The Performance of a Multi-stage System of Constructed

8/4/2019 The Performance of a Multi-stage System of Constructed

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Ecological Engineering 16 (2001) 501517

The performance of a multi-stage system of constructedwetlands for urban wastewater treatment in a semiarid

region of SE Spain

R. Gomez Cerezo *, M.L. Suarez, M.R. Vidal-Abarca

Departamento de Ecologa e Hidrologa, Uni6ersidad de Murcia, Campus de Espinardo, 30100 Murcia, Spain

Received 11 August 1999; received in revised form 16 May 2000; accepted 12 June 2000

Abstract

This paper describes the results obtained in an experimental multi-stage system of created wetlands in Mojacar,

semiarid SE Spain, operating from June to October 1997. We compare the removal efficiency of four different seri

of treatments each consisting of three stages, using different flow rates of sewage, flow regimes, types of substrate an

influents. Pretreated water from an anaerobic stabilization pond and treated water from the last pond of a lagoo

system were used, the latter to test the systems suitability as a complementary system for removing nitrogen an

phosphorus. In spite of the initial high wastewater concentrations, the effluent conforms to the strictest Europea

norms (directive 91/271) for primary and secondary retention. A net treatment area of 2.3 m2/PE showed a hig

performance for SS (9096%), COD (87%) and BOD5 removal (90%) during the early stages of operation; howeve

nutrient removal was lower than was expected as compared with other studies. The addition of iron to the substraimproved phosphorus retention significantly (from 55 to 66%). The decrease of the net treatment area to 1.2 m 2/P

did not significantly affect the wetland performance, with the exception of COD removal (78%). Series fed wi

treated water from the lagoon system (1.6 m2/PE) noticeably improved the quality of the effluent (average values

7 mg/l total-N and 3 mg/l total-P). 2001 Elsevier Science B.V. All rights reserved.

Keywords: Constructed wetlands; Multi-stage systems; Nitrogen; Phosphorus; Phragmites australis ; Typha dominguensis; Purificati

efficiencies; Urban wastewater treatment; Tertiary retention; Semiarid areas

www.elsevier.com/locate/ecole

1. Introduction

The capacity of some helophytic plants to pu-

rify domestic and agricultural wastewaters has

been demonstrated by several studies (Seidel,

1976; Dykyjova, 1977; Radoux and Kemp, 198

1988; Gersberg et al., 1983; Brix, 1987; Denn

1987; Reddy and de Busk, 1987; Brix anSchierup, 1989; Martn and Fernandez, 1992; An

sola et al., 1995). The common reed and cattai

are the emergent plants most used in constructe

wetlands. Phragmites australis is the favoure

plant in European systems and Typha is one o

the dominant species in most of the constructe* Corresponding author.

0925-8574/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 5 - 8 5 7 4 ( 0 0 ) 0 0 1 1 4 - 2


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R. Gomez Cerezo et al. /Ecological Engineering 16 (2001) 501517502

wetlands in the United States (Cooper et al.,

1996). Both are capable of adapting to different

environments and waterloads, showing a high

growth rate in a short time.

In the same way, wetlands have the ability to

transform, retain and remove nutrients, and in

recent years natural wetlands have been used to

control nonpoint source pollution (Lowrance et

al., 1985; Cooper et al., 1986; Whigham et al.,

1988; Baker, 1992; Mitsch, 1992; Van der Valk,

1992; Puckett et al., 1993; Weller et al., 1994).

Constructed wetlands have been used for wastew-

ater treatment as well (e.g. Hammer, 1989;

Cooper, 1990; Brix, 1994; Green, 1997; Urbanc-

Bercic, 1997; Vymazal, 1997). The latter, particu-

larly, are widely recognized as an economical,

efficient and environmentally acceptable means of

treating many different types of wastewater.

In general, the performance of constructed wet-

lands is good in terms of the removal of sus-pended solids and organics. However, as regards

nutrient retention, constructed wetland perfor-

mance is not so good, since only $30% of nitro-

gen and phosphorus is removed (Brix, 1994).

For small urban areas and rural villages to

conform with European Union norms concerning

outflow quality from wastewater treatment plants,

adequate technology with low investment and op-

eration costs but with a high performance level

must be found, especially as regards nitrogen and

phosphorus removal. With the techniques avail-able, nutrient outflow concentrations are far from

the compulsory quality standards of the European

Union for sensitive areas (1015 mg N/l and 1 2

mg P/l).

Although most of the key processes involved in

wetland-based wastewater treatment are qualita-

tively well documented (e.g. Howard-Williams

1985; Kadlec and Hammer, 1988; Kadlec, 1994;

Richardson et al., 1997), quantitative information

on the rates of these processes and the factors

which affect them is scarce.As regards nutrient removal pathways, the ni-

trification denitrification complex and phospho-

rus precipitation and adsorption to soil Ca, Fe

and Al are especially important (e.g. Gersberg et

al., 1983; Richardson, 1985; Reddy et al., 1989;

Richardson et al., 1997). The physico-chemical

characteristics of substrates and flow regimes pla

an important role in all these processes, and stud

ies focused on groundwatersurface water inte

action in natural systems have emphasized th

importance of hydraulic conductivity and su

strate porosity (McIntyre and Riha, 1991; Vervi

et al., 1992; Triska et al., 1993; Holmes et a

1994; Pinay et al., 1994; Brunke and Gonse1997; Palmer, 1997). For example, in natural we

lands, nitrification may occur in shallow plac

with a coarse medium, whereas in places contain

ing the finest sediments and in saturated cond

tions denitrification may take place.

In this experimental study, we attempt to simu

late the environmental heterogeneity of natur

wetlands. For this, we use a multi-stage syste

where the cleaning processes are separated in

different steps. Such a system that optimizes trea

ment performance in relation to specific needs habeen used in other studies (Radoux et al., 1997

The experimental design is in some aspects sim

ilar to that which has been used by Radoux an

colleagues in Arlon, Belgium, since 1977 (Radou

and Kemp, 1982; Radoux et al., 1997) and b

Ansola et al. (1995) in Leon, Spain. Howeve

besides different patterns of flow water, we com

bine different types of substrate and water colum

depths.

The combination of the different factors (wat

flow, substrate type and water depth) at eacstage of treatment is determined by the process w

wish to favour: organic matter oxidation, nitrific

tion, denitrification or phosphorus fixation to th

substrate. Thus, the use of different particle siz

substrates at each stage of treatment is done

order to increase or decrease substrate hydraul

conductivity. In the same way, water colum

depth determines the major or minor oxygen di

fusion to the substrate.

Other new contributions are the use of inverte

vertical flow and vertical subsurface flow, thaddition of iron filings to the substrate and th

use of two different sources of water (pretreate

and treated water). Climatic conditions are al

different. Arlon (Belgium) and Leon (Spain) hav

a temperate climate while Mojacar has a semiari

climate. High temperatures and insolation, bot


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R. Gomez Cerezo et al. /Ecological Engineering 16 (2001) 501517 5

characteristics of semiarid climates, are factors

that affect microbial activity and plant

productivity.

The objective of this study was to analyze the

use of constructed wetlands as secondary and/or

tertiary treatment in a semiarid zone, in this case

in SE Spain. This paper describes the early stages

of four treatment systems operating from June toOctober 1997 that were used to treat two types of

urban wastewater: a pretreated wastewater and an

effluent from a lagoon system.

Results obtained from the treatment of the

lagoons effluent are used to discuss the usefulness

of constructed wetlands to improve the perfor-

mance of conventional treatment plants, espe-

cially for N and P removal. In the southeast of

Spain the lagoon is the most common system for

wastewater treatment, although it must be admit-

ted that the effluents do not always conform tothe EU norm concerning SS and organics re-

moval. The main aim was to attain the most

efficient secondary and tertiary treatment using

the smallest amount of area per person equivalent

(PE).

2. Materials and methods

The experimental wetland plant was con

structed at Mojacar wastewater treatment plan

(Almera, SE Spain). This municipal plant uses

lagoon system to treat water from the village

Mojacar and other nearby villages. As it is

typical tourist resort, there are pronounced fluctu

ations in hydraulic and organic load through th

year.

Located on the coast, Mojacar is characterize

by a semiarid Mediterranean climate. The averag

annual temperature is 21C, with average low an

high temperatures of 10 and 34C, registered

January and July, respectively. Annual rainfall

low, with an average of 237 mm.

The experimental plant at Mojacar is made u

of 24 tanks (Fig. 1), each with a surface area of

m2 and a volume of 0.8 m3 (0.71.43 m and 0

m depth) and laid out in four series on thrlevels. Each series was replicated to obtain mo

reliable results. Different hydraulic loads we

supplied automatically at regular 60-min interval

Series 1 and 2 received a hydraulic load of 19

l/day (hydraulic loading of 19.2 cm/day), series

Fig. 1. Diagram of the experimental system.


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received 372 l/day (hydraulic load of 37.2 cm/day)

and series 4 received 288 l/day (hydraulic load of

28.8 cm/day).

The tanks were filled with different types of

substrate of different particle size: sand (02

mm); fine gravel (12 mm); coarse gravel (22 mm)

and stones (\40 mm). All materials were cal-

careous. Substrate distribution among tanks is

shown in Fig. 1. Tanks with coarse materials were

topped off with fine gravel (upper 20 cm) in order

to secure a good distribution of the water.

The theoretical hydraulic retention time of each

series was: 7, 5.5 and 3 days for series 1, 2 and 3,

respectively and 4 days for series 4.

The different types of substrate were chosen on

the basis of their different hydraulic conductivity.

A coarse medium was used in those tanks where a

high oxygen concentration was required (i.e.

stages 1 and 3), whereas sand was used because of

its phosphorus fixing capacity and to ensureanaerobic conditions for nitrogen removal (stage

2).

The performance of each series was assessed by

chemical and microbiological analyses of the

inflow and outflow at 30-day intervals from June

to October, after a period of stabilization. In

June, measurements were taken at 15-day inter-

vals (six measurements in total). To assess the

performance of each individual treatment, analy-

ses of inflow and outflow from each tank were

made in June, July and October. It should bepointed out that the sampling period comprised

almost only part of the growth period. To moni-

tor subsurface conditions a piezometer, 60 cm

long, was located in all tanks.

Series 1, 2 and 3 (Fig. 1) were fed with pre-

treated water from an anaerobic stabilization

pond, whereas the water fed into series 4 was the

effluent from the lagoon system at Mojacar plant.

The experimental system tested at Mojacar

plant is a multi-stage system where the treatment

process is separated into three different steps (Fig.1). Stage 1 focuses on primary and secondary

retention and on obtaining an oxygenated effluent

in which nitrification can take place.

Stages 2 and 3 are designed to enhance the

results of primary and secondary retention and

for tertiary retention. In the second stage, we

recreate continuous water saturated conditions t

favour denitrification, followed by an oxygenate

medium at stage 3. In this last stage, series

contained a thin layer of iron filings (approx. 2

cm thick) because of their high capacity for pho

phorus fixation.

All tanks were planted with the most commo

rooted emergent macrophytes from nearby we

lands (P. australis and Typha dominguensis

Tanks were planted in April. Rhizome cuttings

Phragmites were collected from existing wetland

and planted at spacing of about eight rhizom

per tank. From the same wetlands young plan

of Typha (approx. 20 cm high) were directly tran

planted to the tanks (six plants per tank).

The first tank (stage 1) in series 2 and 3, wa

designed with an upflow vertical flow and plante

with P. australis. In both series, water was con

ducted to the bottom of the tank by a T waste

pipe located at the center of the tank. Thtreatment was compared with the design in th

first tank of series 1, where substrate was absen

and Phragmites was grown on a floating plast

net. This subsystem was designed with surfa

flow. The first tank in series 4 was also plante

with P. australis but a horizontal subsurface flo

was used in this case because of the higher oxyge

concentration of the influent.

The second tank in each series (stage 2) w

designed with a horizontal subsurface flow an

planted with T. dominguensis. Sand was used iseries 1 and 4, instead of the fine gravel used i

series 2 and 3. Water flowed over the substra

and rose to a height between 15 cm (series 1 an

4) and 5 cm (series 2 and 3).

The third tank (stage 3) in each series w

designed with a vertical subsurface flow an

planted with P. australis. To ensure homogeneou

water distribution over the substrate surface, tw

parallel channels (2 cm wide and 140 cm lon

were used approximately 20 cm over the su

strate. This design improves water oxygenatioSubstrate consisted of coarse gravel and stone

combined with a layer of iron filings in series 1

The net treatment area (stages 1, 2 and 3) pe

person equivalent in terms of hydraulic loadin

rate was 2.3 m2 for series 1 and 2; 1.2 m2 for seri

3 and 1.6 m2 for series 4. Calculations were mad


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Table 1

Characteristics of the raw and pretreated wastewater that was fed into series 1, 2 and 3 a

BOD5 (mg/l) SS (mg/l)COD (mg/l) Total N (mg/l) Total P (mg/l) Chl a (mg/l)

Raw wastewater

Average 212607 200 63 9

Pretreated wastewater

192 160Average 46449 8 0.49

132 30 9228 1S.D. 0.13

85 130 36 7 0.34Minimum 416

443 230 59 11484 0.58Maximum

a Values for raw wastewater were obtained from an integrated sample taken over a 24-h period. Sampling was made in the we

of the maximum hydraulic load of the year. Values for pretreated wastewater were obtained from June to October 1997 (n=

according to a volume of wastewater per inhabi-

tant per day of 150 l.

The parameters analyzed, using procedures out-

lined in the Standard Methods APHA (1989)

were: suspended solids, COD, BOD5, NH4+, NO3

, NO2, total-N, SRP, total-P, Chl a and fecalcoliforms. Other parameters monitored in treated

water were temperature, dissolved oxygen, salinity

and conductivity.

An ANOVA was done to detect differences in

the removal efficiencies of the analyzed parame-

ters according to treatments (series and individual

treatments) and sampling dates. Tukeys test was

applied as a contrasting hypothesis test.

3. Results and discussion

3.1. Plant establishment

With the exception of the first tanks of series 1,

2 and 3, plant growth was good, especially for T.

dominguensis. The healthiest plants and highest

densities were observed in series 4, which was fed

with treated water from the lagoon system.

In stage 1 of series 1, 2 and 3, the organic load

was too high (Table 2) for healthy P. australis

growth; the maximum height of the stems notexceeding 20 cm and the plants showed yellowing.

The growth of Phragmites in the rest of the

tanks was considerable. From April to October,

plants reached 70 cm in height with a cover of

approx. 7590% for series 1 3 and 100% for

series 4. In this period, Typha grew to an average

height of 300 cm with a cover of 100% for th

four series and its growth was vigorous, especial

in series 4. Radoux and Kemp in the plant

Viville (Belgium, 1982) also described bett

growth for Typha compared with Phragmites.

3.2. Purification efficiencies of series 1, 2 and 3

Table 1 shows the characteristics of the raw an

pretreated wastewater from the stabilization pon

that was fed into series 1, 2 and 3. Table 2 show

the inflow suspended solid (SS), organic and nu

trient loadings to the series. The increase in th

number of inhabitants during the summer seaso

(especially at the end of July) increases organ

loading, nutrient content and fecal pollutio

indicators.

3.2.1. Suspended solids, COD and BOD5 remo6a

Table 3 shows the mean outflow concentration

and removal percentages of the different param

ters studied from the three series. The removal o

suspended solids (SS), COD and BOD5 was ver

high in all series.

Table 2

Inflow SS, organic and nutrient loadings to the series (g m

day1)

Total-NCOD Total-PBOD5 SS

86.2 36.9Series 1 30.7 8.8 1.5

1.586.2 8.8Series 2 30.736.9

17.159.5 3.071.4167.0Series 3

86.7Series 4 17.6 51.8 9.8 2.3


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Table 3

Mean values of outflow concentrations and series performance during the experimental period (JuneOctober 1997)a

BOD5 (mg/l) SS (mg/l)COD (mg/l) Total N (mg/l) Total P (mg/l) Chl a (mg/l)

192 160Inflow 46449 8 0.49

69CV 396 20 14 27

Series 1

18 17Outflow 2958 3 0.03

90 90 3887 66Removal (%) 95

5S.D. 83 29 20 2

5 8 764 30CV 2

Series 2

15 6 2859 4Outflow 0.05

90 96 41Removal (%) 5587 90

6 3 225 12S.D. 7

6 3 53CV 225 7

Series 3

23 7Outflow 35100 4 0.17

88 96 2378 48Removal (%) 61

8 2 26S.D. 1210 34

9 3 11512 26CV 55

61 180 34Inflow 8301 0.57

57 46 1823 16CV 12

Series 4

7 7 783 3Outflow 0.02

90 95 78 60Removal (%) 9670

10 10 1618 13S.D. 1

26CV 11 11 20 22 1

a Standard deviation (S.D.) and the coefficient of variation (CV) of the mean removal percentage are also shown.

In spite of the different treatments used, the

two-way variance analyses (ANOVA) (Table 4)

did not show significant differences between series

1 and 2, with the exception of SS retention. The

average of SS retention from series 2 was higher

(96%) than from series 1 (90%). In both cases the

effluent conformed, during the experimental pe-

riod, to directive 91/271 of the European Union

(EU) in terms of SS, COD and BOD5 removal. In

series 1 and 2, the EU norm for removal of COD

(125 mg O2/l) was always met even at the end of

the second stage (Fig. 2).The increase of the flow rate to 16 l/h in series

3 slightly decreased the removal efficiency of or-

ganics (COD and BOD5) but had no effect on SS

retention. Although differences were observed,

they were only statistically significant for COD

removal (PB0.05).

A general increase in outflow concentration

from series 3 was also observed. Mean COD an

BOD5 concentrations at the end of the series we

very close to the limits imposed by the EU norm

unlike series 1 and 2 (Fig. 2).

At the temporal scale, the ANOVA analys

showed significant fluctuations of wetlands c

pacity for SS and organic removal (Fig. 3). Th

maximum values of removal of COD and BOD

from series 1 and 2 were detected in Augus

coinciding with the highest temperatures. At th

time, the retention percentages for series 1 and

were 90 and 93% for COD and 94 and 96% foBOD5, respectively.

In spite of the variations observed, over th

study period the removal fluctuations of SS an

organics from series 1 and 2 were much low

than the fluctuations of inflow SS and organ

concentrations (see CV, Table 3). Only in series


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was the CV of COD removal higher than the CV

of COD inflow. These results show that the wet-

land maintained a high purification efficiency for

a pollutant load of varying constituent concentra-

tions. In the case of COD removal, results seem to

indicate that this affirmation is true within a

range of hydraulic loads.

3.2.2. Nitrogen and phosphorus remo6al

The efficiency of series 1 and 2 for retention of

nutrients was low, especially for nitrogen (Table

3). During the study period, the wetland per-

formed better for phosphorus removal than for

nitrogen. Although there were no significant dif-

ferences between the series (P\0.05), the pres-

ence of a layer of iron filings in series 1 had

positive effect on phosphorus removal. The pe

centage of phosphorus retention in series 1 w

20% higher than in series 2.

The increase of the flow rate decreased nutrien

removal in series 3, although differences were no

significant (P\0.05). The greatest difference

compared to series 2 was in nitrogen removal. Thmean percentage of nitrogen retention decrease

by 44% from series 2 to series 3. Outflow nutrien

concentrations were also higher than in series

and 2 (Fig. 4).

The EU norms for nitrogen (10 15 mg/l o

70 80% reduction) and phosphorus removal (1

mg/l or 80% reduction) for sensitive areas was m

Table 4

Results of the two-way variance analyses among series

df Mean square F-test df P-value P-valueMean square F-test

SS

SourceSource

0.4000.846157.2291Series 2-3 (A)Series 1-2(A) 0.01811.818410.8431

0.0005 3521.981 19.458 0.000 Date (B) 5 4488.570 226.208Date (B)

5 34.763 0.192 0.960AB AB 5 185.921 9.370 0.001

19.8431212 ErrorError 181.006

BOD5SourceSource

1 0.058 0.005 0.945Series 1-2 (A) Series 2-3 (A) 1 104.179 5.331 0.069

Date (B) 5 93.851 12.192 0.000 Date (B) 5 179.835 30.555 0.000

1.42610.9755 3.320 0.04119.541AB 5AB0.284

12 7.698Error Error 12 5.886

COD

SourceSource

9.552 0.0274.975Series 1-2 (A) 0.2191 0.659 Series 2-3 (A) 1 207.891

0.856 0.54234.466 8.113 0.004 Date (B)Date (B) 55 60.490

5AB AB0.0155.33922.680 0.8970.30821.7645

4.2489Error Error 10 70.698

Total-N

Source Source

2024.9791Series 2-3 (A)0.655 6.3740.22546.5231Series 1-2 (A) 0.053

5 0.0042232.300 12.285 0.000 Date (B) 5 1589.420Date (B) 6.403

1.280 0.335206.920AB 1.1395 0.392 AB 5 317.699

181.715Error 12 Error 12 248.248Total-P

Source Source

1 643.961 2.194 0.199Series 1-2 (A) Series 2-3 (A) 1 268.836 1.132 0.336

5 0.006Date (B) 6.207286.1325Date (B)0.3071.137330.578

0.0115.152237.5115AB AB0.3631.217293.4635

46.09811Error241.04911Error


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Fig. 2. Mean outflow concentrations from the different stages of series. Broken line shows the EU quality standards (Directiv91/271: SS=35 mg/l \10 000 PE and 60 mg/l if 200010 000 PE).

only occasionally. Over time, wide fluctuations in

nitrogen removal were observed. The maximum

values of nitrogen retention (74, 67 and 57% for

series 1, 2 and 3, respectively) were observed at

the end of July (Fig. 5), when mean outflow

concentrations were 11, 14 and 19 mg/l, respec-

tively. During the rest of the study period, outflow

nitrogen concentrations fluctuated sharply.The maximum retention for phosphorus in se-

ries 1 and 2 was observed at the beginning of

August. As with nitrogen, the lowest outflow con-

centrations (1.6 and 2.2 mg/l for series 1 and 2,

respectively) were detected during the warmest

days of summer.

Over time, the variability in nutrient remov

was high but fluctuations of inflow concentratio

were even greater (Table 3). There was no relatio

among nutrient removal and nutrient inflow con

centrations. However, both series showed simila

temporal patterns of nutrient removal (Fig. 5

suggesting that nutrient retention is more depen

dent on environmental conditions (temperaturstate of vegetation, growth of bacterial popul

tion, etc.) than on changes in inflow nutrien

concentrations.

To determine which factor or factors are r

sponsible for the temporal pattern observed

nutrient retention is not easy. The effect of tem


9/17


perature on denitrification rates is well docu-

mented (e.g. Knowles, 1982), so the loss of nitro-

gen via denitrification could explain the increase

of nitrogen removal as temperature increases.

However, it is difficult to explain the nitrogen

removal dynamics from wetlands on the basis of

only one parameter. Adsorption, ionic exchange,

volatilization, plant absorption and uptake and

nitrification denitrification complexes are the

most important removal pathways. On the other

hand, the most important process responsible for

phosphorus removal in wetlands is precipitation

with soil Ca, Fe and Al, and redox potential

and pH are important factors controlling the pro

cess similar to variable factors (Richardso

1985).

3.2.3. Treatment efficiencies

Table 5 shows the performance of the individ

ual treatments used within the series. Differenc

observed were not statistically significant, but th

low number of dates used in the ANOVA analy

ses (df=6) has to be noted.

At the first stage, neither a surface flow nor a

inverted upflow represented the optimal design fo

SS removal. In both cases, the SS concentration i

Fig. 3. Removal percentage of COD, BOD5 and SS from series during the experimental period.


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Fig. 4. Mean outflow concentrations from the different stages of series. Broken line shows the EU quality standards for sensiti

areas (Directive 91/271: TN=10 mg N/l \100 000 PE and 15 mg/l if 10 000100 000 PE; TP=1 mg P/l \100 000 PE and 2 m

P/l if 10 000100 000 PE).

the outflow water was higher than in inflow water,

especially at the first stage of series 1 with surface

flow. The low plant density in the surface flow

treatment led to a significant growth of phyto-

plankton, which, in turn, increased particle content.

In series 2, in spite of the presence of a substrate,

the inverted upflow forces water to flow from thebottom of the tank to the top, which is devoid of

vegetation, resulting in an increase of outflow SS

concentration as well.

Organics removal was also low in both treat-

ments for the same reasons.

The second stage performed better as regards

both SS and organic removal. Despite the results

obtained by other authors (Radoux et al., 1997), SS

retention was not very high, regardless of whether

sand or gravel was used as substrate. Compared to

the inflow water to the series, at stage 2 removal ofSS reached only 4414%, from series 1 and 2,

respectively. At this stage, the anaerobic conditions

created (fine substrate and a water column of 515

cm over the substrate) determined the formation of

insoluble ferrous sulfide (FeS) which, in turn,

increases outflow SS concentration.

Although differences between treatments we

not statistically significant (F=16.0, P=0.057

the use of sand improved SS retention during th

study period. However, the risk of clogging shou

not be forgotten for longer periods of operation

COD and BOD5 removal at stage 2 was high an

noticeably higher than SS retention. There was ndifference between sand or gravel substrates; how

ever, the increase of hydraulic load (series

significantly affected (F=30.962, P=0.031) th

COD removal (Table 5). The COD retention d

creased to 4941%, as compared with series 1 an

2, respectively.

The highest SS removal was observed in the thir

stages, where P. australis was planted in a coar

medium and a vertical subsurface flow was used. A

has been demonstrated in other studies (Radoux

al., 1997), vertical subsurface flow systems showhigh capacity for SS retention. From stage 2 t

stage 3 the percentage of SS retention, as compare

to inflow water to the series, increased to 89% fo

series 1 and to 96% for series 2.

The addition of a thin layer of iron filings t

series 1 resulted in an increase in the outflow S


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Fig. 5. Removal percentage of total-N and total-P from series during the experimental period.

Table 5

Performance of the individual treatments (series 13) as regards SS and organics removala

series Stage COD (mg/l)SS (mg/l) BOD5 (mg/l)

Outflow Removal RemovalInflow OutflowInflow Outflow Removal Inflow

(%)(%) (%)

449 (11)26143 (17) 27327 (42)1 192 (54)1 160 (10) 209 (10) 31

192 (54) 173 (20) 10 449 (11) 298 (20)2 1 160 (10) 190 (20) 3419

137 (19) 29 449 (11) 271 (37)3 1 160 (10) 140 (30) 13 192 (54) 40

44169 (9)301 (28)6261 (14)4 23 (4)1 180 (34) 59 (17) 67

143 (17) 34 (7) 76 327 (42) 102 (7) 691 2 209 (10) 89 (18) 57

61115 (9)298 (20)7445 (11)2 174 (20)2 190 (20) 138 (10) 26

137 (19) 39 (7) 72 271 (37) 173 (21) 363 2 140 (30) 110 (30) 21

169 (9) 90 (14) 4723 (4) 654 8 (2)2 59 (17) 58 (14) 234 (7)* 18 (4)* 47 102 (7)* 58 (6)*1 3 89 (18)* 17 (4)* 4378

4959 (6)*115 (9)*6715 (2)*2 45 (11)*3 138 (10)* 7 (2)* 95

39 (7)* 23 (3)* 41 173 (21)* 100 (10)*3 3 110 (30)* 6 (1)* 4291

8 (2)* 7 (2)* 13 90 (14)* 83 (11)*4 3 58 (14)* 7 (2)* 888

a In parentheses are the S.E. of the mean concentration, n=6.

* n=12.


12/17


Table 6

Performance of the individual treatments (series 13) as regards Total-N and Total-P removala

series Total-N (mg/l)Stage Total-P (mg/l)

Inflow Outflow Removal (%) Inflow Outflow Removal (%)

1 1 46 (4) 40 (5) 13 8.4 (0.5) 5.8 (0.7) 31

46 (4) 36 (6) 22 8.4 (0.5) 6.7 (1.5) 202 1

46 (4) 40 (5) 131 8.4 (0.5)3 6.6 (0.6) 2114 34 (3) 26 (4) 24 8 (0.5) 5 (0.5) 38

40 (5) 32 (5) 201 5.8 (0.7)2 4.5 (0.2) 22

36 (6) 28 (6) 222 6.7 (1.5)2 4.7 (0.4) 30

23 40 (5) 36 (5) 10 6.6 (0.6) 4.9 (0.3) 26

26 (4) 9 (3) 65 5 (0.5) 3 (0.6)4 402

32 (5)* 29 (5)* 91 4.5 (0.2)*3 2.9 (0.5)* 36

2 3 28 (6)* 28 (4)* 0 4.7 (0.4)* 3.8 (0.4)* 19

36 (5)* 35 (4)* 3 4.9 (0.3)* 4.3 (0.3)*3 123

9 (3)* 7 (2)* 22 3 (0.6)* 3 (0.4)*3 04

a In parentheses are the S.E. of the mean concentration, n=6.

* n=12.

concentration. The outflow water showed the typ-

ical orange color of oxidized iron and the percent-

age of SS reduction decreased from 95% in series

2 (without iron) to 78%.

In stage 3, organics removal was less consider-

able than at the second stage, in spite of the lower

BOD5 and COD inflow concentrations. The con-

tribution of stage 3 to the additional removal of

the BOD5 and COD load was low; 7 10% for

series 1 and 2, respectively.Nutrient retention was low in all treatments. At

stage 2, the presence of a high density of plants

did not lead to much improvement in nutrient

retention, as compare to stage 1. The same oc-

curred when nutrient removal among stage 1 (low

density of plants) and stage 3 (high density of

plants) are compared. Other results, however (e.g.

McIntyre and Riha, 1991; Ansola et al., 1995;

Radoux et al., 1997), show that the presence of an

actively growing helophyte has a positive effect on

nutrient removal, regardless of the water flowsystem.

Results indicate that at Mojacar plant, the hy-

draulic load is too high (19.2 37.2 cm/day) to

observe a deleterious effect of plant uptake on

nutrient removal. Richardson et al. (1997) hy-

pothesize that once phosphorus loadings exceed 1

g m2 year1, short-term mechanisms are satu

rated. Sediment/peat accumulation is the majo

long-term phosphorus sink and microorganism

and vegetation are a short-term sink.

Differences of substrate (sand or gravel) had n

effect on retention of nutrients; however, the pre

ence of iron filings (series 1) increased the pho

phorus retention from 19% (series 2) to 36%

stage 3.

Taking into account the helophyte species usein each treatment, results of removal of organic

are similar to those obtained by Radoux an

Kemp (1982) and later by Ansola et al. (1995

The mean values of BOD5 and COD retention

Mojacar plant were 75 and 65% for Typha and 5

and 46% for Phragmites, respectively. Howeve

the authors cited obtained higher removal efficie

cies for nutrients.

Table 7 compares results from the first tw

stages of the Mojacar plant with those obtained b

Radoux et al. (1997) at Viville Plant. Both experments have the same surface treatment, 2 m2 (tw

tanks of 1 m2 disposed in series), and a simil

combination of treatments. The differences lay i

the hydraulic load and inflow water compositio

Although differences exist, to compare both expe

imental plants yields interesting results.


13/17


Table 7

Mean inflow load (g m2 day1), stage 2 outflow load (g m2 day1) and removal percentages in the first and second stages

Viville (Radoux et al., 1997) and Mojacar plants

Mojacar plantbViville planta

Hydraulic load: 72 l/day Hydraulic load:

192 l/day (series 1 and 2) 372 l/day (series 3)

In Out Removal (%) In Out Removal (%) In Out Removal (%)

2.2 87 30.7 14.8SS 5217.5 59.5 40.5 32

8.4 60 86.2 20.9COD 7621.0 167.0 64.4 61

2.2 67 36.9 7.76.8 79BOD5 71.4 14.5 79

Total-N 1.3 1.0 26 8.8 5.8 35 17.1 13.4 22

0.2 45 1.6 0.93.8 45Total-P 3.1 1.8 42

a Date 1992/93; treatment surface 2 m2.b Date 1997; treatment surface 2 m2.

Pollutant load was higher at Mojacar plant,

although the system performed better as com-pared to Viville plant in organics and nitrogen

removal, at least during the experimental period

(first 5 months of systems operation). However

SS retention was lower at Mojacar plant. With the

exception of SS removal, the performance of both

systems was similar when the hydraulic load in-

creased from 192 to 372 l/day at Mojacar plant.

Climatic conditions might explain the high per-

formance observed, although we have to be cau-

tious in this respect. The high efficiency observed

at Mojacar plant could be explained by its age, inspite of the fact that the best results from con-

structed wetlands are often obtained after the first

year of operation (e.g. Brix, 1987; Green, 1997).

Longer periods of operation, with a well-estab-

lished microbial population and vegetation, might

improve efficiency, although the risk of clogging

also increases.

3.3. Purification efficiencies of series 4

3.3.1. Suspended solids, COD and BOD5 remo6alTable 3 shows the characteristics of the inflow

water to series 4 (treated water from a lagoon

system) and the performance values of the series.

In comparison with the other series, the main

differences of the inflow water was its lower

BOD5 and higher SS load as compared to series 1

and 2 (Table 2). The chemical composition of th

inflow water did not meet the EU norms fsecondary treatment, even though the water cam

from a secondary treatment plant.

The efficiency of the series was very high an

the effluent always conformed with EU norms fo

SS, COD and BOD5 even at the end of the secon

stage (Fig. 2).

With the exception of COD removal, the effi

ciency of series 4 was similar to those registered

the other series. The mean COD removal percen

age decreased from 8780% (series 1, 2 and 3) t

70%. However, the inflow COD load to series (86.7 g m2 day1) was similar to series 1 and

and even lower than in series 3 (Table 2). Resul

seems to indicate that residual COD from a se

ondary treatment is more refractory to degrad

tive processes. The same reasons could explain th

decrease of COD removal at stage 3 in series

and 2 (Table 5).

During the study period COD retention varie

widely (Fig. 3), unlike in series 1, 2 and 3, ind

pendently of the inflow load fluctuations. How

ever, COD retention increased from the beginninto the end of the study.

The performance of the series as regards BOD

removal was very good, with retentions over 90%

BOD5 outflow concentration, even at the end o

stage 1, was below the limit imposed by the E

norm. Removal of SS was also high and similar t


14/17


series 1 and 2 efficiency, in spite of the higher

inflow SS load to series 4 (Table 2). However, a

third stage of treatment was necessary to ensure a

high performance level whose outflow conformed

to the strictest European norms (Fig. 2).

3.3.2. Nitrogen and phosphorus remo6al

Series 4 performed especially well for nitrogenremoval. The mean retention percentage was high

(78%) and there was almost no fluctuations in

nitrogen removal over time (Fig. 5). Most of the

values were between 72 and 90%.

A more nitrified influent in series 4 can explain

differences observed as regards series 1 and 2,

because the influent load to series 4 was even

higher than in the other series (Table 2). NH4-N

diffusion from the anaerobic soil layer to the

aerobic soil layer and nitrification in the aerobic

soil layer are limiting steps in controlling nitrogenloss (Reddy et al., 1980) and especially in

wastewater treatments.

The mean nitrogen outflow concentration from

stage 2 was 9 mg/l and at the end of the series, 7

mg/l. Both values complying with the EU norms

(Fig. 4).

The performance of series 4 in phosphorus

retention was also better than in other series. The

mean phosphorus removal percentage (60%) did

not differ from series 1 and 2; however, the

influent load to series 4 was higher.Mean phosphorus outflow concentration (3

mg/l) was close but did not reach the effluent

standard of 12 mg/l imposed by EU norm. The

minimum and maximum values observed were 0.7

and 5.1 mg/ P/l, respectively.

3.3.3. Treatment efficiencies

The combination of the two horizontal subsur-

face flow systems at stage 1 and 2 (differences

were type of substrate and water column depth)

performed more than satisfactorily for BOD5 andCOD removal. Both treatments showed similar

organic retention percentages (Table 5). Com-

pared to the inflow water to the series, organic

removal at the end of stage 2 was 87% for BOD5and 70% for COD. The third stage did not greatly

increase the removal of organics; however, it was

necessary to reduce SS outflow concentration un

der the EU limit of 35 mg/l (Fig. 2).The efficiency of the second stage in BOD5 an

COD removal was slightly lower in series 4 tha

in series 1 and 2. Results seem to indicate th

importance of organic compounds quality raththan quantity. In the same way, organic retentio

at stage 3 was almost negligible compared wi

the results obtained in series 1 and 2.In accordance with the results from the oth

series, the effect of the second stage on SS r

moval was practically nil (2%), while the thir

stage produced a high SS removal at the level othe first one.

As could be expected and unlike series 1 and

nitrogen removal at stage 2 was high. Differencebetween series (Table 6) can be explained on th

basis of nitrate availability to denitrification,

has already been commented.

That plant uptake is not the main mechanismin wetland nutrient retention (e.g. Gersberg et al

1983; Richardson, 1985) is clear when perfo

mance of treatments at stage 1 is compared (Tab6). The presence of a high density of plants grow

ing at stage 1 of series 4, had no significant effe

on nutrient removal. However, nitrogen removincreased significantly at stage 2, where denitrifi

cation could take place. As is well known, den

trification is the main factor in nitrogen remov

in a flooded substrate (e.g. Reddy et al., 198Gersberg et al., 1983).

Phosphorus retention took place exclusively

stages 1 and 2. The higher efficiency of series 4 phosphorus removal can also be explained,

part, by the higher oxygenation of the inflo

water. Sediment/peat accumulation is the majolong-term phosphorus sink (Richardson, 1985

but under anaerobic conditions phosphorus m

bility increases. Compared to the inflow concen

trations to the series, the mean nitrogen anphosphorus removal at the end of stage 2 was 7

and 63%, respectively.

4. Conclusions

During the early stages of operation, a create

wetland with a net treatment area of 2.3 m2/P

was enough to ensure a high performance for S


15/17


and organics removal; however, nutrient removal

was lower than was expected as compared with

other studies.

As primary and secondary treatment, the

decrease of the net treatment area to 1.2 m2/PE

did not significantly affect the wetland

performance with the exception of COD removal.

The hydraulic retention time is an importantfactor controlling COD removal from wetland.

However, even with an area of 1.2 m2/PE, effluent

conformed to directive 91/271 of the European

Union for SS and organics removal.

Wetland maintained a high purification

efficiency for SS and organics removal for a

pollutant load of varying constituent con-

centrations, whereas the efficiency of nutrient

removal fluctuated widely. The efficiency of COD

removal depended on both inflow loadings and

COD quality.Climatic conditions (high temperatures and

high insolation rates) might explain the

performance observed at Mojacar plant as

compared with Viville plant. Removal efficiencies

observed at Viville plant, with a net treatment

area of 4.2 m2/PE, were obtained at Mojacar

plant with a treatment area of 0.8 m2/PE.

The performance of the surface flow and the

inverted upflow systems in the first stage of

treatment was not good as regards SS and organic

retention.Subsurface flow systems contribute significantly

to COD and BOD5 removal, regardless of water

column depth. However, the performance of the

system as regards SS retention depends on it. At

stage 2, the water column rose to a height

between 5 and 15 cm and SS outflow

concentration was increased by the formation of

insoluble ferrous sulfide (FeS). The use of sand

improved SS retention; however, because of the

risk of clogging, the advantages of the use of sand

have to be evaluated.Although the performance of this treatment for

organics removal is high, the subsurface flow

system does not offer conditions for nitrification.

So, if nitrogen has to be removed in the next

treatment stage, water oxygenation has to be

ensured.

At Mojacar plant, the hydraulic load is to

high to observe a deleterious effect of plants o

nutrient retention (short-term sink), as is observe

in other studies. It would be necessary to increas

the hydraulic retention time to improve th

performance of the wetland as regards nutrie

removal. Denitrification (long-term sink) w

limited in series 1, 2 and 3 by nitrate availabilitThe addition of iron filings is a useful an

cheap method to improve phosphorus retentio

however outflow has to be treated to reduce S

concentration.

To improve the water quality from the lagoo

system, a total area of 1.6 m2/PE was necessary t

obtain an effluent conforming with the stricte

European norms for secondary treatment plan

and for sensitive areas. Unlike in the case

nitrogen, the wetland system did not general

reduce phosphorus outflow concentrations below the EU limit for sensitive areas, althoug

they were very close to the limit.

Acknowledgements

We are grateful to Gema Ansola and the D

partment of Ecology of Leon University (Spai

for their comments; to the company GALASA fo

supporting the construction of the experiment

plant at Mojacar wastewater plant and to thanonymous reviewers for their critical review

the manuscript. Funds were provided b

GALASA and the Ministerio de Educacion

Ciencia of Spain.

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