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TasWaterBlackmans Bay STP DPEMP

Near-Field Modelling of Effluent

August 2016

GHD | Report for TasWater - Blackmans Bay STP DPEMP, 3218107 | i

Table of contents1. Introduction.....................................................................................................................................1

1.1 Purpose of this Report .........................................................................................................1

1.2 Assumptions ........................................................................................................................1

1.3 Scope and Limitations..........................................................................................................2

2. Edge of Mixing Zone Criteria..........................................................................................................3

2.1 Water Quality Objectives .....................................................................................................3

2.2 Effluent Quality.....................................................................................................................5

2.3 Dilution Needed at the Edge of the Mixing Zones ...............................................................5

3. Model Description, Input Data and Sufficiency of Currents for Dilution.........................................7

3.1 Model Description ................................................................................................................7

3.2 Input Data ............................................................................................................................7

4. Results .........................................................................................................................................11

4.1 Sufficiency of Ambient Currents to Dilute Effluent Discharge............................................11

4.2 Mixing Zone Assessment...................................................................................................12

5. Conclusions..................................................................................................................................23

6. References...................................................................................................................................24

Table indexTable 1 Summary of adopted edge of mixing zone trigger values and ecosystem

guideline values outside of the mixing zone ........................................................................4

Table 2 NHX trigger values on the basis of pH .................................................................................5

Table 3 Blackmans Bay STP current and upgrade 90th percentile effluent quality ..........................5

Table 4 Dilutions required at the edge of the mixing zones for toxicity and ecosystemeffects thresholds.................................................................................................................6

Table 5 Calculations to confirm sufficient currents to dilute STP effluent discharge tomeet toxicant dilution needs ..............................................................................................11

Table 6 Modelling run specifications...............................................................................................12

Table 7 Near-field modelling dilution plots and assessment against dilution needs fortoxicants at the edge of the mixing zone and nutrient enrichment ecosystemguidelines. In the dilution plots red and blue are for predictions for diffusernozzles that are aligned with and against the current, respectively, the linesrepresent average dilution of the plume and the symbols the centreline dilution..............13

GHD | Report for TasWater - Blackmans Bay STP DPEMP, 3218107 | ii

Figure indexFigure 1 Ambient monitoring locations in the Derwent Estuary and location of the

Blackmans Bay STP diffuser (blue circle). (Source DEP, 2010) .........................................3

Figure 2 Salinity measurements at the four reference stations from 2004-2015 where reddot shows salinity range of profiles on 19 November 2013 .................................................8

Figure 3 Salinity profiles at the four reference stations and the idealised strong salinitystratification profile on 19 November 2013 ..........................................................................9

Figure 4 Temperature measurements at the four reference stations from 2004-2015where red dot shows highest surface water temperatures on 16 February 2010 ...............9

Figure 5 Temperature profiles at the four reference stations and the idealised andpotential global warming thermal stratification profiles on 16 February 2010 ...................10

Figure 6 Salinity profiles at the four reference stations and the idealised salinitystratification profile on 16 February 2010 ..........................................................................10

GHD | Report for TasWater - Blackmans Bay STP DPEMP, 3218107 | 1

1. Introduction1.1 Purpose of this Report

In this report GHD Pty Ltd (GHD) assessed the mixing zones for the Blackmans Bay SewageTreatment Plant (STP) on the behalf of TasWater for the following cases:

Existing;

Proposed upgrade maximum dry weather flow; and

Proposed upgrade maximum dry weather flow augment by stormwater flow.

Modelling was used to estimate the spatial extent of the mixing zones on the basis of thedilution needed to meet the water quality objectives of the marine/estuarine receivingenvironment.

1.2 Assumptions

Assumptions in this report are generally stated throughout relevant sections. Some of the keyassumptions include:

The Commonwealth Scientific and Industrial Research Organisation (CSIRO) DerwentComputational Informatics (CCI) model provides reasonable characteristic current speedsand directions of the receiving marine/estuarine environment;

Ambient receiving water quality data from four long-term monitoring sites (stations B1,B3, B5 and C) serve as adequate reference sites to characterise the background waterquality and site specific trigger values to estimate the dilution that is needed at the edgeof the mixing zone;

Water quality data of the existing effluent is of sufficient duration, frequency and accuracyto characterise the 90th percentile concentrations to estimate the existing dilution that isachieved at the edge of the mixing zone. The 90th percentile was adopted to provide areasonable level of conservatism (i.e. the precautionary principle) into this assessment.Proposed 90th percentile effluent values provided by TasWater for the upgraded STPwere used for future scenarios;

Estimates of NOx 90th percentile future design concentrations were provided by TasWaterand are assumed to be suitably conservative for use in this study; and

The information on the diffuser configuration for the STP (e.g. outlet depth andspecifications) is sufficiently accurate to model near-field mixing and predict the spatialextent of the mixing zone.


1.3 Scope and Limitations

This report: has been prepared by GHD for TasWater and may only be used and relied on by TasWater forthe purpose agreed between GHD and the TasWater as set out in section 1.1 of this report.

GHD otherwise disclaims responsibility to any person other than TasWater arising in connection with thisreport. GHD also excludes implied warranties and conditions, to the extent legally permissible.

The services undertaken by GHD in connection with preparing this report were limited to those specificallydetailed in the report and are subject to the scope limitations set out in the report.

The opinions, conclusions and any recommendations in this report are based on conditions encounteredand information reviewed at the date of preparation of the report. GHD has no responsibility or obligationto update this report to account for events or changes occurring subsequent to the date that the report wasprepared.

The opinions, conclusions and any recommendations in this report are based on assumptions made byGHD described in this report (refer Section 1.2 of this report). GHD disclaims liability arising from any ofthe assumptions being incorrect.

GHD has prepared this report on the basis of information provided TasWater and others who providedinformation to GHD (including Government authorities)], which GHD has not independently verified orchecked beyond the agreed scope of work. GHD does not accept liability in connection with suchunverified information, including errors and omissions in the report which were caused by errors oromissions in that information.


2. Edge of Mixing Zone Criteria2.1 Water Quality Objectives

Long-term water quality monitoring has been undertaken by the Derwent Estuary Program(DEP) at sites throughout the top, middle and lower reaches of the estuary for over a decade;monitoring sites are shown in (Figure 1). Sites C, B1, B2 and B3 in the lower reach of theestuary are in relative proximity to the diffuser site and therefore serve as appropriate referencesites for this analysis. Long-term (2004-2015) monthly monitoring of sites is comprised of grabsamples (NHX, Chla, DOC, PO4, NOX, Filtered TN, TN, TP, TOC, TSS, Zn and POC), sondeprofiles (temperature, salinity, pH, DO) and secchi depth (DEP, 2015).

Figure 1 Ambient monitoring locations in the Derwent Estuary and locationof the Blackmans Bay STP diffuser (blue circle). (Source DEP,2010)


In Table 1 a summary of ANZECC (2000) toxicant trigger values and site specifictrigger/guideline values (on the basis of the 80th percentile of the surface waters from the fourreference stations) is provided. Given the primary purpose of this investigation is to estimate thespatial extent of the mixing zone, the use of the ANZECC & AMRCANZ (2000) toxicity triggervalues for a 95% level of species protection and default trigger values for recreational uses (i.e.secondary contact) are deemed appropriate to define the edge of the mixing zone. The sitespecific trigger values (SSTVs) developed from the DEP dataset (particularly for nutrients) areappropriate to evaluate potential eutrophication of the estuary, which will be addressed to somedegree with the limited far-field modelling presented here.

However, the primary aim of this analysis is to define the edge of the mixing zone forecotoxicological and/or human health risks, this is done using NHx as the most significanttoxicant. NOx has also been evaluated at the request of TasWater, as nitrate is considered atoxicant also (although not to the extent of ammonia in marine waters).

Table 1 Summary of adopted edge of mixing zone trigger values andecosystem guideline values outside of the mixing zone

AnalyteANZECC (2000)

Trigger/GuidelineValue

80th %ileSSTV

Site SpecificMedian ANZECC (2000) and Site Specific Notes

Toxicant Trigger Values (Mixing Zone Criteria)

NHX0.91 mg/L at pH=8.00.75 mg/L at ph=8.1

0.013 mgN/L 0.006 mg/L

ANZECC (2000) MTV 95% LSPSite specific 80th %ile of surface waters at 4 stationsbelow ANZECC (2000) MTVRECOMMEND: Adopt ANZECC (2000) MTV

NO3 0.7 mg/L 0.035 mgN/L 0.003 mg/L

ANZECC (2000) Freshwater TV 95% LSP isrecommended by ANZECC (2000)Site specific 80th %ile of surface waters at 4 stationsbelow ANZECC (2000) FWTVRECOMMEND: Adopt ANZECC (2000) FWTV

Secondary Human Contact Trigger Values (Mixing Zone Criteria)Thermotolerant

Coliforms 1,000 CFU/100 ml NotApplicable

10 CFU/100 ml(assumed) ANZECC (2000) median for secondary contact

Estuarine Ecosystem Guideline Values (Eutrophication/Ecosystem Effects Criteria)

TP 0.03 mg/L 0.04 mg/L 0.033 mg/L

ANZECC (2000) EGV for SE Australian estuariesSite specific 80th %ile of surface waters at 4 stationsgreater ANZECC (2000) EGVRECOMMEND: Adopt site specific value

TN 0.3 mg/L 0.316mg/L 0.270 mg/L


NOX 0.015 mg/L 0.035mg/L 0.003 mg/L


NH4+ 0.015 mg/L 0.013

mg/L 0.006 mg/L


*Notes: MTV = Marine Trigger Value, FWTV = Freshwater Trigger Value, LSP = Level of Species Protection, EGV =

Ecosystem Guideline Value


The ANZECC & ARMCANZ (2000) toxicity trigger value for total ammonia at a 95% level ofspecies protection is 0.91 mg N/L at a pH of 8.0. pH corrections of the NHX trigger value wereapplied over the small pH range of the Derwent Estuary (7.8-8.1) (Table 2). As salinity at theSTP outfall site is >10 ppt, the receiving waters are well buffered so that ammonia (NH3) levelsare controlled by characteristic estuarine pH rather than the effluent pH. The median pH of thereference sites is 8.06 (~8.1), which sets the NHX trigger value to 0.75 mg/L (Table 2).

Table 2 NHX trigger values on the basis of pH

pH Marine Trigger Value (μg/L)7.8 13207.9 11008 910

8.1 750

2.2 Effluent Quality

As a conservative measure, the proposed and existing 90th percentile treated effluent qualityvalues were adopted to evaluate the mixing zone extent as summarised in Table 3. For thepurpose of this assessment, a conservative precautionary approach has been adopted for theupgraded effluent water quality and the higher winter 90th percentile values are adopted todefine the spatial extent of the mixing zone.

Table 3 Blackmans Bay STP current and upgrade 90th percentile effluentquality

Parameter Existing 90th %ile STP Upgrade 90th %ileSummer Winter

NHx (mg/L) 41.9 2 5NOx (mg/L) Not Measured 5.5* 17.5*TN (mg/L) 55 10 25TP (mg/L) 9.5 12

Thermotolerant Coliforms(cfu/100 ml) 694 500

*Derived values from TN

2.3 Dilution Needed at the Edge of the Mixing Zones

The dilution needed at the ‘edge’ of the mixing zone (D) was estimated as:

Dilution=(CEFF–CTV)/(CTV–CBG)where: CEFF is taken to be the 90th percentile concentration of the treated effluent; CTV is trigger value at the edge of the mixing zone; and CBG is the median concentration from the reference monitoring sites.

Estimates of the dilution needed at the edge of the mixing zone are provided in Table 4. Inshort, the following dilutions are needed:

For the NHX toxicant mixing zone:

– A 55-56 fold dilution is needed for the existing STP effluent case;

– A 6 fold dilution is needed for the winter upgraded STP effluent case (defined as June,July, August);

– A 2 fold dilution is needed for the summer upgraded STP effluent case (all monthsoutside of ‘winter’).

For the ecosystem effects mixing zone, TP is the critical nutrient for dilution, that requires:

– Nearly a 1,357 fold dilution for the existing STP effluent case;


– A 1,709 fold dilution for the upgraded STP effluent case;

Table 4 Dilutions required at the edge of the mixing zones for toxicity andecosystem effects thresholds

Parameter ToxicityNHX

ToxicityNO3

EcosystemTP

EcosystemTN

EcosystemNOX

EcosystemNHX

ConcentrationsExisting EffluentConcentration 41.9 NA 9.5 55 NA 41.9

Upgrade Concentration(Winter in brackets)

2(5)

5.5(17.5) 12 10

(25)5.5

(17.5)2

(5)Toxicant Trigger or

Ecosystem Guideline Value 0.75 0.7 0.04 0.316 0.035 0.013

Background EstuaryConcentration 0.006 0.003 0.033 0.270 0.003 0.006

Dilutions RequiredExisting Dilution Needed 55.3 NA 1,351 1,189 NA 5,934Upgrade Dilution Needed

(Winter in brackets)1.7

(5.7)6.9

(24.1) 1,709 211(537)

171(546)

284(712)


3. Model Description, Input Data andSufficiency of Currents for DilutionThe spatial extent of the mixing zone for the effluent discharge specifications (Section 3.2.1)was simulated over the range of currents based on characteristic outputs from the CSIRO CCImodel in the region in relative proximity to the Blackmans Bay diffuser (Section 3.2.2) andrepresentative temperature and salinity profiles (Section 3.2.3). Generally, the STP effluentwhen discharged from a particular nozzle of the diffuser is characterised by an initial jet ofrelatively fresh water relative to the receiving marine waters, that is discharged horizontally tothe seabed into the ambient marine waters. As the initial momentum (i.e. velocity) of the jetdecreases, it transitions into a buoyant plume (effluent fresh water less dense than receivingmarine waters) that rises to the surface. Both jet and buoyancy driven mixing are collectivelyreferred to as near-field mixing. Upon reaching the surface further mixing is governed by naturalprocesses that are generally much less energetic, and thereby dilute the effluent at a muchlower rate (i.e. the far-field mixing region).

3.1 Model Description

Predictive modelling of the effluent discharges into the ambient waters was carried out with theUS Environmental Protection Agency’s (EPA’s) Visual Plumes suite of models (Frick et al.2001). The UM3 near-field model was applied to simulate near-field mixing (e.g. dilution) duringthe jet (momentum or velocity dominated) and plume (buoyancy dominated) phases of thedischarge. The near-field model terminates when the plume contacts the surface of the watercolumn. Afterwards, UM3’s simplistic Brooke’s far-field module was applied to predictsubsequent dilution due to the background turbulence of the ambient waters. A typicalconservative (i.e. low rate of natural turbulence) value for the Brooks’ far-field diffusioncoefficient of 0.0003 m2/3/s2 was used here (Frick et al. 2001).

3.2 Input Data

3.2.1 STP Outlet Configurations

The existing pipeline and diffuser will be used for the upgraded plant with specifications of:

600 m long pipeline extends perpendicular to the shoreline with diffuser at end alsooriented perpendicular to the shoreline and the predominant current directions;

Diffuser in 13.0 m depth; and

The diffuser has 21 ports with 80 mm nozzle diameters that are spaced at 4 m intervalswith alternating horizontal discharge directions parallel to the shoreline. The last nozzle atthe end of the pipe is oriented perpendicular to the shoreline. The nozzles areapproximately 1 m above the seabed.

Three STP effluent discharge cases are considered, namely:

Existing case: average 2014-2015 discharge of 4.0 MLD;

Upgrade design Average Dry Weather Flow (ADWF) case: 8.53 MLD; and

Upgrade wet weather design case: 34.12 MD (4x ADWF).


3.2.2 Current Speeds

Current speeds through the water column are assumed to range from 0.01 m/s during slacktides to 0.1-0.2 m/s during spring tides on the basis of CSIRO CCI hydrodynamic modelling(GHD 2013) and recent ADCP measurement (GHD 2015) at Rosny STP. A value of 0.15 m/shas been used as a representative upper value. This current speed range was also cross-referenced with a previous Mike 3 model application, which simulated currents in the range of0.01-0.15 m/s at the Blackmans Bay outlet location (GHD 2010).

3.2.3 Representative Density Profiles of Receiving Estuarine Waters

Temperature and salinity profiles through the water column have been collected monthly by theDEP at each of the four reference sites (i.e. station B1, B3, B5 and C). The two extremes ofdensity stratification conditions at the Blackmans Bay diffuser site can be characterised as:

Elevated winter salinity stratification; and

Well mixed conditions throughout the water column in the autumn-winter withouttemperature stratification.

Temperature has a secondary influence on density stratification through the water column. Aconstant temperature through the water column of 10°C was used for these two extremeconditions, which is representative of winter temperatures when high winter riverine run-offleads to salinity stratification and during low flow winter periods where there is minimaltemperature and salinity stratification.

The largest range in salinity profiles at the four reference stations occurred on 19 November2013 (Figure 2). An idealised salinity profile representative of worst-case stratification conditionswas derived on the basis of the salinity profiles at the four reference stations for use in the near-field modelling (Figure 3).

Figure 2 Salinity measurements at the four reference stations from 2004-2015 where red dot shows salinity range of profiles on 19November 2013


Figure 3 Salinity profiles at the four reference stations and the idealisedstrong salinity stratification profile on 19 November 2013

In order to evaluate the effect of potential global warming on strengthening thermal stratificationduring the summer, a 2°C temperature rise was added to peak thermal stratification thatgenerally occurs during the summer. The warmest surface temperatures over the 2004-2015monitoring record at the four reference stations occurred on 16 February 2010 (Figure 4). Anidealised temperature and salinity profile was derived on the basis of the profiles at the fourreference stations for use in the near-field modelling (Figure 5). Further a 2°C temperature risein the surface waters (0-4 m), and 1.5, 1.0 and 0.5 °C temperature rises at 6, 8 and 10 m,respectively, were added to the idealised temperature profile as an analogue of the potentialeffects of global warming / climate change. The idealised salinity profile on 16 February 2010was also based on the salinity profiles on this date from the four reference stations (Figure 6).

Figure 4 Temperature measurements at the four reference stations from2004-2015 where red dot shows highest surface watertemperatures on 16 February 2010


Figure 5 Temperature profiles at the four reference stations and theidealised and potential global warming thermal stratificationprofiles on 16 February 2010

Figure 6 Salinity profiles at the four reference stations and the idealisedsalinity stratification profile on 16 February 2010


4. Results4.1 Sufficiency of Ambient Currents to Dilute Effluent Discharge

Whether the ambient currents provide sufficient volumetric discharge past the diffuser (QAMB) todilute the three STP effluent discharge (QEFF) cases to attain the needed dilution at the edge ofthe mixing zone (D) was estimated with a mass balance approach as QAMB-NEEDED=D QEFF –QEFF. The actual ambient discharge was estimated QAMB-ACTUAL=vAMB zDIFFUSER LDIFFUSER on thebasis of the low current speed estimate (vAMB), depth of the outfall (zDIFFUSER) and the length ofthe diffuser (LDIFFUSER). If QAMB-ACTUAL is greater than QAMB-NEEDED then there is sufficient ambientwaters flowing past the diffuser to meet the dilution needed at the edge of the mixing zone.Clearly, for the ammonia toxicant concern, there is more than sufficient currents to meet thedilution needs to meet the edge of the mixing zone criteria.

Table 5 Calculations to confirm sufficient currents to dilute STP effluentdischarge to meet toxicant dilution needs

LDIFFUSER= 80 m

vAMB = 0.01 m/s

zDIFFUSER = 13 m

QAMB-ACTUAL = 899 MLD

NHX Requirements Existing Case Winter Upgrade Case Summer UpgradeCase

D = 56 6 2

QEFF (MLD) = 4 9 34

QAMB-NEEDED (MLD) = 220 43 34


4.2 Mixing Zone Assessment

This section presents the predicted mixing zones from the near-field modelling of 10 modelruns.

4.2.1 Overview of Modelling Runs

Table 6 provides the specifications for the ten modelling runs undertaken, the runs weredesigned to meet the Project Specific Guidelines for the Blackmans Bay Upgrade Project.

Table 6 Modelling run specifications

Model Run QEFF (MLD) Stratification Currents Summary

1 8.53 Well-Mixed HighDry weather maximum upgrade

effluent discharge with weakstratification and high currents

2 8.53 Well-Mixed LowDry weather maximum upgrade

effluent discharge with weakstratification and low currents

3 8.53StrongSalinity

StratificationHigh

Dry weather maximum upgradeeffluent discharge with strongstratification and high currents


StratificationLow

Dry weather maximum upgradeeffluent discharge with strongstratification and low currents

5 8.53

NormalClimateSummer

Thermal andSalinity

Stratification

Low

Dry weather maximum upgradeeffluent discharge with ‘normal’

summer thermal stratification and lowcurrents

6 8.53

ClimateChangeSummer

Thermal andSalinity

Stratification

Low

Dry weather maximum upgradeeffluent discharge with climate

change increase of 2°C in surfacewaters with stronger thermalstratification and low currents

7 34.12 Well-Mixed LowWet weather maximum upgrade

effluent + stormwater discharge withweak stratification and low currents


StratificationLow

Wet weather maximum upgradeeffluent + stormwater discharge withstrong stratification and low currents

9 4.0 Well-Mixed Low Dry weather existing effluent withweak stratification and low currents


StratificationLow

Wet weather maximum upgradeeffluent + stormwater discharge withstrong stratification and low currents

4.2.2 Modelling Results

The modelling results are presented in Table 7 along with an assessment against the toxicanttrigger values for the mixing zone analysis, and also an assessment of the degree of dilutionthat is predicted in terms of the ecosystem nutrient enrichment guidelines.


Table 7 Near-field modelling dilution plots and assessment against dilution needs for toxicants at the edge of the mixingzone and nutrient enrichment ecosystem guidelines. In the dilution plots red and blue are for predictions for diffusernozzles that are aligned with and against the current, respectively, the lines represent average dilution of the plumeand the symbols the centreline dilution

Model Run Plot Assessment

Scenario 1

Upgrade NormalDischarge – Well Mixed

Upgraded STP 8.53 ML/day ADWF Well Mixed High Currents

Toxicants

NHx: Achieves Winter dilutionneeded (5.7-fold) <5 m of the diffuser

NO3: Achieves Winter dilutionneeded (24.1-fold) within <5 m of thediffuser

Ecosystem Effects (Nutrients)

TP: Does not achieve the dilutionneeded for TP to reach ambientconcentration (1,709- fold) at 100 m

NHx: Achieves the Winter dilutionneeded for NHX to reach ambientconcentrations (712-fold) within100 mPh



Scenario 2

Upgrade NormalDischarge – Well Mixed

Upgraded STP 8.53 ML/day ADWF Well Mixed Low Currents

Toxicants




TP: Does not achieve the dilutionneeded for TP to reach ambientconcentrations (1,709- fold) at 100 m

NHx: Achieves the Summer dilutionneeded for NHX to reach ambientconcentrations (284-fold) within100 m. Winter dilution notachieved



Scenario 3

Upgrade NormalDischarge – Stratification

Upgraded STP 8.53 ML/day ADWF Strong Salinity

Stratification High Currents

Toxicants





NHx: Achieves the Summer dilutionneeded for NHX to reach ambientconcentrations (284-fold) within100 m. Winter dilution notachieved



Scenario 4

Upgrade NormalDischarge – Stratification

Upgraded STP 8.53 ML/day ADWF Strong Salinity

Stratification Low Currents

Toxicants





NHx: Does not achieve the Summerdilution needed for NHX (284-fold) toreach ambient concentrations within100 m.



Scenario 5

Climate Change

Upgraded STP 8.53 ML/day ADWF Normal Temperature


Toxicants





NHx: Does not achieve the Summerdilution needed for NHX to reachambient concentrations (284-fold)within 100 m.



Scenario 6

Climate Change

Upgraded STP 8.53 ML/day ADWF Climate Change

StrengtheningTemperatureStratification

Low Currents

Toxicants








Scenario 7

Wet Weather Conditions

Upgraded STP 34.12 ML/day

(4 x ADWF) Well Mixed Low Currents

Toxicants








Scenario 8

Wet Weather Conditions

Upgraded STP 34.12 ML/day

(4 x ADWF) Strong Salinity


Toxicants








Scenario 9

Existing STP

Existing STP 4 ML/day Average Well Mixed Low Currents

Toxicants

NHx: Achieves Winter dilutionneeded (55.3-fold) <5 m of thediffuser


TP: Does not achieve the dilutionneeded for TP to reach ambientconcentrations (1351-fold) at 100 m

NHx: Does not achieve the dilutionneeded for NHX to reach ambientconcentrations (5934-fold) within100 m.



Scenario 10

Existing STP

Existing STP 4 ML/day Average Strong Salinity


Toxicants

NHx: Achieves Winter dilutionneeded (55.3-fold) <5 m of thediffuser


TP: Does not achieve the dilutionneeded for TP to reach ambientconcentrations (1351-fold) at 100 m

NHx: Does not achieve the dilutionneeded for NHX to reach ambientconcentrations (5934-fold) within100 m.


5. ConclusionsWhen making conclusions from the results of the near-field mixing assessment, it must beremembered that statutory mixing zones are set based on the dilution of toxicants only.Therefore, any observations made regarding ‘ecosystem effects’ nutrient dilutions in the near-field are discussed here to assist in far-field mixing discussions only (i.e. the ecosystem effectsresults presented in this study are not used to set statutory mixing zone boundaries).

In summary:

The toxicant trigger value for total ammonia (and for nitrate) is readily met within 5 m ofthe diffuser in all modelled upgrade scenarios. This is also the case for the existingeffluent discharge, therefore no change to the 15 m existing mixing zone (either side ofthe pipeline) is required.

Even under high effluent flow events (4xADWF), required dilutions for toxins are met wellwithin the existing mixing zone.

High current speeds are seen to greatly enhance dilution, and for the upgrade cases,current speeds in excess of 0.15 m/s may result in sufficient dilution to meet ecosystemnutrient enrichment dilution targets for a number of nitrogenous compounds.

Dilution to meet the site specific guideline values for nitrogenous compounds occurs inthe near-field for the well mixed and high current scenario (Scenario 1) for the upgradedplant. This is a vast improvement to the existing plant, where total near-field mixing tomeet the site specific guideline values is highly unlikely to occur under anycircumstances.

However, for the higher probability / frequency lower currents the ecosystem nutrientenrichment dilution targets generally will not be met within 100 m of the outfall.

The required dilution for ecosystem effects for phosphorus are not met under any of themodelled conditions for the upgraded plant in close proximity to the outfall. However, fornutrients, nitrogen enrichment generally poses a greater eutrophication risk inestuarine/marine environments than phosphorus.

The climate change scenario (Scenario 6) show that higher temperature stratificationrelative to normal summer stratification (Scenario 5) results in slightly changes to dilution,but not materially compared to the large changes between well-mixed and strong salinitystratification during the winter.


6. ReferencesANZECC & ARMCANZ (2000) National Water Quality Management Strategy: Australian andNew Zealand Guidelines for Fresh and Marine Water Quality.

DEP (2015) Coughanowr C, Whitehead S, Whitehead J, Einoder L, Taylor U and Weeding, B,2015. State of the Derwent estuary: a review of environmental data from 2009 to 2014. DerwentEstuary Program

DEP (2010) Whitehead J, Coughanowr C, Agius J, Chrispijn J, Taylor U, Wells F., 2010. Stateof the Derwent Estuary 2009: a review of pollution sources, loads and environmental qualitydata from 2003 – 2009. Derwent Estuary Program, DPIPWE, Tasmania.

Frick, W.E., Roberts Roberts, P.J.W., Davis, L.R., Keyes J., Baumgartner, D.J., George, K.P.,2001. Dilution Models for Effluent Discharges, 4th Edition (Visual Plumes) Draft. USEnvironmental Protection Agency, Georgia. July 2001.

GHD (2010) Electrona WWTP Outfall Mixing Assessment: Hydrodynamic Modelling. ForSouthern Water. Doc No 61/15269/97106.

GHD (2013) Derwent Estuary WWTP Investigation: Preliminary Near-Field Modelling of MixingZones. For Southern Water. Doc No 32/16561/126099.

GHD (2015) Derwent STP Rationalisation: Near-field and Far-field Modelling of Outflows. ForTasWater. Doc No 32/17676/1152334.

GHD

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© GHD 2016This document is and shall remain the property of GHD. The document may only be used for thepurpose for which it was commissioned and in accordance with the Terms of Engagement for thecommission. Unauthorised use of this document in any form whatsoever is prohibited.3218107-11576/https://projects.ghd.com/oc/Tasmania/blackmansbaystpdpemp/Delivery/Documents/157555_REV 0_SL.docxDocument Status

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