State of Art Australian Wastewater Treatment Design and Operation

Yu Dai*, Peter Griffiths, Shane Morgan CH2M HILL Pty Ltd, Brisbane, Queensland, Australia *yu.dai@ch2m.com.au

Abstract Three full-scale Australian wastewater treatment plants are reviewed to present the ‘lessons learnt’ so far in developing an improved design and operating strategy able to efficiently achieve biological nitrogen removal and phosphorus removal. The success of the ‘state of the art’ wastewater treatment plants is based on the development and refinement of existing nutrient removal process models, the improved understanding of the operation of BNR plants, and a fundamental approach to design for microbial behavioural impacts. A number of key design and operating features are identified for the achievement of low effluent nutrient concentration, including optimal bioreactor configuration, adequate internal recirculation, plant DO control and appropriate equipment selection.

Keywords Biological phosphorus removal; dissolved oxygen control; full scale; internal recirculation; nitrogen removal; process optimisation

INTRODUCTION The Australian wastewater treatment industry has some of the most complex requirements for treatment design, including some of the tightest environmental regulations, a high strength wastewater influent and reducing per population wastewater flows. Over the past decade the industry has also begun to focuss on the total cost of treatment, the carbon footprint of doing business, and the need to drive operational efficiency and control. There is still much to learn but through case examples from successfully operating plants this paper presents the ‘lessons learnt’ approach to developing an improved design and operating strategy able to efficiently achieve very low effluent nutrient levels in future designs. This success has relied on the development and refinement of existing nutrient removal process models, the improved understanding of the operation of these types of plants, a deep understanding of the common ‘operational culture’ of the workforce, and a respect for the biological impacts of engineering decisions made at every stage of design, commissioning and operations. A treatment requirement to achieve a mean effluent standard of 5 mg/L Total Nitrogen (TN) and 1 mg/L Total Phosphorus (TP) is becoming common in both inland and coastal areas of Australia. This standard is driven by a need to protect coastal aquatic eco-systems and to eliminate any human impacts on what are often drought prone streams and waterways. Over the past decade there has been a trend to minimising chemical consumption used in wastewater treatment, optimising power usage and reducing the carbon footprint of operations. Achieving a TN 3/TP 0.3 effluent standard without chemical phosphorus treatment and with a very low energy footprint is now the proxy standard of ‘state of the art’ design. Coupled with this, any design needs to ensure operations are safe, reliable and easily controlled. Australian sewage is characterised as having a relatively high nitrogen and phosphorus content,

typically in excess of 55 mg/L as TN and 10 mg/L as TP. This is attributed to a diet high in red meat and the preservation of these meats with nitrate-based preservatives. Whilst they are gradually being phased out of use, phosphorus-based detergents are still in use by commerce and industry. The sewage ratio of TKN:COD generally falls into a range of 0.09 to 0.11, which should be favourable for biological nutrient removal. However, the fraction of non-biodegradable inert organics is found to be exceptionally high, ranging from 0.20 to 0.25, compared to commonly used levels around 0.10-0.15 for other countries (Ekama, 1984; Metcalf and Eddy, 2003). This often results in a COD:BOD5 ratio of 2.3 to 1 or even higher, which can be associated with substrate limitation and hence poses challenges to maximising nutrient removal. Dealing with these treatment conditions requires a deep understanding of the biological system mechanisms. Wastewater flows are also highly variable. Tropical storms and changing weather conditions mean that peak flows are commonly in excess of five times average dry weather flow (ADWF) and in some coastal communities can be more than ten times ADWF. These flows can appear rapidly at plants due to sudden and massive tropical storms and cyclones. The ability of a plant to maintain treatment through these operational conditions is essential, and that relies on not only the understanding and design of appropriate treatment bypass arrangements, but also the optimal control of wastewater biology to ensure the solids inventory can be maintained in the plant. To date there have been a number of quantitative mathematical models developed for the design of biological nitrogen and phosphorus reduction systems (Ekama, 1984; Henze et al., 1987; Henze et al., 1995; Henze et al., 1999; Gujer et al., 1999). Interestingly different denitrification kinetics have been reported for the nitrification/denitrification only system and biological nitrogen system incorporating biological phosphorus removal, however reasons for the kinetics variation has remained unclear (Clayton et al., 1991). Subsequently a general denitrification model has been developed (Griffiths, 1994), which builds on the hypothesis that two distinctive groups of denitrifiers exist, namely readily biodegradable COD (RBCOD) consumers and slowly biodegradable COD (SBCOD) consumers. The model satisfactorily interprets the denitrification kinetics observed by Clayton et al. (1991) and correlates well with the research outcomes published by Dold and Marais (1986). Based on this model, the anoxic mass fraction is the key factor determining the denitrification performance rather than actual hydraulic retention time. Provided sufficient anoxic mass fraction is present, very low effluent nitrate concentration should be theoretically achievable by operating increased internal recirculation (from aerobic to anoxic). This principle has been applied to the design of a number of wastewater treatment plants in Australia which are now in operation. This paper presents operational data for these full-scale Australian wastewater treatment works, from start-up through current operation. The design and optimisation of these plants have been further reviewed based on the further understanding of biological process models and microbial behaviour.

METHODS Three full-scale Australian wastewater treatment plants have been analysed for system performance and operational behaviour. These plants all require biological nitrogen and phosphorus removal to achieve tight effluent discharge standards. Process optimisation and operational improvement were explored in an attempt to fulfil the design intent and achieve the most optimal system performance. The elemental characteristics, COD, BOD5, TKN-N, NH3-N, NO3-N, TN, PO4-P, TP and TSS have all been monitored (APHA, 2005) to provide support for the reported outcomes.

Merrimac WWTP Stage-4 Upgrade Merrimac Stage-4 WWTP is a 60,000 EP biological nutrient removal (BNR) plant constructed in a configuration based on the variation on the modified UCT process (Griffiths, 1990). The median influent TN concentrations were in the range of 55 to 60 mg/L and TP in the order of 10 mg/L. By provision of additional pre-anoxic zones and employing a high internal recirculation rate of approximately 16 times ADWF (Griffiths, 1994), the plant was designed to treat municipal wastewater to a standard of 5 mg/L TN and 1 mg/L TP without the aid of chemical coagulants. The schematic process flow diagram along with the site layout is presented in Figure 1.

Figure 1. Merrimac Stage-4 BNR Plant Layout and Process Configuration Commissioned in 1995, the initial plant performance in phosphorus reduction was poor. Despite a high dissolved oxygen (DO) set point of 3 mg/L for all the aerobic zones (based on the successful operating experience (Jennings et al., 1993) at the Bendigo BNR plant), phosphorus reduction appeared minimal. An action was then taken to validate all the measured field DO probes. Correction to the DO set point at each individual aerobic zone was then made (shown in Table 1) based on the probe error measurements. Table 1. Initial and Corrected DO Set Points for Merrimac Stage-4 BNR Plant Commissioning. Aerobic Zone Initial DO Set Point Corrected DO Set Point after Probe Validation

Zone 1 3.0 mg/L 3.4 mg/L Zone 2 3.0 mg/L 3.6 mg/L Zone 3 3.0 mg/L 3.0 mg/L Zone 4 3.0 mg/L 3.2 mg/L

Significant performance improvement was not observed following the above changes to the DO set points. A decision was then made to further validate the DO probes by measuring DO in each aerobic zone using a calibrated hand-held DO probe. This identified a major error with DO probe #2. The DO set point at Aerobic Zone #2 was subsequently rectified to remedy the device flaw. A gradual improvement of phosphorus reduction was observed following the operation change. This is demonstrated by the effluent TP monitoring results (Table 2).

Clarifiers

Influent

RAS

Aerobic Zone 1

Aerobic Zone 2

Aerobic Zone 3

Aerobic Zone 4

Anoxic Zone 4

Anoxic Zone 3

Anoxic Zone 2

Anoxic Zone 1

Anaer. Zone 4

Anaer. Zone 5

Pre-Anoxic Zone 1

Pre- Anoxic Zone 2

Anaer. Zone 3

Anaer. Zone 2

Anaer. Zone 1

Pre- Anoxic Zone 3

A Recycle R-Recycle

Table 2. Optimisation of DO Set Points for Enhanced Phosphorus Reduction at Merrimac WWTP. Date DO Set Point at Aeration Zone #2 Effluent TP Concentration

13/4/1995 3.6 mgDO/L 5.8 mgTP/L 19/4/1995 4.1 mgDO/L N/A 28/4/1995 4.1 mgDO/L 3.7 mgTP/L

2/5/1995 4.1 mgDO/L 2.2 mgTP/L 4/5/1995 4.1 mgDO/L 1.7 mgTP/L

16/5/1995 4.1 mgDO/L 0.5 mgTP/L

Wetalla WRP Stage-4 and Stage-5 Upgrade The Wetalla Water Reclamation Plant (WRP) services the city of Toowoomba located in South East Queensland. The 240,000 EP plant receives a very high trade waste component resulting in influent TN concentrations typically of the order of 120 mg/L, BOD5 concentrations around 550 mg/L, and TP of 14 mg/L. A high internal recirculation rate in excess of 50 times ADWF was therefore designed to achieve a high degree of nitrogen removal. Accompanying the high recirculation rates large DO carryover to the anoxic zone would inevitably result impeding denitrification performance. Alterations to the modified UCT / Johannesburg process configuration were accordingly made to minimise potential DO carryover whilst not compromising the aeration condition favoured by polyphosphate-accumulating organisms (PAOs). As diagrammatically shown in Figure 2, the modified process configuration involves:

1. Addition of a de-aeration zone to the internal recirculation flow path prior to the primary anoxic zone;

2. Inclusion of a post-aeration zone (with high DO set point 3 mg/L) immediately before the secondary clarifiers and separated from the high internal recirculation flow.

Figure 2. Wetalla WRP Stage-4 and Stage-5 Plant Process Configuration By incorporating a separate high-DO operated aeration zone at the end of the configuration, the majority of the biological phosphorus uptake is expected to occur within this post-aeration zone. Consequently the risk associated with secondary phosphorus release within low DO regions (i.e. de-aeration zone or low-DO operated aeration zones) should be substantially reduced. A photo of the plant site is presented in Figure 3. The Stage-4 and Stage-5 plants are mirror images except that the Stage-5 plant (to the right of the picture) incorporates a four-cell aerobic digester into the main bioreactor structure.

De-aeration

High DO

Medium DO Anoxic Pre Anoxic

Anaerobic

Pre Anoxic

Figure 3. Wetalla WRP Stage-4 and Stage-5 Plant Layout

Loganholme WPCC Stage-7 Upgrade Loganholme Water Pollution Control Centre (WPCC), servicing the city of Logan in South East Queensland, is rated with a treatment capacity of 220,000 EP (42 ML/d as ADWF). Upon the completion of the Stage-7 upgrade the Loganholme WPCC was expected to accommodate the forecast population growth and meet the current discharge licence requirement of 5 mgTN/L and 5 mgTP/L and should have the potential to be further upgraded to achieve longer term target of 3 mgTN/L and 3 mgTP/L. The Stage-7 Loganholme WPCC consists of two oxidation ditches, two bioreactors and eight secondary clarifiers (site layout shown in Figure 4).

Figure 4. Loganholme Stage-7 WPCC Plant Layout The oxidation ditches, divided into anoxic zone (34%), aerobic zone (50%) and de-aeration zone (16%), were designed to perform high level nitrification and denitrification with diffused aeration and a high internal recirculation rate of 25 time ADWF. The bioreactors, configured with pre-anoxic zone (5%), anaerobic zone (12%), anoxic zone (43%) and aerobic zone (40%), were designed for nitrification/denitrification and biological phosphorus removal. Weekly effluent composite/grab samples were collected to monitor the system performance.

RESULTS AND DISCUSSION

Operational data for the three Australian WWTPs has been reviewed and analysed to demonstrate the continued performance in process and operations.

Merrimac Stage-4 WWTP The poor biological phosphorus reduction at the commissioning of the plant was investigated, and DO set points discrepancy (particularly at Aeration Zone #2) was identified as the main cause to the process failure. An operational procedure was thus set up to undertake regular calibration of all the DO probes and adequately frequent replacement of the DO sensor heads. A satisfactory system performance on nitrogen and phosphorus removal was achieved as shown in Figure 5. The effluent TP “spike” of 1 mg/L was associated with a major wet weather event of approximately 4 times ADWF. This may be ascribed to the corresponding diluted RBCOD concentration in the anaerobic zone which potentially impedes efficient biological phosphorus removal (Ekama, 1984). Nevertheless, the plant was able to recover rapidly as demonstrated by the low effluent phosphorus concentrations during the following period.

0

0.2

0.4

0.6

0.8

1

1.2

Con

cent

ratio

n m

g/L

Total Phosphorus as P Ortho Phosphate as P

Storm Event

0

0.5

1

1.5

2

2.5

3

Con

cent

ratio

n m

g/L

Effluent Ammonia Nitrogen Effluent Nitrate Nitrogen Effluent Total Nitrogen

Figure 5. Merrimac Stage-4 WWTP Effluent N and P The Merrimac Stage-4 WWTP has since consistently achieved low effluent TN below 5 mg/L with a median value of 2.9 mg/L and very low TP at 0.1 mg/L (partly due to introduction of waste alum sludge from a nearby water treatment plant). The optimised BNR performance demonstrates the importance of accurate DO control and criticality of calibrating DO probes at regular intervals. This also demonstrates very clearly the detrimental effect to biological phosphorus removal from any form of “DO dip” in the DO control profile, as reported by others (Watts and Clews, 2008). It is however unclear as to the theory behind this observed phenomenon. A hypothesis is that a sudden DO drop in the early phase of the aerobic zones (where nitrate concentration is extremely low) may cause a pseudo “anaerobic” condition which promotes secondary phosphorus release without sequestration of a carbon source. Further investigation of the metabolic behaviour of PAOs under such operating conditions is warranted to better understand the impact of DO control on phosphorus reduction efficiency.

Wetalla Stage-4 and Stage-5 WRP Under the modified process configuration incorporating de-aeration and post-aeration zones (Figure

2), both Stage-4 and Stage-5 Wetalla plants reliably achieves effluent nitrate of 3.3 mg/L, ammonia below 0.05 mg/L and TP below 0.3 mg/L without tertiary filtration or any supplementary chemical dosing. Consistent system performance is demonstrated in Figure 6. Note that the elevated level of soluble non-biodegradable nitrogen content in the effluent (in the order of 2.5 mg/L) introduced by the high trade waste loads is not shown in the plot.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

160.0

6/27

/200

7

7/27

/200

7

8/27

/200

7

9/27

/200

7

10/2

7/20

07

11/2

7/20

07

12/2

7/20

07

1/27

/200

8

2/27

/200

8

3/27

/200

8

4/27

/200

8

5/27

/200

8

6/27

/200

8

7/27

/200

8

8/27

/200

8

9/27

/200

8

Efflu

ent N

O3-

N o

r NH

3-N

Con

cent

ratio

n (m

g/L)

Influ

ent T

N-N

Con

cent

ratio

n (m

g/L)

Influent TN (Left) Effluent Nitrate (Right)Effluent Ammonium (Right)

0.0

0.5

1.0

1.5

2.0

2.5

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

6/27

/200

7

7/27

/200

7

8/27

/200

7

9/27

/200

7

10/2

7/20

07

11/2

7/20

07

12/2

7/20

07

1/27

/200

8

2/27

/200

8

3/27

/200

8

4/27

/200

8

5/27

/200

8

6/27

/200

8

7/27

/200

8

8/27

/200

8

9/27

/200

8

Efflu

ent T

P-P

Con

cent

ratio

n (m

g/L)

Influ

ent T

P-P

Con

cent

ratio

n (m

g/L)

Influent TP (Left) Effluent TP (Right)

Figure 6. Wetalla Stage-5 WRP Influent and Effluent N and P As shown in Figure 6, the effluent ammonium nitrogen concentration has been consistently below 0.05 mg/L. The variation of the nitrogen reduction performance, decoupled from the variation of influent nitrogen loads, was attributed to variable availability of influent organic substrate. The occurrence of higher effluent nitrate concentration coincided with limitation of available biodegradable carbon in the feed. This substrate constraint was exacerbated in June 2009 as one of the trade waste discharge point was shut down leading to a considerable decrease in the influent organic carbon. The effluent nitrate nitrogen concentration was subsequently elevated at both plants. As a remedial measure, one of the four internal recycle pumps in Stage-4 plant was turned off in an attempt to reduce DO carryover to the primary anoxic zone thus making more carbon source available for denitrification. This approach successfully addressed the denitrification issue caused by carbon deficiency. The internal recirculation operation change and resultant performance improvement is provided in Table 3. Table 3. Improvement of Nitrogen Reduction by Optimising Internal Recirculation Rates Date Stage-4 Wetalla WRP Stage-5 Wetalla WRP 17/06/2009 Effluent NO3-N 8.8 mg/L Effluent NO3-N 5.7 mg/L 22/06/2009 One internal recycle pump off No changes to operation made 24/06/2009 Effluent NO3-N 5.6 mg/L Effluent NO3-N 5.6 mg/L 01/07/2009 Effluent NO3-N 1.8 mg/L Effluent NO3-N 5.0 mg/L 08/07/2009 Effluent NO3-N 3.7 mg/L Effluent NO3-N 4.6 mg/L The effectiveness of reducing internal recirculation rates on enhancing nitrogen removal performance was further supported by site measurement, which showed a residual DO concentration of the order of 0.6 mg/L in the de-aeration zone. This suggests that in the context of carbon constraints for BNR treatment the internal recirculation rates need to be carefully controlled to ensure that only treatable nitrate is sent to the primary anoxic zone, preventing excessive DO carryover consuming the limiting carbon source. A mathematical model can be used to simulate the denitrification potential and predict the optimum internal recirculation rates (Ekama, 1984).

As shown in Figure 6 stable biological phosphorus removal was also achieved. The sporadic effluent phosphorus “spikes” were associated with severe wet weather events, a similar observation to that for Merrimac Stage-4 WWTP. Despite provision of a de-aeration zone in the aeration profile, no apparent secondary phosphorus release was observed. This appeared counter to what was discovered from the “DO dip” in Merrimac Stage-4 WWTP. Possible explanations for this different system behaviour are:

1. Abundance of nitrate in the de-aeration zone may have prevented secondary anaerobic phosphorus release;

2. Low DO operating concentration (1.0 to 1.5 mg/L) in the prior aeration zones likely suppressed PAOs’ activities, resulting in majority of the phosphorus uptake occurring in the post-aeration zone downstream the de-aeration zone.

Further studies towards the microbial behaviour of PAOs under such a process configuration are required to identify the underlying mechanisms. Some additional findings from the operation of the Wetalla WRP are:

1. Excellent solids settleability demonstrated by a stirred sludge volume index (SSVI) less than 80 mL/g. This was attributable to the optimum aeration control resulting in very low effluent ammonium concentration (<0.1 mgNH3-N/L) and hence controlled proliferation of filamentous bacteria (Casey et al., 1992; Griffiths et al., 1997);

2. Low nitrous oxide emissions measured from the anoxic zone (Foley et al., 2009). This was again attributed to the adequate aeration control giving rise to low ammonium re-circulated to the anoxic zone;

3. Efficient power use by means of pumping internal recycle flow straight through the reactor wall; ascending DO profile was another contributing factor to the economic power consumption as aeration demand was reduced along with the ascending DO profile.

Loganholme Stage-7 WPCC Despite not originally being designed to remove phosphorus via biological treatment, the oxidation ditches #1 and #2 have been consistently achieving substantial biological phosphorus reduction (from influent 8 mgPO4-P/L to effluent 3 mgPO4-P/L) as shown in Figure 7. This superb “unusual” PAOs effect is related to the excellent nitrification and denitrification performance achieved (Figure 7). The following reasons were considered as plausible causes for the unexpectedly high level of phosphorus removal:

1. Part of the anoxic zone was likely transformed into “anaerobic” state during selective periods of the days due to depletion of nitrate. This was possible as the influent contained exceptionally high level of volatile fatty acid (VFA) (100 mg/L) and high RBCOD. During the time when nitrate re-circulated to the anoxic zone was of extremely low level, there may be considerable amount of VFA/RBCOD that remained in the anoxic zone after depletion of nitrate, which would become available to PAOs for anaerobic metabolism;

2. The low effluent ammonium concentration indicated sufficient aeration capacity and hence potential to provide the required aeration demand for phosphorus uptake;

3. The fact that only partial biological phosphorus removal was achieved, suggests that low-DO operation (DO set points of 1.0-1.5 mg/L) maybe the limiting factor. It is possible that if an additional high-DO operated post-aeration zone was provided further improvement of biological phosphorus reduction may be achieved. Alternatively, improvement may be realised by raising the DO set points to 3 mg/L for part of the aeration zones.

0

1

2

3

4

5

6

12/3/2009

12/13/2009

12/23/2009

1/2/2010

1/12/2010

1/22/2010

2/1/2010

2/11/2010

2/21/2010

3/3/2010

3/13/2010

3/23/2010

4/2/2010

4/12/2010

4/22/2010

5/2/2010

5/12/2010

5/22/2010

6/1/2010

6/11/2010

6/21/2010

7/1/2010

7/11/2010

Efflu

ent N

and

P C

once

ntra

tion

(mg/

L)

Oxidation Ditch #1 Effluent Ammonium Oxidation Ditch #1 Effluent NitrateOxidation Ditch #1 Effluent Phosphate

0

1

2

3

4

5

6

7

8

9

10

12/3/2009

12/13/2009

12/23/2009

1/2/2010

1/12/2010

1/22/2010

2/1/2010

2/11/2010

2/21/2010

3/3/2010

3/13/2010

3/23/2010

4/2/2010

4/12/2010

4/22/2010

5/2/2010

5/12/2010

5/22/2010

6/1/2010

6/11/2010

6/21/2010

7/1/2010

7/11/2010

Efflu

ent N

and

P C

once

ntra

tion

(mg/

L)

Oxidation Ditch #2 Effluent Ammonium Oxidation Ditch #2 Effluent NitrateOxidation Ditch #2 Effluent Phosphate

0

5

10

15

20

25

30

12/3/2009

12/13/2009

12/23/2009

1/2/2010

1/12/2010

1/22/2010

2/1/2010

2/11/2010

2/21/2010

3/3/2010

3/13/2010

3/23/2010

4/2/2010

4/12/2010

Efflu

ent N

and

P C

once

ntra

tion

(mg/

L)

BNR #3 and #4 Effluent Ammonium BNR #3 and #4 Effluent NitrateBNR #3 and #4 Effluent Phosphate

Figure 7. Loganholme Stage-7 WPCC Oxidation Ditches and Bioreactors Effluent N and P In contrast, Bioreactors #3 and #4 showed unstable and minimal biological phosphorus removal behaviour in spite of the presence of anaerobic and pre-anoxic zones (Figure 7). This may be correlated to poor nitrification performance, highlighting the aeration/SRT constraints which can be the issues for PAOs’ metabolic reaction.

CONCLUSIONS Three Australian wastewater treatment plants are showcased to demonstrate the evolution of modern Australian wastewater treatment design practices. Each shows advancement or improvement on the previous design, culminating in new designs (not yet built) which are believed to demonstrate the current ‘state of the art’ in Australian wastewater treatment design. The design and optimisation of these plants identifies a number of key design features and operating features that appear critical to the achievement of low effluent nutrient concentrations, including (1) optimal bioreactor configuration, (2) optimised internal recirculation, (3) plant DO control, (4) appropriate equipment selection.

REFERENCES

APHA, AWWA and WEF. (2005). Standard methods for the examination of water and wastewater, 21st edition. Port City Press, Baltimore, Maryland, USA.

Clayton, J.A., Ekama, G.A., Wentzel, M.C. and Marais, G.v.R. (1991). Denitrification kinetics in biological nitrogen and phosphorus removal activated sludge systems treating municipal waste waters. Wat. Sci. Tech. 23, 1025-1035.

Casey T.G., Wentzel, M.C., Loewenthal, R.A., Ekama, G.A. and Marais, G.v.R. (1992). A hypothesis for the cause of low F/M bulking in nutrient removal activated sludge systems. Water Research. 26(6), 867-869.

Dold, P.L. and Marais, G.v.R. (1986). Evaluation of the general activated sludge model proposed by the IAWPRC task group. Wat. Sci. Tech. 18, 63-89.

Ekama, G.A. (1984). Theory, Design and Operation of Nutrient Removal Activated Sludge Processes: A Collaborative Information Document. Water Research Commission, University of Cape Town, South African Council for Scientific and Industrial Research.

Foley, J., De Haas, D., Yuan, Z. and Lant, P. (2009). Nitrous oxide generation in full-scale biological nutrient removal wastewater treatment plants. Water Research. 44(3), 831-844.

Griffiths, P. (1990). Optimisation flow configurations for biological removal of nitrogen and phosphorus. Australian Water and Wastewater Association, International Association on Water Pollution research and Control, First Australian Conference on Biological Nutrient Removal, Bendigo, Victoria.

Griffiths, P. (1994). Modifications to the IAWPRC task group general activated sludge model. Water Research. 28(3), 657-664.

Griffiths, P., Stratton, H.M., Brooks, P. and Seviour, R.J. (1997). Problem organisms in activated sludge – causes and cures. Australian Water and Wastewater Association 17th Federal Convention, Melbourne, Victoria.

Henze, M., Grady, C.P.L.Jr, Gujer, W., Marais, G.v.R. and Matsuo, T. (1987). Activated Sludge Model No.1. (IAWPRC Scientific and Technical Report No.1). London: IAWPRC.

Henze, M., Gujer, W., Mino, T., Matsuo, T., Wentzel, M.C. and Marais, G.v.R. (1995). Activated Sludge Model No.2. (IAWPRC Scientific and Technical Report No.3). London: IAWQ.

Henze, M., Gujer, W., Mino, T., Matsuo, T., Wentzel, M.C., Marais, G.v.R. and van Loosdrecht, M.C.M. (1999). Activated Sludge Model No.2d. Wat. Sci. Technol. 39(1), 165-182.

Jennings, J., Pulbrook, C. and Griffiths, P. (1993). Bendigo biological nutrient removal plant startup and commissioning. Australian Water and Wastewater Association, 15th Federal Convention, Gold Coast, Queensland.

Gujer, W., Henze, M., Minto, T. and van Loosdrecht, M.C.M. (1999). Activated Sludge Model No.3. Wat. Sci.Technol. 39(1), 183-193.

Metcalf and Eddy Inc. (2003). Wastewater Engineering: Treatment and Reuse, 4th Edition. New York, NY, USA: McGraw Hill [Revised by Tchobanoglous G, Burton FL, Stensel HD].

Watts, J. And Clews, S. (2008). Optimisation of the Morpeth WWTW BNR Plant. 2nd Annual WIOA NSW Water Industry Engineers & Operators Conference, Newcastle, Australia.