Page 1


Official Journal

water

ISSN 0310-0367

Volume 17, No. 6, December 1990

AUSTRALIAN WATER AND WASTEWATER ASSOCIATION

My Point of View

3

Lake Number D. M. Robertson, J. lmberger-and K. Boland

Association News President's Message It Seems to Me

4 4

Foaming in Activated Sludge Plants L. C. Blackall, A. E. Harbers, P. F. Greenfield, A. C. Hayward

34

Book Reviews

41

Plant Product and Equipment

43

CONTENTS

IAWPRC

14

Conferences and Seminars Reports AWRC National Workshop on Sludge Disposal

16

Sludge -

The Looming Problem

18

Environment in the South West

19

Integrated Catchment Management and Tariff Structures

20

Waste Management

21

Focus on Northern Territory Water Quality Perspective N. R. Allen

29

22

Water Reuse and Future Potential G. Jackson and M. Steitieh

24

Water Treatment Directions D. A. Day

26

OUR COVER Our cover shows the installation of a solar powered production bore by members of the Baniyala community of 120 peop le, in East Arnhem Land, Northern Territory There are approximately 80 communities in the Northern Territory where so lar power is utilised for water pumping and water treatment processes such as ul traviolet disinfection and reverse ' osmosis. The installation at Baniya la cons ists of a 28 panel, 60 watt solar power supply, inverter and 3 phase submersible pump to supply the community via a 3km rising main. Power and Water Authority provided financial support and technical assistance for the project. The local community undertook all installation works including hand excavation for the uPVC rising main. Photo courtesy of Denis Se th, Power and Wa ter Authority.

28

Conference Calendar

FEDERAL SECRETARIAT PO Box 460, Chatswood NSW 2057 Facsimile (02) 410 9652 Telephone (02) 413 1288 Office Manager - Margaret Bates

FEDERAL PRESIDENT Peter Norman Telephone (08) 226 2249

EXECUTIVE DIRECTOR Peter Hughes Telephone (02) 410 9654

FEDERAL SECRETARY Greg Cawston Telephone (042) 29 0236

FEDERAL TREASURER John Molloy Telephone (03) 615 5991

BRANCH SECRETARIES Canberra , ACT Peter Cox PO Box 306, Woden 2606 (062) 498 522

New Soulh Wales David Hope, PO Box 460, Chatswood 2057 (02) 269 5212

Victoria John Park C/- Water Training Centre, PO Box 409 , Werribee 3030 (03) 7 41 5844

Queens land Don Mackay, PO Box 412, West End 410 1 (07) 840 4844

South Australia Rob Townsend , C/- State Water Laboratories, E&WS Private Mail Bag , Salisbury 5108 (08) 259 0244

EDITORIAL CORRESPONDENCE E.A. Swinton, 4 Pleasant Vi ew Crescent,

Glen Waverley 3150 Office Phone and Autofax (03) 560 4752 Home (03) 560 9306

Western Aust ralia Steve Gibson CMPS , 200 Adela ide Terrace Perth 6000 (09) 325 9366

Tasmania Annette Nichol GPO Box 503E Hobarl 7001 (002) 28 2757

ADVERTISING Ann Sykes-Smith Appita, 191 Royal Parade, Parkville 305 2 (03) 347 2377 Fax (03) 348 1206

PRODUCTION ED ITOR J . Grainger,

Northern Ter ritory Lindsay Montei th PO Box 351 Darwin 0801 (089) 81 5922

Appita, 191 Royal Parade , Parkville 305 2 (03) 347 2377 Fax (03) 348 1206

WATER December 1990


Northern Territory Water Quality Perspective by N.R. Allen SUMMARY The Power and Water Authority (PAWA) with its enabling legi slation has responsibility for the management and control of water resources in the N.T. This control extends not only to protection and development of water resources, extractions, treatment and distribution, it also covers waste discharges to the environment. The Authority has various management strategies/ policies for the protection of water resources. This paper endeavors to present an overview of the application of these strategies for the future protection and maintenance of drinking water quality. The key element in the evaluation of management strategies is found in appropriate monitoring programs, the future requirements of which are based on past and current practices.

HISTORICAL PERSPECTIVE In the 1960s and early 1970s the key thrust was the provision of a plentiful, reliable water supply for consumption, hygiene and sanitation, hence the emphasis was on resource development and supply. The management and monitoring of the water supplies lacked cohesion due to separate government agencies having responsibility for urban and community water supplies. The resource development skills were in one department and the water treatment and operational skills in another. Both departments maintained monitoring programmes for their respective needs.

WATER QUALITY GUIDELINES · The absence of National Guidelines meant that in the period in question water quality assessment was based on a plethora of guidelines. The USEPA guidelines commonly referred to as the red book and blue book were used for compliance assessment together with the 1970 WHO guidelines for water treatment practices. In addition, methodology was based on the UK Ministry of Health guidelines. The use of these various guidelines had a significant impact on management practices and monitoring programs. As the N.T. was considered to be a developing region the WHO recommendations for such regions were implemented for the communities. All water supplies were chlorinated using gas chlorine and a routine bacteriological monitoring program was carried out on a quarterly basis. The monitoring programs involved field trips of up to ten days duration and in addition to vehicles, chartering of light aircraft and helicopters was required . Field monitoring test kits based on USEPA methodology for remote areas were used . The monitoring of urban centres was based on the UK practice with samples collected in the field and dispatched to a central li3-boratory. The frequency of sampling was at least monthly depending on the size of the centre with the analytical methodology being based on the determination of E Coli content. With these two separate monitoring programs, approximately 40 towns and communities were monitored with any degree of frequency. The nett effect of the various water treatment and monitoring programs was best summarised in the 1974 review which showed that 90% of water supplies providing water for 90% of the population failed to comply with any recognised guidelines in terms of bacteriological content. A detailed review of the data base together with specific studies indicated that the problems with supply could be attributed to either or all of the following: • water resources • chlorination • storage • sampling and analysis. The shortcomings in these areas resulted in a review of management policies and strategies. It was generally found that ground waters were free of bacterial contamination, hence, where possible all surface water supplies were replaced with groundwater supplies. 22

WATER December 1990

Norm A lien is currently Supervising Chemist Water Quality, PAWA, Norm is responsible for monitoring and assessment of all potable water supplies together with water and wastewater treatment processes. Norm graduated from Adelaide in 1966, in Microbiology and has more than 25 years experience in the water industry including laboratory services and process control.

Minimum standards for construction of production bores to prevent bore head contamination were implemented. Training programs on the handling and operation of chlorination equipment were developed and implemented. All storage tanks were to be roofed and made animal-proof. Where feasible the bore supply direct to the elevated storage tank was replaced by a bore to ground level tank and then transfer to elevated storage tank. This was to enable more effective treatment to be put in place. Monitoring programs were modified to include samples from the source water, storage and reticulation. Where possible samples were to be dispatched to a laboratory for analysis. The advent of self government in 1978 enabled the Government departments responsible for water resources, supply and operations to be amalgamated into a single department. This single department was in turn formed into the PAWA. The development of a single department allowed for the wholistic management of water resources and supplies in the N.T. Given the problems with hygiene and sanitation in the 1960s, monitoring programs were primarily targeted at microbiological content. Whilst data was available on inorganic parameters, there was no evidence to suggest that ,there were any health related problems.

CURRENT STATUS OF DRINKING WATER QUALITY National Drinking Water Quality Guidelines In 1979, National Drinking Water Quality Guidelines were prepared jointly by the National Health and Medical Research Council (NHMRC) and the Australian Water Resources Council (AWRC). The production of these guidelines allowed water quality goals to be set and appropriate monitoring programs to be developed. The guidelines were reviewed and the upgraded edition was made available in 1987.

COMPLIANCE WITH 1987 GUIDELINES By 1987 approximately 90 towns and community water supplies were being monitored. Whilst in many instances the frequency was below that recommended in the guidelines, significant advances had been made. The analytical services had been combined and bacteriological field analysis terminated. Authority personnel or their agents were trained in the collection of samples and bacteriological sampling programmes developed around airline schedules. In the 10 years that this system has been in place there have been significant benefits in terms of economics, water quality improvement and utilisation of resources. In 1989, 85% of the N.T. population received water that satisfied the NHMRC guidelines with respect to bacteriological quality and chemical parameters. Nevertheless, more than 90% of water supplies failed to comply with aesthetic and/ or chemical parameters. This high percentage of failure to comply can be attributed to the following: • climatic and hydrological conditions • homeland movement. The following tables summarise the problems facing PAWA with respect to management and monitoring of reticulated water supplies.


Table 1. Summary of Community Water Supply Centres and Source Types in the Northern Territory Classification

Major Urban

Minor Urbans Aboriginal Communities Aborigina l Outstat ions

Population

Suppl!' Centres

1,000-75 ,000

8

< 1000 100-1 ,000

10 57

< 100 %

643

\Vater

Source

Water Treatment •

Monitoring

3 Surface 5 Ground

7

8

8

IO

28

57

6

15

IO Gro und IO surface 47 ground 206 Surface 437 Ground

• Water treat ment inclu des pH cor rect ion and/ or c hlor in a tio n. % Th e major ity of these com munities have less th a n 50 people.

(2) Use of Guidelines

Table 2. Water Quality Compliance of Reticulated Water Supplies GUIDELINES 1987 Parameters

Aes1he1ic pH Iron TDS Ha rdness

0/o Failure to comply (Approximate)

< 6.5 70.3 mg / L > 1000 mg/ L >500

· Health Related Bacteriological Ni tra te Fluoride Radiological

developed for the larger urban populations and fail to recognise the problems associated with small com1nunities. This problem is recognised in Volume 3 of WHO Drinking Water Quality Guidelines and the following quote should be noted: "In the case of small-community supplies, particularly in developing countries, the parameters used in assessing the measuring the quality of water intended for public supply must necessarily be limited in number. Similarly, the guideline values given have often to be considered as long-term goals rather than rigid standards that have to be complied with at all times and in all supply systems."

500Jo 70%

<50Jo <50Jo 880Jo* 20Jo < IOJo < 0.5 0Jo

• Routine bacteriolog ical sa mples have only been do ne on the urban ce ntres and major co mmunities. No sa mpling is included as fa ilure 10 co mpl y.

Aesthetic Parameters pH (<6.5) The low pH is common to groundwater of the coastal communities in the Top End. Surface waters in Arnhem Land also exhibit low pH . Analyses in the laboratory indicate pH readings less than 5.0 units in many supplies. pH at the bore head in some cases is less than 3. The low pH is due to high carbon dioxide content together with low dissolved solids. The water is extremely corrosive to cement and metal fittings and pipework. Where appropriate, given skill level and resource the pH is corrected by application of alkali. In many communities the adverse quality effects of the corrosive water are minimised by use of corrosion resistant materials e.g. PVC pipes, polybutylene internal plumbing, corrosion resistant valves with ceramic seats. Iron (> 0.5 lng/ L) The indicated number of water sources with unacceptable iron concentrations is very high compared with actual reticulated water supplies. This high number is generally the result of sampling technique. Samples of the source water have in many instances been collected just after bore construction and results subjected to interference by drilling muds, air lift procedures etc. In reality very few water supplies have an iron problem and those that do are usually the resultant effect of local mineralisation or when sourced from spears and billabongs. TDS (>1,000) In Central Australia elevated dissolved solids occur. The majority of the waters have dissolved salts in the range 1000-1500 mg/ L although there are a few systems with dissolved solids > 2,000 mg/ L.

Health Related Nitrates (>45 mg/ Las NO) There are several supplies in Central Australia where nitrate is in the range of 45-90 mg/L. The majority of the elevated nitrate levels occur in waters with a TDS > 1500 mg/ L. FUTURE WATER OUALITY MANAGEMENT STRATEGIES The provision of services and water quality management for small communities, which may only be occupied for six months of the year, is a major task for the next decade. The key issues that need to be addressed can be summarised as follows :

(1) Development of Appropriate Water Quality Guidelines Generally speaking, Water Quality Guideli.nes have been

The rationale for the use of the guidelines needs to be clearly stated particularly in terms of risk assessment.

(3) Monitoring Monitoring programs will need to be developed that satisfy both short and long term assessment requirements. Priority areas will need to be developed .

(4) Regulation and Enforcement The development of appropriate guidelines and monitoring programs by themselves whilst useful as management tools will be ineffective unless they are regulated and / or if necessary, enforced.

(5) Corrective Action Whilst there may be an understanding of the problem, the development of appropriate technology that takes into account skill level, human resources and cost is an emerging issue. A key issue that needs to be addressed in future strategies is one of Education and Communication. In this general area the following items need to be addressed: • how to understand water quality • appropriate involvement process • level of service which would include: demand management (needs) quality cost community perceptions and expectations dual water supplies.

CONCLUSIONS In summary, the N.T. and in particular the Authority, is facing some major challenges with respect to the provision of services and the management of water quality. These challenges will involve not only the Authority but also medical researchers, the water industry for development of new technology, educators and most importantly the community (or customer).

AWRC SEMINAR Continued from page 17 • Compost Manufacturers are in a precarious position in light of very strict NSW Guidelines for products containing slud~e. ~hey are convinced that a healthy market exists but fear that gmdelines, unless uniform across the nation and more in line with agricultural use, could ruin their operation. • Inorganic fertilisers will also soon be subject to guidelines on contaminants (e.g. cadmium in superphosphate) in NSW. • Draft "Australia Guidelines for the Assessment and Management of Contaminated Sites" (NHMRC/ANZEC - June 1990) also have significant implications for sludge disposal practices. • Concentrations of copper, and to a lesser extent zinc, are high in most Australian sludges.

CONCLUSIONS As a result of the Workshop, there was general consensus that Australia should have National Guidelines for the disposal of sludge and that work should commence immediately. The organising committee of the Sludge Disposal Workshop will issue a draft planning framework by late 1990 for discussion. The format will give positive emphasis to the concept that sludge is a resource and should be beneficially used. The formal will be based on, but not necessarily limited by, the United States Environmental Protection Agency Guidelines. The guidelines should assist all involved in sludge collection, use and disposal. WATER December 1990

23


Wastewater Re-use and Its Future Potential in N.T. by G. JACKSON and M. STEITIEH SUMMARY The Northern Territory covers an area of some 1,346,000 sq. km. and has a population of 155 000 inhabitants. The climate varies from tropical monsoon in the north (Top End) with high summer rainfall, but with a warm dry winter, to the arid desert type of the centre. The objective of this paper is to bring together the many considerations necessary for successful reuse of treated effluent in the N.T. and to present some ideas and concepts. To further aid our understanding and to put it in a local context some Territory examples are briefly presented and discussed . Finally the question of future reuse potential in the N.T. is raised .

CURRENT REUSE PROJECTS DARWIN REGION The amount of effluent that can be reused is affected by the availability of fresh water, transportation and treatment costs and water quality standards. The current potential volume of water from the treatment plants in Darwin is listed in table 1. The available land area that can be irrigated by treated effluent from Darwin Wastewater Treatment Plant and Leanyer/ Sanderson is shown in table 2.

Graham Jackson graduated from University of Queensland in 1973 with an honours degree in civil engineering and received his masters degree in public health engineering, from the University of NSW in 1986. Graham is currently the supervising engineer, water and sewerage for the northern region, PAWA.

Graham Jackson

Mohammed Steitieh is currently Senior Design Engineer, Northern Region, PAWA. Mohammed graduated in Civil Engineering from Manchester in 1982. He returned to Jordan for military service, then worked with the Water Authority of Jordan until returning to London in 1987 for his MSc. in Environmental Engineering. He immigrated to Australia in 1989 and joined PAWA in 1990. Mohammed Steitieh

Table 1

Treatment Flow

Leanyer/ Sanderson Palmerston DWWTP Berrimah

Inflow (ML/D)

Effluent (ML/D)

Possible Area for Irrigation (ha)**

Type of Treatment

14.0 1.8 7.8 0.89

12.6 1.6 6.8 0.88

176 22 95 12

WSP WSP Physical/Chemical WSP

Based on the fact that the inflow is reduced by about 10% due to evaporation . * * Based on precipitation rate of 50mm/ week . WSP = Waste stabilisation ponds DWWTP = Darwin wastewater treatment plant. *

Table 2 Treatment Plant

Leanyer/ Sanderson DWWTP

Irrigable Area

122 50 54 45 43

Possible Reuse Site

Parks & ovals North Lakes East Point Reserve Parks Botanic Gardens

Tables I and 2 show that there is a possible potential for reuse, but more research is required to determine the suitability of certain ares for reuse. Leanyer The Leanyer Ponds were established in 1969 to serve the Northern Suburbs of Darwin. They had a design population of 20000.Their location was on the edge of the coastal mangroves, 1000m from the nearest planned development. The effluent discharged into the adjacent Buffalo Creek which meanders through the coastal mangroves and enters the sea several hundred meters downstream. This area was generally remote and inaccessible except for the more adventurous picnickers and fishermen . At the beginning there was interest in reuse. A joint project between CSIRO and the Department of Works was developed whereby a tree plantation was commenced on adjacent RAAF land to ascertain the potential for using the effluent. Cyclone Tracey caused severe damage in 1974 and soon after the project folded. The original 1000m buffer zone would have afforded the 24

WATER December 1990

opportunity to develop reuse schemes adjacent the ponds but this was reduced when the Northern Suburbs were expanded into the zone. The current method of disposal is cheap and effective. It could be argued that the effluent has actually enhanced the area through increased mangrove productivity. There has been little reason to consider reuse projects . North Lakes Treated sewage effluent has been used for irrigation of the North Lakes Golf course and a number of playing fields in the Marrara sporting area since 1983. The effluent is pumped from Leanyer/ Sanderson Stabilisation Ponds to a lake within the Golf course, From there it is pumped, chlorinated and distributed via an automatic sprinkler system . When the scheme started in 1983, a few odour complaints commenced and by 1987, when North Lakes housing development â&#x20AC;˘ had taken place, large number of complaints arose that led to a community odour impact assessment being carried out. The assessment showed that the storage lake was the main source of odour and the golf course irrigation system was causing part of the problem. The lake is now aerated and mixed with a small floating pump aerator to maintain viabl~ algal biomass. Odours generated by the irrigation system are controlled with the introduction of a fresh water system. This injects fresh water into the whole system at the end of the daily spray cycle to displace effluent, thus preventing stagnation and intense odours which used to occur on spray startup. Bores provide fresh water for the system. The effluent which is currently supplied to the North Lakes meets NHMRC /AWRC Guidelines. The median ÂŁ.Coli obtained from samples of the water in the storage lake is 210 per 100 ml. The average supply to the North lakes during the dry period of 1989 was 56 ML per month. Humpty Doo The Humpty Doo community, 40K south of Darwin, with a population of several hundred, was sewered in the early 80's. One of the problems to be solved was what to do with the effluent. The traditional options of discharging to an estuarine river or creek was not a realistic proposition 'principally because of distances. Evaporation basins were a possibility but if not carefully managed and designed can cause problems. The method adopted was one of irrigation of the surrounding bushland buffer zone through a


series of sprinklers. Apart from losing sprinkler heads the system has worked satisfactorily although it has not yet reached its full design capacity.

ALICE SPRINGS Alice Springs has two pond systems, the old and the new, and both have reuse systems attached to them . The old ponds are adjacent to Batheskite Park and effluent is pumped and used to irrigate lucerne which is regularly harvested and sold for fodder. The new ponds, some 2km to the west of the old ponds, provide effluent for a forestry plot. There are two ponds in the system and effluent is diverted from the second half of the first one and runs by gravity through a series of earth channels. The scheme was commenced soon after the new ponds were commissioned some 15 years ago. The idea behind it was to provide a commercial source of firewood for the Alice as the surrounding areas were rapidly being exploited. While the trees grow reasonably well they have not yet been harvested to any extent. It should be noted that not all the effluent in Alice Springs is reused. Much is discharged into what has become a permanent wetland known as Ilpapa swamp. YULARA The tourist town at Ayers Rock in Central Australia is perhaps the best example of the reuse of sewage effluent in the N.T. Good water quality is a very scarce resource at Yulara and this was recognised right from the beginning in the planning for the new town. The groundwater has a minimum TDS of 1500 ppm .. A dual water supply distribution was installed with each consumer receiving two classes of water: Desalinated water with TDS 500 ppm: Raw water. The raw water is connected to the toilets and outside taps. The sewers receive a mixture of raw and treated water and have a TDS of around 1200 ppm. Treatment is through an activated sludge plant and the effluent is stored in a small sealed pond before final disposal. In order to make the best use of this expensive resource the effluent is chlorinated and pumped back to the town and used to irrigate the main community oval. Plans are now underway to extend this system to other areas of the town which are currently being irrigated with either raw or treated water or a mixture of both. Excess effluent is discharged to the surrounding bushland . ABORIGINAL COMMUNITIES There are several examples of effluent being reused in Aboriginal Communities, All of these have come about because of the problems of final effluent disposal in the dry interior. After treatment the effluent has been disposed to a low area or watercourse through an open drain. This method has created problems, and options to better contain the effluent were considered. Evaporation basins are feasible but themselves can be difficult to manage. The solution adopted in the cases of Kulguringi, Lajamanu, and Yuendumu was to spray irrigate nearby land to encourage vegetation to grow or in the case of Yuendumu to water the local oval.This latter instance has not been successful because of insufficient effluent being available.

THE FUTURE OF REUSE IN THE NT GENERAL Factors which make reuse more favourable are: • Water demand is approaching the safe capacity of the developed sources. • The capital cost of new source and headworks upgrading could be substantial. • There is scope for more reuse schemes. On the other hand, factors that make reuse less feasible include: • Adequate water resources are available for development. • The current systems for disposal of wastewater are acceptable. • The cost to construct pipelines for reuse could be quite high. • The price of water does not reflect the true economic cost to supply. • Odour complaints. On balance , it is considered that Darwin is unlikely to have major increases in reuse, whereas in Alice Spring the balance is shifted more in the direction of reuse. This is because of the relatively high cost of developing additional water sources. The situation could, however, change if some or all the following occur: • National Standards are introduced severely restricting discharge of sewage direct to the environment unless it is of tertiary standards.

• The price of water is increased signic{icantly. • Effective and efficient means for the removal of algae are found. • Social or political pressures force the 'subsidisation' of reuse schemes. • Town Planning takes into account and designs urban and regional plans around the reuse scenario. Town Planning can be used to influence the way water and treated effluent can be used. For example the following are possible outcomes. • Treatment Plants be located near areas that have agricultural uses. eg inland rather than by the shore. • A number of smaller plants be constructed in areas where the effluent can be used, so as to avoid the need for long transport distances. • WSPs might be designed to make them into wetland type area which can be developed as reserves and recreation areas. • Alternatives to WSPs might be built in areas where a particular quality of effluent could be used. • Public parks, open spaces and schools etc, might be located closer to treatment works.

FUTURE SCHEMES Given the appropriate circumstances what may be the possible reuse schemes in the NT? Some ideas are as follows: • Darwin could have the East Point Reserve fully irrigated . The Botanical Gardens, and in fact the whole of the foreshore area of Fannie Bay, provides a good opportunity for reuse. In the Northern Suburbs the area to the south of Vanderlin Drive could be put to recreational use through use of water from Leanyer. In the other direction the current RAAF land offers prospects for forest growth. Nurseries on the 2 Ha allotments in Leanyer could all use lagoon effluent. The new airport development may have viable requirements for irrigation water. Looking to the proposed Hudson Creek ponds, this would be an ideal place to locate industries which have high water uses. In Palmerston local aquifers might be recharged from the Palmerston Ponds. In the coastal zone the productivity of the mangroves systems might be increased by allowing the effluent to be spread over large areas instead of just into a creek. • Alice Springs offers some interesting prospects, although the high salinity may present problems. The I!papa swamp could be developed into a wetlands area-with tourism potential. Effluent could be pumped back into the town for irrigating public park lands. Desalination of a mixture of effluent and town basin water would provide a high quality effluent suitable for extensive public and private use. We could even see an extension of the Yulara concept. The point with Alice Springs is that great benefits could be derived from taking a totally integrated approach into the sources and uses of water and developing the planning process around it. • Business and commercial centres might individually or collectively separate out the 'grey' water for recycling to toilets before discharge to sewers. • Water reticulation will be planned with reuse in mind if not for the present then with the thought that a line may need to be changes to a reuse main in the future. • Road corridors have space allocated for reuse mains to be built at a later date.

CONCLUSIONS The collection, treatment and disposal of sewage should be viewed not as a separate but as an integral component of the overall water supply picture. In this way maximum and efficient utilisation of the communities water resources will be possible. The Northern Territory is in a good position to make the most of the emerging awareness of the potential and need for reuse. It is still a young society with scope to incorporate a reuse strategy into its planning framework. The qualities of treated effluent ,in general terms, can be classified as good from the irrigation point of view. Furthermore, the total faecal coliform present in the final treated effluents would not cause any public hazard when these effluents are reclaimed. As water resources become scarcer it is inevitable that reuse will become an economic necessity. Better to plan for it rather than have it forced upon one in an adhoc manner.

Continued on page 41 WATER December 1990

25


Water Treatment Directions in N.T. by D. A. Day SUMMARY Current treatment practices for community water supplies in the Northern Territory are reviewed. Limited development has enabled a high level of water supply catchment and aquifer protectionto be established. The large number of remote, small community water supplies require innovative solutions for water treatment and good holistic management practices. Future water treatment practices will be required to address issues of appropriate technology, remoteness, seasonal demand fluctuations and treatment of the more difficult waters.

Darryl Day is currently regional engineer AES, PAWA, responsible for power generation, water supplies and sewerage services for major aboriginal communities and homeland centres in the northern region. Darryl has been employed by PAWA and its predecessors for approximately 10 years, working throughout the northern territory. He graduated from the University of Adelaide in 1978, in Civil Engineering.

Darryl Day

INTRODUCTION The Northern Territory has a relatively small population of 156,000 (Gardner, 1990) with 78.5% living in the eight major urban

centres (Darwin, Jabiru, Nhulunbuy, Alyangula, Katherine, Tennant Creek, Alice Springs, and Yulara). The remaining 21.5% reside in 710 minor urban , major Aboriginal communities and minor Aboriginal communities or outstations. The majority in number of water supplies are for minor Aboriginal communities, in remote locations, with limited technical expertise and support services, generally with populations less than 50 people. It should be noted that not all the outstations are permanently occupied, and a bore or spear may be worked by a hand pump only. The 1987 National Health and Medical Research Council (NHMRC) /Australian Water Resources Council (AWRC) "Guidelines for Drinking Water Quality in Australia" (NHMRC/ AWRC, 1987) and the 1985 World Health Organisation (WHO) "Guidelines for Drinking-Water Quality" (WHO, 1984) both advise that for smaller communities, parameters should be considered as objectives rather than rigid standards that have to be complied with at all times and in all supply systems. The degree of compliance with guideline values should depend on local circumstances, including the water supply source, degree of source protection, culture, dietary conditions and consumer acceptance. Holistic water supply management, appropriate treatment technologies and operational practices must be developed and refined to meet the challenges of the Territory environments from arid to sub-tropical. Water treatment objectives will vary with the level of service provided to a community depending on a number of factors, including: • Water source (bore, well, soak, dam, creek etc.) • Degree of source protection and likelihood of non-seasonal changes in level and/ or range of seasonal changes in level. • Level of community expertise and availability of backup support to maintain the treatment system. • Capital and recurrent costs. In the Territory, the majority of water supply sources are well protected from contamination. Source protection, through such practices as minimum standards for bore construction, cannot be overstressed in the management of water supplies. "Prevention" is easier to manage and less expensive than any "cure" required (via water treatment) as a result of poor protection and/or management practices. A significant mis-conception is that smaller community water supplies require less expertise in treatment considerations than large community supplies. In fact, the reverse is true since a broader range of expertise is required to provide innovative, appropriate and varied treatment solutions. Microbiological safety of drinking-water supplies is the foremost important parameter. There is a very limited number of physiochemical parameters of significance in the Territory, and appropriate treatment technologies will be discussed for the most common parameters encountered.

MAJOR URBAN SUPPLIES The major urban water supplies in the Territory comply with health related parameters of the NHMRC/AWRC guidelines following a variety of treatment systems, which are generally conventional in nature, with the exception of the tourist resort of Yulara where membrane desalination (electrodialysis reverse polarity) is used for the potable supply of a dual supply system . 26

WATER December 1990

TREATMENT OBJECTIVES • • • •

Treatment objectives fall into four categories: Microbiological quality Physical quality Chemical quality organic Chemical quality inorganic.

Microbiological Quality As discussed, microbiological quality is the prime consideration for treatment of potable water supplies to ensure that ideally no microorganisms known to be pathogenic are present. Generally all community supplies, excepting minor Aboriginal communities, are regularly monitored (monthly) for microbiological quality in accordance with NHMRC/AWRC guidelines. However, as shown in table 1, not all water supplies monitored are provided with continuous disinfection. In fact, only major urban centres (excepting Tennant Creek) and other communities with surface, surface/ groundwater or poorly protected groundwater supplies, are reliant upon continuous disinfection. The long term objective is to provide disinfection facilities for and monitor all surface water supplies, and problem groundwater supplies (with inadequate aquifer protection), to meet current NHMRC/AWRC guidelines. Table 1. Community water supplies in the N.T. with facilities for continuous disinfection. Major Urban•

Gas Chlorination Liquid Chlorination Ultraviolet Reverse Osmosis Distillation TOTAL

7 0 0 0 0 7

Major Minor Aborigina l Minor Aboriginal Urban % Communitiest Communities

7 I 0 0 0

8

26 3 2 0 0 31

Total

0 0 I 2

33 4 3 2

3

42

• Darwin, Jabiru , Nhulunbuy, Alyangula, Katherin e, Tennant C ree k, Alice Springs a nd Yulara. % Batchelor, Adelaide River, Pin e cree k, Timber C ree k, Mataranka , Larrimah, Borroloola, Newcastle Waters, Elliott and Ti ~Tree. t N.T. Interim Guide lin es, Communities generall y over 100. N.T. Interim Guidelines, communities ge nerally under 100.

Physical Quality Treatment required to improve physical aspects of water quality in the Territory which generally result from natural causes and not contamination of the water source. pH Many communities adjacent to the northern coastline have soft acidic water supplies with an extremely low pH (between 4.0 and 5.5), which are highly corrosive to copper, brass, mild steel, galvanised iron, asbestos cement and cement lining of pipes and fittings . The low pH results from high levels of dissolved carbon dioxide, belie\'ed to originate from the high carbon dioxide respiration rate of tree and plant roots in the catchments and aquifer recharge areas during the wet season. The acidity is coupled with very low alkalinities due to sandstones being silica rather than carbonate based (Marks and Jolly, 1987). Since the technology available for raising pH is judged inappropriate for small communities, and low pH water has no known adverse effects on health, the direction taken has been to provide the majority of these


communities with an infrastructure comprised of inert materials to eliminate corrosion. Turbidity High turbidity generally is associated with surface or contaminated groundwater supplies and protects micro-organisms from disinfection. It is recommended by NHMRC to reduce turbidity to I Nephelometric Turbidity Unit (NTU) for disinfection by Ultraviolet (UV) radiation and 5 NTU for disinfection by chlorination. Colour, Taste and Odour The level of service established for colour, taste and odour generally relate to consumer acceptance. Generally taste and odour complaints (with the exception of total dissolved solids) will result from undesirable contaminants of the source which should be investigated.

Chemical Quality -

Organic

Within the Territory, contamination of community water sources from industries and agricultural practices (or malpractices) is negligible. NHMRC/AWRC guidelines for maximum limits of organic compounds are appropriate.

Chemical Quality -

Inorganic

There are areas in the Territory with naturally occurring high levels of nitrate, fluoride, iron and total dissolved solids (TDS). Water treatment of these sources must be assessed against an appropriate level of service for the community. The current moves to excise land from pastoral stations for residential living areas and the establishment of new Aboriginal homeland living areas is demanding consideration of treatment processes for the more difficult groundwaters, often with high levels of nitrates, fluorides and total dissolved solids.

Nitrate Nationally there is very strong support for ra1smg of the NHMRC/AWRC recommended maximum level of nitrate from 45 mg/ L to 90/ 100 mg/ L for all consumers above the age of 3 months and for water not used for formula preparation. This issue is currently on both the Territory and National agendas. Fluoride Although the NHMRC/AWRC guideline for fluoride sets a maximum level of 1.7 ppm, comment is made that levels up to 3 ppm show no adverse health effects. Within the Territory there is a small number of water supplies with a natural level of fluoride in excess of 1.7 mg/ L to 2.5 mg/ L. Total Dissolved Solids (TDS) Taste generally provides the threshold for TDS. A level of service of 1200 ppm is considered reasonable for small communities, however levels up to 1500 ppm are acceptable at locations where a higher quality water is not locally available and where other supplies cannot be obtained at a reasonable cost.

MICROBIOWGICAL TREATMENT Treatment Options The following options (Wade, 1990) are available for typical water disinfection systems:

Physical process Coagulation - multimedia filtration, Ultrafiltration, Microfiltration, Ultraviolet light irradiation, Heat sterilization, Dissolved air floatation, Reverse osmosis, Distillation. Biocidal agents Chlorine, Bromine, Chloramine, Chlorine dioxide, Ozone, Hydrogen peroxide, Silver and copper ions, Quaternary ammonium compounds. Table I shows the range of disinfection facilities used in the Territory. Gas chlorine is by far the most common disinfection facility resulting from familiarity with the technology, ability to assess the performance (measurement of the residual level and microbiological testing) and affordable capital and operational costs. Communities not provided with disinfection treatment facilities generally have protected sources from which the supply is drawn, and secure transfer, storage and reticulation facilities .

Chlorine Gas and liquid chlorination treatment facilities installed for major

Aboriginal communities are normally operated in the "standby mode" to provide disinfection on an "as and when required" basis, dictated by regular monitoring programmes. Consumers in Aboriginal communities have shown significant resistance to even low levels of residual chlorine in the drinking water supply. The taste and odour has frequently resulted in communities not operating chlorination facilities unless an "inspector" is visiting the community. At Ngukurr the resistance to the taste and odour of chlorine resulted in a slow sand filter being selected to provide treatment of the Roper River supply for turbidity and disinfection. (see below)

Ultraviolet (UV) Light Irradiation UV disinfection is used in the Territory (on a limited basis) solely or in conjunction with gas chlorine on reticulated supplies or as "a point of use" treatment. UV does not provide residual disinfection. Future directions will involve the assessment and possible wider use of UV disinfection on "point of use" water supplies for small, remote communities with solar or solar/ battery power supplies. UV disinfection has a negligible effect on the chemical composition and taste of water. Over-dosing presents no danger and is commonly used as a safety factor in the system design. There remains considerable disagreement within the industry on performance claims, especially relating to usage on waters with greater than I NTU. UV disinfection is seen as being "environmentally friendly" .

Slow Sand Filtration As mentioned earlier, slow sand filtration was installed to provide for treatment of the Roper River supply for the community of Ngukurr. Unfortunately, as a result of turbidity levels being greater than expected, the presence of colloidal partial and operational difficulties (filter cleaning) this plant is no longer being utilised. Australian and international research indicates that with modifications and provision of pretreatment using a horizontal roughing filter, this process can achieve the desired reduction in turbidity and disinfection. Slow sand filtration (depending on criteria selected) remains a viable, low technology treatment process.

Micro Filtration New developments in micro fil,tration show potential for wide use of this technology for disinfection of surface water supplies.

TREATMENT TECHNOWGY FOR OTHER THAN DISINFECTION The following treatment technologies are used in the Territory or are being assessed as future technologies suitable for the Territory environment.

pH Correction As discussed earlier, provision of a pH correction treatment systems using chemicals such as lime, sodium hydroxide or soda ash for Aboriginal community water supplies is generally considered inappropriate technology and uneconomical as opposed to a long term plan of using inert materials for storage, reticulation and plumbing. Experimentation with drip tray aerators in the late 1970s established that the "soft" acidic coastal water supplies have insufficient buffering capacity to hold the pH level above 4.5 to 5.5. Commercial neutralising filters are currently being trialled to assess performance and establish unit production costs. Sodium hydroxide dosing is used for the communities of Alyangula and Angurugu with, arguably, mixed success.

Biological Denitrification Biological denitrification of drinking water has been successfully developed in many parts of England and Europe, where nitrate, added to the soil as fertiliser, is a serious contaminant of the groundwater supply. In contrast, however, the high groundwater nitrate levels in many parts of Central Australia are not from applied sources, but are the result of natural events occurring over many thousands of years. The nitrate originates from nitrogen fixed in the soil by termites and other biological agents, and flushed through the soil profile to the water table. A variety of strategies is available to remove nitrate from drinking water. These strategies include desalination, reverse osmosis, electrodialysis, ion-exchange and solar distillation. However, many of these processes require a ready supply of power and / or skilled WATER December 1990

27


maintenance personnel, neither of which are generally available in many small, remote, Aboriginal communities. Biological denitrification is an alternative strategy, and probably the most appropriate to suit the needs of these communities. Unlike the other options this biological method is the only treatment which does not leave a concentrated residue as a by-product for later disposal. Environmentally it is the most attractive option. By combining the ability of denitrifying bacteria to reduce nitrate to nitrogen gas under anaerobic conditions, with some simple plumbing, a low maintenance 'bioreactor' system can be employed to effectively remove nitrate from excessively nitrate-rich groundwaters. As a result of research in 1989-90 (Smith, Barnes and Jacobson, 1989), a bioreactor has been tested at the Centre for Appropriate Technology at Alice Springs. The bioreactor effectively contains a desired denitrifying bacteria, in a reactor chamber, so that nitraterich water can be passed through and the nitrate removed . The current system utilises the denitrifying bacterium Hyphomicrobium. The bacteria are immobilised on a polyurethane support and maintained on a low concentration of methanol, which is their carbon energy source. Laboratory tests at the Australian National University (ANU), indicate the system totally reduces nitrate, which is given off as gas, and results in nitrate-free water. Further development of an operational prototype is required to provide this technology to Aboriginal and other communities.

Reverse Osmosis Reverse osmosis (RO) technology has become highly refined and is well proven. RO can be used for removal of hardness, nitrates, fluorides and suspended solids from bore waters. The inhibiting factors are high capital cost, high unit production cost and specialist skills required for operation and maintenance. A three month trial at the remote Aboriginal community of Tara, confirmed good performance of RO in the Territory environment.

(__O_O._fl._'F._'ER_E_N_C._'E_CA_L_E._'ND_!AR _ _) For further information please contact AWWA Secretariat P.O. Box 460, Chatswood NSW 2057 Phone: (02) 413 1288 Fax: (02) 410 9652

AUSTRALIA March 13-15 - Perth Site Remediation March 17-22 - Perth AWWA Biennial Convention April 7-11 - Hobart National Conference Institution of Engineers Australia August 18-19 - Cairns Mining and Mineral Processing/ Environment August 25-30 - Hobart Local Government - Management of assets and environment October 2-4 - Perth International Hydrology and Water Resources Symposium October 9-11 - Adelaide Engineering Management November 27-28 - Perth Appropriate Waste Management Technologies 1992-March - Sydney 4th National Conference on Hazardous Wastes 1992- November - Sydney Constricted Wetlands for Wastewater Treatment

OVERSEAS February 26-March 1 - Belgium, Gent Congress on Characterisation & Treatment of Sludge March 3-8 - USA, Los Angeles 3rd Symposium on Off-Flavours in the Aquatic Environment IAWPRC March 4- 7 - USA, California, San Diego 1991 Internation Oil Spill Conference April 3-5 - Portugal, Lisbon Marine Disposal Systems IAWPRC April 14-17 - USA Houston Computer Specialty Conference American Water Works Association April 15-19 - Spain, Canary Islands, Tenerife, Puerto de la Cruz XXIll International Congress of the Internation Association of Hydrologists - Aquifer Over exploitation April 15- 20 - Malta Desalination & Water Reuse

28

WATER December 1990

However, the operational skills required ami the support backup for the specific unit/ arrangement has limited acceptability for remote Aboriginal communities. Recent investigations have established that with appropriate use of leading edge control system technology and solar power supply, RO may be suitable for remote community applications. There are currently two solar powered units proposed for installation in the Territory on a trial basis. The disposal of the concentrate waste remains a major concern.

CONCLUSIONS The evaluation and development of water treatment systems must be undertaken in conjunction with the establishment of appropriate levels of service and consideration of all aspects of the water supply management for the individual community situation. Disinfection of surface and contaminated ground water supplies will remain the most important treatment consideration. Future directions will see a concentration of efforts towards appropriate technology, improved training of operators and maintenance personnel and better utilisation of financial and human resources in the monitoring and evaluation of treatment system performance. REFERENCES I. 2.

3. 4. 5. 6. 7.

GARDNER PM (Stati stician Northern Territor y). Northern Territor y Statistical Summary 1990. Australian Bureau of Statistics - Northern Territory Office. MARKS A R and JOLLY P. Extreme Corrosivity of Northern Territory Coasta l Groundwater Supplies - Ori gin , Effects and Materials of Construction.1987 Engineer Con fe rence, Darwin, 11th May - 15th May, 1987. The Institution of Engineers, Australia Reprints of Papers pp. 334-342. (The Institution of Engineers, Australia National Conference Publication No. 87/1) WHO (World Health Organisation) 1984. Guidelines for Drinking-Water Quality, Volume I, Recommendations. WHO (World Health Organisation) 1985 . Guidelines for Drinking-Water Quality, volume 3, Drinking-Water Quality Control in Small-community Supplies. WADE A , 1990, Water Quality, Health and Water Disinfection Practice. Australia Water and Was tewater Association 1990 Summer School. Melbourne 12-16 February 1990. NHMRC/AWRC, 1987 . Guidelines for Drinking Water Quality in Australia. SMITH G, BARNES C AND JACOBSON G. Biological Denitrification of Drinking Water. AWRAC Project 86/ 32 - Interim Report .

April 22- 25 - Belgium, Ostend Environmental Biotechnology April 24-26 - England, Manchester Urban Waterside Regeneration Conference & Exhibition April 30-May 2 - England, Birmingham IWEM 91 International Conference & l!xhibition Water & The Environment May TBA - South Africa, Johannesburg Water Institute of Southern Africa & 2nd National Water & Wastewater Conference & Exhibition May 12-18 - USA, Texas, Houston 4th International Symposium on Land Subsidence May 12-16 - Brazil, Sao Paulo Sixth International Symposium on Anaerobic Digestion May 13-19 - Morocco, Rabat Vllth World Congress on Water Resources, IWRA May 21-24 - Hungary, Budapest Measurement of Water Quality May 25- 31 - Denmark, Copenhagen 18th International Water Supply Congress & Exhibition June 3-6 - USA, New Hampshire, Durham 2nd International Conference on Watermatex 91 IAWPRC June 9- 15 - West Germany, Frankfurt ACHEMA '91 June 23-27 - USA Philadelphia American Water Works Association Annual Conference August 12- 15 - Sweden, Stockholm Water Resources in next century August 26-30 - Czechoslovakia, Prague Design & Operation of Large Wastewater Treatment Plants, IAWPRC September 8-11 - USA Atlanta Distribution System Symposium, American Water Works Association September 24-26 - Spain, Costa Brava, Castell Platja d'Aro International Symposium on Wastewater Reclamation & Reuse, IAWPRC October 6-10 - Canada, Toronto 64th Annual Conference WPCF October 8-9 - France, Paris International Conference on Groundwater Protection November 10-14 - USA Fla, Orlando Water Quality Technology Conference American Water Works Association The Association welcomes details of conferences and exhibitions for inclusion in the diary pages of Water. Please send all relevant details, including date, venue and contact to: AWWA Secretariat, PO Box 460, Chatswood NSW 2057 Facsimile (02) 410 9652.


LAKE NUMBER An indicator of reservoir mixing: a water quality management tool D.M. Robertson*, J. lmberger* and K. Boland!"* ABSTRACT A goal of water quality management is to develop an easily measured parameter which reflects the dynamic physical behavior of density-stratified lakes and reservoirs, and which can be used to predict changes in specific water quality parameters. One such parameter is the Lake Number, defined as the ratio of the moments, about the centre of volume of the water body, of the stabilising force of gravity to the destabilising force supplied by wind stress. Changes in Lake Number values are shown to be directly related to changes in in deep water dissolved oxygen, iron, and manganese concentrations and surface chlorophyll concentrations in two Australian reservoirs and one lake in the USA. After developing empirical relationships between Lake Number values and various water quality parameters, it is possible to use continuously measured Lake Number values, calculated using water temperature profiles and surface wind velocities, to estimate changes in water quality. Based on the estimated changes in water quality it is then possible to suggest the next time to sample, and to forecast deterioration in water quality so as to allow time to select management options.

INTRODUCTION All lakes and reservoirs experience a certain degree of stratification. The length of the stratified periods vary greatly from system to system, ranging from a few hours in very shallow ponds to permanent stratification in some very deep lakes with small surface areas. However, in most lakes and reservoirs, the length of stratification is between these two extremes and results in the deep water being temporarily isolated from surface mixing. This isolation can result in the depletion of dissolved oxygen in the deeper water and is often accompanied by elevated iron and manganese levels. Anoxic deep water is often the cause of fish kills and if the body of water is being used as a water supply; elevated iron and manganese levels are often the cause of poor tasting or coloured water. The extent of oxygen depletion and dissolution of iron, manganese, and other nutrients in the deep water are dependent on the length of stratification and the chemical composition of the sediments. In many systems, the extent of mixing and the deterioration in water quality occurs on a seasonal basis; however, in other systems, the extent of mixing and the changes in water quality are dynamic and are difficult to ascertain without a rigorous monitoring program. Various techniques have been used to try to eliminate poor water quality. One approach is to try to maintain permanent oxygenated conditions using destratification devices such as aerators or mechanical mixers. This technique is very costly and must be implemented when stratification first develops. Another technique is to simply use the best quality water as a supply source by ·monitoring the water quality at various depths, and by using a system of variable offtakes, selectively withdrawing the water of best quality. This requires a great deal of monitoring to assure that the best water is being used. One goal in water quality management is to develop an easily monitored index which reflects the dynamic stability and the extent of mixing within a lake or reservoir and which can be directly related to changes in specific water quality parameters. Then, by continuously monitoring the change in that index, it may be possible to estimate the extent of mixing and predict changes in specific water quality parameters. Schmidt stability values, St, defined in Equation 1, represent the stability of the lake by the amount of work required to mix the entire water body to a uniform density without adding or subtracting heat (Schmidt 1915; 1928). However, the absolute values of St are dependent on the size of the water body and do not describe the amount of mixing actually occurring in the lake. Even with very low S1 values, a lake can experience very little mixing if very low wind velocities are present. Footnote: • Centre for Water Research, University of Western Australia, Ned/ands WA 6009 •• Australian Centre for Tropical Freshwater Research, James Cook University of North Queensland. Townsville. QLD, 48/1

Dale Robertson is a research fellow at the Centre/or Water Research of the University of Western Australia. He obtained his MSc and PhD from the Oceanography and Limnology Graduate Program of the University of Wisconsin-Madison, USA. In addition to the research discussed here, he is also involved in developing practical means of improving water quality through aeration, and in modelling the movements of particles in the upper mixed layers of lakes and reservoirs.

Dale Robertson

Jorg Imberger is a professor in the Department of Civil and Environmental Engineering at the University of Western Australia and is Director of the Centre for Water Research and the Special Research Centre for Environmental Fluid Mechanics. A graduate of the University ofMelbourne, he went on to obtain his MEngSc from UWA and PhD from the University of California, Berkeley. His main research interests lie in the motion of stratified fluid .. and also in the quality of water in estuaries Jorg lmberger and lakes, turbulent, buoyant jets, sewage outfalls, surface hot water discharges and natural and forced convection in the environment. The interaction of the biological system and water motion is also a primary focus. Kevin Boland has been involved in Water Research in North Australia for the past ten years. His current interests focus' on two North Queensland lakes near Mt. Isa. This project has been undertaken in conjunction with The Centre for Tropical Freshwater Research located at James Cook University of North Queensland. Prior to this he had worked extensively on the limnology of lakes in the Northern Territory as a Senior Scientist with the N.T. Power & Water Authority and will return to this position upon completion of his Doctorate.

s,

~IO z. (,-ZJ'

Where: z

Zm Zg A(z) p(z)

Kevin Boland

A(,)' p (,)' d,

(1)

Depth above the bottom Maximum depth of the lake Centre of volume of the lake Area of the lake at depth Density of the water at depth z

An index which does represent lake stability and the extent of mixing is the dimensionless Lake Number, formalized and discussed by lmberger and Patterson (1990). The Lake Number, LN, is a quantitative index of the dynamic stability of the water column and is defined as the ratio, about the reservoir's centre of volume, of the moment of stabilising force of gravity (resulting from the stratification) to the moment of the destabilising forces due to the wind, inflow, outflow, and destabilising devices. Assuming the effects of inflow, outflow, and destabilising devices are minimal, i.e., wind is the dominating force for mixing, LN can be defined by Equation 2. WATER December 1990 29


(2)

. Po u,2

A)/ 2 ( 1 -

z

zg) m

where g is the acceleration of gravity, S, is the Schmidt stability (Equation 1), Z, is the thermocline height in meters from the bottom, Zm is the maximum depth of the water body, p is the water density at the surface, u, is the water friction velocity due to _wind stress (Equation 3), A is the surface area, and Zg is the height to the centre of volume of the reservoir, in meters off the bottom. u* = {l.612E-6 * Uw 2) 1/2 (3) 0

0

Where: Uw = Wind velocity at 10m above the water surface A LN = 1 indicates that the wind is just sufficient to force the thermocline to be deflected to the surface at the upwind end of the lake. For LN> > 1, the stratification will be strong and dominate the forces introduced by surface wind stress. Under these circumstances, the stratification is expected to be primarily horizontal, with little seichingt of the thermocline and little turbulent mixing in the metalimnion and hypolimnion. Note: Above a certain value, increases in LN represent very little difference in terms of deep mixing. For LN < < 1, the stratification will be weak with respect to the forces introduced by wind stress. Under these circumstances, the thermocline is expected to experience strong seiching and much turbulent mixing is expected in the metalimnion and hypolimnion (Imberger, 1989). In this paper, we examine Darwin River and Manton River Reservoirs, N.T., which experience dynamic, i.e., highly episodic, mixing and Lake Mendota, U.S.A, which undergoes very predictable seasonal stratification, to demonstrate the relationship between changes in the LN of the systems and changes in their water quality. A procedure is then presented which describes how real time measured LN values can be collected and used in a water quality control system.

STUDY SITES Darwin River Reservoir (DRR) and Manton River Reservoir (MRR) are located in the monsoonal region of Australia's far north (12°54'S, 131 °00'E). DRR is the main supply storage for city of Darwin in the Northern Territory and MRR is a reserve emergency supply. DRR has a maximum depth of 22m at its full supply level. Throughout this study, the water level of DRR was approximately 2m below this level. At this elevation, DRR has a surface area of 3,175 ha., a mean depth of 5.6m, and a maximum length of 8km. MRR has a maximum depth of 14.5m at its full supply level; throughout this study, the water level was very near this depth. At this elevation, MRR has a surface area of 444 ha., a mean depth of 4.3m, and a maximum length of 5km. Boland (1990) reported that both reservoirs are susceptible to wind mixing which results in two mixing periods in the year. One is a mid-year mixing period ("dry" season) and the other is a more variable and less predictable period coinciding with the monsoonal troughs, cyclones and storm events during Dec. to Mar. (the "monsoonal" period). Lake Mendota, by contrast, is a dimictic, eutrophic lake in southcentral Wisconsin, U.S.A (43 °40'N, 89°24'W) which undergoes seasonal stratification and is frozen over approximately 100 days on average during the winter. The lake has a surface area of 3,940 ha., a mean depth of 12.4m, and a maximum fetch of 9.8km and experiences very little fluctuations in water level (Wis. Dept. of Natural Resources, 1961).

METHODS During 1986 and 1987, both D RR and MRR were sampled near the dam walls at their deepest point. Fortnightly profiles at 2m intervals were conducted using a Martek Mk XV submersible water quality analyser for the determination of temperature, conductivity (E.C. 25°C), and dissolved oxygen. Samples were retrieved at 2m intervals using a Van Dorn bottle and analysed fortnightly for iron and manganese and monthly for chlorophyll 'a' (corrected for phaeophyton). Total iron and manganese were determined using atomic absorption spectrophotometry after acid digestion. Chlorophyll concentrations were determined using a fluorom"eter after ultrasonic-assisted extraction in 90% acetone. t Seiching is a low imernal oscillation

30

WATER December 1990

Average daily wind speeds for MRR and.pRR were measured at the Darwin Airport located approximately 50km north of the storages. Fortnightly profiles of temperature and oxygen were collected at the deepest location in Lake Mendota by the Wis. Dept. of Natural Resources using a YSI Model 57 underwater temperature and dissolved oxygen meter. The depth interval between measurements was variable and dependent upon the change in temperature and oxygen with depth . Average daily wind speeds for Lake Mendota were measured at Dane County Airport located approximately 2km northeast of the lake. LN values were computed for each temperature profile using Equation 2. The friction velocity, u., was estimated using the average wind speed for the three previous days. Wind speeds were adjusted to a 10m height assuming a logarithmic profile (Sellers, 1974). Thermocline depths, Z" were estimated as the depth from the bottom to center of the metalimnion. Specific data presented for dissolved oxygen, iron, and manganese were those measured at 7m above the bottom reference depth in DRR and 5m above the bottom reference depth in MRR and Lake Mendota. These depths were chosen because they were the depths of the deepest consistently measured chemistry data and were located near the bottom of the hypolimnion of each water body.

RESULTS Lake Number LNvalues for DRR and MRR for the period from Jan. 1986 to Dec. 1987 were computed and are shown in Fig. 1. Low LN values correspond to periods of relatively high turbulence in the deeper water and high values correspond to periods of relatively low turbulence (Imberger, 1989; Imberger and Patterson, 1990). The LN time series for each reservoir show very similar patterns, indicating that even though the reservoirs are much different in size, the response to wind mixing is very similar. However, during the monsoonal period, Jan. and Feb., LN values for DRR reached slightly lower values for longer periods, which indicated that the reservoir had longer periods of mixing than MRR. LN values for the reservoirs are relatively low throughout the year, which demonstrate the very dynamic mixing regimes found by Boland (1990). DRR had four periods of extended stratification during the two year period: a period from Feb. to Mar. in 1986, a longer period from Aug. 1986 to mid-Jan. 1987, a brief period from Mar. to midApr. 1987, and a longer period from mid-Aug to Dec. 1987. MRR had longer periods of stratification, with much less mixing during the monsoonal periods. Even during the stratified periods, the relatively low LNvalues indicate that only moderate winds would be required to completely mix the reservoirs. Lake Mendota, in comparison, has very periodic mixing regimes separated by relatively strong summer (Jun. to Oct.) and weak winter stratification (Jan. to Apr.). Only LN values during summer stratification are demonstrated in Fig. 2, because no wind stress is applied during the ice-covered period. During summer stratification, the larger LN values suggest Lake Mendota has much less turbulence and less mixing in its deeper water than the two reservoirs. Dissolved Oxygen A comparison between deep water dissolved oxygen concentrations and LN values are given in Fig. 3a for DRR and Fig. 4a 5.0

ORR MRR

4.0

cii

.c

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z

.,,_GI 2. 0

.,

..J

1.0

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M

J

1986

S

N

Jan

M

M

J

S

N

J

1987

Fig. 1 - Comparison between Lake Number values for DRR and MRR for 1986 and 1987.


120

12

., ... o····

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,

100

0 ~

10

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80

i c......f

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+----,-.,.........q,==,...,---.---r-..... F--<><><><>QO<,.._,.,_--+ M J S N Jan M M J S N J

Jan M

1988

1987

Fig. 2 - Time series for percent dissolved oxygen concentration at Sm above the bottom and Lake Number values for Lake Mendota during 1987 and 1988.

for MRR. In general, a strong inverse relationship is evident for both reservoirs, with low oxygen concentrations linked to high L N values (low turbulence) and high oxygen concentrations linked to low LN values (high turbulence). L N values in both reservoirs fluctuate greatly from Jan. to Mar. in 1986, the monsoonal period, and are most pronounced in DRR. During this period, the minimum LN values for DRR were lower than those in MRR, reflecting more turbulence and mixing occurring in DRR. This greater response in DRR to variable winds results in larger shifts in dissolved oxygen, and only a short period of anoxia in March . In comparison, MRR remains anoxic from Feb. to May. A similar pattern is evident during the monsoonal period of 1986 - 1987, when DRR has an extended period of mixing from mid-Jan. to mid-Mar. resulting in high

dissolved oxygen concentrations; on th~otherhand, MRR has only a brief period of mixing which only very temporarily raised dissolved oxygen concentrations. Both reservoirs have similar "dry" season mixing and elevated dissolved oxygen concentrations. The dissolved oxygen concentrations in both reservoirs respond very quickly to turbulent mixing. However, as stratified conditions develop in DRR deep water dissolved oxygen is gradually depleted, i.e. anoxia lags the onset of stratification; whereas deep water dissolved oxygen in MRR is depleted very quickly, i.e., a very small lag time. Deep water dissolved oxygen concentrations in Lake Mendota have almost a perfect inverse relationship with the LN time series (Fig. 2). Dissolved oxygen is gradually depleted over approximately 10 weeks after stratification is developed in early summer; however, dissolved oxygen concentrations respond almost immediately to turbulent mixing in fa ll. Iron and Manganese

The dissolution of iron, manganese, and other nutrients in many water bodies is directly dependent on deep water dissolved oxygen concentrations. Therefore, given the strong relationship between LN values and dissolved oxygen concentrations and given that inflows do not significantly contribute to in-reservoir or -lake concentrations, it may also be expected that deep water total iron and manganese concentrations would be related to LN values. Fig. 3b,c and Fig. 4b,c illustrate the deep water manganese and iron concentrations for DRR and MRR, respectively. Clearly, concentrations of manganese and iron in MRR are much higher than in DRR and fluctuate more radically; however, the positive relationship with L N values is very similar. After stratification (high LN values) and anoxia develop, concentrations of manganese and iron begin to increase. Then, shortly after turbulent mixing

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0

E

20

A.

........

100

M

J

s

1986

N

Jan M

M

J

s

N

J

1987

Fig. 3 - Time series comparison between Lake Number values and A) Dissolved oxygen; 8) Manganese; and C) Iron at 7m off the bottom in DRR during 1986 and 1987.

Jan M

C.

M

J

s

1986

N

Jan M

M

J

s

N

J

1987

Fig. 4 - Time series comparison between Lake Number values and A) Dissolved oxygen; 8) Manganese; and C) Iron at Sm off the bottom in MRR during 1986 and 1987. WATER December 1990

31


occurs (low LN values) concentrations of manganese and iron drop off quickly. More mixing (lower LN values) occurs.in the monsoonal period in DRR, which results in a shorter period of anoxia and lower elevations in manganese and iron concentrations than in MRR. During the monsoonal period in early 1986, turbulent mixing frequently occurred in DRR resulting in only a brief period of anoxia and correspondingly low levels of iron and manganese. Extended anoxia occurred in MRR resulting in very high concentrations of manganese and iron. Similarly, during the monsoonal period of 1986 - 1987, an extended period with complete mixing (low LN values) occurred in DRR which lowered the concentrations of both manganese and iron . However, MRR remained stratified during this time, except for a very brief period, which resulted in concentrations of both manganese and iron remaining elevated with only a slight reduction associated with the period of mixing. Prior to mixing during the monsoonal period in both years, concentrations of manganese and iron increased rapidly associated high LN values (low turbulent mixing). Chlorophyll In many systems, the growth of phytoplankton is limited by low concentrations of available nutrients (Cole, 1979). Therefore, if turbulent mixing of deep water allows higher concentrations of nutrients to be made available to surface phytoplankton, it may be expected that surface chlorophyll concentrations should increase after complete mixing following an extended period of stratification. Therefore, changes in surface chlorophyll concentrations may also be linked to changes in LN values. Fig. 5a,b illustrates the comparison between surface chlorophyll 'a' concentrations and LN values for DRR and MRR, respectively. In DRR, surface chlorophyll concentrations are inversely related to LN values, similar to that described above for dissolved oxygen. Concentrations consistently increase and remain elevated during periods of low LN values (much mixing) following a period of high LN values (limited mixing). A similar but much weaker relationship is found in MRR, with higher concentrations of chlorophyll occurring during periods with low LN values. However, in general, concentrations in MRR demonstrate a smaller change throughout the year and remain higher even during periods with limited mixing.

DISCUSSION Several water quality parameters for systems with very different mixing regimes have been shown to be strongly related to LN values. Therefore, if a quantitative relationship between LN values and specific parameters can be determined, real time measured LN ,---,,---,,-----,~-----------3 ..... .,,,,, Surface Chlorophyll

.:;- s E

C)

E

2

~

E ::,

z

.,.Cl> CII

...J

0

A.

Jan

Jan

C)

:5

C/l

~

.2

r-..

8

/V\

6

; .0

2

.,. Cl>

CII ...J

2

Jan M

DO1 _1

LN value at time T, with an upper threshold of 2.5 = Dissolved oxygen concentration at time T-1 (other than the initial value, this is a predicted value)

Equation 4 was determined using an auto-regressive time series analysis (Snedecor and Cochran, 1973) applied to the two years of data for Lake Mendota. Under very stable conditions, a very low wind velocity results in very large LN values. Therefore, a large L value and a very large LN value both reflect that little mixing is occurring in the deep water. This is unable to be distinguished in a statistical model; therefore, an upper threshold of 2.5 was used in this analysis to represent that the lake was very stable and little deep mixing would occur. This suggests above a LN value of approximately 2.5, increases in LN values represent very little difference in terms of deep mixing. Equation 4 was used, with only measured LN values (with an upper threshold of 2.5) and an initial dissolved oxygen concentration, to predict the entire dissolved oxygen time series (Fig. 6). This model simulates the entire dissolved oxygen time series very well including both the depletion of dissolved oxygen over approximately 10 weeks in early summer, as well as, the rapid increase in concentration occurring with turbulent mixing in fall. Changes in dissolved oxygen dpring ice covered conditions were not attempted to be simulated with this model. The results suggest that for a relatively simple system, such as Lake Mendota, it is possible to predict most of the changes in hypolimnetic dissolved oxygen solely from changes in LN values. DRR and MRR's have much more dynamic mixing regimes than Lake Mendota; however, given the strong relationships with LN values a similar approach to that for Lake Mendota is expected to be able to be used to identify the relationships for these water quality parameters. Currently, work is being conducted to identify these more complicated systems and to determine whether the upper threshold used in this analysis has global characteristics. One of the benefits of using LN values over Schmidt stability values, St, values can be seen in Fig. 4. From May to Sep. 1987, very little stratification was present in MRR; therefore, St values remained very low throughout the period. But, in late June, very low winds were present for a few weeks resulting in very little deep turbulent mixing and higher LN values. With this limited mixing,

-

Ice

~E

--

C

C

o ::: CII

o 0

-

100

M

J

S

1986

N Jan M

M

J

S

N Jan

1987

WATER December 1990

Ic e

80

_!l

60

I

CCI

:50 cu C/l E

-Ji.

Fig. 5 - Time series comparison between Lake Number values and surface chlorophyll 'a' concentrations for A) ORR and B) MRR during 1986 and 1987. 32

at time T

= Measured

LKT1

40

-

LI)

o-1--,~~~ir-.-----~~~.....-.~--.-:-'r--"~L....-~~-~o

B.

::,

z

.;

4

E

(4)

= Dissolved oxygen concentration (5m off bottom)

Where: DO1

120

.s::.

(.)

DO1 = 57.33 - 23.18* LKT1 + 0.34* DO1_1

.... ., ... , Surface Chlorophyll

E Cl>';. <.> .s::. .!11 a.

System Identification The mixing regime in Lake Mendota is very simple in comparison to DRR and MRR and it has a very strong relationship between its LN values and deep water dissolved oxygen concentrations. Therefore, we are able to present a very simple model for Lake Mendota which relates changes in deepest consistently measured dissolved oxygen concentrations (5m above the maximum depth in the lake) to only changes in the LN values (Equation 4) .

~~,-----------------""T3

10

.:;E

values can be used not only to represent a quantitative indication of the dynamic mixing regime of the lake or reservoir but also to estimate changes in water quality and ultimately aid in the refinement of decision-making procedures for effective lake management.

20

/ ······•······

0

Jan

M

M

J

1987

s

N

Jan

M

M

J

s

N

J

1988

Fig. 6 - System identification for deep water dissolved oxygen concentrations in Lake Mendota. Predicted values were computed using Equation 4.


the oxygen concentrations quickly dropped . This change in dissolved oxygen would be able to be predicted using LN values, but unable to be predicted using S,. values. Real Time Lake Number as a Management Tool Once a system is identified, a few instruments and a simple procedure can be used to measure real time LNvalues and estimate changes in the specified water quality parameters. The equipment required consists of a buoy placed at the centre of the lake or reservoir with a thermistor string, suitable to describe the thermal structure of the water body, suspended beneath it, an anemometer placed on top of it. The thermistor string and anemometer are connected to a data recorder and transmitter located within the buoy. The water temperatures and wind velocities are then recorded within the buoy and transmitted back to a base computer where real time LN values are computed. The LN values are then continuously input into a model, similar to that presented for Lake Mendota, and changes in water quality are continually estimated. The initial system identification requires detailed information of coinciding LN values and specific water quality parameters. For most systems, a full year of detailed information is thought to be sufficient; however, additional data collected after the initial system identification can be used to improve and refine the model as well as to adapt the model for long-term changes in the storage. It should be noted here that even with very detailed information collected over several years it may be impossible to identify certain systems. This may be caused by the specific water quality parameters not being primarily related to the amount of turbulent mixing, but to some other factors. For example, it appears that only part of the variability in the chlorophyll time series for MRR (Fig. 5b) is related to LN values. System identification using LN values in conjunction with chemical and biological analysis programs presents the opportunity for refinement of decision-making procedures for effective lake management (Fig. 7). Estimated changes in specific water quality parameters can be used to optimise sampling strategies, aid in the selection of depths of offtake in water supply storages, assist in the timing for pump-back systems, help in determining criteria for the operation of destratification devices, and used as an early warning of potential deleterious changes in water quality. Real time implementation of Lake Number models should not be used to completely eliminate water quality sampling, but as a means to more efficiently sample a storage. To optimise sampling efficiency, field sampling should occur when the model suggests a rapid change in water quality or when critical thresholds for specific parameters are being approached. The measured data may be then used to improve and refine the model as well as adapt the model for longterm changes in the storage. The specified depths at which water quality parameter for DRR, MRR, and Lake Mendota were rather arbitrary and represented the deeper region of the hypolimnia. In the applied situation for water supply storages, however, the depths selected might coincide with particular offtakes. Then, by continually estimating the water quality at those specific depths, a better offtake protocol may be established to optimise the apparent water quality of the storage.

CONCLUSION Lake Number represents a quantitative index of the dynamic stability, i.e., the extent of turbulent mixing, in a body of water. Several water quality parameters for systems with very different mixing regimes have been shown to be strongly related to LN

Real Time Lake Numbers As A Management Tool Mnsurtd LaktNumbtts

Sys1em Identification

Eslimatlon or WaterQualily

Para met en • l>iMOl•NI (h)RH

• Ml"Alflnr •lrOII

Conlrol Stral~les • s•.,,~.,.T;.,...,, • 11ffuli.f l ,o,1 • Ot~1,11it,ntion ........ ,l1lKli.

Compare Wi 1h Measured Data To Improve System lden tlncation

Fig. 7 - Flow chart demonstrating how continuously measured Lake Numbers can be used in water quality management.

values. Once a quantitative relationshi~ between LN values and specific parameters is determined, such as was shown for Lake Mendota, real time measured LN values can be used to represent a quantitative indication of the dynamic mixing regime of the lake or reservoir and also to estimate changes in water quality. The estimated changes in water quality may then be used to optimise sampling strategies, aid in the selection of depths of offtake in water supply storages, assist in the timing for pump-back systems, help in determining criteria for the operation of destratification devices, and used as an early warning of potential deleterious changes in water quality.

ACKNOWLEDGEMENTS We thank the Northern Territory Power and Water Authority and R. Lathrop of the Wisconsin Department of Natural Resources for making the reservoir and lake data available for this study, and G. Ivey, J. Patterson, and R. Lukatelich, whose comments and suggestions have helped improve the manuscript. This work has been financially supported by the Centre for Environmental and Fluid Dynamics at the University of Western Australia, and the Australian Water Research Advisory Council under the Centres of Concentration in the Water Research Program.

REFERENCES Boland, K. T. 1990. The limnology of the Darwin River and Manton River Reservoir. Northern Territory Power and Water Authority. Report 32/ 88. Cole, G. A. 1979. Textbook of Limnology. Second Edition. The C. V. Mosby Company, London. 426 P. Jmberger, J. 1989. Vertical heat flux in the hypolimnion of a lake. Proceedings of the Tenth Australian Fluid Mechanics Conference, Melbourne. p.2.13-2.16. Imberger, J. and Patterson, J.C . 1990. Physical Limnology. In: Advances in Applied Mechanics. Academic Press. 27:p.303-475. Schmidt, W. 1915. Uber den Energie-gehalt der Seen. Mit Beispielen von Lunzer Untersee nach Messungen mit einen einfachen Temperaturlot. Internal. Rev. Hydrob iol. Suppl. 5. Leipzig. Schmidt, W. 1928. Uber Temperatur und Stabilitatsverhaltnisse von See. Geogr. Ann. 10: p.145-177. Sellers, W. D. 1974. Physical Limnology. University of Chicago Press. ' Chicago, IL. 272 P. Snedecor, G. W. and Cochran, W. G. 1973. Statistical Methods. Sixth edition. The Iowa University Press. Iowa . 593 P.

QUEENSLAND SEMINAR Continued from page 18 Paul Greenfield from the Department of Chemical Engineering at University of Queensland presented a paper on novel approaches to sludge treatment. It included a description of new processes, and also other issues such as the effect of different sludges on process outcomes such as yields from oil-from-sludge plants and ash melting points for making clay pavers fo llowing incineration. He also described cocomposting with municipal refuse, starved-air combustion, the neutralysis process and mobile plasma incinerators. In a summary on future trends, he said that reliability would be a key point, and foreshadowed greater source control, including removal of contaminants form domestic wastes which might inhibit sludge re-use. Tim Kempton of Amgrow Pty Ltd brought together several issues in his paper on beneficial sludge use. He basically said, "If you want beneficial re-use of sewage sludge, here's how to do it," and he then evaluated all the various processes, marketing issues for products, and the advantages and disadvantages of the various alternatives. He also provided additional information on one of the more innovative processes, microwaving. In a comprehensive paper, analytical aspects, pathogens, land application rates and costs ere all covered. Jeff Brown of the Public Works Department, New South Wales, outlined a range of options provided to his client councils (280). These are usually biological filter plants or intermittently decanted aeration tanks, and simple, low-cost appropriate technology is provided for sludge disposal, for example; lagoons and mobile dewatering units with land disposal, turf farming, soil improvement and municipal landfill. This contrasted with many of the high technology processes being canvassed for major cities such as Sydney and Newcastle. The paper also included a useful tabulation comparing the dewatering performance of filter belt presses and centrifuges which appeared to be about equal. WATER December 1990

33


Foaming in activated sludge plants: field scale trials to evaluate control strategies by L.L. BLACKALL, A.E. HARDERS, P.F. GREENFIELD, and A.C HAYWARD ABSTRACT Field scale trials are described which evaluated means of controlling actinomycete foaming in activated sludge plants. The effects were assessed of: altering the rate of recycle of return activated sludge, dosing of the aeration tank with anaerobic digester products, addition of a commercial biological product specifically designed to control foaming, altering sludge age, and varying the levels of mixed liquor suspended solids (MLSS) as well as altering a number of other operational or design features . In general these strategies had a marginal effect on the control of foaming. Some operational and design features, however, were highlighted as either exacerbating or reducing the problem. In particular, eradication of.regions of low turbulence on the aeration tank surface was found to be important as these zones allowed the initiation and stabilization of a foam which then acted as an inoculum source; this generally led to an increase in the frequency of severe foaming events.

INTRODUCTION The formation of a stable, dark, viscous foam or scum on the surface of the aeration tank section of an activated sludge process was first reported in 1969 (Anon, 1969). Bacteria among the nocardioform actinomycetes have been found to accumulate selectively in this foam, however, the reasons for the overgrowth of these organisms are unknown. Although foaming is not considered as serious a problem as sludge bulking, it is unsightly, can lead to effluent quality problems and generally increases operating costs. Hence, control of such foaming is generally attempted. There is no uniformly accepted cause of the problem (Blackall et al., 1990a), and this leads to many possible "solutions", few of which have been tested systematically. The intermittent nature of foaming in a number of plants and the complexity of the activated sludge treatment system exacerbate the problems of establishing a c·ausal relationship and of proving the success of a particular control strategy. · Some plants appear to suffer continuously from the problem while in others it appears at irregular intervals, not obviously associated with any single factor. Ho and Jenkins (1990) have recently shown that the presence of small amounts of slowly biodegradable non-ionic surfactants can induce previously nonfoaming Nocardia to foam significantly. They have inferred from this result that the presence of such surfactants and their degradation products in sewage streams can lead to a rapid onset of foaming. Because nocardioform actinomycetes are thought to be slow growing bacteria, one much-mooted control strategy has been to reduce sludge age which generally leads to a reduction in the mixed liquor suspended solids (Pipes, 1978; Sezgin & Karr, 1984, 1986; Sezgin et al., 1988). Return of anaerobic digester supernatant to the aeration tank (Lechevalier, 1975), dosing of the influent with ferric chloride (Dhaliwal 1979), or pickle liquor (Heath & Chan, 1985), modification of plant configuration (Hiraoka & Tsumura, 1984), physical removal of scum (Nelson, 1981; Hoffmaster, 1981; Ludwig, 1981), using the flotation characteristics to effect separation (Pretorius & Laubscher, 1987), and exposure of the mixed liquor to periodic anoxia (Gasser, 1987) have also been reported as control strategies. Surface spraying with chlorine solutions is currently the most commonly used control technique in Australia. A recent report by ATV Technical Committee 2.6 in West Germany (1989) reflects the current lack of certainty on causes and control of foaming. Their principal recommendation is to remove the foam hydraulically, mechanically or by vacuum. We attempted to verify rigorously two reported control methods and to explore one untried control strategy. Design and operating features that could affect foam formation and stabilisation were investigated at a number of plants and the results of these are also reported. The paper attempts to explain biologically the bases of the different foaming control strategies and reports on their efficacy. 34

WATER December 1990

Linda Blackall is a microbial ecologist with a PhD degree from the University of Queensland (1987); thesis entitled ''Actinomycete Scum Problems in Activated Sludge Plants." A CSIRO Postdoctoral Award allowed her to spend 18 months at the University of California, Berkeley, and 12 months at the University of NSW. She is currently working at the Animal Research Institute, D.P.l., Qld. Linda L. Blackall

Anne Harbers gained her first degree from the Queensland University of Technology in 1982. As a Research Assistant at The University of Queensland, she worked on Actinomycete in Anaerobic Sewage Treatment Plants and studied part-time to gain MAppSc-Research and Thesis. Following this she moved to private industry, to work on the scale-up of a biotechnology processes. Anne Harbers

Paul F. Greenfield is Professor and Head of Chemical Engineering at The University of Queensland where he has been since late 1975. Together with Chris Hayward he supervised this project on behalf of the Queensland Department of Local Government and Water Research Foundatio~ of Australia.

Paul F. Greenfield

Chris Hayward completed his PhD at the University of Birmingham, England and then worked for two years at the Microbiological Research Institute, Port-of-Spain, WI. After six years as Bacteriologist at the Commonwealth Mycological Institute, he joined the staff of the University of Queensland in 1965, where he is at present a Reader in Microbiology. Chris Hayward

MATERIALS AND METHODS Sewage treatment plants (STPs) in south-east Queensland, Australia, between latitudes 24 ° and 29° South and longitudes 150° and 154° East were studied. Plants in the trials were visited once per fortnight with more regular contact being made directly with the operator who kept daily records of the location and area of foam coverage. Samples of mixed liquor, return activated sludge and foam were collected for microscopic analysis (Blackall et al, 1990a) and for foaming and surface tension measurements (Blackall et al, 1990b). Foaming was measured in a piece of equipment constructed from a glass cylinder (500 mm high, 40 mm diameter) into which was poured 50 mL of liquid sample and through which compressed air was bubbled at 200 mL/ min. The gas bubbles were forced through a sintered glass disc (maximum pore size 40-90 J.Lm)


and rose though the liquid. Foam so generated on the surface was assessed. Foam generation and stability were recorded in terms of foam volume, bubble size, speed of formation and time taken for the foam to collapse after aeration ceased. A classification system to assess the foams was developed (Table 1).

foaming. Plant performance was monitorecl and samples were collected on a monthly basis. Plant operating data were collected from the plant operators' records. Table 3. Dosing Schedule of Sybron/Gamlen Bi-Chem Products for Carole Park STP Dosage kg/day_,

Table 1. Classification of Foams Generated in the Laboratory Foam Rating 0

Reaction to aeration as for pure water. Bubbles break surface but are unable to foam or have no stability.

1.0- 3.0 cm of foam with fragile, ill-formed bubbles. Insufficient stability to form films. Immediate collapse on aeration being halted . 2

Intermittent films sufficiently stable to last for > 5-10 seconds. Usually generated from a fragile foam structure of limited height. Films unstable on aeration being halted.

3

Foam of some substance (ie bubbles about 1.0 cm dia.) to 3-8 cm height. Infrequent - regular film formation, with both film and foam semi-stable on aeration being halted. Films have 10-30 seconds stability.

4

Initially 8-15 cm of foam (about I cm dia. bubbles) with stable films being formed at regular intervals. Body of the foam and films stable for 3-5 min once aeration ceases.

5

Condition of stable foam 5-10 cm in height in 2 min, after which collapse to 3- 5 cm height, which is stable when aeration is halted . No films.

6

Stable foam 15- 30 cm in height with no films. Bubble size about 0.5 cm during production and only increases to 2.0-3.0 cm dia. in 3-5 min from time that aeration is halted. Dense stable foam > 30 cm over 2 min aeration. Bubble size about 0.3 cm during production of foam and max. 1.0 cm dia. in 3-5 min after aeration is halted. Foam is sufficiently stable to show no change in height In 10- 15 min after aeration is halted.

7

Volatile fatty acids, nitrates and nitrites were determined in some samples (APHA, Standard Methods). Access to plant operation log books was granted and operators cooperated with all aspects of the trials. The following trials were conducted: 1. Return activated sludge flow rate The aim of this trial was to monitor the effects of altering the flow rate of return activated sludge (RAS) from the secondary sedimentation tank (SST) to the aeration tank (AT). Three return rates were evaluated over a 6 month period at the Bundamba STP.

2. Return of anaerobic digester supernatant and solids The effect of returning anaerobic digester supernatant and solids to the aeration tank was investigated. The Caloundra STP had both primary and secondary digesters on site. The trial period was 6 months and 3 different return regimes were evaluated. 3. Addition of commercial bacterial cultures Bundamba and Carole Park STPs were used for these tests. Two Sybron/Gamlen Bi-Chem products were evaluated. They were BiChem 1004TX and Bi-Chem 1008SF. The manufacturer's instructions were followed for dosing and these are indicated in Tables 2 and 3. The daily dosages involved activating the product by mixing 250g of product per litre of water and leaving to stand for 4-6 hours; the mixture was then added directly to the aeration tank. The trial lasted 6 weeks. 4. Other design and operational features Five sewage treatment plants were monitored over a 4-6 month period. Each of them had features which appeared to affect Table 2. Dosing Schedule of Sybron/Gamlcn Bi-Chem Products For Bundamba STP Dosage kg/ day (50:50 mixture of 1004TX and 1008SF)

Day

Day

(50:50 mixture of 1004TX and 1008SF)

Usage (kg)

1- 2 3- 4 5- 11 12- 19 20- 30

4.0 2.0 1.5 1.2 0.4

8.0 4.0 10.5 9.6 4.0

RESULTS AND DISCUSSJON Effect of RAS flow rate At the beginning of this trial Bundambra STP had two oxidation ditches operating at 2500 mg/L MLSS and a sludge age of 20 days, but with only one SST accepting mixed liquor from both ditches. There was no primary sedimentation and a sludge thickener concentrated the waste activated sludge. A large baffle at the mixed liquor exit point prevented foam from leaving the aeration tank. Liquid effluent from the plant was chlorinated before discharge. Three return rates were evaluated (Table 4). As the rate of return is increased the period of time that the sludge spends at the bottom of the SST is reduced. It was felt that this might affect the distribution of organic components in the RAS stream which, in turn, would affect Nocardia growth in the mixed liquor. During Stage 1 of the trials foam coverage ranged from 250Jo to 500Jo of the surface of the aeration tanks and was always about 5-20 cm in depth. After the aerators were non-functional in one of the ditches for a weekend, the foam volume in both ditches was temporarily reduced. Effluent quality was high throughout this phase of the trial and no foam was present on the SST. Negligible amounts of volatile fatty acids were detected in the sludge at the bottom of the SST. Samples of raw sewage, RAS and effluent from the SST contained less than 0.1 mg/L nitrite, whilst the latter two samples consistently contained 17-20 mg/L nitrate. Stage la was plagued by heavy rainfall totalling 208 mm. Actinomycete filaments were always concentrated in the foam compared with the mixed liquor and the RAS. When stage 1 wa{ repeated (Table 4, lb) there was no significant rainfall. The parameters assessed (viz. actinomycetes present, foam coverage, nitrite, nitrate and volatile fatty acids) were unchanged from those in the earlier trial. Table 4. Sludge Return Rates and Periods of Evaluation at Bundamba STP Stage

Dates

Ia lb 2 3

9/ 11/83-7/ 12/ 83 (early summer) 24/ 2/84-8/ 3/ 84 (late summer) 8/ 12/83-23/ 2/ 84 (summer) 9/ 3/ 84-5 / 4/ 84 (early autumn)

4.5 2.5 1.8 1.0 0.6

Tria l

Ditch

MLSS

Stage

number

mg/ L

I I I 2 I 2

(kg)

9.0 5.0 12.6 8.0 6.0

Achieved 217 266 102 155

300 300 100 150

± ± ± ±

('lo)

58 28 29 22

Table 5. Effect of Sludge Return Rates on Foaming at Bundamba STP

2

1-2 3-4 5- 11 12- 19 20-30

Actual Rale*

Desired(%)

• Relative to inlet flow

la Usage

Return Rate*

lb

I 2

2382 2572 2059 2053 2596 2487 2122 2035

Foam Rating ML SST

SVI

± 386 104 ± 12 ND ± 434 108 ± 19 ND ± 382 132 ± 50 I ± 282 ± 140 322 ± ± 91 338 ± ± 198 373 ± ± 137 398 ±

17 14 99 69

2

ND ND 1

ST mN/ n

SST

ML

70 .8 ± 2.3 66.7 ± 8.4 70.3 ± 4.8 71. 1 ± 3. I

2

65.5

71.7

0

72.0

69.8

Table 6. Effect of Return Anaerobic Digester Liquor and Solids on Foaming at Caloundra STP Trial

Stage I 2 3

36

MLSS mg/ L

988 ± 287 14 19 ± 484 1661 ± 504

WATER December 1990

RAS mg/ L

SVI

178 222 328

62 164 183

4604 4385 3995

1871 1424 988

ST mN/ m

Foam Raling

ML

RAS

ND 9

ND ND 2.0 ± 0.6

ND 1.7 ± 0 .5

ML 64.5 57.1 67.7

SST 3.5 8.8 7.2

68.6 57.6 64.4

4.3 12.8 10.5


During stage 2, with only 100% recycle, foam coverage was 25% to 50% of the aeration tank surface and was about 5-20 cm deep. Effluent quality was high during this period. Table 7a. Dosing of Sybron/Gamlen Cultures to the Bumdamba STP Re turn Activated Analysis

Suspended Solids, mg/ L Day I Day 22 Day 29 Day 44 Surface Tension, mN/ m Day 1 Day 22 Day 29 Day 44 Foam rating Day I Day 22 Day 29 Day 44

M ixed Liquor

Sludge

1640 1940 2390 2480

2280 2570 3930 3850

64.3 68.2 71.7 71.1

64.8 68.5 71.3 69.5

1 I 2 2

1 0 2 2

Table 7b. Dosing of Sybro n/Gamlen Cultures to the Carole Park STP Return Activated A nal ysis

Suspended Solids, mg/ L Day 1 Day 22 Day 29 Day 44 Surface Tension, mN/ m Day 1 Day 22 Day 29 Day 44 Foam Rating Day 1 Day 22 Day 29 Day 44

Mixed Liqu or

Sludge

8140 2140 3980 2820

13760 2280 4660 7180

41.2 47.3 61.3 62.9

55.8 62.2 65.7 60.0

3 0 1 0

3 I 2 1

Insignificant levels of volatile fatty acids were present in the samples of sludge from the bottom of the SST, nitrite levels were mostly below 0.1 mg/L while nitrate levels in samples of RAS and effluent from SST ranged from 9-23 mg/L. Raw sewage contained less than 0.1 mg/L nitrate. On two occasions the nitrite levels of the RAS were 0.3 and 0.5 mg/L respectively. Stage 3 of the trial evaluated a return activated sludge flow rate of 155 OJo of the raw sewage flow rate. The sludge age increased to 46 and 49 days respectively for the two ditches. Coinciding with this was an increase in foam coverage which over time reduced slightly. Overall , the foam coverage was relatively unchanged from that during stages 1 and 2 o f the recycle trial. Volatile fatty acids, nitrite and nitrate levels were not determined during this stage. Table 5 summarises some of the data collected during these trials. The actinomycetes observed in the foam were isolated and identified as Nocardia amarae and "Nocardia pinensis" (Blackall et al., 1989). Discussion: Operators at some sewage treatment plants have noted that continuous recycle of sludge rather than intermittent recycle was less conducive to foam formation. As well, Hartley (1982) postulated that foaming may be involved in phosphorus uptake. The anaerobic zone necessary for enhanced phosphorus uptake was thought to occur in the sludge blanket of the SST aeration tank with the aerobic zone being the aeration tank . By increasing the recycle of the RAS, the time spent in the sludge blanket was reduced. Although three return rates were evaluated, there are no real differences in the foam coverage on the aeration tanks while microscopically the population of actinomycetes did not change. The levels of nitrate in the bottom of the SST were consistently high and this has a deleterious effect on phosphorus accumulation (Osborn & Nicholls, 1985). Therefore, the trial results suggest that the phosphorus removal theory does not provide an explanation for the competitive growth of actinomycetes in activated sludge plants. The duration of the various recycle rates may have been too short to elicit any response if a biological foam was produced, return

to a foam-free state via high recycle of Rl'\S would take a long time. Three plants examined by Lechevalier (1975) had recycle rates of about 45% whilst one operated at 25% recycle. Only one of the plants operated at 45% was affected by foam whereas the other two were not. To ascertain the role of phosphorus in the metabolism of actinomycetes, pure culture studies are required. Although operators note less foaming in plants with continuous recycle of the RAS, we could not produce any effect on the aeration basin coverage by maintaining continuous recycle or by altering the rate of recycle. Although continuous recycle of RAS has been correlated with alleviation of foaming, it appears some other variables, perhaps related to sludge age, may be affecting the growth or foaming ability of the actinomycetes.

EFFECT OF RETURN OF ANAEROBIC DIGESTER SUPERNATANT AND SOLIDS Caloundra STP is a conventional activated sludge plant comprising grit removal, pre-aeration, primary sedimentation, aeration, secondary sedimentation and incorporating primary and secondary anaerobic digesters to handle the waste sludge. In plan the aeration tank was square in shape, aeration was by vertical surface aerators and the sludge age was· 10 days. The return of anaerobic digester components to the plant was carried out following three protocols: • return of anaerobic digester supernatant only to the aeration basin for 30 days (i.e. normal operation) • no anaerobic digester components returned to the aeration tank for 62 days, and • return of anaerobic solids with minimal supernatant to the aeration tank for 92 days. The solids came from the bottom of the secondary digester and from the primary digester. Problems were encountered as a result of the increased flow to the plant during a holiday period and there were also plant breakdowns. Before the initiation of each phase of the trial, the foam coverage was at a significant level. Normal operation of the plant included return of the supernatant from the digester to the aeration tank . There were therefore no changes to the plant for the first stage of the trial. The foam prior to the commencement of the trial covered most of the aeration tank surface; the areas free of foam were the highly turbulent regions around the surface aerators. At tlte beginning of the trial, one of the SSTs was taken out of use with concomitant reduction in MLSS in the aeration tank. Foam coverage was reduced to about 30% of the aeration basin surface. Foam filled the central baffled region of the SST but the effluent quality remained acceptable. After some heavy rainfall midway through stage 1 (133 mm in 2 days), the foam virtually disappeared from the plant. It had begun to build up again when the recycle pump failed and RAS was wasted along with the waste sludge. MLSS levels fell dramatically, as did foam coverage. On return to normal operation and as MLSS began to recover, the foaming also increased and blooms of foam followed morning peak flows. Actinomycete filaments were present in the samples with filament concentration in the foam higher relative to values in the mixed liquor or the RAS. The organisms were isolated and identified as Nocardia amarae. When the foam disappeared from the plant, the presence of the actinomycete filaments in the mixed liquor was still very obvious. During this stage of the trial there were few detectable volatile fatty acids in the raw sewage, mixed liquor, effluent from the SST, and the return activated sludge. The levels were insignficant and, in the RAS, only valeric and caproic acids were detected, at levels less than 0.2 mmoles/L. Stage 2 of the trial meant that the plant operator had to waste to the sludge drying beds more frequently to cope with the increased anaerobic digester load. The sludge drying beds became clogged with fine particles in the supernatant and an alternative method of wasting supernatant from the digesters was required. It was pumped to the head of the plant as no other arrangement could be facilitated. The increased holiday flows exacerbated this problem. The SST that had been taken out of use at the beginning of stage 1 was put back into commission at the beginning of stage 2. During this phase of the trial foam was present in the corners of the aeration tank and in strips between the aerators. Foam increased with the peak daily flows and then dissipated with the lower flows throughout the day. Over the period of the trial it was noticed that degree of foam coverage followed the solids levels in the plant. As the solids rose, foaming increased and as the solids levels dropped, the foam also subsided. Although there were changes to the foam coverage WATER December 1990

37


at the plant during stage 2 of the trial, they could not be associated with the changes to the anaerobic digester supernatant flow. Change was linked to the solids levels in the mixed liquor. The microscopic appearance of the foam, mixed liquor and the RAS did not differ from that of the first stage of the trial. No volatile fatty acids were detected in samples taken during this stage. Stage 3 consisted of two phases . Initially, 80 kL of solids and supernatant from the bottom of the secondary digester were returned to the aeration tank at 40 kL per day for 2 days. Foaming was monitored after this addition. After 4 weeks, 80 kL of digester solids were again added to the aeration basin at 8 kL per day for 10 days. Again the foam coverage was monitored. The correlation between foaming and MLSS was again noticed in this phase of the trial. As the anaerobic digester solids were added, the MLSS rose as did the foam coverage. A heavy spell of rain (130 mm over 2 days) midway through this phase eradicated the foam temporarily. Following the rain, the MLSS levels dropped. No volatile fatty acids were detected in samples taken from the mixed liquor, the lower part of the SST, RAS and influent. The microscopic appearance of the foam was unchanged from that in the first stage of the trial with respect to the actinomycetes present. When the solids levels built up as a result of the addition of anaerobic digester solids, the activated sludge did not appear as healthy. There were more free swimming bacteria in the MLSS but the floes were not affected. The average pH of the aeration tank for the duration of the trials was 7.25 ± 0.12 whilst that of the raw sewage was 7.90 ± 0.21. For stages 1, 2, 3 respectively, average daily inflows to the plant were 6566 ± 1235 kL, 7046 ± 889 kL, and 6130 ± 813 kL: sludge ages were 8 ± 2 days, 15 ± 12 days and 14 ± 6 days: DO, values were 2.32 ± 0.95 mg/ L, 1.91 ± 0.20 mg/ Land 1.92 ± 0.12 mg/ L. Other data from the trial are summarised in Table 5. Discussion: Lechevalier (1975) noted a correlation between return of anaerobic digester supernatant to the aeration tank and reduction in actinomycete-based foam . He concluded that a thermolabile component that was toxic to Nocardia amarae was associated with the solids from anaerobic digestion and that the component was poorly soluble in water but soluble in organic solvents. The Caloundra STP conventionally returns untreated anaerobic digester supernatant to the aeration tank, yet the plant is still affected by significant foaming episodes. This indicated the doubtfulness of return of anaerobic digester products as a control strategy. Operational difficulties precluded an efficient test of the effect of no return of anaerobic digester products because of problems in the sludge drying beds. Nevertheless, there appeared to be no alleviation in foaming in the plant as a result of dosing with anaerobic digester products. Solids levels, influent flowrate and rainfall, however, were noted as influencing the rate of formation. The hypothesis has also been tested at other plants (Ferguson, 1980) with mixed success. The poor results imply that this mode of control is not recommended. EFFECT OF ADDING COMMERCIAL BIOLOGICAL PRODUCTS Bi-Chem 1004TX is described by the manufacturers as "non-ionic detergent bacteria" whilst both Bi-Chem 1008SF and Bi-Chem 1004TX are described as" .. . bacteria that have been chemically mutated and, in the case of 1004TX, developed for surfactant degradation". The suggested application of 1004TX is for wastewaters containing foams, ethoxylated phenols, detergents, nonionics, anionics, cationics and Nocardia. The suggested application of 1008SF is to improve settleability and to improve treatment of wastewaters containing H2S, sulfur compounds, foams, oil, grease or increased MLSS. Trials were carried out at two treatment plants. a. Bundamba STP This is a carousel design oxidation ditch receiving more than 4 ML of wastewater per day. The dosing regime was followed as given in Table 2. Initially, foam covered 50% of the aeration basin surface, however, after 4 days of dosing, the foam was reduced to 25% and after 6 days it covered 12.5% of the ditch. After 17 days of dosing, the foam began to increase and at day 31 was up to 40% coverage. Two weeks after dosing was completed the foam had not increased any further. The effluent quality was high for the duration of the trial and no operational difficulties occurred as a result of dosing. A thin foam was present on the SST after dosing had been completed. This had not been present prior to dosing. Data collected during the trial is summarised in Table 7a. The microbiological appearance of the foam and mixed liquor was monitored and the predominant actinomycete was Nocardia pinensis (Blackall et al., 38

WATER December 1990

1989). There were higher numbers of actino~cete filaments in the foam than in the mixed liquor in every situation. b. Carole Park STP This plant is a conventional activated sludge plant that treats around 4 ML of wastewater per day in two aeration tanks. A prolific foam was present on the aeration tank and SST at the beginning of the trial while the predominant actinomycete was N ocardia amarae. There were many other microorganisms present in the foam and in the mixed liquor. Specifically, there were many Gram-positive organisms and free-swimming bacteria in unstained preparations of the mixed liquor. Mechanical problems in operation occurred during the trial. The dosing regime was followed as per Table 3. Foam coverage of the one aeration tank was 70% at the start of the dosing. Two SST's had foam in the central baffled regions and over the surface of the tanks. High levels of suspended solids were present in the effluent as a result of the foam. After day 10 of dosing, there was no change to the area of the foam and it was a dark, oily, heavy mass. Foam gradually reduced to 50% coverage and, on day 16 of dosing, a third SST was made operational. According to the plant operator, the MLSS had increased over the trial duration and the extra SST alleviated this. After 21 days of dosing, a second aeration tank was brought on line and foam covered 30% of the old basin and was negligible on the new. MLSS levels were again reduced as a result of the extra aeration tank. After 29 days of dosing, foam covered 70% of the old basin and 35% of the new one even though the MLSS levels had not risen. The SST foam coverage had gradually reduced over the trial period and at day 29 only floating surface matter was present. The central baffled regions were still full of foam. Two weeks after dosing was completed the foam covered 70% of both aeration tanks. Results for the trial are summarised in Table 7b. Discussion: Although the exact specifications of the two products used could not be obtained, we were able to isolate a wide range of bacteria and fungi from both. The products contain mutated organisms which apparently cannot compete in terms of growth rate with the activated sludge organisms and are steadily washed out of the system. Consequently, the product has to be added continually to produce its effect. At the time' that the trials were carried out, the costs of addition of the products to Bundamba and Carole Park were A$68 and A$61 per day, respectively. Although reduction of foam coverage at Bundamba was observed during the initial high dosing period, t he foam returned when the recommended lower maintenance doses were applied . Operational problems at Carole Park provided this control strategy with a good test. Because many plants have operational problems, control strategies need to be effective under such situations. Surface tension results at Carole Park indicated that surfactants were present in the mixed liquor and RAS and that, as the trial progressed, the surfactant levels in the mixed liquor dropped (Table 7b). In the laboratory foaming apparatus, however, mixed liquor samples foamed over a range of surface tension values (Blacka,11 et al., 1990b). Higher doses of the product may have been more efficacious, however, this would increase the cost of the operation. Other products similar to the Sybron/ Gamlen ones have been evaluated in the UK (Goddard & Forster, 1986), however, more rigorous field scale data is required for these products. Our conclusion is that this method of control may help to remove foam, however, by itself it did not appear to be cost-effective. EFFECT OF OTHER DESIGN AND OPERATIONAL FEATURES a. Brackenridge STP At this plant there are two aeration ba!lins. One is a conventionally operated, square, surface-aerated tank while the other is a circular, fine-bubble submerged aeration aerobic digester. The aerobic digester was converted to conventional activated sludge operation with a sludge age of 20 days whilst the other was maintained at a 2-3 day sludge age. The MLSS of the 20 day sludge age plant was maintained at about 2700- 3000 mg/ Land the 2-3 day sludge age plant at 1000- 1500 mg/ L. The foam coverage of the longer sludge age plant was consistently greater, and the effects on plant operation were more noticeable than on that of the shorter sludge age plant. The foam in the 2-3 day sludge age plant was virtually non-existent. Actinomycetes were predominant in the foam on the 20-day sludge age plant.


b. Redbank STP

This is a plant that incorporates a square aeration tank with surface aerators. The calculation of the sludge age was very . unreliable as the flow meter for the waste activated sludge pump was inoperative and the amount of waste sludge from the plant was unknown. Bulking was a problem at this plant most of the time. Although the MLSS values were in the range 800-1200 mg/ L with an average of 1032 Âą 160 mg/ L, the plant consistently suffered actinomycete foams. The plant was monitored for 6 months with approximately two visits per month and the foam was always a significant problem. Alleged sludge ages were in the range 5 to 15 days with an average of 8 Âą 3 days. Actinomycete filaments were predominant in the foam at each visit.

e

AERATORS

IN

SCU M IN REGIO NS OF LOW TURBULENCE

c. Capalaba STP

This plant is of conventional design operated at a sludge age of greater than 8 days. The aeration tank is square while dissolved oxygen is provided by submerged, coarse bubble diffusers operated from a variable speed compressor in an intermittent manner. The air imparts a spiral flow pattern to the mixed liquor that directs it to the overflow weir. The aerators are located along the sides of the aeration tank and push the liquor toward the centre, then in a spiral flow down the tank to the weir. Although most aeration tanks are baffled to prevent the foam from entering the SST transfer channel, this plant is unbaffled. The wasting and return of sludge from the bottom of the SST is continuous and the sludge age is accurately and regularly calculated. DO is measured with a DO probe and meter that are connected to a chart recorder, which is constantly monitored. A subjective foam rating system is maintained. During the period of observation, a primary sedimentation tank and two centrifuges for sludge handling were commissioned. The sludge wastage was no longer continuous and the sludge age was significantly increased to greater than 50 days. Although solids levels were maintained at about 3500 mg/ L, the mixed liquor did not foam in the laboratory foaming apparatus (Blackall et al., 1990b), the surface tension of the mixed liquor was similar to that of water and there was no foam on the aeration basin surface. Actinomycete filaments could be found in the mixed liquor frequently although there was no foam at the plant. Operation of the plant by intermittent aeration allowed the foam to collapse during times when the aerators were off, while the lack of baffles allowed the foam to leave the aeration tank as rapidly as it was formed. The control over DO and sludge wastage and return, and the strict monitoring of the sludge age assisted in the efficient operation of the plant and the relatively foam free status that it enjoyed.

Figure 1. Schematic diagram of Carina No. 1 aeration tank and clarifier showing regions of low turbulence with scum accJJmulation and regions of high turbulence with no scum.

e

AERATORS. NO REG ION WITH LOW TURBULENCE

OUT

IN

Figure 2. Schematic diagram of Carina No. 2 aeration tank and clarifier showing that with no regions of low turbulence, the tank is scum free.

d. Thornside STP

This plant shares some of the design features of the Capalaba Sewage Treatment Plant such as aeration control and diffused aeration. The mixed liquor was not directed toward the overflow weir because the aerators were located down the centre of the aeration tank and the liquor was directed to the sides of the tank. A large amount of foam was present on both the aeration tank and the SST. When it was observed microscopically, however there were no actinomycetes and the foam resembled concentrated mixed liquor. The MLSS was about 6000 mg/ L, the mixed liquor surface tension was 72 mN/ m and it did not foam in the laboratory foaming apparatus. The operator noted that the aeration was not satisfactory for the load to the plant and that the solids were being wasted erratically. It appeared that there was excessive air being introduced to the mixed liquor which led to the foaming. Better control over the aeration coupled with reduction in the MLSS and regular sludge wastage alleviated the problem.

Figure 3. Schematic diagram of Tingalpa No. 1 aeration tank and clarifier showing few regions of low turbulence with accumulation of foam.

e. Carina and Tingalpa STPs

At each of these sites there are two circular package plants employing coarse bubble diffused aeration and treating identical wastewaters (Figures 1-4). The central portion of these plants is used for secondary clarification. At each site, one tank appeared to be significantly more affected by foaming than the other. At Carina the foam affected plant was referred to as Plant Number 1 whilst the unaffected plant was Plant Number 2. At Tingalpa the reverse coding was adopted. Solids levels, surface tension and foaming ability in the foaming apparatus were not markedly different (Table 8). The arrangement of the aerators differed between the tank with foam and that without (Figures 1-4). In the tank which was affected by the foam, surface "dead zones" exhibiting low turbulence were observed. The foam was being established and stabilised in these regions whereas, in the tanks that were turbulent 40

WATER December 1990

Figure 4. Schematic diagram of Tingalpa No. 2 aeration tank and clarifier showing many regions of low turbulence with accumulation of foam.

over the entire surface, no foam initiation or stabilisation was possible. The foam reservoir provided by the quiescent areas is clearly a major factor in allowing the foam to build up at regular intervals. Discussion: At Brackenridge STP, a tank operated with a longer sludge age and higher MLSS levels suffered from more severe and frequent foaming episodes. This confirms one of the first observations made by Lechevalier (1975), that the foam problem


Table 8. Parameter Values for Carina and Tingalpa STPs Sample

Suspended Solids mg/ L

Surface Tension

m Im

Foam Rating

2870 4990 2380 3930

69.3 65.9 67.8 71.1

2 2 I I

2380 4350 3330 5670

67.3 71.8 72.3 62.9

I 2 2 2

Carina MLI RAS! ML2 RAS2

Tingalpa MLI RASI ML2 RAS2

occurred when the mixed liquor was concentrated. This observation has been made since then and is a frequent comment by the operators. Although Redbank STP was operated at short sludge ages, it still suffered from foaming. Therefore, although the trend is toward fewer problems at low sludge ages and low MLSS levels, this is not always the case. Other operational parameters must be considered and obviously the foam is able to be present at low MLSS levels and shorter sludge areas. Capalaba STP was operated at long sludge ages and relatively high MLSS levels. This shows that although operation under these conditions generally produces foam, it is not always the case. It proves that if treatment objectives dictate a long sludge age then it can be achieved in a foam-free manner. Operational features that assisted foam control included: intermittent aeration; an aeration design configuration which produced specific hydraulic flow patterns that precluded the foam from being stabilized and/or trapped; accurate and regular sludge age calculations and strict adherence to a preset value; continuous wasting of sludge from the SST, and accurate, continuous measurement of DO and amendment to the aeration if the setpoints were not being maintained. The above parameters were arrived at after experience had been gained in operating the plant. This proves that foam free operation can occur at supposedly undesirable values of sludge age and MLSS. Flexibility, for example in aerator operation and sludge recycling, assisted the control of foaming problems. Thornside STP showed that attention to detail in design is important. The location of the aerators in the centre of the tank significantly affected the control of the foaming. The formation of low surface turbulence was shown to be a most important and often overlooked feature of the aeration tanks. Correlation between these zones and foam formation was conclusively shown at the Carina and Tingalpa treatment plants (Figures 1-4).

CONCLUSIONS In conclusion, most control strategies that were evaluated were unsuccessful in eradicating or controlling the foaming events. Model studies incorporating the implementation of the more promising strategies should be carried out as well as the field scale studies. Trials at full scale are complex and efficient plant operation is required for accurate evaluation, of cause and effect. While some of the strategies appeared to assist in the control of foaming, none by itself was sufficient to ensure trouble free operation. Operation at long sludge ages and high MLSS levels can occur without foaming, but greater attention to process control is required under such conditions. Attention to the presence of quiescent zones on the surface of the aeration tank is important and eradication of these regions is encouraged.

ACKNOWLEDGMENTS We acknowledge the financial assistance of the Queensland Department of Local Government and Water Research Foundation of Australia. Linda Blackall was supported by a Commonwealth Postgraduate Research Award.

REFERENCES APHA,AWWA and WPCF (1985) Standard Methods/or the Examination of Water and Wastewater. American Public Health Association, American Water Works Association and Water Pollution Control Federation, Washington, D.C. Anon (1969) Milwaukee mystery: unusual operating problem develops. Wat.Sew.Wks. 116, 213. ATV Working Group 2.6.1 Report (1989) Prevention and control of bulking sludge and scum. Korrespondenz Abwasser 13th edition, 36, 165- 177.

Blackall, L.L. , Harbers, A.E., Greenfield, P.F. and Hayward, A.C. (1990a). Foaming in activated sludge plants: A survey in Queensla~, Australia. Accepted by Water Research. Blackall, L.L. , Harbers, A.E. , Greenfield, P.F. and Hayward, A.C. (1990b), Activated Sludge Foams: "Effects of Environmental Variables on Organism Growth and Foam Formation''. Accepted by Environmental Technology. Blackall, L.L., Parlett, J.H., Hayward, A.C., Minnikin, D.E., Greenfield, P.F. and Harbers, A.E. (1989) Nocardia pinensis sp. nov., an actinomycete found in activated sludge foams in Australia, J. Gen. Microbiol. Dhaliwal, B.S. (1979) Nocardia amarae and activated sludge foaming. J.Wat. Poll. Cont. Fed. 51, 344-350. Ferguson, E.H. (1980) Actinomycetes of sewage treatment plants. Paper presented to the Queensland Branch of Aust. Wat. Wastewat. Assn., 7pp. Gasser, J.A. (1987) Control of Nocardia foam in activated sludge by periodic anoxia. J.Wat. Poll. Cont. Fed. 59, 914. Goddard, A.J. and Forster, C.F. (1986) Surface tension of activated sludges in relation to the formation of stable foams . Microbios 46, 29-43. Heath, C.W. and Chan, J. (1985) Laboratory and plant scale trials using pickle liquor for Nocardia control in activated sludge plants. In Proc. /Ith Federal Conv. Aust. Wat. Wastewat. Assn, April 28-May 3. 1985, pp 346-353. Hiraoka, M. and Tsumura, K. (1984) Suppression of actinomycete foam production - a case study at Senboku wastewater treatmeni plant, Japan. Wat. Sc. Tech. 16, 83-90. Ho, CF. and Jenkins, D. (1990) The effect of surfactants on Nocardia foaming in activated sludge. Water Science and Technology 23, 879- 887. Hoffmaster, W.W. (1981) Foam control in aeration tanks Part II. Wat/ Engin. and Management. May, 34 & 39. Lechevalier, H.A. (1975) Actinomycetes of sewage treatment plants. U.S. Dept of Commerce NTIS Report No PB 245 914. Ludwig, K.L. (1981) Foam control in aeration tanks Part III. Wat/ Engin. and Management April, 34 & 39. Nelson, E.D. (1981) Foam control in aeration tanks Part 1. Wat/ Engin. and Management April, 63-64. Osborn, D.W. and Nicholls, H.A. (1985) Biological nutrient removal in South Afica. In Proc. llth Federal Conv. Aust. Wat. Wastewat. Assn, April 28-May 3. 1985, pp, 361-368. Pipes, W.O. (1978) Actinomycete foam production in activated sludge processes. J.Wat. Poll. Cont. Fed. 50, 628- 634. Pretorius, W.A. and Laubscher, C.J.P. (1987) Control of biological foam in activated sludge plants by means of selective flotation. Wat.Sc.Tech. 19, 1003-1011. Sezgin, M. and Karr, P. (1984) Controlling foam by lowering sludge age. Op Forum August, 28-29. Sezgin, M. and Karr, P. (1986) Control of actinomycete foam on aeration basins and clarifiers. J. Wat. Poll. Cont. Fed. 58, 972-977. Sezgin, M., Lechevalier, M.P. and Karr, P.R. (1988) Isolation and identification of actinomycetes present in activated sludge foam. Proc IAWPRCConf Microbial. Wat. and Wastewat. Newport Beach, California, USA. February 8-11, 1988. pp41-l- 41 - 7.

G. JACKSON and M. STEITIEH Continued from page 25 The relevant authorities have a responsibility to their constituents to be well prepared for reusing effluent at the appropriate time . To do this it is important that research, experiments and knowledge be improved so as not only to be aware of what is happening in this field but also to be active in areas of particular interest. The NT has over the past twenty odd years undertaken trials in the reuse of effluent and has several current schemes in operation. These need to be actively monitored. The public deserve to be kept informed of what is happening and prepared for changes. Community support will be required if reuse is to take its place in the water resources of the community. The economic situation will ultimately be the principal determining factor in reuse programmes so a detailed and accurate analysis of the overall true cost of 'fresh' water and reuse water is required. Pricing policies will ultimately need to reflect this economic reality unless distortions in the market are to be allowed to continue. Finally while it is argued that reuse is inevitable its timing will depend upon the relative strengths of the influencing variables for each location. For some this could be well into the next century while for others it should have been yesterday.

SHORT COURSE FILAMENTOUS BACTERIA IDENTIFICATION - with Professor David Jenkins and Bendigo staff Enough interest has already been shown, so the courses will be planned for the capital cities where final numbers warrant. People who would be interested are therefore asked to contact Bob Seviour at Bendigo CAE, PO Box 199, Bendigo, 3550, or telephone (054) 447 459 so that plans can proceed. Deadline: - mid February 1990

WATER December 1990

41

Profile for australianwater

Water Journal December 1990  

Water Journal December 1990