Solley David 2000

7/30/2019 Solley David 2000

1/86

THE WINSTON CHURCHILL MEMORIAL

TRUST OF AUSTRALIA

Report by: David Solley - 2000 Churchill Fellow

Project: To study the Upgrading of Large

Wastewater Treatment Plants for

Nutrient Removal


2/86

Churchill Fellowship 2000 Report David Solley

Upgrading of Large Wastewater Treatment Plants for Nutrient Removal Final Report 20/03/02

C:\WINNT\Profiles\so7sbw.000\Desktop\working\Churchill Report.doc Page 2 of 39

CONTENTS

1 EXECUTIVE SUMMARY 4

2 INTRODUCTION 5

2.1 BACKGROUND 5

2.2 PURPOSE 5

2.3 ACKNOWLEDGEMENTS 5

3 PROGRAMME 6

4 UPGRADING OF LARGE WASTEWATER TREATMENT PLANTS 7

4.1 NUTRIENT REMOVAL 7

4.1.1 Biological Nutrient Removal 74.1.1.1 BARDENPHO AND UCT BNR PROCESSES 84.1.1.2 THREE STAGE VERSUS FIVE STAGE BNR PROCESS 114.1.1.3 SUBSTRATE UTILISATION 114.1.1.4 PREFERMENTATION OF PRIMARY SLUDGE 134.1.1.5 HYDRAULIC RETENTION TIME (HRT) 14

4.1.1.6 SOLIDS RETENTION TIME (SRT) 154.1.1.7 PLUG FLOW AND COMPLETE MIX REACTORS 154.1.1.8 CYCLIC/BATCH OPERATION 164.1.1.9 ATTACHED GROWTH PROCESSES 174.1.1.10 SECONDARY RELEASE OF PHOSPHORUS 184.1.2 Sludge Settleability, Scum and Foam 194.1.3 Control and Instrumentation 204.1.3.1 AERATION CONTROL 214.1.3.2 RECYCLE FLOWS CONTROL 224.1.3.3 SLUDGE SOLIDS CONTROL 234.1.3.4 EXTERNAL CARBON DOSE CONTROL 234.1.4 Separate Sidestream Treatment 24

4.1.4.1 SHARON PROCESS 254.1.4.2 PHOSPHORUS STRIPPER 264.1.5 Chemical Nutrient Removal 264.2 BIOSOLIDS HANDLING, TREATMENT AND DISPOSAL 27

4.2.1 Digestion 274.2.2 Sludge Hydrolysis 274.2.3 Dewatering and Thickening 284.2.4 Incineration, Agricultural Reuse and Final Disposal of Biosolids 294.3 EFFLUENT POLISHING, DISINFECTION AND REUSE 30

4.4 ODOURCONTROL 30

4.4.1 Chemical Scubbing 314.4.2 Biofilters 31

4.4.3 Disposing Odorous Air into Diffused Aeration Systems 324.5 PRIMARY TREATMENT AND PLANT BYPASS 32


3/86




4.6 OPERATION 33

4.6.1 Operator Training 334.7 DESIGN DETAILS 34

5 CONCLUSIONS 35

6 RECOMMENDATIONS 37

7 REFERENCES 38

TABLES

TABLE 3.1 PROGRAMME HIGHLIGHTS 6

TABLE 4.1 COST COMPARISON OF SIDESTREAM N REMOVAL PROCESSES 24

LIST OF FIGURES

FIGURE 4.1 3 STAGE BARDENPHO BIOLOGICAL NUTRIENT REMOVAL PROCESS...........................8

FIGURE 4.2 UCT BIOLOGICAL NUTRIENT REMOVAL PROCESS............................................................9

FIGURE 4.3 OXIDATION DITCH PROCESS CONFIGURED FOR P REMOVAL......................................10

FIGURE 4.4 BCFS

PROCESS.........................................................................................................................10FIGURE 4.5 5 STAGE BARDENPHO BIOLOGICAL NUTRIENT REMOVAL PROCESS.........................11

FIGURE 4.6 BIO-DENIPHO PROCESS.........................................................................................................17

APPENDICES

APPENDIX A ITINERARY

APPENDIX B - CONTACTS

APPENDIX C TREATMENT PLANT DETAILS

APPENDIX D PHOTOGRAPHS


4/86




1 EXECUTIVE SUMMARY

Name: David Solley

Address: 200 Kadumba St, Yeronga, Qld 4014

Position: Senior Process Engineer, Brisbane Water

Telephone: 07 3403 3325Email: [email protected]

Project Description: To study the upgrading of large wastewater treatment plants for

nutrient removal in Europe and North America, with particular

reference to overseas design and operations experience.

This fellowship involved an eight week programme including visits to 40 wastewatertreatment plants and two international water industry conferences in six countries (France,

the Netherlands, Denmark, Sweden, USA and Canada) and two continents.

In the Netherlands, the BCFS oxidation ditch plants and SHARON process wereinspected courtesy of Mr. Rob Vromans (BDG Engineers) and Mr. Jan Mulder (Hollande

Eilanden en Waarden) respectively.

In Denmark, Kruger A/S (Dr. Marinus Neilsen & Mrs. Tine Onnerth) introduced theiradvanced control technology, as well as inspecting the Cambi process.

Low effluent nitrogen BNR plants operating in similar climatic conditions to those inBrisbane were inspected in Hillsborough County (Tampa Florida, USA).

The pioneer for BNR technology in North America, Prof. Cliff Randall (Virginia Tech.)organised an itinerary including inspection of various BNR upgrades in the Chesapeake

Bay region (USA) and discussed his past and current BNR research.

Tours of BNR plants in Western Canada were organised by Dr A. Warren Wilson (ReidCrowther P/L). These plants included some of the earliest and recent North American

BNR upgrades, including those incorporating primary sludge fermentation.

Where very low effluent nitrogen concentrations (less than 5 mg/L) are required, a fivestage Bardenpho process (with primary settling) or oxidation ditch process (without

primary settling) is recommended. These processes are also suitable for achieving both

low phosphorus and nitrogen concentrations.

Advanced control and instrumentation offers great potential for achieving lower effluentnutrients and improved treatment efficiency. Key areas for improved control include

aeration control, recycle stream flow control and sludge solids/SRT control. This control

will be implemented at the Luggage Pt, Gibson Is and Oxley Ck WWTPs in Brisbane.

Effective utilisation of substrate is key to obtaining maximum nutrient removal in BNRprocesses. Reduction of aeration to that required for complete nitrification, maximising

EBPR, step feed of influent wastewater between anaerobic and anoxic zones and

prefermentation of primary sludge, will all contribute to achieving this objective. Side stream treatment of digestion dewatering liquors can be implemented for removal of

15 to 30% of the nitrogen load. The SHARON process is the most cost effective and

simple of the proven technologies. Research and pilot trials of alternative sidestream

treatment processes will be conducted over the next two years. Sidestream phosphorus

removal is likely to be unnecessary for most plants.

Multi-level training and qualifications for operators of advanced wastewater treatmentplants would be a positive development for the wastewater treatment industry. The Open

Learning Institute will be contacted to determine how their operations training

programme and qualifications could be further developed.

The information gathered from this fellowship will be disseminated through this report,presentations to colleagues and the wider water industry, working with colleagues toimplement the recommended improvements, and presentation of future case study results.
mailto:[email protected]:[email protected]


5/86




2 INTRODUCTION

2.1 Background

Wastewater treatment plants (WWTPs) in Australia are being required to remove

nutrients (nitrogen and/or phosphorus) from the wastewater to prevent degradation of

water bodies and the environment. Many new small to medium sized nutrient removalplants have been built in Australia in the last decade, but large WWTPs built in the

1970s for large cities are only now being upgraded for nutrient removal. Large

existing plants have typically been upgraded because of the value of the existing assets

and the high cost of new infrastructure.

This Fellowship provided the opportunity to visit upgraded large WWTPs in regions

where these plants have been operating for a number of years. No such plants and

first hand design or operations experience currently exists in Australia. The plants and

countries visited represent a selection of centres where significant developments have

taken place and an extensive operating history exists. The fellowship and this report

do not attempt to comprehensively cover all worldwide developments in wastewater

treatment plant upgrading and BNR technology.

2.2 Purpose

The purpose of this fellowship was to:-

Study the upgrading of WWTPs overseas; Investigate nutrient removal upgrades;

Focus on both design and operational aspects; Increase personal skills and knowledge; Benefit Australia; Pass on the skills and knowledge gained.

2.3 Acknowledgements

The support of the Winston Churchill Memorial Trust of Australia in making this

fellowship possible is gratefully acknowledged. Thank you also to my employer

Brisbane Water, for their assistance and providing time to undertake the fellowship.

The hospitality and freely given time and knowledge of each person who hosted me

overseas is greatly appreciated. Prof. Cliff Randall (Virginia Tech.), Dr A. Warren

Wilson (Reid Crowther P/L) and Mr. Rob Vromans (BDG Engineers) were

exceptionally generous.

Special thanks to Dr Peter Dold, Dr A. Warren Wilson and Mr. Nick Reid for planting

the seed and strongly encouraging me to travel overseas to see how things are done

elsewhere.


6/86




3 PROGRAMME

The fellowship involved an eight week programme and included visits to:-

40 wastewater treatment plants; 2 international water industry conferences; 6 countries in two continents.

The two conferences attended were:-

The First World Water Congress of the International Water Association, heldin Paris, France;

The Second International Symposium on Sequencing Batch ReactorTechnology, held in Narbonne, France.

Countries visited included:-

France; The Netherlands; Denmark; Sweden; USA (Florida, Atlanta, Chesapeake Bay region); Canada (Calgary, Okanagan Valley, Vancouver).

Table 3.1 below, lists the highlights of the fellowship programme.

TABLE 3.1 PROGRAMME HIGHLIGHTSLocation Activity Organisation/Contact

France 1st World Water Congress

2nd Int. Symposium on SBR Technology

3 WWTPs

IWA

IWA

Vivendi Water/OTV

The

Netherlands4 BCFS

oxidation ditch WWTPs

2 SHARON sidestream processes

BdG Engineers

Jan Mulder & Grontmij

Denmark 4 BioDenipho plants & Cambi process Kruger A/S

Sweden Trickling filter WWTP

ANOX AB (research activities)

Gryaab/Peter Balmer

Asa Malmqvist

Florida, USA 3 WWTPs

4 WWTPs

Hillsborough County

Orange County

Atlanta, USA 3 WWTPs City of Atlanta/Joe Porter

ChesapeakeBay, USA

6 WWTPsVirginia Polytechnic (research activities)

Prof. C. RandallProf. C. Randall

Canada 7 WWTPs Reid Crowther Ltd

Appendices A to D contain greater detail on the itinerary, personal contacts and

wastewater treatment plants visited for this fellowship.


7/86




4 UPGRADING OF LARGE WASTEWATER TREATMENT PLANTS

Prior to the last decade, wastewater treatment plants were constructed primarily for

organic carbon removal. During the last decade, the process of upgrading wastewater

treatment plants to meet more stringent environmental requirements has tended to be

almost continuous. The main driver for these upgrades has been increased

eutrophication and pollution of receiving water bodies, though water reuse

requirements are beginning to have an impact on treated wastewater quality. The

process of upgrading is likely to continue as effluent requirements become more

stringent and treatment technology develops, driven by increasing urbanisation and

increased understanding of the detrimental environmental effects of wastewater

discharges.

4.1 Nutrient Removal

The primary objective of any nutrient removal upgrade is that the nutrient removal

requirements be met, usually for nitrogen and/or phosphorus. Elements to be

considered in achieving this objective include the process configuration, design, andthe control and operation of the process.

Secondary considerations for an upgrade include the capital and operating costs, the

degree of flexibility incorporated for unexpected conditions, the reliability and

robustness of the process and the ease of operation. Capital cost is largely influence

by the size of the tankage required for the process. Operating costs are mainly

influenced by personnel, energy and chemical usage. For activated sludge processes,

the settling characteristics of the sludge will have the greatest influence on reliability

and operability.

4.1.1 Biological Nutrient Removal

Almost all wastewater nutrient removal processes developed in the last decade have

been based on biological nutrient removal (BNR) principles. This is largely due to the

overall cost advantages of BNR in most cases. Chemical treatment to further remove

phosphorus is sometimes applied, when the nutrient removal achieved by BNR is

insufficient to meet effluent requirements. Non-biological removal of nitrogen from

municipal wastewater is difficult to achieve (in comparison to BNR). Therefore, the

usual strategy is to remove all the required nitrogen and as much phosphorus as

possible with BNR, then remove the remaining phosphorus requirement with chemical

precipitation of phosphorus (refer Section 4.1.5).

Biological nitrogen removal is achieved by the nitrification/denitrification process,

where the influent ammonium is oxidised to nitrite/nitrate, which in turn is reduced to

nitrogen gas. Two different groups of bacteria carry out the two step process. The

ammonium oxidising bacteria operate in an aerobic environment and the nitrate/nitrite

reducing bacteria operate in the absence of dissolved oxygen. The denitrifying

bacteria also require substrate/carbon for the reduction reaction. The processes can be

achieved in a number of formats, including attached growth/fixed film systems and

suspended culture/activated sludge systems.

Enhanced biological phosphorus removal (EBPR) is achieved by bacteria capable ofstoring substrate/carbon and releasing phosphorus in an anaerobic zone (that is, in the

absence of dissolved oxygen and nitrate/nitrite). These bacteria are then able to utilise


8/86




the stored substrate to store excess amounts of phosphorus in an aerobic environment.

By wasting excess biosolids from the aerobic zone of the process, excess phosphorus

is removed from the wastewater by these phosphate accumulating bacteria. Recent

research also shows that excess phosphorus uptake can occur in the presence of

nitrate/nitrite and absence of dissolved oxygen (anoxic conditions) (refer Section

4.1.1.3). EBPR is conventionally achieved in suspended culture/activated sludge

processes, but recent research has successfully demonstrated EBPR with attached

growth processes (albeit with some difficulty).

For biological removal of both nitrogen and phosphorus, nitrification/denitrification

and EBPR are combined in the one process, usually using a single sludge system. For

low effluent concentrations of both nitrogen and phosphorus, a conflict usually exists

in municipal wastewater, where the amount of substrate available for EBPR and

denitrification is limiting on one or both of the processes (refer Section 4.1.1.3).

Additional substrate can be added to the process to make up the substrate deficit, for

either the denitrification or EBPR processes. Alternatively, chemical precipitation of

the required phosphorus deficit can be applied.

A large variety of process alternatives have been developed on the above principles,

each with a particular advantage or designed to overcome the limitations of another.

The choice of process type is largely dependent on the nutrient removal requirements,

site constraints and the influent wastewater characteristics amongst other factors.

Some of the significant process alternatives examined as part of the fellowship are

presented below.

4.1.1.1Bardenpho and UCT BNR ProcessesOf the processes investigated, most fall into two main groups; the Bardenpho processand the University of Cape Town (UCT) Process. In the three stage Bardenpho

process (refer Figure 4.1), influent wastewater combines with return sludge from the

clarifier in an unaerated reactor (anaerobic zone). In this zone, phosphorus is released

and substrate stored by the phosphorus accumulating bacteria, in the absence of

nitrate. The process flow then enters an anoxic zone, where it combines with a nitrate

recycle from the end of the aerobic zone. In this unaerated zone, the returned nitrate is

reduced to nitrogen gas. The flow then enters an aerobic zone, where ammonia is

oxidised to nitrate. Phosphorus is removed from the process with the excess biosolids,

which are wasted either from the clarifier underflow or the aerobic zone. Nitrogen is

removed as nitrogen gas bubbling from the anoxic zone.

Figure 4.1 3 Stage Bardenpho Biological Nutrient Removal Process

Return Sludge

Mixed Liquor Recycle

Anaerobic Anoxic Aerobic


9/86




One limitation of the Bardenpho process, is that nitrate returned to the anaerobic zone

with the return sludge from the clarifier can reduce the effectiveness of the phosphorus

release/substrate storage mechanism. The magnitude of this effect is directly related

to the levels of nitrate in the return sludge stream. In a process designed for low

nitrogen, low nitrate levels are likely achieved, minimising the mass of nitrate returned

with the sludge to the anaerobic zone. Endogenous denitrification of the return sludge

in the clarifier sludge blanket, return channels or pipe work can also reduce the mass

of nitrate returned. In Johannesburg/Westbank type modifications, a specific

predenitrification zone can be included for nitrate removal from the RAS. In some

plants, a proportion of the influent wastewater (say 10%) can be fed to the

predenitrification zone, to increase the denitrification rate that would be achieved by

reliance on endogenous respiration (Bonnybrook and Summerland WWTPs, Canada).

In order to better protect the anaerobic zone from nitrate, the UCT (University of

Cape Town) configuration returns sludge from the end of the anoxic zone to the

anaerobic zone, which can be controlled to have low or zero nitrate (refer Figure 4.2).Disadvantages cited for this configuration include the need for an additional recycle

stream (and associated pumps, pipes etc), and diluted mixed liquor solids in the

anaerobic zone resulting in larger anaerobic volume. Some proponents of the UCT

configuration argue that a larger anaerobic zone is not required, as the process favours

a greater mass proportion of phosphorus accumulating bacteria.

Figure 4.2 UCT Biological Nutrient Removal Process

Two VIP (Virginia Initiative Project) plants were inspected near Chesapeake Bay in

the USA. These plants are essentially high rate UCT processes that achieve excellent

phosphorus removal and good nitrogen removal with a low hydraulic retention time(HRT).

The oxidation ditch process for biological phosphorus and nitrogen removal is

essentially a three stage Bardenpho process with a very high mixed liquor recycle rate

(refer Figure 4.3). Typically these plants are configured without primary settling, with

the process usually operated at long sludge retention time (SRT) and long HRT (so

called extended aeration). The advantage of this process is that effluent total

nitrogen of less than 3 mg/L is possible for almost any wastewater (Randall et.al.,1992). With very low nitrate levels in the return sludge, good phosphorus removal is

Return Sludge



Anoxic Recycle


10/86




also possible. The disadvantages include large tankage requirements and the

disadvantages associated with not including primary settling (such as higher aeration

demand, and no primary sludge available for anaerobic digestion and subsequent gas

utilisation; refer Section 4.1.1.3).

Figure 4.3 Oxidation Ditch Process Configured for P Removal

The BCFS oxidation ditch plants (Netherlands) included a UCT type configuration

with an oxidation ditch (refer Figure 4.5). This may be unnecessary from a

phosphorus removal viewpoint, because with an oxidation ditch it should be possible

to operate the process with low nitrate in the sludge return stream. However, the

proponents of this system cite the advantages of better settling sludge and better

utilisation of substrate (by denitrifying dephosphating bacteria) for this system (refer

Sections 4.1.2 and 4.1.1.3 respectively for discussion of these aspects).

Figure 4.4 BCFS Process

The Bonnybrook plant (Calgary, Canada) has the capability of operating in either a

Bardenpho or UCT configuration. Experience at this plant was that the UCT

configuration gave greater variation in the phosphorus removal performance comparedto the Bardenpho configuration (the current operating configuration).



Anoxic Recycle

Return Sludge

AnoxicSelector

Anaerobic

Anoxic

Return Sludge

Influent

EffluentAerobic


11/86




4.1.1.2Three Stage Versus Five Stage BNR ProcessFor greater removal of nitrogen, five stage processes are proposed to give greater

nitrate removal. Typically, a secondary anoxic zone is included after the primary

aeration zone, where endogenous denitrification occurs. Alternatively, additional

external substrate can be dosed into this zone for higher denitrification rates.

Following this zone, a secondary aeration zone is usually included to reaerate thewastewater and nitrify any ammonia produced as a result of the endogenous activity in

the secondary anoxic zone. Figure 4.4 shows the five stage Bardenpho process

configuration.

Randall (1997) concluded from his investigations, that an effluent TN limit of 3 mg/L

cannot be achieved with a three stage BNR process. Accounting for the

unbiodegradeable TKN in the effluent (usually greater than 1 mg/L) would require

effluent ammonia and nitrate levels totalling 2 mg/L. This is not typically achievable

with a three-stage process. Process options for low nitrogen then include a five stage

BNR process, or post-denitrification in a denitrification filter for example. As

discussed in Section 4.1.1.1, Oxidation ditches without primary settling (essentially a

three stage process), have demonstrated in South East Queensland and overseas that

they are another alternative for effluent TN less than 3 mg/L.

Figure 4.5 5 Stage Bardenpho Biological Nutrient Removal Process

4.1.1.3Substrate UtilisationTo achieve low effluent concentrations of both nitrogen and phosphorus, a conflict can

exist between the amount of substrate available and required. This is often the case

with municipal wastewater. It follows then, that the appropriate utilisation of

available substrate for the various biological processes is a critical success factor for

BNR.

As discussed in Section 4.1.1, for low concentrations of both nutrients, the focus is

usually to first achieve the nitrogen removal requirement. Therefore, the required

amount of substrate available for denitrification should be ensured. As demonstrated

by the BCFS plants (Netherlands), this can be achieved in a couple of ways (van

Loosdrecht, 1998).

The use ofdenitrifying phosphorus removing bacteria (DPB) can maximise the useof substrate. These bacteria remove 1 mg/L of P for every 4-5 mg/L of nitrate

removed, essentially utilising the same substrate for both objectives (about 20

Return Sludge


Anaerobic Anoxic Aerobic Anoxic Aerobic


12/86




mgCOD/L). Kuba et. al. (1996 & 1997) report that these bacteria require recycling of

activated sludge through anaerobic and anoxic conditions for a good accumulation,

and are found in abundance in many UCT-type processes. A maximum reduction of

50% of the COD required, compared to conventional aerobic phosphorus

uptake/nitrogen removal processes is reported. DPB observations are widely supported

(Randall, 1997).

At the Genemuiden WWTP (Netherlands), the DPB activity was found to be not as

high as at other BCFS

plants. This was possibly due to low VFA in the wastewater,

nitrate or oxygen recycle to the anaerobic zone and less recycle of nitrate to the anoxic

zone. That is, the BNR process was not operating as well.

It is not known whether other process configurations with suitable anoxic and

anaerobic fractions and recycles can achieve enhanced phosphorus uptake under

anoxic conditions. Results from the Westbank plant (Kelowna, Canada), suggest that

significant phosphate uptake is taking place in the anoxic zones of this three stage

Bardenpho process; the DPB mechanism a likely cause. Randall suggests that theUCT process favours DPB because the UCT process provides the most reliable EBPR.

Because the UCT process best protects the anaerobic zone from nitrite/nitrate, the

VFA present in the wastewater is all used for EBPR, thus maximising DPB. It also

follows then that the VFA/PO4 ratio is also an important factor in determining the

degree of DPB possible.

Step feed of influent was applied at a number of plants inspected. One variation

found at the Bowie plant (Maryland, USA) and the Kelowna, Lake Country, Westbank

and Summerland plants (British Columbia, Canada), was to step feed influent

wastewater, part to the anaerobic zone and the remainder to the anoxic zone. This

ensured that the required proportion of the substrate required for EBPR anddenitrification was applied to the respective zones. An added benefit is that the solids

concentration in the anaerobic zone is kept higher, and its size could therefore be

reduced. Although this process would result in readily biodegradable substrate being

fed directly to the anoxic zone (with its low F/M ratio), there was no evidence of a

significant deterioration in sludge quality or rise in SVI (refer Section 4.1.2). The

percentage of wastewater fed direct to the anoxic zone was 50 to 60% in the Bowie

plant and up to 30% for the Canadian plants. In New York City, 13 of the 14

wastewater treatment plants are to be converted from plug flow aeration processes to

step feed anoxic/aerobic processes for nitrogen removal.

At the Bjergmarken WWTP (Roskilde, Denmark), a proportion of the raw wastewatercould be bypassed around the presettling and sent direct to the bioreactors. The

proportion of wastewater fed direct to the bioreactor was controlled to ensure

sufficient substrate for denitrification.

Endogenous substrates are a significant contributor to the requirements for

denitrification. These are produced and utilised throughout the BNR process.

Therefore, it follows that the denitrification process benefits if the aerobic retention

time and oxygen provided are minimised, providing the maximum endogenous

substrate for the anoxic process. Usually, the objective of the aeration zone is

complete nitrification, so excess aeration provided above that required for complete

nitrification is wasted. This additional aeration also consumes endogenous substrate

that should be saved for the anoxic zone and denitrification, and also causes


13/86




unnecessary release of nutrients through the cell destruction of the aerobic endogenous

activity. These minimum aeration objectives were the focus of many processes

inspected (Netherlands, Denmark, Chesapeake Bay/USA and Canada). The objectives

were often achieved through control of the aerobic fraction and oxygen supplied.

Further detail of this control is given in Section 4.1.3.1.

Van Loosdrecht et.al. (1997) supported maximising the fraction of nitrifying bacteria

in the BNR process to minimise aerobic endogenous activity, through:-

Operating at long SRT Primary settling to minimise the accumulation of inerts and heterotrophic

bacteria

Not combining chemical phosphorus removal with the activated sludgeprocess, to prevent accumulation of chemical precipitates in activated sludge.

Primary settling of wastewater is often incorporated in treatment plants to minimise

energy requirements and operating costs. The primary sludge removed is usuallystabilised in an anaerobic digester in combination with waste activated sludge (WAS)

(refer Section 4.2). Methane gas produced in the digestion process can be used for

various energy needs at the plant. Energy is also saved in reduced aeration and costs

are saved with lower excess biosolids production in the BNR process. However,

plants lacking sufficient substrate or with a poor TKN/COD ratio may require the

substrate available from primary sludge for the BNR process (primary sludge usually

contains a more favourable TKN/COD ratio than that for settled wastewater).

Oxidation ditches without primary settling usually produce very low effluent nitrogen

levels through utilisation of all the available substrate and a favourable TKN/COD

ratio. Prefermentation of the primary sludge to obtain VFA and readily available

substrate for the BNR process is discussed in Section 4.1.1.4.

Of course, there is always the option to meet substrate needs from an external source;

whether it is commercial grade methanol, acetate or a waste product from another

industry. Of the plants inspected that dosed an external carbon source, methanol was

most commonly utilised for denitrification. One disadvantage is that the activated

sludge culture needs to adapt to utilising methanol, whereas dosing with acetate

requires no acclimatisation. This is most likely due to the natural occurrence of

acetate substrates in most municipal wastewaters.

4.1.1.4Prefermentation of Primary SludgePrefermentation of primary sludge is another method for gaining readily

biodegradable substrate for the BNR process. The substrate gained is in the form of

volatile fatty acids (VFA), which is the required substrate for EBPR. For this reason

prefermentation is most often applied to EBPR, where the prefermenter substrate is

dosed to the anaerobic zone. The critical ratio in assessing the VFA requirements for

EBPR is TP/VFA, and prefermenters are usually sized/designed on this basis.

Prefermenter generated substrate could be dosed to anoxic zones to assist with

denitrification, but this wasnt found at the plants visited. Fermentation of primary

sludge will lead to the release of nitrogen and phosphorus, which had previously been

removed from the effluent with the primary sludge. For this reason, careful

consideration must be given to this additional nutrient load when assessing thebenefits of prefermentation (important for nitrogen, more so than for phosphorus).


14/86




There are various types of prefermenter configurations available, but complete mix

and static fermenters were the types incorporated at the plants inspected (Canada). In

the static fermenter, primary sludge is discharged to a reactor/tank, which is similar in

configuration to a gravity thickener (sludge loading approximately 30 kg/m2/d). A

thick sludge blanket is maintained in the reactor, corresponding to the required sludge

age. The fermentation products are elutriated from the sludge blanket into the

supernatant by the thickening action and stirring of the scraper/picket fence rake.

These systems are favoured for their simplicity and easy control (i.e. maintenance of a

sludge blanket level). Static prefermenters can be controlled to various sludge blanket

levels, corresponding to the required sludge age for a particular temperature/season

(Kelowna plant, Canada). Complete mix fermenters incorporate a complete mix

reactor ahead of a separation process, which is usually a gravity settler.

At the Bonnybrook plant (Calgary, Canada), the formation of methogenous bacteria is

controlled by periodically aerating the fermenter reactor for short periods, using a

course bubble diffuser system installed in the reactor. At Penticton WWTP, the staticfermenters are fully mixed for an hour a day (and at times, on a couple of other

occasions throughout the day also), using vertical shaft mixers. This is also to control

the formation of methogenous bacteria and elutriate the VFA. Odour control is

necessary for prefermenters and those inspected were fully covered, with odorous gas

removed to odour treatment systems (refer Section 4.4 and Appendix C).

The Nansemond (Virginia, USA) and Summerland (Kelowna, Canada) plants operate

their primary clarifiers to maintain deep sludge blankets, in order to ferment the settled

solids. The VFA formed is elutriated from the sludge blanket to the primary effluent

in much the same way as a static fermenter; by action of the sludge thickening and

stirring of the sludge blanket by the scraper mechanism. In doing so, the Nansemondplant is able to consistently reduce the effluent total phosphorus from about 1.1 mg/L

to less than 1 mg/L. Reid Crowther P/L recommended reducing the primary settling

tank (PST) peak overflow rate from the normal 4.5-5.5 m/hr to less than 3.5 m/hr in

this mode of operation, to allow thickening of the primary sludge in the PST. (This is

similar to the approach that has been adopted at Luggage Pt WWTP in Brisbane).

4.1.1.5Hydraulic Retention Time (HRT)Hydraulic retention time has perhaps the greatest influence on the capital cost of a

treatment plant. The greater the HRT, the larger the tankage required. The HRTs of

the plants inspected varied from 6 hours to greater than 24 hours. Generally, theplants achieving greater nitrogen removal operated at a higher HRT (apart from

exceptions such as Largo WWTP, which incorporates post-denitrifying filters). There

is often a link between HRT, SRT and MLSS concentration in the bioreactor (refer

Section 4.1.1.6). For example, oxidation ditches without primary settling and long

SRT often require a high HRT to maintain MLSS in a workable range (say 4000 to

5000 mg/L). Phosphorus removal does not appear to be influenced significantly by

HRT in the normally applied range.

The VIP plants (Virginia, USA), based on the UCT configuration, were designed and

are operated with very low HRT (about 6 hours). These plants achieved low

phosphorus levels (less than 1 mg/L) and moderately low nitrogen levels (8 mg/L). Itshould be noted that even lower nitrogen results (say 6 mg/L) could probably have be


15/86




obtained by increasing the nitrate recycle (to 2 times the inlet flow rate), but this

performance was not required to meet licence at these plants. The wastewater treated

at these plants is quite dilute compared to Australian conditions. So when comparing

performance, the effluent concentrations should not be considered in isolation to

percentage removal and the effluent nutrient mass figures, which are not quite as

impressive.

4.1.1.6Solids Retention Time (SRT)The solids retention time is one of the key parameters in designing and operating a

BNR plant. The SRT defines the average time which bacteria are retained in the

process, and with the wastewater loading, effectively determines the mixed liquor

solids level in the process. The maximum SRT value (and hence solids level) possible

for the process, is defined by the capacity of the final clarification step. That is, the

limit for solids in the bioreactor corresponds to the maximum allowable flux loading

on the clarifiers. Put another way, the SRT determines the size of the clarifiers for the

process and has a large influence on capital cost of a plant. It follows then, that an

improvement in solids settleability can lead to greater flexibility and possible increase

in the SRT of the process or lower capital cost (refer Section 4.1.2).

Wastewater temperature also has an influence on the required SRT for a process, with

lower temperatures requiring higher SRTs. This is most important with respect to

nitrification. Many plants in cold temperature have difficulty fully nitrifying during

winter months, and this is often reflected in their nitrogen licence (Canada and

Denmark).

The generally held view is that for nitrogen removal, the longer the SRT, the better the

performance; and for phosphorus, the shorter the SRT, the better the P removalperformance. So to achieve both N and P removal, a compromise is often required.

The approach is usually to ensure the required biological N removal and supplement

the biological P removal with chemical precipitation. For complete nitrification, a

minimum SRT is required, dependent on the aerated fraction in the bioreactor and the

wastewater loading and temperature. The SRT then required to achieve the desired

nitrate level is dependant on the substrate available. If there is adequate readily

available substrate in the raw wastewater, then the reliance on endogenous

denitrification is diminished and the SRT could be quite low. If the TKN/COD ratio is

unfavourable, then the plants reliance on endogenous denitrification increases and

generally the SRT is increased. Of the plants inspected, many are achieving very low

effluent phosphorus and low nitrogen with quite a low SRT (e.g. the Canadian plantssuch as Kelowna and Summerland and the VIP processes in Virginia, USA). On the

other hand, many long SRT plants achieved very low nitrogen and phosphorus, albeit

often with the addition of precipitant chemicals.

Control of SRT is fundamental to the performance of most BNR process. It provides a

stable control parameter, which allows adjustments in mixed liquor solids to meet

varying wastewater loadings. Many plants have different SRT set points for different

seasons of the year; or may more directly control SRT to wastewater temperature

(Summerland WWTP, Canada). Further details on SRT and solids control are given

in Section 4.1.3.3.

4.1.1.7Plug Flow and Complete Mix Reactors


16/86




Each zone in a BNR process can be designed either as a plug flow reactor, complete

mix reactor, or some variation between the two extremes. A plug flow reactor

provides the most efficient arrangement, where all the flow remains in the reactor for

essentially the same duration; that is, a uniform residence time. Plug flow reactors can

generally be sized smaller than complete mix reactors. Plug flow reactors also allow

for better control. For example, when recycling from a plug flow anoxic zone to the

anaerobic zone in the UCT process, there is a much better chance that the recycle

stream has low nitrate in the plug flow reactor than in a complete mix reactor.

Selectors in particular are reported to give much better performance in the plug flow

configuration (refer Section 4.1.2).

However, maintaining mixed liquor solids in suspension in a plug flow reactor can be

difficult to achieve and can promote short-circuiting when mechanically mixed.

Unless the reactor is configured such that the forward velocity of the mixed liquor

through the reactor is greater than about 0.27 m/s (as in an oxidation ditch), then the

best that can be achieved will be a number of complete mix reactors in series. The

greater the number of complete mixed reactors, the closer the approximation to plugflow. The efficiency and power requirements of the mechanical mixing system also

needs to be balanced against the desire for plug flow. Each complete mix zone will

require at least one mechanical mixer. Very efficient mixing can be provided for large

complete mix reactors, using large diameter impeller, slow speed mixers (Kelowna

WWTP, Canada).

4.1.1.8Cyclic/Batch OperationVarious alternatives exist for cyclic/batch operation BNR processes. In these

processes, the aeration and wastewater feed applied are cycled to produce varying

conditions in the reactor. The Danish developed Bio-Denipho

process has beenadopted throughout Denmark and applied in various parts of the world (Florida, USA

and Australia). Figure 4.6 shows the various phases of the process, as usually applied

(alternatives exist with phases where both reactors operate in an anoxic mode). Mixed

liquor is fed alternatively to the two main bioreactors, which are sequentially operated

in anoxic and aerobic modes.

The process is reported to give increased flexibility over continuous processes, in that

the HRT and mass fraction of the anoxic and aerobic zones can be adjusted by

variation of the various phase lengths. A greater degree of on-line instrumentation is

usually applied to these types of processes, aimed at optimising this available

flexibility. The process also eliminates the requirement for a nitrate recycle, leadingto some potential energy savings.

Of the plants inspected, the performance of the cyclic processes did not exceed the

performance of many continuous processes, despite this increased flexibility. This is

likely either because there has been an increasing focus on providing flexibility in

continuous processes, or the degree of flexibility is not required due to the continuous

processes being quite well matched to their loading conditions. The SRT and HRT

adopted were at the long end of the range for the examined plants, indicating that the

increased flexibility had not lead to great capital cost savings. At the South County

plant (Tampa, USA), the process has not been operating in the cyclic BioDenipho

mode for over six months. The operators have preferred operating the plant in acontinuous mode, with the two oxidation ditches operating continuously in series. A


17/86




low DO concentration is maintained at this plant, which would be promoting

substantial SND (simultaneous nitrification/denitrification).

Figure 4.6 Bio-Denipho Process

4.1.1.9Attached Growth ProcessesAs an alternative to suspended growth processes (e.g. activated sludge BNR), attached

growth BNR processes are being investigated and adopted for a number of plants.

These process types include:-

trickling filters, suspended and fixed carrier processes, biological filters.

Suspended and fixed carrier processes are usually combined with activated sludge inthe same process stream. Biomass grows on the carriers, as well as forming activated

sludge flocs. The carrier media are retained in the bioreactor and the activated sludge

flows through to the clarifier for separation. For many attached growth projects, the

aim of installing the media in the aerobic zone is to improve the plants nitrification

capacity. The biomass growth for nitrification is usually more difficult to establish in

an activated sludge process, and also forms only a thin biomass film on the carriers,

avoiding the bulky heterotrophic growth that often leads to clogging of the carrier

media. The effectiveness of clogged media is severely reduced due to limited contact

between the wastewater and the biomass, plus the media can sink to the bottom of the

reactor floor.

Anoxic

Aerobic

Return Sludge

Anaerobic

PHASE A

Aerobic

Return Sludge

Anaerobic

PHASE B

Anoxic

Aerobic

Return Sludge

Anaerobic

PHASE C

Aerobic

Return Sludge

Anaerobic

PHASE D

Aerobic

Aerobic


18/86




Trials using LinPor

sponges overcame clogging problems by utilising airlift pumps

to squeeze out inert material (Chesapeake Bay, USA). At the Annapolis plant

(Maryland, USA), ring-lace attached growth media are installed in the last two thirds

of the aerobic zone of the bioreactor. Most of the nitrification at the Annapolis plant

was complete in the first third of the aeration zone, so it was difficult to gauge the

success of the trial. From this and similar trials carried out in Canada and Australia, itseem appropriate to install media throughout the whole aeration zone in processes

with up-front anaerobic/anoxic zones (e.g. MLE, Bardenpho or UCT processes). Most

of the readily available substrate has been absorbed onto the activated sludge flocs in

the unaerated zones and nitrifier growth on the carriers should predominate for the

whole aerated zone.

Worm growth on ring lace or carrier media is also a common problem with attached

growth processes. This has been successfully controlled in aeration zones by turning

the air off each day for a period, and also by chlorinating the RAS (Annapolis, USA).

Carrier media have also been successfully used for separate denitrification reactors orwithin unaerated zones with an activated sludge process. Mechanical mixing is then

required to keep the media mobile and any activated sludge in suspension. Estimated

of the mixing power require for carriers in unaerated reactors is estimated between 10

to 13 W/m3 (Maurer, 2000). This was confirmed by Anox AS (Sweden), as between

10 to 20 W/m3.

Trickling filters were included in a couple of plants inspected (Goteburg, Sweden and

Vancouver, Canada). The reported advantages of trickling filters, which lead to their

inclusion in these plants, included their robust performance, low operating cost,

simplicity and higher nitrification rates (for Goteburg). None of the trickling filter

plants examined achieved low effluent nitrogen.

Post-denitrification filters, with the addition of substrate for denitrification, have been

included at several plants throughout Europe and the USA. Two French water

companies (OTV/Vivendi and Degremont/Lyonnaise) have implemented biological

filter processes as the main biological treatment process in the Paris region (France).

The biological filter processes adopted there are the companies proprietary products

(such as BIOSTYR

and BIOFOR

) which are filters configured for nitrogen

removal. Methanol addition to the unaerated section of the filter provides suitable

conditions for denitrification. These processes are compact and achieve substantial

nitrogen reduction, though not matching that of BNR processes elsewhere and at

higher capital and operating cost. The BAF process (biologically aerated filter) at the

Roanoke plant (Virginia, USA), has experienced media retention and inlet screening

problems. The second stage filters also clog with algae regularly. The BAF is taken

off-line twice a day for cleaning.

4.1.1.10 Secondary Release of PhosphorusSecondary phosphorus release occurs under anaerobic conditions, when phosphorus is

released by the EBPR bacteria without substrate storage. This is believed to have a

detrimental effect on EBPR, as excess phosphorus uptake by these bacteria cannot

then occur in the aerobic zone. Low effluent nitrate can cause secondary release of

phosphorus in the final settling tanks (FSTs) and RAS return stream. To overcomethis effect, the preference is often to operate with some effluent nitrate, when low


19/86




concentrations of phosphorus are required. Secondary release can also occur if

anaerobic or secondary anoxic zones are too large, such that for a significant part of

the zone, the nitrate and VFA are zero. These effects may be controlled by providing

flexibility in the sizing of these zones and through control of nitrate levels (i.e. recycle

rates, DO and carbon dosing control; refer Section 4.1.3).

An alternative view is that secondary release in an anoxic environment may not

necessarily be a disadvantage, as substrate storage has still been measured with this

release (Randall, Virginia Tech.). The mechanism could be associated with glycogen

storage, as the glycogen may be converted to PHA at the end of the anoxic zone.

4.1.2 Sludge Settleability, Scum and Foam

Activated sludge settleability and the degree of scum formation and foaming can be

the most influencing factors in determining the ease of operating a BNR plant. Sludge

settleability governs the final clarification or separation step in the activated sludge

process. A sludge that has poor settleability results in difficulty with separating the

biological solids from the treated effluent. In the worst case, process failure occurs

when the sludge solids are lost with the effluent. Poor sludge settleability usually also

indicates that the activated sludge will have poor thickening and dewatering

characteristics (that is, low thickened sludge and cake solids content from thickening

and dewatering processes such as gravity thickeners, belt presses and centrifuges

refer Section 4.2).

The most widely adopted measure of sludge settleability is the sludge volume index

(SVI). BNR plants are notorious for sludges that have poor settling characteristics and

high SVI values, usually greater than 150 mL/g and often in the 200 to 300 mL/g

range. The result of this is that the final clarifiers in a BNR plant are usually sizedconservatively, to avoid process failure when the sludge is settling poorly. If plants

can be designed to reliably produce a good settling sludge, then the reliability of the

process is increased and/or the size of clarifiers can confidently be reduced, resulting

in capital and operational savings.

Of the plants inspected, the BCFS processes (Netherlands) and VIP processes

(Virginia, USA) consistently produced the lowest SVI sludges. The features of these

processes proposed to lead to this excellent sludge settleability include a UCT

configuration and plug flow or compartmentalised anaerobic selectors. The BCFS

processes use a carousel type selector configuration and the VIP processes, two to

three complete mix reactors in series. Some complete mix anaerobic selectors alsoproduced good settleability, so this is obviously not the only influencing factor.

Selectors are designed so that a sufficiently high F/M (food/micro-organism) ratio

exists for the floc forming bacteria to out-compete filamentous bacteria. In theory,

anaerobic selectors are sized such that all the readily available substrate is adsorbed

into the activated sludge flocs in this high F/M environment. This substrate is then not

available in a low F/M environment where filamentous bacteria have a competitive

advantage. Good settling sludges were produced at the plants where the anaerobic

zones were greater than 10% of the bioreactor volume.

The benefits of the UCT process in producing good settling sludge is attributed to that

processs complete elimination of nitrite from the anaerobic selector. The presence ofnitrites in the aerobic zone is reported to favour filamentous bacteria (Randall,


20/86




Virginia Tech.). Nitrite is a powerful oxidiser and is thought to inhibit floc-forming

bacteria. In the UCT process it is possible and best to control the nitrate recycle to

have little or no nitrate at the end of the anoxic zone, as the presence of nitrate usually

indicates the present of some nitrite. Oxidation ditches (without upfront anaerobic

selectors) could have nitrite contact conditions quite easily with their high

recirculation rates, and this may be the cause of their notoriously poor settlebility.

Low selector nitrate/nitrite is also possible with other process configurations (e.g.

Bardenpho and BioDenipho processes), by maintaining low effluent nitrate and/or by

completely denitrifying the return sludge.

In the BCFS

processes, the first part of the anoxic reactor is also compartmentalised

to provide an anoxic selector (refer Figure 4.4). This selector is provided for

absorption of the soluble hydrolysis products that remain after the anaerobic zone.

Introduction of this anoxic selector reportedly lead to reductions in SVI from around

150 mL/g to between 80 and 100 mL/g at one of the BCFS

plants, which is typical

for the process type (van Loosdrecht, 1998).

Many plants reported severe scum problems, particularly when mixed liquor

concentrations were high (generally during winter). One successful control method

was to remove the scum from the bioreactor (Canada and Atlanta, USA). The

approach is to remove the scum forming organisms from the surface of the bioreactor

where they are growing. Several of the Canadian plants selectively wasted sludge and

scum together from the end of the bioreactor.

Foam control using poly-aluminium chloride dosing (e.g. PAX-14 by Kemwater) has

been successful practised in the Netherlands and in Denmark (Naestved WWTP)

against microthrix filamentous bacteria. The chemical is dosed at 70 to 80

mgPAX/kgVSS. The theory is that the alum coats the filaments and decreases theircompetitiveness. Improvements are reported to occur in 2 to 3 days from

commencement of dosing.

4.1.3 Control and Instrumentation

The degree of control applied to the plants inspected varied from manual adjustment

of equipment without the aid of on-line instrumentation (e.g. no on-line DO meters at

Falkenburg WWTP, Florida), to state of the art control systems with automatic set-

point adjustment (Denmark). More advanced control systems generally allowed

plants to achieve improved performance, rather than lower staffing levels. In essence,

the more advanced control systems have allowed monitoring and tuning of thetreatment process to a degree that would never have been considered manually. It is in

this area of advanced control and instrumentation that the greatest improvements

could be made in achieving low effluent nutrients in Australia.

Improved instrumentation is partly responsible for the improvements in control.

Reliable nutrient meters are becoming available, and advances are focused on reduced

maintenance, low running costs (e.g. less reagents) and higher sample frequency. For

example, the Dr Lange Nitrax probe is used extensively in Europe for online nitrate,

utilising the UV measurement principle. This meter is capable of giving true on-line

readings, rather than the reactor chemistry meters, which have a sample frequency

of about fifteen minutes or so.


21/86




All plants with on-line nutrient analysers, incorporated sample filtering (using simple

cross-flow membrane systems).

4.1.3.1Aeration ControlControl of aeration in BNR plants has been achieved in many ways, the usual

objective being to control to a DO set-point in the aerated zones, sufficient to removeammonia-nitrogen. By controlling the aeration to the minimum required to achieve

complete nitrification (say ammonia less than 1 mg/L), the following objectives can be

achieved:-

The substrate utilised aerobically is minimised, providing the maximum fraction ofsubstrate for denitrification,

Aeration energy is minimised, Aerobic degradation of the biomass is minimised.

Essentially, aerating the process after all ammonia is removed is wasted effort and islikely to be detrimental to the process.

At the Genemuiden WWTP (Netherlands), the aeration in the main aerobic reactor is

controlled to a DO set point at the reactor outlet. In the anoxic/aerobic swing reactor,

the surface aerator is controlled by turning the aeration on when the ORP value at the

outlet of the reactor is less than a set point (say < -100 mV); and off when the DO at

the outlet of the main aeration reactor reaches above a high set point. Controlling the

ORP in the swing zone between zero and 150 mV can ensure that suitable conditions

for simultaneous nitrification/denitrification (SND) occur. In this way, the aeration

mass fraction is optimised at this plant.

Sen and Randall (1990) successfully controlled aeration at the Bowie WWTP using

feedback from an on-line effluent alkalinity meter. The effluent alkalinity was used to

control the oxygen supplied by increasing when alkalinity rose above a set range and

vice versa. The control method is appropriate for systems with long HRT, low F/M

and high recycle rates (e.g. oxidation ditches). Low DO levels were also maintained

(0.5 mg/L), to reduce the oxygen applied and maximise SND. Sen and Randall also

showed that effluent phosphorus levels rose along with turbidity, when the amount of

oxygen supplied was insufficient for substrate stabilisation and nitrification. In this

way, turbidity has been used to control aeration in the Schreiber Corporation plants

(Maryland, USA).

In Denmark, the aeration system DO set points were controlled to ammonia and OUR

(as determined by the on-line DO and air flow rate measurements). Similar automatic

DO set point adjustment to on-line ammonia values has been implemented at the

Beckton WWTP (U.K.). The operational mode of aerobic/anoxic swing zones can

also be controlled to effluent ammonia values.

The Summerland plant (Kelowna, Canada) automatically changes the aeration system

DO set point through 12 different set point values during each day. The set-point

values are manually entered by the operator, with the objective of maintaining

ammonia between 0.6 and 0.8. Grab samples are taken from time to time, to monitor

the effluent ammonia achieved. The samples are analysed using the commonlyavailable test kits that utilise pre-packaged reagent vials (Merck, Dr.Lange and Hach).


22/86




Set point adjustments are manually made on the basis of these results and wastewater

temperature.

At the end of an aerobic zone, over-aeration is often caused by low oxygen demands

mismatched with the inability of the aeration system to sufficiently reduce the airflow.

Also, the requirement for mixing in this zone (i.e. to keep the mixed liquor in

suspension) often requires the airflow to be higher than that required by the biological

process. This can lead to carryover of oxygen into the anoxic zone and reduced

efficiency of the denitrification process. Solutions to the problem include:-

Installing supplemental mechanical mixing in these aeration zones (Utoy CkWWTP, Atlanta USA).

Installing coarse bubble diffusers for the low aeration demand sections of theaeration zone (Westbank WWTP, Kelowna Canada).

Pulsing the aeration system on and off in these sections, to provide minimumaeration but sufficient mixing.

4.1.3.2Recycle Flows ControlThe mixed liquor recycle flows in a BNR process have also been successfully

controlled for improved nutrient removal at many plants. Control of the nitrate (or

A) recycle, aims to return the optimum mass of nitrate to the anoxic zone to

maximise removal. In the UCT type processes, this is particularly critical, in that the

nitrate at the end of the anoxic zone (where the mixed liquor is recycled to the

anaerobic zone) should be minimised preferably to zero.

Most plants keep the mixed liquor recycles at a constant rate, while the most basic

control paces the mixed liquor recycles to the plant inflow (Florida, USA). TheBCFS plants controlled the nitrate and anaerobic mixed liquor recycles to ORP

values from meters located at the ends of the anoxic and anaerobic zones respectively.

When the redox value becomes greater than a set-point value (-100 mV at the end of

the anoxic and 300 mV at the end of the anaerobic), the respective pump rate is

reduced linearly with increasing redox value. In Denmark, Kruger AS control the

nitrate recycle rate to the nitrate level at the end of the anoxic zone. In their

experience, they found ORP to be too sensitive a control parameter.

Control of the return sludge flow from the FSTs has also been optimised at some

plants. Kruger A/S (Denmark) control the return sludge flow to a constant suspended

solids level in the return stream. Two different set point flow rates are adopted for dayand night (for sludge dewatering reasons). Control of the sludge return flow rate to

maintain a constant sludge blanket level in the FSTs, has also been successfully

adopted at some plants in Denmark and Florida. Dr Lange Sonatax ultra sonic

sludge blanket detectors are used in Netherlands successfully. As an indicator of the

efforts being made to reduce maintenance, the instrument has a wiper to automatically

clean biofilm growth off the element, with an alarm indicated when the wiper needs

replacing.

At the South County plant (Florida, USA), the return sludge flow is paced to the main

stream effluent flow rate. This gives a much better response to the flow being applied

to the FSTs than using plant inflow. However, pacing of the return sludge flow to

plant flow should be applied cautiously, as this may have disastrous results during


23/86




peak flow conditions. Although the objective of returning solids to the bioreactor

during the peak load is desirable (from a F/M ratio perspective), the solids flux

loading on the FSTs is greatly exacerbated by higher sludge recycle flows at the time

when the loading is critical.

4.1.3.3Sludge Solids ControlThe two usual choices for solids control in the activated sludge process are either to

maintain a mixed liquor suspended solids (MLSS) level, or to control to a solids

retention time (SRT). Control to a set point MLSS range is often favoured, due to its

simplicity. The operators of a treatment plant often become comfortable with a solids

level that is suitable for the BNR process. This may work very well, especially for

plants where the loading does not vary significantly. Different MLSS levels may be

adopted for different mixed liquor temperatures or seasons of the year. Some of the

BCFS

plants (the Netherlands) control to solids and are greatly aided in this by the

use of on-line suspended solids meters.

Control to a set point SRT provides control of the MLSS appropriate to the process

requirements and independent of plant loading. While arguably a technically superior

control method, in practice the control can be erratic when wasting solids from the

FST underflow, due to variations in RAS solids concentration. When mixed liquor

wasting from the bioreactor is practised, control to an SRT set point becomes simple

and easy to control (the volume of sludge wasted per day is equivalent to the

bioreactor volume divided by the SRT in days). Mixed liquor sludge wasting from the

end of the BNR bioreactor also has the advantage of:-

Wasting sludge with the highest phosphorus content.

Providing the opportunity to selectively waste collected scum with the mixedliquor (Canada).

As with MLSS control, different SRT set points are often adopted for different mixed

liquor temperatures or seasons of the year.

On-line solids meters were widely used throughout Europe and America for control,

with reported success. Most instruments used an infrared measurement principle.

This is different to the experience in Brisbane, where the accuracy of readings was

poor unless the meters were frequently calibrated (at least weekly). This may be due

the highly varying sludge quality at the plants where these meters were installed (these

meters are optical instruments and varying floc/filament sizes and proportions arelikely to require very frequent calibration).

4.1.3.4External Carbon Dose ControlControl of external carbon dosing to an on-line effluent nitrate value is often applied

to post-denitrification processes (France, Netherlands & USA). Kruger A/S

(Denmark) control external carbon dosing to the main BNR process through

monitoring of nitrate levels and the calculated OUR (from DO and air flow meters).

The capacity for denitrification is calculated (as the OUR/4), and the denitrification

need is calculated (as the measured nitrate in the effluent or recirculation point).

When the need exceeds the capacity, then carbon dosing is implemented.


24/86




4.1.4 Separate Sidestream Treatment

Significant quantities and concentrations of nutrients can be found in the sidestreams

of most wastewater treatment processes. The most common of these is the dewatering

liquid from digested sludge, which is very high in ammonium and phosphorus. This

sidestream includes centrate/filtrate from the cake dewatering process and any digester

supernatant removed. Other high nutrient sources may include tankered industrial orseptic wastes. Most of these sidestreams are conventionally returned to the head of

the main process, where they are combined with the raw wastewater. Estimates of the

nitrogen load from this sidestream return are between 15 and 30% of the total nitrogen

load on the process. The phosphorus load is largely dependent on the degree of EBPR

and the degree to which struvite and similar compounds form in the sludge digesters

and surrounding pipe work.

A promising recent development is the separate treatment of these high nutrient

sidestreams, which is reported to be more efficient and lower cost (Vandaele et al.,

2000). The effect of sidestream treatment is to unload the main nutrient removal

process, resulting in lower effluent nutrient concentrations.

Sidestream treatment methods for nitrogen removal include:-

Air stripping, Steam stripping, MAP (magnesium ammonium phosphate or struvite) process, Ion exchange processes, Conventional biological nitrification/denitrification processes, Nitrification/denitrification variations involving sequencing batch reactors (SBR),

membrane bioreactors and attached growth systems.

Mulder (2000) gives cost estimate for the various different techniques available for

nitrogen removal from sidestreams, shown in Table 4.1 below.

TABLE 4.1 COST COMPARISON OF SIDESTREAM N REMOVAL PROCESSES

Process Cost estimate (Euro/kg N)

Air stripping

Steam stripping

MAP/CAFR process

Membrane bioreactorBiofilm airlift reactor

SHARON process

6.0

8.0

6.0

2.85.7

1.5

Sidestream treatment processes for phosphorus removal include MAP and chemical

precipitation processes. Very few plants incorporating anaerobic digesters were found

to carry out sidestream phosphorus removal, and yet many achieved excellent effluent

phosphorus results. This is due to the formation of struvite, calcium phosphate and

vivianite in the digester, which usually removes the necessity for P removal from

sidestream flows. Studies show that greater quantities of magnesium are also taken up

by the EBPR bacteria, which then becomes available in anaerobic digesters forstruvite formation (Pattarkine and Randall, 1999). Typically, up to two thirds of the

released phosphorus remains in the digested sludge in these various crystaline forms.


25/86




To maximise the effect, it is recommended that removal of supernatant from anaerobic

digesters is not practised. The Danish experience is also that phosphorus release from

anaerobic digesters is generally not a problem.

4.1.4.1SHARON ProcessThe SHARON (Single reactor for High activity Ammonia Removal Over Nitrite)

process has been developed in the Netherlands for the treatment of high strength

ammonium wastewater from the dewatering of digested sludge (Mulderet. al., 2000).

Two plants utilising the SHARON process were inspected in the Netherlands; the

Dokhaven plant in Rotterdam and the Utrecht WWTP. In this process, the ammonium

is nitrified to nitrite as the intermediate, followed by denitrification of nitrite to

nitrogen gas. This has the following advantages over conventional

nitrification/denitrification (via nitrate):-

75% of the aeration energy is required to oxidise ammonium to nitrite, and 60% of the carbon addition is required for denitrification starting with nitrite.

The ammonium oxidation is simply controlled to nitrite by operating at elevated

temperature (30 to 40 C) and low solids retention time (SRT). The process is

essentially achieved without sludge retention, such that biomass growth and washout

are in equilibrium. That is, the SRT is equal to the hydraulic retention time (HRT), at

about 1 to 2 days. The process generates excess heat, usually requiring a heat

exchanger for cooling. A heat exchanger may also be required for heating during

startup. pH control is very important and can be achieved with the addition of alkali.

However, experience has shown this is unnecessary and that addition of methanol with

the resulting recovery of alkalinity through denitrification is sufficient to maintain a

stable pH. The Dokhaven plant operates with a simple single reactor system operatingin a aerobic/anoxic cycle, which is superior to the two reactor continuous system at

Utrecht WWTP. The process is very stable and simple to operate.

Greater than 90% removal of ammonium is achieved with the SHARON process,

which is fairly independent of the initial ammonium concentration. Therefore, a

greater mass of nitrogen is removed with higher initial nitrogen concentrations,

favouring the process for high concentration applications (around 1 g.NH4/L or higher

at the Dokhaven plant).

The SHARON process has been evaluated in a number of studies as being the most

cost effective (Mulder et al.,2000) and most robust (Vandaele et al.,2000) sidestreamtreatment for nitrogen removal. Evidence from the two sites inspected indicated that

the SHARON process was highly effective.

The new ANAMMOX (aerobic ammonium oxidation) process is currently being

developed as a variation on the SHARON process, in which the ammonium is

oxidised by serving as the electron donor in a denitrification reaction. In effect, the

ammonium meets the energy requirements of the denitrification process, removing the

need for substrate addition. By combining the process with a process in which nitrite

is produced (e.g. SHARON), a nitrogen removal process is possible which requires

much less oxygen (half that required for the SHARON process), no additional

substrate and very low sludge production (Van Loosdrecht and Jetten, 1998). This,and similar processes are being developed by a number of different research groups


26/86




and shows promise for lower cost and more effective nitrogen removal from

sidestreams. (Brisbane Water in co-operation with the University of Queenslands

Advanced Wastewater Management Centre, Redland Shire Council and the

Queensland Governments Advanced Wastewater Treatment Technology Scheme will

be researching and pilot-trialing ANAMMOX type processes over the next two years).

4.1.4.2Phosphorus StripperOne innovative alternative developed for the BCFS process (Netherlands), puts a

twist on sidestream treatment by specifically creating a high strength phosphorus

stream from the main BNR process. Where chemical phosphorus removal is required,

they have implemented a system that withdraws pre-settled phosphorus rich liquor

from the end of the anaerobic zone (that is, following EBPR phosphorus release). The

phosphorus is then chemically precipitated from this high strength stream. Greater

efficiency is achieved by chemically dosing in the sidestream rather than the main

process stream, due to the high phosphorus concentrations. An in-tank settler has

been developed for decanting the phosphate rich liquor from the anaerobic zone (van

Loosdrecht et.al., 1998).

4.1.5 Chemical Nutrient Removal

BNR has been chosen for most nutrient removal upgrades in order to minimise

chemical usage and costs. Where chemicals are added, they are often in the form of

substrate for the BNR process. The most common substrate chemicals added are

methanol, ethanol or acetate. Acetate is usually more expensive, but has the

advantage of being suitable substrate (VFA) for EBPR and not requiring biomass

acclimatisation for nitrogen removal. Acclimatisation is likely not required for acetate

because it occurs natural in most domestic wastewaters, whereas methanol or ethanol

does not.

As discussed in Section 4.1.1, chemical phosphorus removal is often applied to BNR

plants to remove additional phosphorus that is not removed in the biological process.

This is usually achieved by dosing precipitant chemicals either into the mixed liquor

entering the bioreactor, or the mixed liquor stream feeding the FSTs. This was

widely practiced at the treatment plants inspected. The FSTs need to be adequately

sized for the additional chemical sludge load, which can be significant.

Alternatives precipitant chemical dosing points include:-

dosing to the PST feed (which places an additional chemical sludge load on thePSTs) (France),

dosing the effluent from the FSTs (which would require a downstream separationprocess such as settling or filtration),

dosing into the sewers, which is often practised for odour control, dosing to a separated sidestream (refer Section 4.1.4.2).

The two main precipitant chemicals used for phosphorus removal are alum and ferric

salts. There is some concern that ferric salts can have a detrimental effect on EBPR

processes, due to some inhibition of the phosphorus accumulating bacteria. This has

recently been summarised and studied in a series of papers by de Haas et.al. (2001).At the Bonnybrook plant (Calgary, Canada), it was also reported that iron salts caused


27/86




the UV disinfection to become ineffective, due to increased fouling of the UV lamps

and reduced efficiency from adsorption. It is also important not to remove too much

phosphorus (through overdosing of chemical precipitants), as this has lead to nutrient

deficiency in BNR processes (Virginia, USA).

4.2 Biosolids Handling, Treatment and Disposal

The management of biosolids is a major challenge for modern wastewater treatment

plants. A variety of techniques are used for the handling, treatment and disposal of

biosolids. Mainstream BNR processes place greater demands on the sludge stream of

nutrient removal plants.

New biosolids stabilisation and treatment processes have been introduced to increase

the destruction of volatile solids and pathogens (refer Section 4.2.1 and 4.2.2). In

doing so, biosolids disposal options are improved. With greater volatile solids

destruction comes greater release of bound nutrients, often requiring separate

treatment of sludge return liquors for nutrient removal (refer Section 4.1.4).

4.2.1 Digestion

Many plants include digestion processes for sludge stabilisation. Aerobic digestion is

not common, but anaerobic digestion is widely used. In Denmark, the numbers of

plants incorporating anaerobic digestion is still increasing, with those using aerobic

digestion remaining unchanged over the past few years. This seems fairly typical of

Europe and America. Aerobic digestion is often chosen with the objective of retaining

excess phosphorus bound with the EBPR bacteria. However, many plants have

demonstrated this in unnecessary, due to formation of struvite and similar compounds

from the released phosphorus in the anaerobic digester (refer Section 4.1.4).

As an alternative to conventional mesophilic anaerobic digestion, thermophilic

anaerobic digestion is being adopted at many plants. The defining difference between

the two processes is the operating temperature; about 37 C for mesophilic and 55 C

for thermophilic. Different anaerobic bacterial groups dominate at either operating

temperature. Reported characteristics of thermophilic digestion include:-

HRT lowered to 12 days or less (mesophilic time almost halved), Disinfects/pasteurises the sludge, increasing disposal options, Suitable for 100% WAS,

Struvite precipitation increased at higher temperature, Volatile solids destruction not greatly increased over mesophilic digestion, May need steam to heat sludge effectively (scaling of heat exchangers reported

otherwise, though a few plants still used heat exchangers).

4.2.2 Sludge Hydrolysis

Sludge hydrolysis processes cause the rupture of biosolids cell structures, releasing

the water tied up in the cells and dissolving organic cell matter. Essentially, the

sludge is cooked prior to digestion. This released substrate results in greater overall

volatile solids destruction when applied prior to anaerobic digestion. Various

hydrolysis processes have been developed including Cambi, Krepro, Porteus, Krampol(Alfa Laval) and Zimpro.


28/86




In the Cambi process, steam is used to directly heat sludge under pressure prior to

digestion. At the Naestved WWTP (Denmark), the Cambi process heats dewatered

waste activated sludge (at about 14 to 16% DS) to a temperature of between 160 to

180 C and pressure of between 8 to 10 bar. The sludge is subject to these conditions

for between 30 and 60 minutes. Following cooling, the sludge is pumped to the

anaerobic digesters for stabilisation (a mesophilic process). The Cambi processappears much simpler than many of the alternatives.

The Cambi thermal hydrolysis process is reported to give the following advantages

over conventional anaerobic digestion:-

Increased biogas production (by approximately 60%); Increased volatile solids destruction; Improved dewaterability of digested biosolids (from say 18% to greater than 35%

using a belt filter press, though very dependent on the dewatering equipment);

Less polymer usage for sludge dewatering;

Increase in the digester reaction rates and hence digester capacity; Improved stability of the digested sludge cake; Pasteurisation of the waste biosolids (suitable to meet the USEPA standards).

Thermal hydrolysis may also be applied to primary sludge, but the improvements in

biogas production and solids destruction during digestion are not nearly as significant

as those for waste activated sludge. However, thermal hydrolysis is often applied to

primary sludge for the pasteurisation benefits.

The Cambi process is reported to usually give an energy benefit. That is, the energy

available from the increased biogas produced exceeds the energy requirements for the

Cambi process. This would be dependent on the nature of the biosolids raw product

and the thickening/dewatering equipment used. No figures were provided for the

Naestved plant (as the plant had only recently been commissioned - March 2000).

Other plants report significant savings in energy and biosolids disposal costs, leading

to payback periods as short as four years for some case study plants.

Sludge hydrolysis processes create a higher nutrient mass in the sludge dewatering

filtrate return stream, caused by the increased biosolids destruction. This higher

nutrient return to the main process increases the requirement or possibility for

sidestream treatment (refer Section 4.1.4), and options are

Solley David 2000

Documents