. Tin Can Bay WPCW and Lee
Fishing discharge into the waters surrounding the study
site. The analysis undertaken for this report indicates
that Tin Can Bay WPCW is not contributing to eutrophication
of the waters of Snapper Creek. Results of DEH monitoring
confirm that the discharge has no effect outside the zone of
initial mixing.
From this we can conclude that there is no significant
point source discharge affecting the waters of Snapper Creek
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and the Tin Can Estuary. Fluctuations in the density and
extent of seagrass beds in the upper reaches must be due to
causes other than anthropogenic influences
(Thorogood,pers.comm.).
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PART TWO
7.0 Constructed Wetlands
7.1 Introduction
"Natural" methods of wastewater treatment began to
attract attention in the 1970's with the introduction of the
Clean Water Act in the United States and the expression of
similar environmental concerns here in Australia, although
at that time regulatory issues were less significant,
especially in Queensland. Land disposal of wastewater had
been practiced in the nineteenth century, the first paper
being written by E. and F. Spon, "Sewage Irrigation by
Farmers", in 1879. This method of treatment lost favour as
community concerns about insect-borne diseases grew with the
advancement of engineered wastewater treatment facilities
(Reed,1995). Land disposal has been practised at Werribee in
Victoria since 1897.
Natural methods of wastewater treatment include aquatic,
terrestrial and wetland systems. JWP (1993) considered the
potential for overland flow systems at Tin Can Bay in their
report for Cooloola Shire Council. Constructed free water
surface (FWS) wetlands are considered in this thesis as a
viable option for tertiary treatment of effluent, following
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an evaluation of the proposed site at the Tin Can Bay WPCW
and the existing natural wetlands presently receiving
effluent.
7.2 The existing Tin Can Bay WPCW
The existing sewage treatment works have been in
operation since 1975, and have a design capacity of 292
kL/day. On average, however, the plant receives 445 kL/day
and is overloaded. In 1983 the Department of Local
Government (DLG) recommended augmentation of the Tin Can Bay
WPCW to 5000 equivalent population (EP), with an expected
peak flow of 1250 kL/day in 2005 (JWP,1993).
The Tin Can Bay WPCW discharges effluent to a series of
ponds and earth basins. Effluent passes through the three
ponds and six basins into a forest dominated by Melaleuca
quinquenervia with a dense understorey of Typha species.
From there it passes through the intertidal zone which is
now dominated by Juncus species, with emergent freshwater
tolerant mangroves. The effluent plume then flows into a
mangrove forest. The dynamic process of the effects of the
effluent on the natural wetlands are considered in the first
part of this thesis. Effluent is also piped to a nearby
Golf Course and used for irrigation. The quantity of
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effluent pumped to the Golf Course increases significantly
in dry weather.
John Wilson & Partners (1993) considered the
augmentation of the treatment works and recommended
upgrading the plant to secondary treatment, followed by
public open space watering and non-potable household re-use
by dual reticulation. This would involve the distribution
of potable and non-potable water through two separate
networks of pipes for each household. This thesis considers
another option, that of using constructed wetlands for
tertiary treatment of sewage. This option offers a number of
advantages over existing treatment methods:
• the system involves less use of energy and
resources, and is less complex
• wetlands support many life forms including
microbial, invertebrate and vertebrate animals, and
microscopic and macroscopic plants, thereby increasing
biodiversity and habitat value in the area (Hammer,1989)
• water quality is improved beyond secondary
treatment quality, therefore groundwater and waterways
are protected from contamination and eutrophication
• carbon, sulphur, iron, manganese and heavy metals
are stored in sediments
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• wetlands for treatment of domestic sewage provide
an opportunity for environmental education and research;
larger forested wetlands provide opportunities for
outdoor recreation, and habitat enhancement.
• wetlands have a greater buffering capacity than
other treatment methods and potentially handle wide
fluctuations in input flows and influent quality without
the problems of mechanical plant adjustments.
Analysis of secondary treated effluent being
discharged into the receiving site adjacent to the plant was
undertaken by Aquatech Pty. Ltd. prior to the commencement
of this study. Samples collected by the author were
collected during low flow and peak flow periods. Laboratory
analysis was carried out by Envirotest Pty. Ltd. for
Cooloola Shire Council, prior to interpretation of results.
A grab sample was also collected directly from the trickle
filter during a holiday period. This sample produced the
following results with BOD5 and suspended solids levels
comparable to that of raw sewage, indicating that the plant
is performing well below expectations:
Parameter Units Result
pH Standard 7.5
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Total
nitrogen
mg/L 50
Ammonium-
nitrogen
mg/L 30
Nitrate-
nitrogen
mg/L 0.1
Total
phosphorous
mg/L 7.4
BOD5 mg/L 270
Total SS mg/L 225
Table 10: Analytical results of grab sample from
trickle filter during holiday period, October 1995.
Analysis of effluent being discharged from the final
pond has been undertaken this year by Aquat-tech Pty.
Ltd and Helen Hillier. Although results of these tests
indicate that effluent discharged to the natural
wetlands exceeds licence limits at every sampling event,
these results must be taken in the context of the method
of treatment employed.
Suspended solids in particular were very high and
composed mostly of algae, a problem common to secondary
and tertiary lagoons such as those in existence at Tin
Can Bay.
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Levels of nitrogen and phosphorous were high,
however nutrient concentrations in discharges are not
the subject of licence conditions at this point. The
nutrients are removed during sheet flow through the
existing wetlands, indicating that proposed future
tertiary treatment of sewage in constructed wetlands
would result in an effluent which complies with
licencing limits for concentration of nutrients in
wastewater prior to discharge into receiving waters.
7.2 Proposed Augmentation of WPCW at Tin Can Bay
JWP (1993) proposed upgrading the plant to include
aerobic digesters to achieve secondary treatment of
effluent, followed by a tertiary treatment process such
as overland flow and public-space irrigation. Aerobic
digestion is similar to the activated sludge process and
achieves a high rate of BOD5 removal. The disadvantage
of this form of treatment is that very little nitrogen
or phosphorous is removed. Discharging nutrient rich
effluent to receiving waters can result in algal growth
and depletion of dissolved oxygen in receiving waters
which then affects marine life and can become a public
health hazard. High nutrient levels also affect the
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potential of the water for re-use through household dual
reticulation and open-space watering, an option for
tertiary treatment proposed by JWP (1993)(Metcalf &
Eddy,1991).
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7.3 Biological Nutrient Removal (BNR)
Biological nutrient removal focusses on removal of
Ammonia and organic nitrogen. These nutrients are
oxidised to nitrate through a two-step autotrophic
process involving two micoorganisms, Nitrosomas and
Nitrobacter. Nitrate is then reduced through a process
of denitrification which requires a carbon source and
energy in anoxic conditions.
Phosphorous is removed by exposure of the effluent
microorganisms to anaerobic and aerobic conditions. A
number of proprietary processes have also been developed
for combined removal of nitrogen and phosphorous
(Metcalf & Eddy,1991).
Prior to the Queensland State Government election in
1995, the Labor Government committed considerable sums
of money towards assisting local authorities to
implement BNR plants in order to improve effluent
quality prior to discharge into receiving waters. This
costly option may not be required if the plant is
upgraded and tertiary ponds installed.
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DPI have stated that one third of the cost of a BNR
plant or constructed wetlands will be funded by State
Government this financial year. Next financial year, an
EMP must be provided in order to meet new guidelines for
funding.
7.4 Phosphorous removal
JWP (1993) included projected cost estimates for
alum dosing for removal of phosphorous in their report.
In this process, aluminium sulphate is added during
primary treatment, resulting in precipitation into
primary sludge. Precipitated phosphorous is removed from
the system with the sludge. This process is employed at
West Byron Treatment Plant (Bavor et al,1994).
Phosphorous (P) removal is never assured but nonetheless
efficient in natural treatment systems such as
constructed wetlands or overland flow. Initial P
removal is usually excellent, although it will take at
least two years for equilibrium to be reached.(Reed et
al,1995;Bavor et al,1994).
The P removal efficiency of the mangrove ecosystem
receiving the treated effluent from the Tin Can Bay WPCW
cannot be overlooked. The results obtained from
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sampling through the transect set up within the effluent
plume indicate that this forested wetland achieves
excellent P removal (82.5% in waters, pre holiday and
95% removal post holiday period.)
The possibility of Department of Environment and
Heritage (DEH) and Queensland Department of Primary
Industries(QDPI) relaxing the water quality guidelines
for effluent P levels following the implementation of
the EPA (1994) was discussed at a recent conference on
"Wetlands for Water Quality Control" hosted by QDPI in
Townsville.
The potential for natural treatment methods to
achieve phosphorous removal without alum dosing prior to
tertiary treated effluent reaching estuarine receiving
waters should be considered in the light of the
excellent results obtained from the natural wetlands at
Tin Can Bay. Bearing in mind that the mangrove ecosystem
has been receiving effluent for twenty years, it is
possible to conclude that the mangrove forest has
successfully adapted to increased nutrient levels within
this system(Odum,1984).
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Whether Cooloola Shire Council could eliminate alum
dosing from the proposed treatment process to achieve
phosphorous removal by relying on a constructed
broadacre wetland system followed by flow through the
mangrove forest would be a matter for consideration by
DEH. The results obtained from samples taken during this
project would indicate that this may be a viable option.
7.5 Potential for constructed broadacre wetland
With the proclamation of the Environmental
Protection Act (1994) and the (Draft) Water Quality
Guidelines(EPP) (1995), the discharge of secondary
treated effluent to natural wetlands and tidal waters
will no longer be permitted, except under certain
conditions.(Refer 'Regulatory Issues').
The Tin Can Bay WPCW site appears well suited to
construction of "reed beds" and the construction of a
forested wetland dominated by Melaleuca quinquenervia
with an understorey of Typha species, Phragmites
australis, Schoenoplectus species and Cyperus species
native to this area.
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As discussed earlier in this thesis, Melaleuca
quinquenervia and various reeds and sedges have
overtaken the area inundated by effluent adjacent to the
lagoon discharge pipes . At West Byron Treatment Plant,
Melaleuca quinquenervia trees, also endemic to that
area, are thriving in the constructed broadacre wetlands
despite permanent inundation (E.Andell, pers. comm.).
They appear to have adapted to the permanent
inundation by developing extensive root systems and
thicker trunks. The extensive adventitious root system
which can be seen above the water line increases the
area available for biofilm formation and the pollutant
removal potential of the system. A thorough
investigation of the effect of effluent inundation on
Melaleuca quinquenervia is presently being undertaken by
Keith Bolton, a PhD student at Griffith University
(Bolton and Greenway,1995).
Forested wetlands offer the advantages of high
biomass-sink volume, potential for harvesting and the
opportunity to produce nectar, oil and timber
(Bolton,pers.comm.). The harvestable resources available
in forested wetlands have been exploited in Asian
countries for hundreds of years (IUCN,1993,Davie,pers.
comm.).
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7.6 Sub-surface flow wetlands
The biological reactions in sub-surface flow (SF)
wetlands are similar to those of the FWS wetland,
relying on the development of attached growth organisms
on plant stems and roots. They offer a number of
advantages in areas where land is limited and where
public access and safety are an issue. A SF wetland is a
plastic or clay lined earth basin filled with a regular-
sized porous media such as gravel. Some species used in
FWS wetlands are also grown in the gravel media. The
increased surface area available for reaction with the
wastewater means that the SF wetland is often much
smaller in total land surface than FWS.
The advantages of a SF wetland must be offset
against the cost of procuring and placing the gravel in
the trenches, and the long term costs of ensuring the
system does not become clogged. Influent suspended
solids concentrations need to be monitored in order to
prevent clogging of void spaces which can result in
surface overflow and significant short-circuiting
thereby negating the purpose of using a SF wetland.
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Sub-surface flow wetlands (SF) were considered for
the Tin Can Bay site, however the large quantities of
gravel and associated costs involved preclude them from
being a viable option to a FWS wetland. It may be
desirable to include three SF basins in place of the
deep ponds in existence at Tin Can Bay WPCW. This would
remove BOD and SS as a first step in the tertiary
treatment process. SF wetlands achieve good removal of
BOD, NO3 and SS. Subsequent removal of nitrogen and
phosphorous would then occur in the FWS wetland after
water is reoxygenated using some method of aeration such
as sprays.
The treatment plant proposed by JWP however would
require nitrogen and phosphorous removal during tertiary
treatment to reach licensed discharge limits, which is
best achieved in a FWS wetland if sufficient oxygen can
made made available to the system.
The availability of a large area of land and the
existence of constructed ponds at the Tin Can Bay WPCW
favours the establishment of a FWS wetland. The
pollutant removal capability of the natural Melaleuca
wetland which has developed indicates that ideal site
conditions exist for the construction of a FWS wetland.
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8.0 Constructed wetlands versus natural wetlands
The most important consideration of the preliminary
design phase of this form of pollution control is to
determine the exact function of the wetland. A constructed
wetland for treatment of domestic sewage will provide some
habitat values, however the goal of treating wastewater must
be the main priority. The wetlands must be carefully
designed for each situation and operated according to a
'tailor-made' management plan. Wetlands are not a 'set and
forget' solution to the problems of wastewater treatment.
The influent loading will determine the size and
configuration of the natural treatment process. As with any
other engineered wastewater treatment system, there must be
regular and ongoing monitoring and maintenance.
Constructed wetlands offer a number of functional
attributes which make them more efficient and reliable than
natural wetlands as a method of wastewater treatment.
Constructed wetlands can be carefully graded to suit the
exact requirements of the system designer. The hydraulic
regime can be controlled and freeboard for shock flows can
be built into the system. The type of vegetation growing in
the ponds, trenches or marshes can be pre-determined and the
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water depth and hydroperiod can be controlled. Some
researchers have suggested a constructed wetland which
"mimics" the features of a natural wetland with varying
depths, and vegetated and unvegetated zones is more likely
to be successful (Breen & Spears,1995;Sainty et
al,1995;Mitsch et al,1987).
Free water surface (FWS) wetlands can be thought of as
"attached growth biological reactors". Design models have
been determined for BOD, suspended solids, nitrogen and
phosphorous e.g.Reed et al,1995. Designs are based on the
principles of first-order plug flow kinetics for nutrient
removal. Their performance is based on hydraulic retention
time (HRT) and the temperature in the system. The
preliminary design concepts for Tin Can Bay have been
calculated using the algorithims developed by Reed et al
(1995), which are based on the performance of constructed
wetlands in the United States over the last twenty years.
Other researchers such as Konyha et al(1993) and Roser and
Bavor at the University of New South Wales Water Research
Laboratory, have developed new models, however the most
well-developed of these - such as the SWAMP™ model - are
proprietary and therefore unavailable for comparison (Roser
et al,1995).
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QDPI has supported the development and construction of a
number of wetlands in Queensland in both sub-tropical and
tropical communities. Performance standards to date indicate
that climate plays an important role in the functioning of
the system. Reliable models are slow to develop because data
collection and analysis is costly and requires a concerted
effort over a number of years to develop conclusive
'evidence' regarding the performance of constructed
wetlands. Data to date has been limited to sampling
influent and effluent nutrient concentrations, with scant
regard for spatial and temporal variations which may occur
within the pond, marsh or trench. This oversight is not
necessarily due to sampling design error; the high cost of a
continuous, broad-based sampling program has hindered many
researchers. Both Konyha et al(1993), and Bavor et
al(1994), emphasise the importance of continuous monitoring
in order to achieve realistic influent loading budgets for
each wetland.
8.2 System components
The pH of the water, depth, temperature and dissolved
oxygen have a significant impact on the success or failure
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of the system. In a vegetated wetland, the plant species,
detritus, soils, algae, bacteria, protozoa and higher
animals also influence the treatment process and quality of
the effluent (Mitchell et al,1995)
8.3 Microorganisms in wastewater treatment
Wastewater is principally composed of nutrient rich
organic matter in soluble and colloidal form. The selection
of a suitable wastewater treatment process must take into
consideration the nutritional and environmental requirements
of the microorganisms which reduce the levels of organic
matter and nutrients (Mitchell,1995; Metcalf & Eddy,1991).
The microorganisms which "clean" wastewater require an
energy source, carbon and nutrients to reproduce and
function, and are then classified according to their
metabolic type and oxygen requirements. Some microorganisms
require oxygen, others require anoxic or anaerobic
conditions. Facultative organisms have the ability to
function in anaerobic and aerobic environments.
Manipulating these conditions in the first stages of
treatment and in a wetland facilitates achievement of a
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number of wastewater treatment objectives, specifically
nitrification and denitrification.
8.4 Environmental conditions
Organisms involved in wastewater treatment are sensitive
to a number of environmental parameters, in particular, pH,
temperature and oxygen availability.
Temperature has a significant effect on growth rates.
Climatic conditions in Queensland generally provide optimum
temperature ranges for organism productivity. It is
important to remember in planning for a constructed wetland
that these optimum temperatures also reflect the optimum
growth requirements of the wetland vegetation. In sub-
tropical and tropical areas, wetland vegetation needs to be
carefully managed in order to prevent clogging of FWS
wetlands with uncontrolled clumps or mats of vegetation.
Unmanaged vegetation growth such as mats of Salvinia spp.
will inevitably result in short-circuiting and reduce light
penetration to the water column which then reduces
wastewater treatment potential(pers.obs.).
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8.5 Microbiology and public health considerations
Water is an ideal medium for the growth and transmission
of disease-causing pathogens. Pathogens may include
viruses, bacteria, fungi and protozoa (Mitchell,1995).
Water-borne, water-based and water-washed diseases are
related to faecal-oral contamination. These diseases are
generally debilitating and contagious especially for those
suffering immume system deficiencies, children and elderly
people. Water-related diseases such as malaria and epidemic
polyarthritis involve insect vectors, usually mosquitoes.
Wastewater treatment must therefore remove the pathogens
which affect higher life forms. Insects such as mosquitoes
which breed or feed in the water must also be controlled by
careful management of the polluted water bodies at the
treatment plant. Mosquito control will be addressed in a
later section.
The primary, secondary and tertiary sewage treatment
processes currently employed in Australia achieve excellent
results for bacterial reduction. The West Byron Treatment
Plant has consistently monitored their faecal coliform
levels and observed significant reductions in the SF
wetlands and trenches. Only slight further reductions occurr
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within the Melaleuca broad-acre wetlands due to dense
resident wildfowl populations (Bavor et al,1994). UV
disinfection in a wetland would be expected to remove other
micro-organisms which affect human health.
From the information available, it appears that the Tin
Can Bay WPCW would have similar performance expectations.
8.6 Microorganisms and biological yields
Effective design requires the use of stoichiometric
equations in order to determine maximum possible yields in a
system. Combined with bacterial composition and substrate
yield coefficients, an accurate prediction can be made of
overall quantities of substances consumed or produced. The
two values of most interest are oxygen yield and biomass
yield (Mitchell,1995). Sufficient oxygen must be available
for the preferred organisms to remove organic and/or
nitrogenous compounds.
• Oxygen requirement = mass of oxygen required
mass of BOD or nitrogen removed
• Biomass yield = mass of cells produced
mass of BOD or nitrogen removed
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Yields are inextricably linked to growth rates. High growth
rates correspond to high biomass yield per unit of BOD
consumed (Mitchell,1995,p.21).
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8.7 Biological rates
The yields and rates of biological processes have a
direct impact on the size of the wetland and therefore
the ultimate cost and feasibility. As loading rates are
more likely to change than yields, designs should be
based on expected loading rates. The three parameters of
most importance here are:
• rate of cell production - this determines the length of a
treatment cycle and the vulnerability of the system to
shocks
• rate of nitrogen uptake - determines nitrogen removal
rate
• rate of oxygen transfer - determines whether aerobic or
anaerobic conditions will prevail.
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9.0 Pollutant Removal in FWS Wetlands
9.1 Biochemical Oxygen Demand
Both Reed et al (1995) and Metcalf and Eddy (1991) note
that a limiting design factor for constructed wetlands can
be Biochemical Oxygen Demand (BOD) loading.
BOD is a simple measure of the potential deoxygenating
effect of the biologically oxidizable matter present in an
effluent. It represents the oxygen uptake over the initial
five-day period of the total respiratory curve
(Hawkes,1963). Respiration is an energy-yielding process,
also described as the aerobic oxidation of organic carbon.
Aerobic conditions need to be maintained through surface
aeration or through the oxygen transfer capacity of the
wetlands vegetation via photosynthesis. Metcalf and Eddy
(1991) advise:"care must be exercised in using area (BOD)
loading criteria (mass/area.time) because the actual load is
not applied uniformly but is concentrated at the inlets,
whereas oxygen is supplied uniformly over the surface."
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Surface reaeration can be created by mechanical means
such as sprays, and by taking advantage of prevailing
breezes. In a coastal area such as Tin Can Bay, wind-
induced reaeration can be complemented by placing inlet and
outlet structures at opposite corners of the pond or trench
to increase mixing and reduce short-circuiting.
Emergent macrophytes and tree species in wetlands
transfer oxygen from the leaves to the plant roots, however
most oxygen in the root zone will be consumed by benthic
demand. Therefore the plants themselves do not remove BOD
directly, but act as hosts for organisms attached to the
plants which are responsible for organic decomposition.
(See Figure 9)
Unfortunately, if a wetland is too densely vegetated,
the opportunity for wind reaeration and oxygenation through
photosynthesis is reduced, thereby reducing the BOD loading
rate which can be tolerated. A balance between vegetation
and optimum loading rates must be found preferably by
providing optimum secondary treatment for effluent and
managing the vegetation growth in the wetland through
selection of species and appropriate water levels.
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The main point here is that the BOD level of the
influent must be less than the potential rate of oxygen
transfer to the system, otherwise anaerobic conditions will
prevail and the system will fail and become malodorous. The
organic loading rate for a constructed wetland must be
relatively low, in the order of 20-100 kilograms/hectare/day
(kg/ha.d). The common practice internationally is to begin
tertiary treatment with facultative ponds to settle
suspended solids and reduce BOD. The ponds are aerated by
photosynthetic algae and surface reaeration (Reed et
al,1995). As an example, if the existing deep ponds at Tin
Can Bay were used prior to discharge into reed beds, using
the present treatment system which discharges effluent with
a BOD-5 of 270mg/l during peak periods, the organic loading
rate would be 1313 kg/ha/d (Reed et
al,1995,Envirotest,1995). The required detention time would
be extremely long. By comparison, the wetland influent BOD
concentration at West Byron Constructed Wetlands is on
average 6.4 mg/l with a maximum of 28 mg/l (Bavor et
al,1994).
It would be expected that the primary and secondary
treatment of Tin Can Bay sewage would be improved prior to
constructing wetlands for wastewater treatment. All
calculations related to preliminary design of the
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constructed wetlands are based upon reasonable estimates of
the effluent nutrient concentrations from an upgraded
treatment plant as proposed by JWP in their report for
Cooloola Shire Council (JWP,1995).
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9.2 Suspended Solids
"The most important characteristic of wastewater is its
total solids content, which is composed of floating matter,
settleable matter, colloidal matter, and matter in solution.
Other important physical characteristics include odor,
temperature, density, color and turbidity" (Metcalf &
Eddy,1991,p.50).
The treatment facility proposed by JWP(1993) would be
expected to achieve 90% suspended solids removal during
primary and secondary treatment (Metcalf &
Eddy,1991;JWP,1993). The resultant effluent would be
suitable for further treatment in either a FWS or SF
wetland.
Solids removal is very effective and rapid in both types
of wetlands.
Suspended solids removal is due to the physical
processes of sedimentation and entrapment in the microbial
growths and is influenced only by temperature. Most of the
solids in municipal wastewater are organic in nature and
will decompose leaving minimal residue. For this reason SS
removal is not likely to be a limiting design parameter.
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9.3 Nitrogen removal
Nitrogen removal in a wetland system involves a complex
set of processes with a number of chemical and environmental
conditions required for transformation and removal(Metcalf &
Eddy,1991;Reed et al,1995;Mitchell,1995).
Nitrogen is usually in the form of ammonia or organic
nitrogen, although it may be possible that all of the
Kjeldahl nitrogen (TKN) entering the system will be
converted to ammonia. Ammonia is removed by nitrification in
the presence of oxygen, followed by denitrification.
Denitrification is carried out by facultative bacteria under
anoxic conditions. Since both aerobic and anaerobic
conditions occur in a FWS wetland it is possible for
nitrification and denitrification to occur in the same
reactor volume. The requirement for oxygen severely limits
the potential for nitrification in a SF wetland
(Mitchell,1995).
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9.4 Design for nitrification
The major oxygen source for nitrification in FWS in
wetlands is through atmospheric reaeration, usually by wind
mixing at the water surface. Within the water column,
aerobic conditions exist at the water surface, and even with
a shallow water depth, anaerobic conditions occur in the
bulk of the liquid (Reed et al,1995). Both nitrification
and denitrification require dissolved oxygen and
temperatures ideally above 10 degrees Celcius. Conditions
for nitrification also include sufficient alkalinity and low
carbon. The organic loading of the influent must be
relatively low, as described earlier, in order to allow the
nitrifying bacteria, the chemoautotrophs, to convert
ammonia to nitrite and nitrate. Some ammonia is available
for uptake by vegetation and microorganisms, however the
adsorption capacity of a natural system is finite and
nitrification is necessary to release adsorbed ammonia and
regenerate adsorption sites (ibid,1995).
Where stringent limits apply to nitrogen levels in
effluent, as is the case in Queensland, the limiting design
parameter for calculating the size of the FWS wetland will
be the area required for nitrification. Assuming the Tin
Can Bay WPCW is upgraded to the proposed plant described by
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JWP (1993), then the wetland influent would require a
significant percentage of nitrogen removal. The limiting
design parameter would be ammonia removal.
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9.5 Denitrification
Whilst nitrification is carried out by chemoautotrophic
bacteria, denitrification is carried out by
chemoheterotrophs in the presence of a biodegradable organic
source of carbon and energy and anoxic conditions.
Nitrification occurs before denitrification, however since
both aerobic and anaerobic conditions occur in the FWS
wetland, both steps can occur in the same reactor volume.
Nitrogen gas is liberated from the solution and nitrogen is
removed from the system.
In most cases, "most of the nitrate produced in a FWS
wetland will be denitrified and removed within the area
provided for nitrification and without supplemental carbon
sources" (Reed et al,1995,p.238).
9.6 Phosphorous
Phosphorous removal is a contentious issue in terms of
its impact on the environment and whether constructed
wetlands are the most appropriate form of tertiary treatment
for removal of this nutrient.
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As described earlier, phosphorous removal is not
completely effective in either FWS or SF wetlands, and may
not be reduced to the licensed limit.
In many cases it has been observed that in the first
year of wetland operation, phosphorous removal rates are
excellent, however once the system reaches equilibrium, the
rate of P removal decreases. After construction there is a
greater opportunity for P adsorption on the exposed soil
surface at the bottom of the wetland. As detritus and
sediment accumulates, P is bound in this peat/litter zone as
sediments of iron, aluminium or calcium (ibid,1995).
Phosphorous is required for biological growth, and the
vegetation in a constructed wetland evolves under conditions
of high nutrient availability. Bolton and Greenway (1995)
are presently examining the potential of Melaleuca species
for P removal. They have found Melaleuca alternifolia
particularly promising for removing P from wastewater, and
they can be harvested for "tea-tree oil" production.
Promising results for P removal were obtained from the
natural wetlands at Tin Can Bay. These results can be
interpolated to predict the performance of a constructed
forested wetland at the Tin Can Bay WPCW.
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Since sediment deposition and mineralisation is the
process for removal of phosphorous "the mass removal rate is
a function of the surface area in the wetland and the
phosphorus concentration in the wastewater" (Reed et
al,1995,p.251). The area required for a conventional
constructed wetland to achieve low levels of phosphorous in
the final effluent is relatively large. A cost-effective
approach such as alum-dosing may need to be included in the
treatment process, otherwise licensing limits should be
considered in the light of P removal percentages in the
mangrove forest. If the P removed in the mangrove forest is
included in the determination of licensing limits, no alum-
dosing would be required. For this reason, the wetland
would be sized for specified nitrogen removal using
algorithims appropriate for the climate and conditions of
the site.
As has been stated earlier, the future potential of
using mangrove forests for wastewater treatment is yet to be
explored, even though they have been used as effluent
receiving sites for hundreds of years.
Mackay City Council will be investigating the potential
for irrigating mangroves with effluent in the near future
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and these results may prove very useful for researchers in
the field of wastewater
treatment(Kelso,pers.comm.;pers.obs.).
9.7 Pathogen removal
Public health considerations drive the regulatory
policies related to wastewater management. It is necessary
to remove pathogenic microorganisms from the water and
sludge in order to prevent contamination of groundwater and
surface water, and thereby prevent disease outbreaks.
A significant percentage of pathogen removal occurs
during the wastewater treatment process. Die-off is due to
natural causes, predation, sedimentation and adsorption.
Most helminths, Ascaris and other parasitic cysts and eggs
are mainly held in the sludge. The sludge must then be
treated appropriately if it is removed from the pond or
wetland for disposal. Wetlands also provide the opportunity
for adsorption on to biofilm, and the water column stems and
roots in the FWS system.
The maturation pond system at Tin Can Bay is an example
of the use of ultra violet light to remove pathogens. These
ponds promote algal growth in conditions of relatively high
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pH, which creates an environment unsuitable for most viruses
and pathogens and thereby achieving disinfection objectives.
Detention time in ponds is also an important factor (Reed et
al,1995;Mitchell,1995).
Some wetland plants "exude bacteriostatic and
bactericidal secretions which can further aid pathogen
destruction in wetlands". (Mitchell,1995). The tannins
exuded by Rhizophora mangle are an example of this
bacteriostatic effect.
9.8 Heavy metal removal
Heavy metals are removed via similar mechanisms to those
of phosphorous. Mechanisms include plant uptake,
adsorption, complexation and precipitation. Metals
accumulate in the sediments of a wetland system and wetlands
have in fact been mined for bog iron, as described earlier.
Since the organic and inorganic sediments are continuously
increasing (at a slow rate) in these wetlands the
availability of fresh adsorption sites is also increased
(Reed et al,1995). The concentrations of heavy metals found
in treated municipal wastewaters do not represent a threat
to the habitat values of the Tin Can Bay site.
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Mangroves are particularly suited to storing heavy
metals in their sediments, however developers must then be
cautioned against disturbing effluent receiving sites by
dredging.
Phillips and Greenway (1995) have been studying heavy
metal bioavailability in soils of constructed wetlands.
Their study is yet to be finalised, however findings to date
suggest that short retention times in wetlands may allow
metal-rich waters to pass through and contaminate receiving
waters. The proposed retention time of 20 days should
overcome this problem.
9.8
Wetland influent characteristics at Tin Can Bay
Constructed wetland systems can be expected to treat
BOD, SS, nitrogen, some phosphorous, heavy metals, trace
organics and pathogens. These processes were discussed in
the previous section.
In Australia constructed wetlands are generally used as
a form of tertiary treatment for domestic, municipal and
industrial wastewater. Conservative estimates of performance
expectations can be calculated for a constructed wetland
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receiving municipal wastewater following secondary
treatment. Constructed wetlands, like any other wastewater
treatment system, must be designed, constructed and operated
with due care and attention to site specific influent and
effluent characteristics (Mitchell,pers.comm).
The wetland influent characteristics must be determined
in order to accurately calculate the area required for
treatment. JWP (1993) propose screening and grit removal
followed by aerobic digestion as the primary and secondary
treatment processes. Metcalf & Eddy (1991) predict the
following effluent quality following such a treatment
process:
Parameter Unit Result
BOD mg/L 20
SS mg/L 30
NH3 mg/L 45
TKN mg/L 50
Total P mg/L 10
Table 11: Predicted average effluent quality following
proposed primary and secondary treatment at Tin Can Bay WPCW
These results indicate signiciant removal of BOD and SS.
The limiting design factor (LDF) would then be ammonia
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removal,i.e. design for nitrification. If nitrification
occurs during preliminary treatment, the area required for
the constructed wetland can be greatly reduced. The area
required for denitrification is always smaller than that
required for complete nitrification, since oxygen transfer
is the limiting design factor in nitrification.
The following algorithm is used to determine wetland
treatment area(Reed et al,1995):
KT = temperature dependant rate constant, days-1
T = 20 degrees Celcius - average water temperature Tin
Can Bay
KT = 0.2187(1.048)(20-20),d-1 = 0.2187
Ceffluent = 0.2 mg/L NH3-N
Cinfluent = 45 mg/L NH3-N
Q (design flow) = 1000 KL/d (Predicted average for 2005)
T = 20 degrees Celcius
t = 20 days (theoretical residence time from existing
ponds)
n = 0.65 (porosity of vegetated wetland)
y = 0.5 metres (average depth in wetland)
KT = 0.2187 d-1
As = surface area of wetland,m2
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As = Q ln(Co/Ce)
KT y n
= (1000)ln(40/0.2) m3 d
(0.2187)(0.5)(0.65) d m
= 74,542 m2 = approx. 7.5 hectares
Area required for nitrification = 7.5 hectares
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9.9 Effluent quality
DEH have not been able to supply definite guidelines for
proposed licensed discharge limits(DEH,pers.comm.). A "set
of numbers" have been proposed in line with the limits set
by the New South Wales Environment Protection Authority (NSW
EPA). The Queensland (Draft) Water Quality Guidelines
(1995) have yet to be finalised.
It is important to remember that legal limits are
arbitrary standards set by authorities in order to protect
the environment and human health, and are therefore
subject to change. The following table compares the NSW
EPA water quality guidelines for discharge of effluent to
sensitive waters, the design treated effluent quality
proposed by JWP (1993) using overland
flow and the effluent quality from West Byron Treatment
Plant (Bavor et al,1994).
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Parameter Unit West Byron Proposed NSW EPA
BOD5 mg/L 10 15 30
SS mg/L 21 20 30
Total N mg/L 5 15 15
Total P mg/L 0.7 2 1
Table 12: Comparison of 90 Percentile values for
effluent nutrient concentrations
The West Byron Treatment Plant services a population
comparable to that of the expected peak population of Tin
Can Bay in 2005. Many environmental conditions at the Byron
plant are similar to those at Tin Can Bay, so that plant
serves as a useful indicator of what to expect at the
proposed upgraded treatment facility and wetlands at Tin Can
Bay WPCW.
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10.0 Wetlands and landscape design
The Journal of Ecological Engineering described the
goals of wetland restoration design and construction as:
"(1)the restoration of ecosystems that have been
substantially disturbed by human impacts such as
environmental pollution, climate change, or land
disturbance;
(2)the development of new sustainable ecosystems that
have both human and ecological value and
(3)the identification and protection of the life-
support value of existing ecosystems"(Mitsch,1992).
Mitsch (1992) and Breen (1995) amongst others have
described a number of principles regarding the design of
wetlands which assist in maintaining performance:
(1)Design for function not form. If initial planting
designs and animal habitats do not develop according to plan
but the wetland achieves its water quality control
objectives it can be deemed a success. Natures has its own
design objectives.
(2)Hydrology influences seedling emergence and nutrient
availability influences biomass in aquatic macrophytes.
Ensure enough space is built in to the system to shut down
areas and vary hydroperiod. Being able to influence water
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levels and hydroperiod is an important aspect of vegetation
management. Vegetation will be discussed later.
(3)Utilize natural energies. Large areas of land which
are often available in Australia allow for enough 'head' to
be built into the system, thereby avoiding the need for
pumps and associated civil works
(4)Design the system as an ecotone. The wetlands should
ideally be built with a buffer of vegetation around the
site. The wetlands themselves act as a buffer between the
aquatic and terrestrial ecosystems.
(5)Potential problems such as mosquito breeding and
disease transmission from waterfowl can be controlled by
implementing appropriate design and management guidelines.
Mosquitoes will be discussed in a later section.
(6)Design for unexpected flood events with areas for
overflow.
(7)The system should be designed for minimum maintenance
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11.0 Site selection and evaluation
11.1 Site identification
The most important aspect of site selection is the
determination of land ownership, use and availability (Reed
et al,1995;Hammer,1991). Historical and current uses often
help determine which sites are most suitable for
consideration. The Tin Can Bay WPCW is presently situated
on a large area of land designated for the sanitary reserve,
and lies at a good distance from the community proper,
thereby avoiding problems related to vector-controlled
disease transmission, odours, public access and safety. The
site is flat to slightly undulating and cleared of
vegetation on the side close to the access road. The
obvious suitability of this site eliminated the need to
select any alternative sites for the construction of the
wetlands, although such areas would easily be found if
required in the future.
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11.2 Topography and soils investigation
Topography will influence construction costs, erosion
potential, drainage patterns, access and overall feasibility
(Hammer,1991).
The Tin Can Bay WPCW is located on a relatively flat
piece of land. The existing plant has been constructed on
an earth platform approximately 1.5 metres above natural
ground level in the centre of the reserve. The site does not
lie at the base of a large catchment, thereby eliminating
problems associated with urban or agricultural run-off.
Small streams and groundwater, however, flow through the
site towards the creek inlet. The area which appears to be
most suitable for the broad-acre wetlands would not be
affected by streamflow, however groundwater infiltration
would need to be monitored.
11.3 Soils
Particle size and sediment analysis was carried out
during this study. Apart from those findings, a large hole
has been excavated at the dump site adjacent to the WPCW and
the soil profile can be clearly seen. In 1985, extensive
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studies were carried out at the Wide Bay Training Area
adjacent to Tin Can Bay for Department of Defence (Gillison
et al,1985). This area forms part of the Tin Can Bay "suite"
described by Coaldrake (1961), and these findings were also
useful.
The following is a description of the soils underlying
the area of the proposed first stage of the wetlands
construction:
"These are typical colluvial soils formed in drainage
depressions which are saturated during most of the year and
liable to flooding after heavy rains. They are vegetated by
either dense, low Banksia shrubs with Wallum heath and
sedges or open paperbark forest with an understorey of
Banskia, heath and sedges. The soils consist of uniform
grey or light grey sands which is overlain by a generally
40-60 cm, but up to 120cm thick, black to very dark grey A1
horizon with high organic matter content. The grey subsoil
is often yellowish brown mottled in the upper part and below
80cm may grade into coarse sandy clay. The soils are acid
to strongly acid throughout.."(Gillison et al,1985,p.32).
The significant clay content of the soil substrate
indicates that materials for the 'clay liner' for the
wetland will be available and will not have to be procured
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off-site, allowing considerable savings in construction
costs.
The Sanitary Reserve itself is a promontory of land
jutting into the Snapper Creek estuary. It is a large area
of unused flat land except for the dump site, and could be
used in the future if further expansion of the wetlands were
required. The following description of the soils in this
area indicates their suitability for wetland construction:
"Greyish brown sands ....dark brown to very dark grey A1
horizon with a loamy sand greyish brown A1 horizon merging
gradually into a pale brown (light) yellowish brown sand to
loamy sand transitional A/B horizon which is often gravelly.
Between 60 and 100cm this horizon abruptly overlies a dense,
massive, strongly grey, red and yellowish brown mottled clay
to heavy clay. These soils are imperfectly drained and have
a strongly acid reaction."
These findings were confirmed by borings on site and
particle size analysis.
11.4 Infiltration and permeability
Definition of the groundwater position and flow
direction is essential as the site may be affected by near-
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surface seasonally high groundwater. A number of test pits
will need to be dug and the water levels observed in the
borings. This data will provide information on the general
hydraulic gradient and flow direction for the area (Reed et
al,1995). From personal observation of the area, it appears
that the groundwater flows are directed towards the tiny
inlet marked 'O' on Figure 1. Underdrains are necessary for
a shallow groundwater table, although it does not appear
that they would be necessary at the Tin Can Bay site. With
careful construction of a non-porous clay liner, groundwater
infiltration can be avoided.
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11.5 Hydraulic Configuration
The hydraulic design of the wetland system is critical
to its successful performance. The hydraulic design of the
wetland is just as important as the pollutant removal model,
since those models are based on the critical plug flow
assumption, with uniform flow across the wetland cross-
section and minimal short-circuiting(Reed et al,1995).
Frictional resistance is imposed in a FWS wetland by the
vegetation and litter layer. Tight clumps of vegetation such
as Typha can increase resistance considerably. This matter
will be addressed later. The energy necessary to ensure
continuous flow is provided by the head differential between
the inlet and outlet of the wetland. Constructing a wetland
trench with a sloping bottom is an unreliable method of
achieving the head differential required since resistance to
flow may increase over time and solids and detritus build-up
will affect the slope. Some head differential can be
provided by a sloping bottom "with sufficient slope to
ensure complete drainage when necessary and to provide an
outlet which permits adjustment of the water level at the
end of the wetland. This adjustment can then be used to set
whatever water surface slope is required and in the lowest
position used to drain the wetland" (ibid,1995,p.203).
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The smallest hydraulic gradient possible should be used
to allow for future adjustments to the system and then the
maximum acceptable cell length can be calculated. In early
FWS systems, it was believed that length to width aspect
ratios of at least 10:1 were required in order to ensure
plug flow conditions. It has since been discovered that
resitance to flow increases with length because of the
accumulating vegetative litter. A length to width ratios of
3:1 have been proposed by Reed et al (1995) as cost
effective solutions. Breen and Spears (1995) working in
Melbourne, suggest a minimum of at least 5:1, with width of
flow path limited to reduce short-circuiting. As with most
engineering solutions, the width of the flow path is
probably best determined by the reach of the mechanical
earth-moving equipment used for construction, maintenance
and refurbishment, with berms between cells built wide
enough to be able to allow free passage of maintenance
equipment. Breen and Spears (1995) suggest a maximum width
of <10-15 metres.
The implication from these findings for Tin Can Bay WPCW
is that although the basins on the earth platform perform
well at present, they will have to be reconfigured and
regraded if they are to perform as constructed FWS wetlands.
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Rectangular wetlands work well if they have multiple inlet
and outlet structures installed to minimise short-circuiting
(QDPI,1995).
The general agreement amongst most researchers is that
the optimal wetland design is a number of varied wetlands in
series. With a number of flow paths operating in parallel,
it is possible to have considerable operational flexibility.
One flow path can be off-line at all times to allow for
variations in hydroperiod, maintenance, treatment rate, and
to provide overflow area for flood events and holiday peaks.
If disease strikes the vegetation in one trench, for
example, (not a common problem however) the trench can be
isolated.
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These photos of West Byron Treatment Plant give an idea
of the trench configurations used at that plant which allow
for maximum operational flexibility.
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11.7 Flow path zonation
Mitsch (1992),Breen and Spears (1995), Sainty
(1995),Russell and Kuginis (1995) and Hammer (1991),
emphasise the potential for using natural wetlands as a
model.
Breen and Spears (1995) have outlined the potential
number of treatment mechanisms available when zones range
from open water through herbaceous marshes to woody swamps.
"The zones are established by manipulating surface area to
volume ratios so as to encourage particular vegetation types
and treatment processes. In natural wetlands zonation is
often most clearly observed as being parallel to the flow
path. It is critical in constructed wetlands that zones
occur in series and are perpendicular to the flow path. In
this configuration zones have a uniform cross-section with a
common hydraulic resistance that will reduce the potential
for short-circuiting...Where sufficient slope is available
each zone in the flow path should be capable of independent
hydraulic control" (Breen and Spears,1995,p.209-210).
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Table 13 (After Breen & Spears,1995) Wetland Zone
Characteristics
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Figure 10: Natural Wetland Zones (after Breen &
Spears,1995;Russell & Kuginis,1995)
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11.7 Performance variability
Even with careful planning and rigorous performance
specifications, constructed wetlands, like any natural
system, suffer from performance variability. As constructed
wetlands are a relatively 'new' technology, subject to wide
performance variability in different climates, it will be
some time before reliable models are designed for each
climatic zone. Wetlands are designed at this point in time
using very conservative models. These models were based on
years of data collection from both successful and failed
systems, eventually creating a sufficient body of data to be
able to confidently develop pollutant removal models. The
West Byron Treatment Plant, for example, has scaled down its
hydraulic budget after four years of operation to meet
actual rather than 'estimated' loadings.
There are a number of factors which frequently cause
performance variability:
• seasonal changes in plant growth
• fluctuating water levels and 'shock' loads
• changes in influent water quality e.g.
treatment plant breakdown
• hydraulic short-circuiting
• poor vegetation management and weed infestation.
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This list highlights the sensitivity of a 'natural'
treatment system. Man has no control over seasonal
changes in vegetation, however careful monitoring and
management of the whole system from primary treatment
stage through to outlet weir will help to ensure
performance expectations are met.
11.8 Subgrade construction
Constructed wetlands require some form of impermeable
liner to contain the wastewater and prevent contamination
of groundwater.
Membrane liners such as plastic, asphalt and certain
chemical treatments can be used as artificial liners,
however these increase construction costs considerably.
Ideally a clay liner compacted to a nearly impermeable
state is used if clay is naturally present in the area.
From the soil survey it is clear that clay is present in
large quantities at Tin Can Bay. Clay liners have been
used at the Laminex Industries wetland and appear to be
successful at this stage.
Topsoil should be stripped from the site and
stockpiled for use as the planting medium in the FWS
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wetland. This practice ensures that existing native seed
stock in the soil will be preserved.
The floor of the wetland must be carefully graded
prior to placement of the liner. The precision required
for the construction of a golf course should be employed
in the wetland construction. The floor of the wetland
must be level from side to side for the entire length of
the wetland bed. A slight uniform slope to enable
drainage of individual cells is advisable, bearing in
mind that the hydraulic gradient and water level control
is provided by an adjustable outlet device as described
earlier (Reed et al,1995).
During the final grading operation , the clay liner
is compacted to a degree similar to that for road
subgrade construction to create an impermeable liner.
With this degree of compaction the design surface will be
maintained during subsequent construction activities
including placement of topsoil.
Trucks should not be driven over the finished cell
floor during wet weather!
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11.9 Placement of topsoil
Once the liner construction is complete, the
stockpiled topsoil can be placed directly over the clay
to serve as a rooting media for the vegetation. The
topsoil layer must be approximately 300mm deep. If soil
is to be imported, it is imperative to analyse its
nutrient status, pH and conductivity (Maslen,1995). The
P adsorption potential of the soil will give some
indication of performance expectations during the
establishment phase. Whilst the wastewater comes into
direct contact with the soil interface prior to detritus
build-up, P removal performance can be expected to be
quite good. All data to date suggests that the system
will take at least two years to reach equilibrium.
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11.91 Batter slope and freeboard
Steep cell slopes should be avoided as they do not
create an environment conducive to macrophyte
establishment. Steep slopes also create difficulties in
accessing each cell to remove weeds and maintain the
wetland in general. The groundsperson at Laminex
Industries plant, for example, has had to build a
specialised 'mower' in order to mow the steep slopes on
the wetland cells.
QDPI have suggested a maximum slope of 2:1 depending
on the soil characteristics, for wastewater treatment
wetlands. In areas where public safety is an issue, a
minimum slope of 3:1 should be used.
More gentle slopes encourage mosquito breeding and
stagnant pools amongst the macrophytes. Controlling
slopes and macrophyte growth encourages the movement of
fish and other predators (Russell and Kuginis,1995).
QDPI suggest a minimum of at least 300mm to 800mm
freeboard.
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11.92 Inlet and outlet structures
Uniform flow conditions through each cell are
required to achieve expected performance. Small to
moderate systems may employ "perforated manifold pipes
extending the full width of the cell, for both inlet and
outlets. An above-surface inlet manifold provides access
for adjustment and control and is preferred for most
systems" (Reed et al,1995).
Larger systems usually require concrete inlet and
outlet structures. The adjustment arms and valves must be
easily accessible for both daily use and maintenance of
the structures themselves.
The outlet weir must be adjustable to permit control
of the water level in each cell. It is preferable to
maintain an open water zone in the immediate vicinity of
the outlet weir to minimize clogging and to ensure water
quality samples are not contaminated by duckweed and
algae. A simple baffle structure upstream of the weir is
usually sufficient to prevent clogging.
Inlet distribution points "should not be spaced more
than 15 metres apart; 3 metres spacing will provide
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excellent distribution and uniform collection of the
liquid" (ibid,1995,p.278).
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Photo 6: Inlet at West Byron Treatment Plant
Experimental SF wetland
Photo 7: Final V-shape weir outlet at West Byron. The
baffle structure has degraded and allowed duckweed to
accumulate near the outlet.
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11.93 Aerators
As previously described, the limiting design factor
for the Tin Can Bay wetlands appears to be the area
required for nitrification. Nitrification requires
oxygen, which can be supplied to the system by using
prevailing winds to create surface reaeration, or by the
use of various forms of aeration equipment.
Mixing and aeration are important to (after
Bell,1995):
(1) disperse concentrated or fluctuating loads
(2) distribute oxygen and carbon dioxide
(3) distribute sludge
(4) distribute algae and zooplankton
(5) reduce stagnation or dead space
(6) reduce stratification
(7) ensure good contact with biological materials.
In order to maximise the potential for surface
reaeration, inlet and outlet structures should be
constructed at opposite corners of the cell (Bell,1995).
As a coastal settlement, the WPCW may be able to take
advantage of coastal breezes however this will not aerate
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sufficiently to introduce the required oxygenation of the
system. Spray aerators may prove to be the most suitable
at this site.
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11.94 Buffers and corridors
The Tin Can Bay WPCW Sanitary Reserve lies at some
distance from the residential area of Tin Can Bay. A
minimum distance of 200 metres between the wetland and
human habitation is desirable for construction of FWS
wetlands (Marble,1992).
Corridors and earth berms within the site should be
sufficiently wide and strong to allow the free and safe
passage of machinery and equipment required for
construction, operation and maintenance of the wetlands.
A natural vegetated buffer zone exists around the
site as it has been a designated Sanitary Reserve for
some time. The existence of the industrial estate and
golf course adjacent to the site will ensure that future
residential development does not encroach upon the site.
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12.1 Planting and vegetation management
Although a wetland can be constructed at any time of
the year, the project manager must plan to ensure plant
availability. Some plants may have to be ordered up to
one year in advance, although a number of nurseries now
specialise in the supply of wetland species. Once the
civil works are completed, the project takes on the
difficulties of any landscape construction project,
subject to vagaries of weather, labour supply and damage
caused to plants by humans and predators.
Wetlands can be planted out with seeds, seedlings,
mature plants or rhizome stock. The most reliable methods
appear to be seedlings and rhizome stock.
12.2 Soil Preparation
The topsoil in the wetland must be well-soaked and
loose prior to planting. Wetland plants find their best
growing conditions in moist soils. As the wetland will be
quite large, it is best to plant one section at the time,
with all personnel working together. Once an area is
planted it must be kept moist. Planting a 'sample area'
prior to commencement of the planting phase will give
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some idea if the area is going to be attacked by
waterfowl. If that is the case, then netting must be set
up over each unit after it is planted. As a number of
bird species are resident at the Tin Can Bay WPCW site,
it would be wise to include the cost of netting in the
budget.
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Photograph 8 : Newly planted cells at West Byron
Treatment plant covered with removeable netting.
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12.3 Planting densities and water control
Planting densities will vary according to plant
species, however denser planting (9 plants per m2) will
establish the wetland more quickly than sparse plantings
(1 plant per m2). Woody plants such as Melaleuca spp
will require plantings of only one plant per 6 m2, and a
number of trees will probably grow from seedstock in the
inundated areas adjacent to the plant.
Once the plants have been firmly placed in the
topsoil, the water level should equal half the height of
the shortest plant. For rhizomes and seeds, the soil
should be kept saturated until shoots are evident. The
best results are obtained by keeping the water level as
low as possible for as long as possible. Plants should be
no more than an average of one third their height in
water(Maslen,1995).
It is important to remember that this is a natural
system, and observing the natural tolerance to flooding
of the chosen plants will assist ongoing management.
Water depth is critical for the first year after
planting. Many wetlands have failed because of the
mistaken belief that the plants need or can survive in
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deep water(Hammer,1991). Most emergent macrophytes and
woody species prefer saturated not inundated soils. It is
preferable to introduce stresses from flooding into the
system slowly so the plants and other components have an
opportunity to adapt to the new environmental conditions
(Hammer,1991). Wetland plants are adaptable and will be
establish themselves quickly in the new environment.
12.4 Plant selection
Over the years a number of plant species have been
used successfully for the treatment of wastewater. Geoff
Sainty, a renowned wetland plant authority, has recently
published a field guide to waterplants in Australia. A
text specific to Queensland will shortly be published by
Ralph Dowling from the Queensland Herbarium, which will
focus on native species suitable for the sub-tropical and
tropical climate. A new list of preferred species has
just been released by DEH (See Appendix 2). Sainty et al
(1995) discussed the performance data available to date
from a constructed wetland in Blacktown, New South Wales
at the recent 'Wetlands for Water Quality Control'
conference. This wetland was built with specific
objectives in mind, including educational potential,
habitat enhancement and species richness, rather than
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simply wastewater treatment. Although these objectives
are broader than the specific objective of wastewater
treatment required at the Tin Can Bay WPCW, the
recommendations for plant selection are valid.
Sainty et al (1995,p.193) recommend a mix of species
in constructed wetlands to increase:
"(1)the probability of successful establishment;
(2)species richness
(3)aesthetics
(4)educational potential
(5)wildlife habitat."
They also suggest careful selection of propagating
material to reduce the susceptibility of the wetland to
disease and predation.
The most successful species to date is Phragmites
australis followed by Bolboschoenus fluviatilis. Typha
spp. were not selected for management and aesthetic
reasons, however a large number of hours have already
been spent to date weeding this plant from the wetland.
From the performance data collected at other
wetlands, it would appear that selection of a number of
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species endemic to the area of Tin Can Bay is the most
appropriate choice. From personal observation it would
appear satisfactory to include Typha spp. in the species
selected. Construction of different zones within the
system and careful vegetation management will eliminate
problems associated with this species and the costs
associated with eliminating it from the wetland.
Wetland plants were abundant in the area inundated
with effluent, where previously wallum heath existed.
Some of these plants would not be suitable for a
constructed wetland, however there were many plants with
reliable performance expectations in the area. The
presence of so many plants indicates that there is
abundant seed stock and propagation material available.
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The following species were noted in the immediate
vicinity of the Tin Can Bay site which would be suitable
for use in the constructed wetland:
Typha orientalis
Typha domingensis
Paspalum urvillei
Juncus articulatus
Juncus kraussii
Bolboschoenus fluviatilis
Baumea teretifolia
Schoenoplectus mucronata
Ludwigia octovalvis
Cyperus sesquiflorus
Cyperus esculentus
Melaleuca quinquenervia
Plant species related to the species listed would
also be suitable. For example, Melaleuca alternifolia may
be selected for its potential as a producer of 'tea-tree'
oil and for its potential for P removal in the light of
the excellent results obtained by Greenway and Bolton
(1995) at Griffith University.
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12.6 Weed control
As with any other crop or garden, weed species may
be regarded as any plant not wanted in the wetland. It
is therefore necessary to determine which plants are to
be eliminated and ensure that those managing and
operating the wetland on a day-to-day basis are aware of
the importance of constant maintenance. During the
establishment phase the wetland should be monitored at
least once a week.
At the time of writing, the most efficient form of
weed removal is physical removal of the plants. An
'invasion' can be controlled by slowly reducing water
levels and using a 'flame thrower'.
Herbicides should only be used as a last resort under
strict environmental controls and with approval of the
regulatory authority!(After Maslen,1995).
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12.7 Pest control
Birds are usually a problem during the establishment
phase and the use of netting is advisable until the
plants are of sufficient height and density to withstand
bird occupation.
West Byron Treatment Plant has had problems with the
introduction of faecal bacteria by the dense wildfowl
populations resident in the broad-acre Melaleuca
wetlands. This problem is seasonal; when the birds are
roosting, water is diverted around this area prior to
discharge into Belongil Creek (Andell,pers. comm.)
Although phytoplankton blooms were observed , chlorophyll
concentrations in the discharge seldom exceeded 200 ug/l
and blue-green algal populations were not dominant (Bavor
et al,1995).
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12.8 Harvesting
Harvesting is not usually practised in FWS wetlands
because of the high labor costs and difficulty of access.
"Studies have shown that harvesting of the plant material
from a constructed wetland provides a minor nitrogen-
removal pathway as compared to biological activity in the
wetland"(Reed et al,1995,p.184).
Some nutrients will be removed with the plants,
however the loss of biofilm can reduce wetland
performance in the short term. There is a net loss of
sediment litter and reduced potential for phosphorous
removal. Nutrients are released by stirring up sediments
(Maslen,1995).
Harvesting is usually only practised when plants have
formed dense clumps or mats and are impeding water flow.
Lemna (duckweed) may produce mats of vegetation.
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The potential for harvesting Melaleuca alternifolia
for 'tea-tree' oil production is an option that may be
considered. The trees will take a number of years to
reach maturity, by which time an assessment of the
potential for harvesting can be made. As noted earlier,
these trees appear to have excellent P removal
potential(Bolton and Greenway,1995).
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13.0 Dewatering
There will be occasions when vegetation management
requires a drawdown period. Phragmites australis, for
example, may die off and require burning off to clean up
the wetland cell. The inlet and outlet structures
recommended in the earlier section will permit control of
water levels.
Water should be released slowly to prevent high flows
causing erosion and downstream sediment release. "Water
levels of 50 to 300mm should be maintained for emergent
and submerged plants respectively if exposure without
moisture for more than short periods (1 to 2 days) is
envisaged. Alternatively irrigation may be necessary"
(Maslen,1995,p.5).
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14.0 Sedimentation Management
FWS wetlands are efficient removers of both fine
solids. The solids are mostly organic in nature and will
decompose over time, however at some stage removal of the
built-up sediments will be required. It could take up to
twenty years before a complete renovation is
required(Reed et al,1995).
The cells should be drained at a time which will
cause minimum impact to the wetland. For example, when
some plants die off over winter and need to be cut or
burnt-off, this would be an appropriate time to desilt.
"Desilting should be done a small section at a time in
bands across the wetland so as to minimise the impact on
the wetland. If replacement soil and plants are to be
placed in the desilted area these should be available to
be installed as soon as the desilting occurs so as to
minimise the down time....Disposal of the silt should be
done with the same approach as with sludge disposal
checking for contaminants before placing this nutrient
rich and possibly heavy metal and chemically contaminated
soil on agricultural or recreation land"
(Maslen,1995,p.4).
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15.0 Mosquito control
Wetlands provide an ideal habitat for the breeding of
mosquitoes. The mosquitoes can become both a nuisance
and health risk to nearby residents. Mosquito-borne
arboviral disease is becoming increasingly prevalant in
Australia (Russell and Kuginis,1995). In the last three
years,for example, epidemic polyarthritis, also known as
'Ross River virus', a mosquito-borne disease, has reached
epidemic proportions during the warmer months in
Queensland. The control of mosquito populations in
constructed wetlands is therefore an important objective
in the planning, construction and maintenance of
wetlands.
With regard to the Tin Can Bay WPCW, it would be
necessary to assess the relative abundance of the
existing mosquito population prior to construction of the
wetlands, in order to be able to assess post-construction
mosquito populations. Community perception of the
proposed wetlands' contribution to the mosquito
population could then be weighed against accurate data.
From personal observation, it would appear that the
existing environmental conditions in this area are
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conducive to the colonisation of mosquitoes, and their
presence can be felt at any time of day during warmer
months in the existing natural wetlands.
The following design criteria and operational
considerations can be used to reduce mosquito populations
in constructed wetlands(after Russell and Kuginis,1995):
Manipulating water levels within the wetland
The need to be able to manipulate water levels within
the wetland has been discussed previously in relation to
vegetation management. Water level fluctuations can be
used to interrupt the breeding cycle of mosquito larvae.
Flooding of the reed bed will interrupt the breeding
cycle of Aedes which require periodic drying of the
habitat. Flooding will also flush larvae sheltering on
the edges into the open water zones where they will be
exposed to predators such as fish and insects.
Appropriate plant selection and site habitat
enhancement
Predation by vertebrates and invertebrates can be
enhanced by creating a diverse environment with a variety
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of plant species and associated water levels.
Dragonflies, damselflies, beetles and bugs are all
important mosquito predators. Fish will inhabit the deep
water zones.
Some species of plants, such as Typha spp., can clump
tightly and inhibit access of predators to mosquito
larvae. In line with other management objectives, it may
be necessary to harvest and burn-off problem areas of
vegetation periodically.
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Providing areas of sufficient depth to discourage
mosquito breeding
Investigation of wetlands with dense vegetation has
found that water depths in excess of 40cm had lower
densities of mosquitoes and higher densities of
invertebrate predators such as fish (Batzer and
Resh,1992). Operating a reed bed zone with a depth of
approximately 50 cm satisfies the plant requirements and
allows fish to move through into open water zones.
Constructing bank gradients to discourage the
development of mosquito habitats
Public safety will not be an issue at Tin Can Bay
WPCW. A batter slope of 2-3:1 will discourage mosquito
breeding. A concrete apron around the edge of the cell
will also assist in preventing the formation of stagnant
pools. A smooth edge of either earth or concrete will
prevent macrophyte and grass growth.
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Chemical Control
Chemical control may be required on occasion in
response to episodes of heavy breeding. This is a short-
term response and should be avoided as a long term
strategy as mosquitoes may develop resistance to the
chemicals. The chemicals can also be toxic to fish, birds
and invertebrates, and treatment must therefore be
carried out with the consent and supervision of the
regulatory authority.
"Chemical control has the advantage that it can be
carried out within a short period of time and can produce
quick results at relatively low cost...environmentally
efficacious products such as bacterial derivates and
hormonal growth regulators can be used as well as more
conventional chemicals such as organophosphates" (Russell
and Kuginis,1995,p.221).
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16.0 Community involvement
During the planning phase of a project, it is
important to canvass community opinion before money is
committed to the project. Project goals should be
carefully and fully explained to gauge public attitudes
and reactions, and to modify opinions through education
if necessary and feasible. Although wetlands are viewed
with enthusiasm by those with an interest in
conservation, the view of wetlands as mosquito-infested
malodorous swamps persists. Community concerns about
snakes, mosquitoes, odors, depressed property values and
aesthetics should be addressed frankly.
As a complex and diverse biological community,
wetlands tend to attract human visitors, even if public
recreation is not a planning objective. The wetlands at
Tin Can Bay WPCW can be designed with the specific
objective of wastewater treatment, as significant areas
of natural wetlands exist in the Tin Can Bay estuary.
Nevertheless, public information and education programmes
should be considered in planning, design and operating
stages. Public information campaigns are vital to the
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acceptance and long-term success of this innovative
technology. Allowing the public to visit the site
increases public support for the allocation of public
funds to wastewater treatment technology.
The abundant birdlife at the existing Tin Can Bay
WPCW has already attracted environmentalists,
birdwatchers and other individuals interested in wetlands
and wildlife. The construction of broad-acre wetlands
will enhance the wildlife habitat and attract waterfowl
of increasing number and diversity.
Planning ahead for restricted visitor access will
avoid potentially dangerous situations. Reasonable
precautions include signage which complies with
Australian Standards Association criteria, prevention of
access to known hazards, and supervision of all visitors.
The proposed wetland for Tin Can Bay WPCW will not be
designed for public recreation. The design criteria for
this wastewater treatment wetland are not compatible with
those for a constructed wetland which is completely
accessible to the general public.
It is important to note that the operators of West
Byron Treatment Plant have found it necessary to
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completely fence and padlock not only the control rooms
and laboratories, but also the wetlands, due to vandalism
and attacks on wildlife (Andell,pers.comm.).
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17.0 Site layout and wetlands design
The proposed site layout and wetlands design is shown
in the following plan. It is envisaged that the first stage
of construction would include adaptation of the earth basins
to reed beds and construction of a 4 hectare broad-acre
wetland. The ready availability of land allows for further
expansion of the wetlands in the future if the nearby
population continued to increase. In view of the limited
availability of potable water and the ecological fragility
of the Cooloola National Park, the population may reach its
peak in the first part of the next decade.
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18.0 Costs
Until the Cooloola Shire Council commits to
augmentation of the treatment plant, it is impossible to
give even an approximation of the potential costs involved.
Needless to say, the construction and operational costs of a
wetland treatment system will be less than the construction
and operational costs of a traditional treatment process,
such as BNR.
Some estimates of costs are made below:
NETTING $8000/ha $48000
CLEAR AND GRUB
EARTHWORKS
CIVIL WORKS:
INLETS
OUTLETS
PLANT
PIPES
CLAY LINER:
EXTRACT,PLACE,
COMPACT
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VEGETATION
LABOR
ENGINEERING, LEGAL,
ADMINISTRATION CONTINGENCIES
LAND COSTS NOT REQD
TOTAL
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PART THREE
19.0 RESTORATION AND MANAGEMENT OF THE MELALEUCA WETLAND
AT TIN CAN BAY WPCW
This report has considered the effect of effluent
discharged from the Tin Can Bay WPCW on a Melaleucaforest
receiving site and an adjacent mangrove forest prior to
discharge into Snapper Creek. A constructed wetland has been
proposed for sustainable tertiary treatment of sewage at Tin
Can Bay.
The mangrove forest has not suffered any visible
negative impacts from the effluent flow after twenty years
of continuous inundation. The wallum heath complex of
vegetation which originally made up the 'receiving site' has
undergone radical changes and has clearly suffered
significant degradation as a result of the effluent flow.
The interaction of edaphic constraints and nutrient rich
freshwater inundation has resulted in the development of an
'artificial' wetland where once a heathland existed.
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It appears that the practice of overland flow is not
sustainable in this situation. If the proposed augmentation
of the Tin Can Bay WPCW is commenced, the fate of the
Melaleucaswamp must be considered. Although of little value
as a habitat, for aesthetics or conservation significance,
the area is Vacant Crown Land(V.C.L) and it cannot be
assumed that it would have a place as a phase in the
'treatment process' once an environmental management system
for Tin Can Bay WPCW is put in place.
The planning of the proposed augmentation of the Tin Can
Bay WPCW should therefore include establishing goals,
objectives and a general approach for restoration of the 1.5
hectare 'swamp'. The process will be relatively simple and
will contribute to the general amenity of the area.
19.1 Project goals
(1) Divert effluent flow from the existing receiving
site to a constructed wetland. Final discharge should
be via the mangrove forest prior to reaching receiving
waters.
(2) Restore original 'wallum' heath habitat.
(3) Clear area of noxious weeds.
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19.2 Project Objectives
(1) To augment the Tin Can Bay WPCW and provide a
sustainable tertiary treatment process.
(2) To ensure existing habitat values are restored
and/or maintained following the implementation of an
EMP
(3) To clear area of weed infestation thereby
assisting the process of natural regeneration and
facilitating maintenace of the site and the constructed
wetlands
(4) To alert and inform those using the site that a
restoration program is in place
(5) To monitor progress of the restoration project and
determine future mitigation objectives.
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1.To augment Tin Can Bay WPCW and provide a sustainable
tertiary treatment process.
The present system is overloaded and the practice of
discharging effluent onto adjacent V.C.L. is not
sustainable.
The construction of tertiary treatment wetlands means
that effluent of a higher quality will be discharged into
receiving waters. The mangrove forest adjacent to the
Sanitary Reserve in Snapper Creek has received the effluent
for twenty years without negative impacts. It is envisaged
that the outlet weir and control structures will direct
water into this area rather than permit overland flow across
wallum heathland.
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200
2.To ensure existing habitat values are restored and/or
maintained following the implementation of an EMP.
The fragile ecological balance of the wallum heathlands
flora and fauna at Tin Can Bay has been assessed by Gillison
et al,(1985) and recently by WBM Oceanics (1985). The
negative impact of effluent overflow and poor landscape
management has been described in this report. The wetland
habitats have proved more resilient coping with
anthropogenic influences.
The management directives described in this report would
give the existing 'swamp' an opportunity to return to wallum
heathland over time. In the short term, eliminating
effluent overland flow will ensure that nutrients are not
exported into the littoral zone, thereby preventing
eutrophication of receiving waters.
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201
3.To clear area of existing weed infestation thereby
assisting the process of natural regeneration and
facilitating maintenance of the site and the constructed
wetlands.
Weeds compete with trees and preferred plants and weed
prevention must be taken seriously. Once established, weeds
require costly control. Weeds reduce tree survival, impair
vegetation growth and reduce habitat quality.
An important part of maintaining a constructed wetland
that is attractive to wildlife and which meets water quality
objectives is weed management. In addition to the control of
succession and cover-water ratios through water level
management or fire more direct methods such as physical
removal, mechanical destruction and herbicides can be
used(Kusler et al,1990). Removing invasive weeds from the
surrounding vegetated buffer zone will reduce the potential
for weed invasion into the constructed wetlands.
Herbicides should only be used as a last resort with the
goal of minimizing losses of preferred growth and protecting
water quality.
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In a forested wetland, first year weed suppression is of
most importance to tree survival and growth. Once the canopy
closes, weed growth is suppressed. The landscape management
program will be most successful if continued for a few years
until 'natural' conditions prevail.
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4. To alert and inform those using the site that a
restoration program is in place.
Until recently, the unused vegetated areas (actually
V.C.L.) surrounding the Tin Can Bay WPCW have been used as
disposal sites for sludge and other waste from the plant. In
the Melaleuca swamp, dumping of waste with a high clay
content has resulted in die-back of trees and serious
invasion of noxious weeds.
All personnel should be informed that a restoration
program is in place and informed of the benefits of
following management objectives. Understanding how the
restoration process attempts to reinstate the original
ecological processes occurring within the coastal forest
assists workers in setting their own priorities for
maintenance tasks on a weekly basis.
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5.To monitor progress of the restoration project and
determine future mitigation objectives.
The data presented in this report can serve as a
baseline inventory for the area to be restored. Further
quantitative evaluation could be undertaken on an annual
basis for up to five years.
Both the constructed wetlands and the area being
restored will need to be monitored at least on a weekly
basis. A record of problems noted, action taken and
subsequent result should be kept to facilitate ongoing
management.
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Appendix One
Fauna survey
"The Terrestrial Vertebrate Fauna of the
Wide Bay Army Training Area
Dec. 1991, completed by Peter Driscoll"
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206
APPENDIX TWO - PLANTS CONSIDERED SUITABLE
FOR ARTIFICIAL WETLANDS IN QUEENSLAND
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