Water Journal March 1987

water FEDERAL PRESIDENT M. Dureau , Kent Instruments P/L P.O. Box 333, Caringbah 2229 5252811.

ISS N 0310-0367

Official Journal AUSTRALIAN WATER AND WASTEWATER ASSOCIATION

Vol. 14, No. 1, March 1987

FEDERAL SECRETARY G. Dooley, Box A232 P.O. Sydney Sth ., 2001.

FEDERAL TREAS URER J . D. Molloy , C/- M.M.B.W. 625 Lt . Collin s St., Melbourne, 3000.

BRANCH SECRETARIES Canberra , A.C .T. M. Sharpin , Willing & Part. ,

P.O. Box 170, (062) 815 811

Curtin , A.C.T. 2605.

New South Wales M. Hannon, P.W.D. Sewerage Branch, 74 Phillip St. , Sydney, 2000.

(02) 228 4488

CONTENTS Viewpoint .............. ...... . .... ............... .... ...... .

5

Association News, Views and Comments ....... ......... ........ .

6

IA WPRC News .............................................. .

9

Index-Water Volume 13, 1986 . ............................ . ... .

10

Studies of the Trophic Status of the Brisbane River Estuary -A. J. Moss . . .... . ..... . ........................ '. ...... .

11

Mixing in Anaerobic Sludge Digesters -C. K. Hertle and M. L. Lever . . .... ........................ .

16

Water Management in Open Cut Coal Mining-A Case Study - T. C. French .............. ... . ............... . ........ .

22

People and Contracts ................ . .. . .................... .

25

Experience with Manganese in Queensland Water Supplies -E.T. Loos . . ... ...... ... . ........................ ..... .

28

Phosphorus Precipitation with Pickle Liquor at Glenfield WPCP -I. Lim and T. Nguyen ................................... .

33

Victoria J . Park, Water Training Centre, P.O. Box 409, Werribee, 3030. (74 1 5844)

Queensland D. Mackay, P.O. Box 412, West End 4101 . (07) 844 3766)

t

South Australia A. Glatz, State Water Laboratories, E. & W.S. Private Mail Bag , Salisbury , 5108 . (08) 259 0243

Western Australia Dr B. Kavanagh , Water Auth . of W.A. , P.O. Bo x 100, Leederville 6007 (09) 420 2452

Tasmania G. Nolan , G.P.O . Box 78A, Hobart 7001 (002) 44 0600

Northern Territory M. Burg ess, P.O. Box 37283 Wlnn el lie, N.T. 5789.

(089) 39 7885

EDITORIAL & SUBSCRIPTION CORRESPONDENCE G. R. Gollin, 7 Mossman Dr., Eaglemont 3084 03 459 4346

COVER PICTURE The cover photo shows the new water treatment plant on the West bank of the Brisbane River at Mount Crosby. The first stage with a capacity of 250 ML/d was opened by the Lord Mayor, Alderman Sallyanne Atkinson on the 29th October, 1986. The treatment plant wh ich utilises the Dissolved Air Flotation process is the largest application of this process in Australia and was designed and constructed by the Brisbane City Council.

Picture and front cover donated by the Brisbane City Council.

The statements made or opinion s expressed in 'Water' do not necessarily reflect the views of the Australian Water and Wastewater Association, its Council or committees.

WATER March, /98 7

ASSOC/A TION • projects • use of new communications media • activities in developing countries • any other activity or initiative relevant to IA WPRC's international role in water pollution control.

The ANC Executive will be preparing a submission and members of ANC (and A WW A) are invited to send suggestions to the Secretariat in Sydney .

FUTURE PLANNING ANC Following the meeting with A WW A Exec. in March, the ANC will be formulating proposals for the future activities of the ANC. As the timetable for ·Biennial Conferences of IA WPRC is determined well in advance, it is unlikely that Australia would be approved as the venue until about the year 2000. Conference activities in the interim are therefore likely to be on the lines of the forthcoming co-sponsored Brisbane Conference or joining with regional ANCs in holding a regional conference. There are, however, other opportunities for members to participate in international activities as indicated by the list to be considered at Munich . Some Australian members are already involved with Task groups and Specialised Groups. The Executive would welcome any suggestions for ANC activities, bearing in mind that they need to be international or cooperative with national organisations such as A WW A.

•

NEWS

Planning for this Conference on Water Quality and Management for Recreation and Tourism is the responsibility of a local committee representative of A WW A and IA WPRC, which reports to the A WW A Council, the ANC and IA WPRC London as well as the Queensland Branch Committee on its progress.

The venue will be Griffith University at which low cost accommodation will be available. Up market accommodation will also be available in the city. There will be a mix of 'scientific' and 'management' papers and workshops . Arrangements for keynote speakers (including overseas specialists) have been made and a brochure calling for offers of papers and interest in attending has been widely disseminated . It is hoped to limit the registration fee to around $300 but still have a conference to international standards.

New Journal A quarterly journal 'Water Quality International' will commence in 1987. Articles concerning new projects, new processes, innovations, etc are invited from the Australian Water Quality area.

Australian manufacturers marketing overseas may take advertising space in the Journal. Members receive a lOOJo discount. Consultants wishing to practice internationally may be listed in the Journal

•

COMMENT

for an annual fee, again with lOOJo discount for members. Enquiries regarding the Journal should be addressed to the Journal correspondent (Leon Henry) or to the Secretariat.

Proposals for New Specialist Groups The following proposals are being considered by IA WPRC: Acid Rain, Agroindu s tries, Compu ter Technology, Estuarine and Coastal Pollution, Flocculation and Filtration, Industrial Water and Effluent Management, Marine Disposal, Macrophytes in Water Pollution Control, Wastewater Reclamation.

Existing Specialist Groups Health Related Water Microbiology, Instrumentation and Automation, Phosphate Removal, Urban Storm-Water Drainage, Systems Analysis, River basin Management, Large Wastewater Treatment Plants, Tastes and Odours , Anaerobic Digestion, Monitoring and Control of Contaminants .

Activated Sludge Model A 76 page report by the Task Force on Mathematical Modelling for Design and Operation of Biological Wastewater Treatment is available for perusal from the Secretariat.

L. HENRY

WATER VOL. 13, 1986

No. 1 - MARCH Australia's Overseas Aid Programme . .. ..... . . R. F. Goldfinch Water Supply for the Singida Region, Snowy Mountains Tanzania ... . .. . ... ..... ... . .. . .. Engineering Corporation Water Engineering in Asia - Problems and Solutions ..... . .... .. .. .. . .... . ... .... . . F. R. Bishop Overseas Projects Corporation of Victoria 'Water' - Index, Volume 12, 1985 Combined Sewer Implementation D. C . Hanrahan and Project - Shanghai .. . . . . .. . . . . . . . ... ... . .... M. F . Oddie Flow Measurement and Analysis in the Chang Jiang . .. ..... . .. . ..... .. .. . .. . . .. . .. ... . . . T . Beer Wastewater Disposal in Three Regional R.H. Edwards Cities in Thailand .......... ..... . .. .. .... and M. J. Hazell Village Water Supplies in Burma .. .. . ..... . ... H.F. Eggington Historical Malacca - Sewerage and Urban Drainage Strategies J. H . Crockett, T . J. Fricke, for the Future . .. ....... . .. . . . J. B. Murray and R. H. Smith No. 2 - JUNE Science and Technology in the Victorian Water Industry ..... . .... ... .. .. ... .. W. M . Drew The Olympic Dam Mining Project P. Nadebaum and - Water Management .... .. .... .. ... ... . ...... T. Amiconi Australia's Hydraulic Infrastructure A.G. Longstaff and - Planning for Renewal . .. . . . . ... . .. . .. .. . .. . F . B. Barnes Staged Oxidation of Sulphides in Wastewater Using Mechanical G. Williams and Aerators ..... . .. . ......... ........ . ... .. : ... . R. G . Shaw WATER March, 1987

VIEWS

Co-sponsored Conference Brisbane, July 10-15

INDEX

10

•

No. 3 - SEPTEMBER Augmentation of the West Pilbara Water Supply-The Harding R. J. Wark, G. C. Meink Dam Project .. .· . . . . . . . ..... . ....... . .. .. . and C . R. Tenby Industrial Waste Disposal in Victoria - The M.M.B.W. Proposal Cape Peron Environmental P. N . Chalmer and Monitoring . ..... . .. . ... . ....... .... ..... L. W. Edmonds Swampy Odour in the J.E . Wajon, B. V. Kavanagh, Drinking Water of R. I. Kagi, R. S. Rosich, Perth, Western Australia . . .. . . .. R. Alexander and B. J. Fleay Water Hammer A lleviation -A Y. H. Ng and Western Australian Case Study . . .. . .......... .. . A. J. Gale

No. 4 - DECEMBER Protecting Alice Springs S. Hancock, T. Fricke, Water Supply .... . . .. J . Crocket, R. Freyling and A . Bowden Water for Leisure . ... . .. . ..... . .. .. ..... . ... . .. J. Lawrence Rum Jungle Rehabilitation Department of Project - Update . . . . .. ........ .. . .. . . .. Mines and Energy Water Resources of the Northern Territory - Management at the Cross-roads ...... H. Watson A WW A - Preliminary Stategic Plan, 1987 to 1991 Water and Sewerage Administration in The Northern Territory - A WWA Submission to the N .T . Government

STUDIES OF THE TROPHIC STATUS OF THE BRISBANE RIVER ESTUARY A. J. Moss ABSTRACT The Brisbane River estuary passes through the centre of the Brisbane urban area and receives a large number of polluting discharges . Concern over the effects of planned increases in these discharges prompted an intensive water quality study of the estuary in 1983-1985. This paper presents results relevant to the estuary's trophic status together with comparative data from unpolluted Queensland estuaries. The main finding was that although much of the Brisbane estuary is highly nutrient enriched, due to adverse physical conditions, principally high turbidity, few if any significant eutrophication effects are apparent. Possible consequences of increased nutrient loading are discussed.

INTRODUCTION The Brisbane River estuary passes through the middle of the Brisbane urban area and, together with its tributaries, receives a large number of polluting discharges. Up to 1983, the Water Quality Council, the State Government body responsible for maintaining water quality in Queensland waters, had carried out low frequency monitoring of the estuary. Results showed that while existing dissolved oxygen levels were generally satisfactory, much of the estuary was significantly nutrient enriched. Little was known of the effects of these high nutrient levels but no serious eutrophication problems were apparent.

At that time the Water Quality Council was faced with the requirement of setting effl uent quality standards for two large planned treated sewage discharges to the Brisbane estuary and for this reason further studies were initiated. These included an 18 month intensive water quality study and some computer modelling of the fate of effluents . The main aims of these studies were: • to predict the effect of the additional BOD loads; • to examine the current trophic status/ behaviour of the estuary; and • to predict the effect of increased nutrient loads.

This paper is concerned mainly with the second aim consideration of the estuary's trophic status - but includes some discussion of the possible effects of increased nutrient loading.

DESCRIPTION OF AREA The Brisbane estuary (Figure 1) extends a distance of 84 km from its confluence with Moreton Bay to its tidal limit. It has one major tributary, the Bremer, which enters 73 km upstream . In its lower reaches the est uary is 300-400 m wide with mid channel depths up to 12 m. In the up per estuary, widths decrease to 100-150 m but depths up to 12 m persist as far upstream as 70 km, decreasing thereafter.

For much of its length the estuary is characterised by large tidal flows and high turbidity.

The lower estuary passes through the Brisbane urban area while further upstream it enters a less densely populated semi-rural zo ne. With the recent completion of the large Wivenhoe Dam, natural inflows from its major upstream catchment (55% of the total) are now severely restricted. Inflows from its other upstream catchments, the Bremer River (16% of the total) and Lockyer Creek

Andrew Moss, B.Sc.(Hons), M.Sc. is senior biologist with the Water Quality Section of the Queensland Department of Local Government. He joined the Department in 1974 and since then has been involved in a wide variety of water quality in vestigations.

A. J. Moss

(18% of the total) are less restric;ted but as both contain areas of intensive agriculture these inflows may be highly turbid.

The lower estuary is subject to continuous dredging for navigation purposes while in the mid and upper estuary there is extensive sand and gravel extraction .

MONITORING Intensive water quality monitoring of the estuary was carried out between October 1983 and March 1985. Dissolved oxygen, salinity, pH, temperature (depth profiles) and nutrients (surface samples for organic N, NO3-N, NH 3-N, filterable reactive P[FRP] and total P) were measured monthly, while surface chlorophyll and Secchi depth were measured twice monthly. A total of 27 sites covering the entire estuary at approximately 315 km intervals were sampled on each run. Net samples for phytoplankton were collected monthly at 10 sites. To ensure uniformity, sampling was always carried out at high tide. Sites and other features in the estuary are located in terms of distaflce (km) from the estuary mouth.

ANN UAL NUTR I EN T LO AD

= 100 000 Kg / Yr NorP 0(0

~

N

"

MORETON

luggage Pt

0

STP

Figure 1. Location map including major point source nutrient inputs.

WATER March, 1987

II

RESULTS AND DISCUSSION --Location of f res hwa t e,/salfwafer inter face - - - Secchi depth <: 0·2m • loca tion of turbidity maximum >< Location of secondary turbidity maximum

Hydrodynamic and Salinity Regime The nature of estuarine hydrodynamic and salinity regimes were strongly influenced by the characteristics of their freshwater inflows. In southern Queensland, climatic conditions give rise to streamflows that are typically episodic, long low flow periods are interspersed with occasional large storm flows. Low flow periods of 2-3 months are not unusual. Thus for much of the time, southern Queensland estuaries fall into Bowdens' (1980) category 3, that is, estuaries in which the ratio of daily inflow volume to tidal prism is less than 1:100. Such estuaries states Bowden, are characterised by a vertically well mixed water column and absence of a salt wedge. The Brisbane estuary clearly falls within this category. Inflows are usually very small in relation to the size of the estuary and salinity monitoring results indicate the water column is vertically well mixed throughout except immediately following storm inflows .

Because freshwater inflows are so relatively small, water movement in the Brisbane estuary is dominated by tidal effects and these provide the only significant mechanism for effluent dispersion. Mean tidal range at the mouth is 1.5 m and there are considerable tidal flows in much of the estuary (peak mid estuary tidal velocity for an average tide is approximately 0.65 m/ s). Such flows can achieve significant dispersion of pollutants but their physical loss from the estuary can only occur through net water exchange at the mouth. Water Quality Council modelling studies indicate that such exchange is only significant in the lower 7 kms of the estuary. Loss of pollutants from further upstream can only occur via dispersive exchange with the lower estuary, a relatively much slower process. Thus, effluents entering the mid and upper reaches are likely to have long residence times and a significant degree of assimilation may occur in the estuary itself. The low rate of new downstream advection in the estuary is illustrated by the fairly even distribution of nutrient concentrations, Figure 3, around the location of the large Donaldson Road sewage treatment plant discharge (47 km) .

Salinity during the study period is shown in Figure 2, expressed as the location of the saltwater/ freshwater interface. Salinity at the estuary mouth was always above 30 g/ L. Major freshwater inflows occurred during December 1983 and January, July and November 1984. Figure 2 indicates that while interface can shift fairly readily between 25 and 60 km, penetration of saltwater further upstream is relatively slow.

Turbidity

Turbidity in estuaries is a function of external particulate inputs and internal cycling and resuspension processes. A signficant fraction of catchment particulate input is trapped within estuaries and, depending on current velocities, a proportion of this trapped load is held in suspension, thus creating turbidity. As a result of estuarine hydrodynamic processes, maximum turbidity usually occurs close to the saltwater/ freshwater interface, regardless of its geographical location (Postma 1967; Morris et al 1982).

Tubidity in Queensland estuaries exhibits both temporal and spatial variation. Highest turbidities occur immediately following storm inflows. During ensuing dry periods, saltwater intrusion moves the turbidity maximum upstream and overall levels decline due to dispersion and coagulation and settlement of particulates.

Superimposed on this general pattern are considerable short term variations associated with the neap/spring tidal velocity cycle.

For the Brisbane estuary, comprehensive turbidity data is not available. Instead, Secchi depth, strictly speaking a measure of light penetration, is used here as a relative measure of turbidity in order to illustrate the estuary' s turbidity behaviour. Figure 2 shows both location of the turbidity maximum (Secchi minimum) and extent of the s0.2 m Secchi zone for individual surveys . The turbidity maximum location is fairly consistent with the saltwater/ freshwater interface but Secchi depths :S0.2 m often extend well up and downstream of this and it is clear that an extensive mid-reach of the estuary is characteristically highly turbid .

The wide extent of this zone is probably due mainly to high tidal velocities in this long estuary but the high absolute levels of turbidity may be a result of the large particulate load held in the estuary. There is historical evidence that in the 1920s/ 30s the estuary was much less turbid . At that time there was less land clearance and development within the catchment and particulate inputs and accumulated loads would have been less.

12

WATER March, 1987

-- -- --- --

0 1983

N

D J ~

8

F

M

...... ......

A

.....

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---

1984

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'

s ~

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1985 F

-

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10

20

30

40

50

> 60

• 70

Brisba ne es t uary, dis tance from mouth (km/

Figure 2. Salinity and Secchi depth data for individual surveys.

It would be expected that turbidity levels would decrease during dry periods but evidence of this in Figure f is inconsistent. During some dry periods, notably December 1984/ January 1985, the :S 0.2 m Secchi zone is reduced but during others, it exhibits considerable variability and on some occasions a second downstream turbidity maximum is present. So;na of this observed variability may be associated with the neap/ spring cycle but the presence of secondary maxima suggests that other factors are operative. One possibility is that the extensive channel and sand and gravel dredging in the estuary is affecting its turbidity regime. Unfortunately to quantify this would require cessation of all dredging for several months.

The turbidity regime of the Brisbane estuary is of interest because turbidity - fine inorganic particulates - is the principal factor affecting light penetration of the water column. The effect of turbidity on light penetration is measured here in terms of Secchi depth and mean Secchi values for the Brisbane estuary are given in Figure 3. Values are :S0.2 in the highly turbid mid estuary between 30 and 70 km. Values increase in the lower estuary but levels remain variable. The :S 0.2 zone sometimes extends as far downstream as 15 km while at other times, particularly during dry periods, Secchi values around 0.8 m occur in the 10-20 km reach . Clarity also increases in the upper estuary but to a lesser extent than the lower estuary . Even as far upstream as 76 km, mean Secchi values are less than 0.3 m individual values above 0.4 m are rare .

Nutrients

Nutrient regime: Under typical low inflow conditions , nutrient inputs to the Brisbane estuary are dominated by large point sources. The locations and relative size of the most important of these are shown in Figure I . These sources interact with hydrodynamic and internal cycling processes to produce a consistent dry weather nutrient regime, Figure 3. The largest point source is Luggage Point Sewage Treatment Plant but being situated at the mouth where net flushing rates are high it has only a relatively small effect on the estuary. In contrast, the second largest discharge, Donaldson Road STP (47 km) gives rise to high concentrations of both NO 3 -N and FRP (Filterable Reactive Phosphorus) in its vicinity . This occurs because the much lower mid estuarine flushing rates allow build up of concentrations over a period of time.

80

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Cheng 1985). In the case of P , some adsorption of soluble forms by inorganic particulates would also be expected and there is evidence,this does occur. Brisbane estuary sediments were found to be significantly p enriched with total p value ar6und 0. 1OJo (dry sediment wt,) between 10 and 70 km. This compares with maximum values of about 0.050Jo in unpolluted Queensland estuaries. The fact that a greater proportion of FRP is not adsorbed suggests that inorganic particulates in the estuary are P saturated.

Significance of nutrient levels: Comparison with a range of unpolluted Queensland estuary mean nutrient concentrations (Figure 3) indicates that much of the Brisbane estuary is highly enriched.

This suggests nutrient limitation of phytoplankton will rarely occur. This is probably true during winter months but in summer, N and P levels downstream of 20 km and upstream of 75 km sometimes fall within the unpolluted range and some degree of limitation may occur.

Inorganic N:Inorganic P ratios (by weight) are generally less than 5 throughout the estuary so that N is more likely to limit growth than P. Chlorophyll

Rang,s of

Chlorophyll data for the Brisbane estuary is given in Figure 3 expressed as log unpolluted mean (log x), upper quartile log mean Qld estuaries (u25 log x, log mean of the upper 250Jo of results) and maximum. Ranges of values Figure 3. Water quality data for the Brisbane River Estuary with characteristic of unpolluted Queensland comparative data from unpolluted Queensland estuaries. estuaries are also shown. Log x values inlevels of both NO,-N and FRP peak adjacent to the Donaldson dicate overall patterns while the u25 log x Road discharge but remain high upstream to the confluence with statistic is a useful measure of chlorophyll levels that occur under the highly enriched Bremer, decreasing thereafter toward the tidal favourable growth conditions. Thesl higher values are of parlimit. Levels also decrease toward the lower estuary. HH 3 -N levels ticular interest in relation to management. are low in the mid / upper estuary but increase in the lower estuary Chlorophyll is a measure of phytoplankton standing crop. In due partly to a large NH 3 -N discharge at 7 km. Neither organic N estuaries, crop size is determined largely through the interaction or particulate P are major effluent constituents and therefore of three factors , nutrients, light and hydrodynamics . Nutrient their distributions are less affected by discharges . Organic N levels availability, expressed here as concentration, has a direct inare only slightly above those of unpolluted Queensland estuaries. fluence on algal growth. Light availability within the water colAlthough low flow nutrient patterns within the estuary are fair- umn, expressed here as Secchi depth, sets a limit on achievable ly consistent, actual levels can vary considerably due to various primary production and hence affects overall growth . These two, factors . The flushing effect of large inflows may reduce nutrients factors interact to determine rate of increase of the standing crop to low levels and although patterns quickly re-establish, it can while hydrodynamic pr.ocesses determine the rate at which it is take several weeks or even months before concentrations build up dispersed. to equilibrium. Seasonal factors can also affect equilibria. NO,-N In the highly turbid mid Brisbane estuary, growth is strongly levels are generally higher in winter months due it is thought to light limited. Figure 3 shows Secchi values s0.2 over an extensive reduced rates of denitrification and to a lesser extent to reduced reach and a corresponding mid estuary chlorophyll minimum . N phytoplankton assimilation at the lower temperatures. A similar and P levels in this reach are high but between 20 and 65 km log x but less marked seasonal variation is evident for FRP. chlorophyll levels are less than 2 µg / L and evidently light limitaNutrient cycling: The high NO 3 / low NH 3 / low organic N status tion is so strong that little use can be made of available nutrient of much of the mid and upper estuary results from the effect of potential, high turbidity levels on N cycling. In most aquatic systems, upTurbidity decreases in the upper estuary and there is a cortake of inorganic N occurs at a higher rate than remineralisation responding increase in chlorophyll levels. However, although N of organic N(Jaworski 1971) . Thus, under equilibrium, organic N and P levels are high, chlorophyll levels lie well within the unis the dominant N fraction. This is certainly true of unpolluted polluted range and again, available nutrient potential is not fully Queensland estuaries and remains largely true in enriched utilised. This is probably because light limitation, although reducestuaries where increases in inorganic forms are paralleled by in- ed, is still significant. As far upstream as 76 km individual Secchi creased organic production. However in turbid estuaries such as values rarely exceed 0,4 m. Beyond 76 km, turbidity decreases furthe Brisbane, phytoplankton production is strongly light limited ther but N and P levels also fall to lower values. In this reach, and nutrient uptake becomes the rate limiting step in the N nutrient rather than light may sometimes limit growth, particular.cycle.Thus under equilibrium, N accumulates as NO 3 , the aerobic ly during summer months .

Turbidity also decreases in the lower estuary, downstream of 30 end point of the inorganic cycle, rather than as organic N. In turbid estuaries receiving large point source inputs, NO 3 may build km, and again there is a corresponding increase in chlorophyll up to high levels, as is the case in the Brisbane. Modelling studies levels indicating reduced light limitation. Log x chlorophyll values (Wofsy 1983) predict exactly this outcome in enriched turbid show only a small increase but µ25 log x and particularly maxwaters. imum values show significant peaks in the 10-20 km reach. This The dominance of FRP over other P forms probably similarly lower estuary chlorophyll pattern is a good example of the inoccurs due to light limitation. Under less turbid conditions, FRP teraction of all three factors (nutrients, light, hydrodynamics) would be rapidly taken up by the phytoplankton-(Simmons and controlling standing crops. Near the estuary mouth, standing Br isbane

Estuary , Di stance From Mouth

/km I

1quiva/ent values in

WATER March, 1987

13

crops are strongly limited by high tidal flushing rates while at 30 km light is strongly limiting. Flushing rates decrease away from the mouth while light limitation decreases downstream from 30 km so that optimum balance of these physical factors occurs in between, usually in the 10-20 km reach . N and P levels also decline downstream of 30 km and below 20 km a degree of nutrient limitation may sometimes occur. However, under present conditions, strong nutrient limitation is unlikely to occur anywhere in the lower estuary and therefore highest standing crops are located in the 10-20 km reach where physical factors are most favourable.

The absence of an equivalent lower estuary peak in the log x chlorophyll values reflects the fact for much of the time even the 10-20 km reach is significantly light limited. High chlorophyll levels develop there only under optimal light conditions, that is during summer months and only when turbidities are particularly low. Such optimal conditions occurred during December 1984/ January 1985. The upstream recession of the :5 0.2 m Secchi zone at this time is clearly visible in Figure 2. Nearly all the high chlorophyll levels recorded in the lower estuary occurred during this short period. At other times during the intensive survey ¡ period, levels were low.

Two significant lower estuary algal blooms recorded in other years exhibited behaviour consistent with that described above .

Both were centred in the 10-20 km reach and both occurred during low turbidity midsummer periods. One of the blooms gave rise to surface chlorophyll levels up to 180 Âľg / L. In general, chlorophyll levels in the lower estuary lie well within the unpolluted range and, apart from the two blooms noted, nutrient enrichment of the area has had little apparent effect.

Phytoplankton The Brisbane estuary net phytoplankton was dominated by Diatoms, these types being well suited to turbulent estuarine conditions . Near the mouth, coastal species were common while in the upper estuary, freshwater types such as Melosia occurred , penetrating right up to the tidal limit. The commonest type throughout was a Coscinodiscus sp.

Blue green algae were expected to occur in the Prich freshwater reaches during low flow summer conditions but were in fact rarely recorded. Blue greens are favoured by quiescent conditions where they can use their buoyancy mechanisms to advantage (Bowles 1980) . The apparently well mixed conditions of the upper estuary, as indicated by the dominance of diatoms, may have been physically unsuitable for them.

While the lower estuary is also normally dominated by Diatoms, the two algal blooms recorded in other years were due to motile species, one a Mesodinium sp. and the other a small (15 m) unarmoured Dinoflagellate. High chlorophyll levels associated with these blooms were due partly to phototactic movement of populations into the surface layers. Such blooms have only been recorded under optimum light conditions. Some degree of transient thermal stratification may also be involved . For most of the time high mixing rates and poor light conditions characteristic of the lower estuary probably prevent these motile types reaching bloom proportions.

Macrophytes No significant macrophyte populations were observed in the freshwater reaches of the main Brisbane estuary. Conditions are probably unsuitable for attached species due to a tidal depth variation of at least I m. Floating macrophyte populations have become established in the highly enriched upper reaches of some small tributary estuaries of the Brisbane River but it is thought that the larger tidal flows and lower nutrient levels in the main estuary prevent establishment of populations there. Certainly seed populations are not lacking as small mats of Salvinia sp. and Eicchornia sp. are frequently washed into the estuary during floods.

Macroalgae The sandy/ muddy substrate of the Brisbane estuary is unsuited to the growth of macroalgae. Some attached growth occurs on walls, piles, etc. but causes no significant problems.

14. WATER March, 1987

CURRENT AND FUTURE TROPHIC STATUS OF THE ESTUARY Based on biological indicators of trophic status, viz phytoplankton chlorophyll or macrophyte/ macroalgae biomass, the Brisbane River estuary cannot be categorised as significantly eutrophied. Much of the estuary is nutrient enriched but due to adverse physical conditions, principally high turbidity, this has had little effect. The only noticeable result to date has been an occasional bloom in the lower estuary when conditions are particularly favourable.

These findings might be used to support the conclusion that the estuary can assimilate unlimited increases in nutrient load without detriment. A similar tentative conclusion was reached by Heath et al (1980) in regard to parts of the Georges [liver estuary, Sydney.

However they suggested such conclusions should be treated with considerable caution because knowledge of the effects of nutrient enrichment of estuaries is limited.

In the Brisbane estuary, the reach between about 20 and 75 km is both significantly light limited and highly nutrient enriched and under current turbidity conditions it is unlikely that increased loading of this reach would have much effect. However, areas outside this zone are both less strongly light limited and less significantly nutrient enriched. In the upper estuary nutrient loading could result in algal blooms or the establishment of nuisance floating macrophyte species. Such upstream regions of Queensland estuaries are in general vulnerable to enrichment as they are poorly flushed in dry weather. Increased loading of the lower estuary could lead to more frequent or extensive blooms of the type already noted. There are also indications from current research on behalf of the Water Quality Council that nutrient rich water leaving the lower estuary may help to promote the red tide type blooms that have occurred in nearby areas of Moreton Bay. These blooms have resulted in fish deaths and any increase in nutrient input to the area would be undesirable.

It is also necessary to consider the possibility of reduced turbidity in the estuary. If for some reason, such as the cessation of sand and gravel extraction or the long term -silt retention effect of the recently completed Wivenhoe Dam, turbidity in the estuary declined towards levels reported early in the century, the degree of light limitation would be reduced throughout. In this situation, increased phytoplankton producti0n could lead to significant reductions in the levels of inorganic nutrients. As a result, some areas of the estuary that are currently light limited/ nutrient saturated might instead become nutrient limited. If this were to occur, and nutrient rather than light became the major growth limiting factor, more stringent controls on nutrient discharges might be considered necessary .

For long term management of eutrophication it would be desirable to have specific nutrient criteria for each section of the Brisbane estuary. However, the current level of understanding of estuaries is insufficient to permit confident definition of such criteria. For the present therefore, it is necessary to maintain a conservative nutrient control policy and to encourage relevant authorities to make use of modern biological nutrient removal processes in the design of new treatment plants .

ACKNOWLEDGEMENT The permission of the Director of Water Quality, Department of Local Government to publish this paper is gratefully acknowledged.

REFERENCES BOWDEN , K. F. (1980). ' Physical Factors: Temperature, Circulation and Mixing Processes in Chemistry and Biogeochemistry of Estuaries'. Ed. E. Olausson and I. Catto. John Wiley and Sons.

BOWLES , B. A. (1980). Physical Factors and Eutrophication. Proceedings of A WRC Workshop, Canberra, December 1980. Australian Government Publishing Service.

HEATH , C. W. , SMALLS, l. C. and CANNON, D . (1980). Some factors involved in the occurrence and limitation of algal blooms in an Australian estuary. Prag.

Wat. Tech ., 12, 421 -443.

JAWORSKI, N. A., LEAR, D. W. and VILLA, D. (1971). Nutrient Management in the Potomac Estuary. Amer. Soc. of Limnol. Oceonog. Special Symp.

MORRIS , A. W. , BALE, A . J . and HOWLAND, R. J . M. (1982) . Chemical liability in the Tamar Estuary, Southwest England. Estuar. Coastal and Shelf Science, 14(6), 649-661.

CONTINUED ON PAGE 37

MIXING IN ANAEROBIC SLUDGE DIGESTERS C. K. Hertle and M. L. Lever ABSTRACT This paper describes investigations carried out at several Brisbane City Council wastewater treatment plants into the effectiveness of mixing in anaerobic sludge digesters. Results are compared with experiences and theories presented in the technical literature.

The paper discusses factors which affect mixing efficiency including sludge characteristics, digester shape, natural and artificial mixing .

A number of conclusions are derived in relation to mixing of digesters, specifications for mixing equipment and a method for comparing the effectiveness of various gas recirculation mixing systems.

INTRODUCTION Concern about the performance of sludge digesters at different treatment plants in Brisbane prompted an investigation into their design and operation. Whilst there had been no digester failures the evidence of less than optimum performance was low gas production, instances of odour from the digested sludge and maintenance problems caused by accumulated silt and debris .

A survey by Swanwick' revealed that 62% of difficulties in operating anaerobic sludge digesters in the U .K. could be attributed to inadequate mixing. More recent publications 23 confirmed the important relationship between successful operation and effective mixing of digester contents .

There have been many reports of the active digester volume being only 500Jo of the geometric volume resulting in a lower true detention period and higher actual organic loadings. This can account for reduced process efficiency and may explain instances of complete digester failure•.

Tracer testing in Brisbane digesters revealed active volumes ranging from 440Jo to 880Jo. Accordingly, in-house investigations and a literature review were undertaken to develop a better understanding of the mixing phenomena. In addition to tracer testing, sludge rheology and design features were examined . The study concentrated on the gas recirculation method of mixing and used a parameter called the root mean square velocity gradient to compare mixing efficiency in different digesters.

No analysis of the costs of different mixing systems and tank design was undertaken .

TRACER STUDIES Tracer testing can be used to measure various mixing criteria . such as turnover time, dispersion time and residence time distribution. The latter is concerned with detection of short circuiting and estimation of the actively mixed volume and hence the true sludge detention period . This is a critical factor for successful digester performance since a minimum period of time is required to prevent process failure. It was possible therefore to explain variable digester performance using the residence time distribution found by analysis of results from tracer testing as described in the following section.

Selection of Tracer

A suitable tracer must be non-toxic, inert, easily detected in small concentrations, not normally present at a significant background level, have a high degree of recovery and be readily available and relatively inexpensive. Lithium hydroxide was found to meet these requirements and was used in the Brisbane tests. The concentration of Lithium in the exit sludge was determined by atomic absorption spectrometry and corrected for the background level.

Method

A pulse dose of tracer was dissolved in water and pumped to the digester together with enough raw sludge to purge the wet well and feed pipe. The weight of chemical added was designed to give an initial concentration (Co) of about 10 mg/ L bf Lithium based 16

WATER March, 1987

Mike Lever, M.Sc., A.R.A . C.l., M.I. W.P.C., is the Officer-in-Charge, Sewage Treatment, with the Brisbane City Council. Prior to joining the Council in 1977 he was employed by the North- West Water Authority and other Local Authorities in the U.K. Mike Lever

Chris Hertle, B.E. (Hons.) Chemical, M.I.E.Aust. is a graduate of the University of Queensland. He has been involved in a number of treatment plant investigations and in 1986 commissioned the sludge belt presses at Luggage Point. For the last two years he has been responsible for operating the Luggage Point treatment plant and is interested in all aspects of wastewater treatment.

Chris Hertle

on the geometric digester volume. For each trial the digester was maintained at a constant volume with daily sludge additions made as close to equal as possible. Samples of exit sludge were taken daily from the digester overflow pipe - more frequently on the first day starting about two hours after injection of tracer to indicate possible short circuiting.

A washout curve of dimensionless tracer concentration ' plotted. A smooth versus time from tracer atldition (t) was first exponential curve indicated the data would fit the definition of a continuous flow stirred tank reactor (CSTR) and that the theoretical mathematical models 1described by Cholette and Cloutier• and Levenspiel 6 for a pulse addition of tracer could be applied.

&

Description of CSTR Models

(b) For a perfectly mixed CSTR the theoretical and actual detention times are equal and the equation of the curve (see Figure 1) is as shown in equation [l] below. t C Co = exp ( - 7 ) [l] Where C = exit tracer concentration r = theoretical detention time based on geometric volume (b) Equation [2] shows the expression derived for a partially mixed CSTR. C V t.V [2] Co = Va exp (- r.va) Where V = total reactor volume Va = actively mixed volume (total volume - quiescent dead region) By plotting ~o against t on semi-log graph paper, Va can be calculated from the slope. (c) For a partial mixing situation with a portion of the flow short circuiting (q 2 ), the following expression was derived for the residence time distribution.

_g_ Co -- y_ Va

· [9J1 Q 2 exp (- !&) Va Where total flow, Q = q, + q,

[3]

By plotting ~o against t on semi-log graph paper, q, and Va can be calculated from the slope and the intercept. It is then possible to calculate the short circuit flow q, .

More complex models are described in references•·•. It should also be mentioned that in a extensive study of mixing models, Hall' has pointed out that difficulties can be encountered in interpretation of data, particularly if tracer recovery is low.

This corresponds to a partially mixed CSTR with a short circuit component and by substituting into equation [3] the active volume was calculated to be 74% with a short circuit component of 21 OJo .

Tests were also conducted at five other treatment plants to measure the residence time distribution.

1-5 ( a I Ideal Mixing ( b I Part ial Mixing ( c I Partial Mixing and Short Circuit

SLUDGE RHEOLOGY

0-S

10

20

30

t Id I

Figure 1. Tracer washout curve.

Results

The No . 1 digester at the Luggage Point treatment plant has unconfirmed gas mixing via two concentric rings of shearfusers. External power input is 2.7 Watts per unit volume (W/ ml) and the gas recirculation flow rate is 0.5 ml per hour per m 2 of surface area (ml/ m 2 .h) .

Results from tracer tests are shown in Figure 1 and the equation for the curve is : C [4] Co = 1.39 exp ( - 0.0437t) This corresponds to a partially mixed CSTR and by substituting into equation [2] the active digester volume was calculated to be 72'1/o.

The No. 1 digester at the Mount Gravatt treatment plant is mixed by unconfirmed gas recirculation through a diffuser manifold.

The unit power is 17 W / ml and the gas recirculation flow rate is 0.9 ml / m2 .h .

Results are shown in Figure 1 and the equation for the curve is: C [5] Co = 1.85 exp ( - 0.023t)

Information about the rheological behaviour of a sludge is an important consideration for the design and operation of digester mixing systems. However, only a limited amount of work has been done. 8 • 15 • The first reported studies were carried out by Hatfield in 1938 8 • He showed that sewage sludge has psuedo-plastic characteristics with slight thixotropic behaviour. Intensive tests by Campbell and Crescuolo• also showed a small but definite degree of thixotropic behaviour.

Buzzel and Sawyer'° agreed on the pseudo-plastic model but in addition found characteristics of an initial yield stress which indicated Bingham plastic behaviour. They found no true thixotropic properties though this is always a matter of degree.

Various other authors 11 • 14 have confirmed the pseudo-plastic behaviour and initial yield stress and this has been termed '' generalised Bingham plastic behaviour''.

Results from Luggage Point illustrate pseudo-plastic and Bingham plastic behaviour as shown in Figure 2. If a sludge is found to behave as a generalised Bingham plastic, continuous rather than intermittent operation of mixers is recommended . Unpublished results from a continuous versus intermittently mixed digester at an interstate plant appeared to confirm this premise .

The observed rheological behaviour will be affected by temperature, solids concentration, volatile content and gas bubble entrapment' l.

Mundoliadis and Bishop 14 showed viscosity was inversely proportional to temperature via an exponential relationship. Thus heated digesters will require considerably less mixing power to achieve turbulent flow conditions than unheated units .

Buzzel and Sawyer' 0 found the viscosity was influenced by the solids content of the feed sludge and digested sludge. For instance, the apparent viscosity was almost twice as high when solids increased from 20Jo to 30Jo. Furthermore, for sludges with a solids content greater than 3OJo , viscctsity increased with higher volatile solids contents . They also found that inflation of sludge mass by entrapped gas bubbles could reduce the effectiveness of any mixing devices which required a constant liquid level within the digester .

Finally, for adequate mixing of any fluids, particularly in a CSTR, the contents must be hydraulically turbulent. Without turbulence, plug flow regime prevails and effective mixing will not

3200

0..

-

Generalised Bingham Plastic

>-

·.; 0

u

VI

>

12 Shear

Rate

I r. p m l

12

Shear

Rate

( r. p. m)

Figure 2. Rheology of Luggage Point digested sludge.

WATER March, 1987

17

occur. Beca use of the initial shear strength of sewage sludges, laminar flow prevails at velocities · much higher than expected, typically up to 1 to 2 mis according to Thompson and Michaelson''.

TANK SHAPE AND SIZE Digesters are usually squat, circular concrete structures. This appears to be an inheritance from E urope and North America where tanks were traditionally built into the ground to conserve heat .

Aspect Ratio To reduce radiant heat losses, the best shape for a cylindrical tank is a height:diameter ratio of 1: 1. Furthermore, digesters with a high aspect ratio promote more efficient self-m ixing by minimising the area at the liquid sur face for sc um formation and maximisi ng the gas mixing flu x i.e. increases the rate of gas generation per unit surface area (m'l m 2.h). This is particularly important for digesters operated at high organic loadings and was a factor in the development of egg-s haped digesters 1 •.

Tank Size Although one large digester is comparably cheaper to construct than a number of small digesters, ideal mixing and minimum temperature and velocity gradients are easier to accomplish in smaller tan ks.

Three types of similarity are relevant for digester mixing, geometric, dynamic properties and kinematic factors. It can be shown by dimensional analysis that when mixing parameters are scaled down with geometric similitude, units are over-sized in terms of power and impeller flow, etc. This may explain the exceptiona l performance of pilot plants and laboratory scale units.

NATURAL MIXING An approximation of the useful power transferred to the liquid by the natural release of gas bubbles can be found using the equation for the work done during isothermal expansion of gas: [6] E = P, Q log, [~ ]

P,

Where E Q P2 P,

power (W) gas flow rate (m' ls) gas pressure at mean bubble release point (Pa) atmospheric pressure (Pa)

TABLE 1. NATURAL GAS MIXING Digester Shape Height (m} 6 6 9.5 9.5 15 15

Loading Rate (kg VM / m'.d)

Gas Flux (m' l m' .h)

Internal Power f rom equation 6 (Wl m')

I 2

0.12 0.20

0. 15 0.24

9.5 9.5

2

0.20 0.32

0.22 0.36

7.5 7.5

1 2

0.32 0.5 1

0.32 0.52

Diameter (m) 12 12

ARTIFICIAL MIXING SYSTEMS Effective mixing depends principally on the size and type of auxiliary mixing system provided. To su pport this, the mixing energy calculated by Zoltec and Gram 1 • in Los Angeles was 0.8 W I m' but this was found to be insufficient to optimise biomass efficiency and external power had to be provided. Similarly in Brisbane, external mixing was the dominant factor in achieving a high active digester vo lume but with nominal loading rates of about 1.0 kg VMl m 3.d the natural rate of gas evolution was only 0.12 m' l m 2 .h compared to a typical external supply rate of 0.8 m3 m2 .h .

The principal methods used for mixing digester contents are hydraulic pumping , mechanical agitation and gas recirculation.

Pumping involves turnover rather than dispersion, and short circuiting is possible - particularly with injudicio us inlet and outlet pipe configuration . Mechanical propeller-type mixers were originally designed to break up surface crust but experience has shown that to minimise operating and maintenance difficulties there sho uld be no moving parts or equipment inside a digester.

Gas recirculation does not have these problems and has been reported to give good dispersion 2°. Because of its apparent advantages, gas recirculation has been the preferred method of mixing digesters in Brisbane and the various alternatives available are discussed in some detail.

Hydraulic Pumping External pumped circulation in volves withdrawal of sludge from the digester by conventional or gas lift pumps and returning it at a lower point through a tangential inlet 21 .

Power requirements recommended for pumped circulation a re 5-8 W I m'. Pumps should have the capacity to turnover digester contents in 24 hours' 6 • Zoltek and Gram 1 • found that hydraulic mixing was not as effective as unconfined gas recirculation. It gave a similar performance to gas mixing using draft tubes but operating experience revealed greater susceptibility to upsets. Hydraulic pumping was fo und to be an unsatisfactory method of mixing digesters in Brisbane a nd is not recommended .

Mechanical Agitators Mixing by propellers, screw pumps and flat -bladed turbines is common in engineering applications and equations have been formulated to determine the rotational speed, diameter of the device and power requirements. However, because of the heterogeneous nature of digester sludge, ratings have been based on practical experience . An empirical rule for the power requirements for mixing digesters is 5-8 W I m' with units distributed in large digesters to avoid high intensity energy dissipation.

Mecha nical mixing is claimed by Nyns 22 to give similar mixing results to gas circulation. However, it is considered that they are better suited to open digesters where they are more accessible for mechanical maintenance a nd clearing rags from impellers. A mechanical mixer was installed in an open digester at Bracken Ridge in Brisbane to augment pumped circulation. T he effective digester volume as measured by tracer tests improved from 44% to 730Jo.

Gas Lift Circulation To illustrate these co ncep ts, the natural mixing intensity for three different digesters shapes have been calculated and are sho wn in Table 1. It is assumed that a plant has six digesters, each with an approximate vo lume of 670 m 3. Initially each is operated at a volatile matter (VM) loading of 1 kg VM l m 3.d and a detention period of 32 days. Half the digesters are then decommissioned and the three remaining digesters (each with a different shape) operate at a volatile solids loading of 2 kg VMl m 3.d and 16 days detention . Using a predictive model' 1 total gas production is estimated to be 2010 m'l d and 1620 m'ld respectively for the two different loading conditions.

It can be seen from Table 1 that self-mixing is maximised at high organic loadings and high aspect ratios. To endorse this point, Brade and Noone'" have also reported that with similar mixing equipment and nameplate power, the highest active digester volumes were found in units operated with high organic loadings.

18

WATER March, 1987

There are three principal gas recirculation systems for mixing digesters. T he first method involves forcing gas in sequence through a series of lances suspended from the digester roof. The second is by unconfined gas circulation via floor mounted diffusers and the final method is by confined release of gas within draft tubes.

Unconfined Gas Mixing Using Discharge Lances M ulti-point sequential gas recirculation uses a rotary valve to inj ect gas in turn into a set of discharge lances immersed to a mean dep th of 3-4 metres and located at about half-tank radius.

This system is intended to control scum and leave the bottom cone area of the tank undisturbed so that thicker sludges can be withdrawn. Alternatively, the system can be modified to complete mix by extending the lances to a greater depth and increasing the rate of gas recirculation - bu t at a cost of a high power input 23 .

With a lo w power operation i.e. 4 W I m', mixing is only effective

in the upper two-thirds of ihe tank and silt accumulation co uld cause problems.

For normal operation the manufacturer of the Pearth Gas Recirculating System (Envirex) recommends a gas flow rate of 2.0 m 3 m 2.h for small tanks using five lances and 0.5 m 3 / m 2.h for large tanks with twelve lances. Brisbane City Council is installing gas recirculation lances in a digester at the Oxley Creek treatment plant to evaluate this system.

Unconfined Gas Mixing Using Floor Mounted Diffusers

Free or unconfined gas circulation can be intensified to provide strong mixing. Open non-clog diffusers are located either in clusters or in a central ring of piers near the tank bottom approximately at the side wall depth. This system is used at Luggage Point Wastewater Treatment Plant.

Since with a free gas circulation system the bubble velocity is zero at the bottom, accelerating to a maximum at the surface, there is no direct pumping of liquid from the bottom of the tank 24 , therefore, it is recommended that the draw-off pipe be located at the bottom of the tank, near the centre, to prevent accumulation of silt. The parameters for good mixing recommended by Walker 25 are 0.3 m 3 gas per hour per m 3 liquid volume (m 3 / m 3 .h) and 8-12 W/ m 3 • Confined Mixing Using Draft Tubes A vertical draft tube is located in the centre of the digester . To minimise point distribution of the circulating sludge in tanks having a diameter greater than 15 m, more than one draft tube should be provided . Gas lift mixers can use either coarse or fine bubbles injected through pipes or diffusers confined within the draft tube 26 • Discharge pipes may be mounted from the digester roof so that they can be removed for maintenance without emptying the tank.

The gas release point can be as shallow as 0.6 x depth below the liquid surface so that in a digester with a liquid depth of about 7 m only a pressure of about 40-50 kPa is required at the discharge point. Turnover or liquid velocity can be increased significantly by increasing the depth of discharge point within the draft tube, e.g. 0 .85 of liquid depth with a minimum depth of gas release of 2.5 m from the bottom, though this requires a greater power input. Nevertheless, the discharge point should not rise above the mid-depth of the draft tube or drop within 0.3 m of the tube bottom. A gas flow rate of 0.4 m 3 / m 3 .h and a unit power of 7.5 W / m3 are considered adequate but the number of draft tubes, their diameter and the unit power input depend on the size of the tank 21 • Draft tube mixing causes pumping and facilitates strong bottom velocities which reduce silt accumulation. However surface turbulence is restricted by the confined gas release and, as liquid flows outwards, floating solids may accumulate in quiescent regions .

Normally the draft tube extends to 1.0 m from the liquid surface but at Oxley Creek Wastewater Treatment Plant the draft tube extends above top water level spraying liquid over the surface to help break up scum layers. The effective volume, measured by tracer tests is 88% and although the gas flow rate is only 0.23 m 3 / m 3 .h the release point is at a depth of 9.1 m, supporting the premise that the depth of gas release is decisive . However, the resultant power input is high , i.e. 11.2 W / m 3 •

a similar condition and the same compressor found that a confined gas lift system provided a 98% effective volume compared to 57% with Pearth unconfined gas mixing.

On the balance of evidence available in the literature and from unpublished studies of mixing in Queensland, including Brisbane installations 29 , together with information from other states, appears that there is little difference between the three systems provided that gas flow rates and power inputs are similar and consideration is given to depth of gas discharge and the number of release points in relation to digester diameter.

POWER AND GAS FLOW RATES Three parameters have been proposed for sizing gas recirculation systems i.e . nameplate power per unit vo ume of digester, gas flow rate per unit area or volume and the root mean square velocity gradient (Gm). Each of these criteria is inter-related and comparisons can be made between them if the gas discharge pressure and sludge viscosity are known 24 • It is claimed that effective mixing is achieved when the unit power input is 5-10 W/ m 3 • Walker 30 claims that a mixing power input greater than 13 W / m 3 may contribute to foaming problems.

Least power is required for sequential discharge lances (5 W / m 3 ) because depending on the depth of submergence they can operate at a lower pressure and manufacturers recommended a low gas flow rate (0.15 m 3 / m 3 .h). The highest power requirement is for free gas circulation. Recommended gas flows range from 0 . 14 to 0.30 m 3 / m 3 .h with a power input of up to 13 W/ m 3 required.

With confined gas mixing the recommended power input is about 8 W / m 3 which is higher than sequential lances because gas flow rates vary from 0.3 to 0.42 m 3 / m 3 .h.

An alternative approach is a recommended gas flow rate based on diameter. The FMC Corporation in the United States recommended 8.4 to 11.4 m 3 / m.h for free gas lift mixers 24 • Flow rates stipulated for Nash and Envirex sequential mixers are plotted on Figure 3 where at the point of intersection the gas flow rate is 13 m3 / m.h.

Bolivar • 800

Nash

600

///

1 ~

/

Woodman Point

>

.s

/

__/-./

200

---~-

/ ' _..-:---:::-

----

----

Envirex IPeartnl

/

!Perth! e

~

0 Luggage Point

FMC

/ 12

16

20 Dia.,eter 1ml

24

28

32

Figure 3. Gas mixing systems performance.

EVALUATION OF GAS RECIRCULATON SYSTEMS A review of the literature revealed conflicting evidence on the efficiences of the different systems. The conclusions from three papers can be summarized as follows.

Zolteck and Gram 19 showed that with equal gas flow rates, free gas circulation was more effective than confined gas mixing using draft tubes. However, for the former the gas was released through floor mounted gas outlets and more power was required i.e . 10 W / m 3 for draft tubes.

Baumann and Huibregtse 28 concluded that uniform solids could be maintained for each of the three systems if designed with the correct gas flow but confined and free gas circulation were restricted with regard to controlling floating solids. Sequential discharge lances were claimed to require least energy for effective mixing.

Carroll and Ross 26 using digesters having the same geometry, in

Degremont 3 ', a French based firm, use gas flow rates designed on surface area and recommended 0.8 m 3 / m 2.h which is in line with the average gas flow rate of digesters in Brisbane.

The root mean square velocity gradient is a more refined parameter as it is based on useful power transferred to the liquid and disregards losses resulting from motor, coupling and other heat losses which may amount to 75% of the nameplate power input18.

The parameter Gm is calculated from the following expression: Where and

Gm = ~ liquid viscosity, Pa .s V liquid volume, m 3 E power from the isothermal expansion of gas as discussed previously (liquid vapour pressure and kinetic energy of the gas are ignored).

/1-

WATER March, 1987

19

CONCLUSIONS From the material covered in this paper and in particular the investigations carried out on sludge digestion at Brisbane City Council Treatment Plants, the following conclusions can be drawn . (a) Good mixing is critical for achieving a high active volume and therefore satisfactory digester performance. It requires an understanding of theoretical factors and knowledge of sludge rheology . (b) Specifications for mixing systems should be directed towards dispersion and residence time distribution rather than inputs such as pumps rates, power and recirculated gas flows. Tracer tests should be conducted in clean water or sludge to check performance specifications. (c) Digesters having a height:diameter asp'ect ratio greater than 1.0 and operated at relatively high loadings with continuous artificial mixing are recommended unless precluded by constraints such as capital costs or ground support difficulties. (d) No one gas recirculation mixing system is superior to another providing they are properly designed in terms of power input and selection of gas release points in relation to tank diameter and depth. (e) The ' root mean square velocity gradient' Gm and actively mixed digester volume are recommended as the basis for selecting power input requirements for gas mixing systems.

80

.,

~

60

e ~ ....

-~ -c,

... .... ·;:; Cl

I!)

>,

40

0

ai

>

..."'

Cl ::,

CT

V)

a 20 "'

:I:

I

REFERENCES 40

80

60

100

Active Volume (¾)

Figure 4. Mixing efficiency.

A plot of active digester volume (found by tracer analysis) versus Gm is shown in Figure 4. It should be pointed out that in calculating Gm, the power input from circulating sludge through heat exchangers and natural internal mixing energy was not included and by convention the viscosity of water at 35°C was used .

The correlation coefficient was 0.96 (8 determinations) and hence the concept can be used as a first approximation to compare gas mixing systems having different gas flow rates and pressures in different sized tanks (see example calculations in Table 2 which are based on a digester having a diameter of 12 m, a depth of 6 m and an assumed compressor efficiency of 250Jo) .

Thus Gm can be used in conjunction with Figure 4 to estimate the active mixing volume without undertaking tracer tests and to evaluate design options such as reducing nominal tank capacity but maintaining the true detention period by selecting a high Gm value to optimise mixing efficiency. For instance, it can be predicted that by increasing the Gm value at the Luggage Point digesters from 37 s- 1 to 71 s- 1 , the active volume would be increased to 95% enabling 32% increase in throughput for the same theoretical detention time based on geometric volume .

It should be pointed out that Gm does not give a measure of shear stress distribution throughout the digester - shear stress will be highest near the mixing device''. Further investigations of tracer dispersion together with comparison of different gas recirculation systems are planned by the Brisbane City Council.

TABLE 2. GAS RECIRCULATION PARAMETERS System

Gas Flow Gas Flux (m'lm' .h) Q (m' l s)

Pressure P2 (kPa)

Nameplate power per unit volume (Wl m')

Gm (S-')

Active Digester Volume (from Fig. 4) %

Sequential Lances

0.028 0 .042

0.9 1.3

130 150

4 .2 10.0

38 59

71 86

Free Release

0.028 0.056

0.9 1.8

150 150

6.6 13.3

48 68

78 93

Confined

0 .063 0.056

2.0 1.8

135 150

11.0 13.3

62 68

88 93

20

WATER March, 1987

I. SWANWICK, J. D. et. al. A survey of the performance of sewage sludge digesters in Great Britain. Journal Water Pollution Control, No. 6 (1969).

2. NOONE, G. P. and BRADE, C. E . Some recent work on anaerobic digestion within Severn Trent Water Authority . Water Services, September (1979) .

3. MONTEITH, H. D . and STEPHENSON, J . P. Mixing efficiencies in fu ll scale anaerobic digesters by tracer methods. Journal Water Pollution Control Federation, Vol. 53 , No . I (1981).

4. TENNEY, M . W . and BUDZIN, G. J. How good is your mixing? Water and Wastes Engineering, May (1972).

5. CHOLETTE , A. and CLOUTIER, L . Mixing efficiency determinations for continuous flow systems. Canadian Journal of Chemical Engineering, June (1959). ' 6. LEVENSPIEL, 0. Chemical Reacton Engineering . John Wiley and Sons , Inc. New York (1972).

7. HALL, E . R. Flow Modelling and Mixing in Anaerobic Sludge Digesters : status report. Wastewater Technology Centre, purlingham , Ontario (1983).

8. HATFIELD , W . D . The viscosity or psuedo-plastic properties of sewage sludge .

Sewage Works Journal, Vol. IO, No . I (1938).

9. CAMPBELL, H . W. and CRESCUOLO, P. J. The use of rheology for sludge characterization. Water Science and Technology, Vol. 14 (1982).

10. BUZZEL, J. C . and SAWYER, C. N. Biochemical vs physical factors in digester failure . Journal Water Pollution Control Federation, Vol. 35, No. 2 (1963).

I 1. MICHAELSON, A . P. et. al. Sewage sludge pumping - recent research and application. Water Pollution Control, 81, No. 2 (1982).

12. PERRY , R. H . and CHILTON, C. H. Chemical Engineers' Handbook. 5th Edition McGraw-Hill, Kogakusha (1973).

13. ROSS, W. R . and SMOLLEN, M . Engineering problems with the anaerobic digestion of soluble organic wastes . In: Anaerobic Digestion . (Ed) Huges, D. E . et. al. Elsevier, Amsterdam (1982) .

14. MUNDOLIADIS, P . and BISHOP, P. L. Temperature effects on rheo logy of sludges. Journal of Environmental Engineering, ASCE Vol. I IO, No. I (1984).

15. THOMPSON, J. L. and MICHAELSON , A. P. Design aspects of the new anaerobic digesters at Bury . In: Sewage Sludge Stabilization and Disinfection.

(Ed) Bruce, A . M . , WRC Ellis Moorwood Ltd, Chichester (1984).

16. NELSON, E. D . and BALEY, J. F. The egg is the shape of things to come.

Water and Sewage Work, February (1979).

17 . LEVER, M. L. Refinements in the anaerobic sludge digestion process. 12th Federal Convention, Australian Water and Wastewater A ssociation (in press) (1987).

18. BRADE, C. E. and NOONE, G . P. Anaerobic sludge digestion - need it be expensive . Water Pollution Control, Vol. 80, No . I (1981).

19. ZOLTEC, J. and GRAM , A. L. High-rate digester mixing using radioactive tracer . Journal Water Pollution Control Federation, Vol. 47 , No. I (1975).

20. RUNDLE, H . and WHYLEY , J . A comparison of gas recirculation systems for mixing of contents of anaerobic digestion. Water Pollution Control, Vol. 80, No . 4 (1981).

21. INSTITUTE OF WATER POLLUTION CONTROL. Sewage Sludge I : Production, Preliminary Treatment and Digestion. Manual of British Practice in Water Pollution Control, Maidstone , Kent (1979).

22 . NYNS, E. J. et. al. Digesters, a worldwide review. In: Anaerobic Digestion.

(Ed) Stafford, D. A ., Wheatley, B. I. , and Hughes, D. E . Applied Science Publishers Ltd London (1980).

23 . MALINA, J . F . and MIHOLITSE, M. New Developments in the Anaerobic Digestion of Sludges. In: Advances in Water Quality Improvement. (Ed) Gloyna , E. F., and Eckenfelder, W. W. University of Texas Press, Austin (1968) .

CONTINUED ON PAGE 37

WATER MANAGEMENT IN OPEN CUT COAL MINING-A CASE STUDY T. C. French PRINCIPLES OF WATER MANAGEMENT

ABSTRACT Moura Mine was the first large scale Open Cut coal mine to operate in Queensland's Bowen Basin. As such, no precedents or procedures had been established in Water Management and a learning, trial and error process developed.

Water Management is now an integral part of the mining operation. This paper deals with the history, planning and operation of Water Management at T.D.M . Coal Pty Ltd, Moura Mine . The consequences of poor planning and benefits of organised planning are highlighted. The influence of environmental constraints on strategy of Water Management is discussed.

GEOGRAPHY T.D .M. Moura leases collectively extend 30 km north south and average approximately 2 km in width. the topography of the lease areas is gently undulating, draining from east to west. One large catchment and six others of significance are intersected by the coal croplines at near to right angles (Figure 1).

Rainfall averages 720 mm/ annum and pan evaporation averages 2200 mm/ annum.

T. C. French

Tom French is Environmental Officer at Thiess Dampier Mitsui Coal Pty Ltd, Moura Mine. He has 20 years experience in surface water management planning and control and for three years has had the responsibility of Water Management and Revegetation of the Moura Mine Site.

22

WATER March, 1987

CURRENT OPERA TIO NS AND PRACTICES In line with the objectives quoted , mining areas are protected from runoff ingress by diversion channels and bunds which guide this runoff to major water corridors through the mining strip. These corridors in the more recent mining areas are designed to cope with major flows (return period of 100 years). An example of one of these water corridors is a major

from inflow of water from the east at times of catchment runoff.

Consultants were engaged in the late 1970s and early 1980s to investigate and formu late a water management programme for the mine area. The reports resulting from these investigations are the basis of today's water management strategy.

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HISTORY When mining operations commenced in 1962, water management planning was minimal. Open Cut pits intersected water courses resulting in pit flooding at times of catchment runoff. Little significant interruption to production occurred in these early years and the situation was tolerated until a major runoff event in 1974 caused serious flooding and major disruption to coal production.

Pit water disposal during this early period was random . Salinity levels were not acknowledged and pumping over the highwall to the west was the usual dewatering practice . During the 1969-1970 drought period, landowners downstream to the mine area were supplied with stock water from pit areas. At that stage, stream pollution was of little concern and only became an issue when an irrigation licence was to be granted to a downstream landowner. It was then recognised that the salinity levels of pit water were in excess of those acceptable in agriculture and controls were applied to further discharges to the environment from mine pits.

In 1978, an agreement was formalised with the then Irrigation and Water Supply Commission for discharge of stored pit water under specified environmental conditions .However, the symptoms were being treated and not the cause and further disruption to coal production resulted

Runoff from catchments east of the mining area is to be guided through the mining area with minimum contamination from mine water.

Runoff from the mining area and thus subject to some contamination by salts of exposed elements will be contained in both disused pits and constructed storage dams for subsequent discharge to the adjacent watercourses subject to the conditions of agreement with the Queensland Water Resources Commission.

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creek diversion constructed in 1974 by use of a Marion 7900 Dragline. This corridor has been and remain s effective in carrying runoff from 30 000 ha through the pit area. The project involved the excavation of several million cubic metres of material and the construction of levees. The diversion is 5 km in length and has a maximum excavated depth of 20 metres.

With the advance in techno logy and market demands, 'old' pit areas previously considered 'worked out' will be reopened for further Open Cut mining.

These areas are not set up with water corridors and major work is necessary to ensure protection fro m eastern catchment runoff. Alternatives available are to cutoff the catchment with large dams, or construct an 'aqueduct' system to carry the watercourses through the mining area.

Both these alternatives are high cost.

Runoff from the mine area is significant and results in an annual accumulation of water in the order of 10 000 mL. This water is typically high in T.D.S. level and therefore cannot be discharged directly to the water courses . This necessitates storage of the water awaiting the specified conditions required for discharge.

STORAGES Three basic types of storage are utilised: (i) Open Cut Pit

Water is pumped from working pits to a disused pit adjacent to a natural watercourse . The pit is equipped with a high capacity pump (200 kW Flygt 52 10) capable of shifti ng 0.2 cubic metres per second . At times of suitable creek flow, the pump is activated and water is discharged to the watercourse.

Th is method has the disadvantage of being unable to discharge significant amounts of water during the limited duration creek flow s. A typical creek flow duration is 48 hours and therefore the maximum discharge possible in that time is 34.5 ML. (ii) Evaporation Pond

This method combines the principles of evaporation and staged discharge storage.

One such system was commissioned in 1980 to dewater a pit area containing approximately 1000 ML of water.

A low profile earth bund was constructed along a contour and closed at each end. Discharge pipes were installed in the bund. T he capacity of the 50 ha surface area pond is 500 ML. In theory, this system is capable of evaporating in excess of 500 ML per annum and has the added capability of discharge to the watercourse at suitable times. This would have ensured the total dewatering of the pit area within two years. In practice , however, unconsidered factors such as catchment area of the ponds, catchment area of the pits concerned and rising salinity of pond water due to evaporation have caused a nett nil loss of water in seven years. (iii) Constructed Dam Storage

T his method is currently in use but as yet not thoroughly tested. The principle is

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to use a large volume deep water storage which can be discharged rapidly when conditions are suitable.

The constructed dam has a capacity of 1300 mL and a discharge capability of 130 mL per day . In practice, this dam will provide sufficient storage for the anticipated perennial dewatering requirements of three pit areas. The dam has particular environmental significance in the fac t that it will provide stock water for neighbou ring graziers (TDS level should not exceed 4000 ppm).

DISCHARGE PROCEDURE As specified in Queensland Water Re sources Commission agree ments, discharge to the watercourses can only occur under definite conditions. These include flow rates of watercourses, T.D.S. levels, adequate water sampling carried out and Queensland Water Resources Commission notification .

To date, major waterco urse flow rates were assessed at a point 45 km from the mine site . This was inconvenient and at many times impossible due to road conditions and local flooding . As such , by the time the visual assess ment was made, many hou rs and at times, days of potential discharge were lost. To remedy this situation , the mining company has supplied a Telemar k interrogator to Queensland Water Resources Commission for installation at a river station nearby the mine site. This will allow telephone access to river height and hence flow rate.

To assess the flow rate of the watercourse in the mine area, a concrete gauging weir is visually inspected. The weir will gauge flows up to seven cubic metres per second . It is essential to accurately assess this flow to determine rate of discharge as the Queensland Water Resources Commission agreement specifies allowable ratios of pit water to creek water for individual storages.

To maximise discharge efficiency, creek flows must be accurately forecast and monitored . The majority of discharge opportunities occur as a res ult of storm rain not necessarily on the main site. To assist in forecasting creek flow, 1 : 'l gauges are

placed within the 30 000 ha catchment .

When storm rains occur, visual inspection of the catchment and rainfall measurements enable flow rate and duration predictions to be made and discharges planned hours ahead of time. As monitoring continues, hydrographs are produced for particular storms and enable more accurate forecasting (Figure 2).

It is planned to install electronic recording devices in place of the present rain gauges to increase the efficiency of this system . Electronic creek flow monitors will also be installed•to greatly assist in the production of flood hydrographs.

T.D.S. levels are monitored and water samples taken throughout the discharge period , upstfeam, within and downstream of the mine area and the Queensland Water Resources Commission officers are notified when discharge is intended.

DAY TO DAY PUMPING Local runoff seepage and natural ground water are dealt with on a day to day basis. Diesel powered Hanson Sykes UV08 HH skid mounted pumps, using 150 mm layflat discharge hose, pump pit water to storage facilities. Generally, the hose is lowered over the 'highwall' of the pit for connection to the pump in pit. The water is then discharged into open drains and gravitates to the respective storages .

LONG TERM PUMPING Situations arise which require prolonged or continuous pumping such as dewatering disused pits for control of seepage or reopening for mining.

In these cases, electric powered, pontoon mounted pumps are favoured. Currently, three such installations are in use and one on standb y awaiting installation.

Apart from the direct cost advantage, labour input is dramatically reduced and availability increased. Power to these installations is generally by trailing cable from an existing field power supply.

COAL WASHERY The Washing Plant water circuit involves the drawing of water from a supply WATER March, 1987 23

dam, the coal washing and wash down process within the plant and the transport of fine washings and slimes to a tailings pit. From the tailings pit, water is recovered after settling has occurred and returned to the storage dam.

The water circuit has a loss of approximately 800 ML per annum . Of this volume, 200 ML is shipped with the coal and the balance evaporated from open storage dams and recovery pits within the circuit (Figure 3).

Replenishment of this circuit normally occurs during summer rainfall on a 600 ha catchment. This has a twofold effect of topping up the volume, and diluting the circuit water. This dilution is an important factor in that rising T .D.S. levels mainly due to evaporation, cause increasing corrosion problem within the Washing Plant itself. In recent years, failing of summer rains, and hence absence of runoff, necessitated the top up of the circuit from pit water. This compounded the rising T.D.S. problem and hence aggravated the corrosion problem in the Washing Plant (Figure 4).

Alternatives for water circuit top-up include drawing from a weir on the river 18 km west of the mine through an existing but inadequate pumping system or treatment of pit water to reduce T.D.S. level and then injecting into the circuit. The treatment of pit water is the preferable but high cost alternative.

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WATER March, 1987

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The introduction of: (i) Transpiration as a tool in Water Management is planned for the near future. Consultant's reports indicate that transpiration through selected Eucalypt species can achieve a disposal rate of water of the order of twice that of evaporation. In a mining situation, planting of trees.on reclaimed areas of spoil is highly desirable as a revegetative practice.

When water disposal can be coupled with revegetation, the project becomes more attractive cost wise and ensures survival of the trees.

An 8 ha plot involving approximately 10 000 trees will be planted early in 1987 to assess actual performance of these solar powered biological pumps . (ii) A series of contour evaporation bays is proposed in a developing area.

This will involve construction of 1.5 metre high contour banks on a vertical interval of 1.0 metres. By design, this will provide maximum surface area for minimum construction costs. The surface area required will be governed by the potential water disposal needs of the particular area.

Preliminary costing indicates this type of storage/ disposal system to be 25 OJo of the cost of dam type storage.

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Potable water is generally trucked from Moura town water supply. This is convenient and acceptable considering the many mobile potable water storages on the mine site, eg. draglines, crib huts. All potable water storages are equipped with activated carbon filters and regularly sampled for bacteriological analysis.

A small sedimentation treatment plant is used for water supplied to a contractors camp adjacent to the mine lease area.

Road Watering

Dust supression is a major task and requires approximately 500 mL/ annum .

Three 70 000 litre capacity water trucks spray roads and working areas regularly.

Water is drawn from pits and dams for this purpose.

CONCLUSION Water Management has ben recognised as a vital facet of coal production.

In recent years, emphasis has been placed on the planning phase of Water Management and capital expenditure on water management projects has increased markedly.

The potential cost savings available by having a sound and integrated water management programme have been recognised and a high priority has been allocated to this need.

ACKNOWLEDGEMENTS The author gratefully acknowledges the effort of those who assisted in the preparation of this paper and particularly the Management of Thiess Dampier Mitsui Coal Pty Ltd for authorising publication of relevant data.

REFERENCES BURROW AND FRENCH (1986). Water Management at Moura Mine. Australian Coal Association Seminar on Water Management, Rockhampton, July 2 and 3, 1986 . â&#x20AC;˘

EXPERIENCES WITH MANGANESE IN QUEENSLAND WATER SUPPLIES E.T. Loos ABSTRACT Manganese-related dirty water problems have been causing increasing concern to Queensland Local Authorities in recent years and have been generating many consumer complaints. The problems have received much publicity particularly in relation to the Gold Coast water supply scheme. At the request of the Local Authorities involved , a Steering Committee organised by the Department of Local Government has been co-ordinating and directing research into the problems. This paper describes the ac. tivities of the Committee, the projects undertaken and some initial results of the research up to December 1986.

INTRODUCTION It has long been recognised that water contammg iron and manganese has the potential to be objectionable to consumers because the precipitation of these metals can cause the water to turn brown or black. Iron and manganese precipitates can stain laundry and plumbing, and can introduce tastes that may make the water unpalatable. As long ago as 1962 an American Water Works Association task group suggested that the 'ideal' limits for iron and manganese in drinking water be set at 0.05 mg/ L and 0.01 mg/ L respectively .

Manganese-related problems in water supplies have been the subject of regular discussion in Local Government engineering circles in Queensland for man y years. This discussion had generally been fairly low-keyed with the occasional request for advice being dealt with as required. Several papers had been published on the specific problems encountered. An impression had developed that Local Authorities were coping quite well with any problems that arose in their schemes. However, over the last four years it has become evident that many Authorities in the south-eastern region of the State have been experiencing an increasing number of complaints and a considerable effort has been applied to overcome these problems. No doubt , part of the explanation for the increasing number of complaints lies in the higher expectations of consumers and the increasing publicity given to the problems.

The water supply scheme serving the Gold Coast, and surrounding areas of Albert Shire, occasionally delivers water containing black/ brown precipitates . This occurs more often in the summertime, when storages are stratified, temperatures are higher, and when there is a greater demand on the system, imposed by a larger holidaying population . It is well known that the Gold Coast is a major destination for tourists and any bad publicity associated with this matter is of great concern to the Authorities involved.

The Gold Coast scheme has experienced problems with dirty water for many years but the problems have worsened since the late seventies, in particular following the use of water from the Hinze Dam storage since that time . The significant nature of the problem was first drawn to the Department of Local Government' s attention in early 1983.

In efforts to overcome the problems, the Gold Coast City Council undertook an extensive mains swabbing programme, experimented with potassium permanganate dosing and artificial destratification, changed disinfectants from chloramination to chlorination and finally to chlorine dioxide, investigated the pipeline sediments and biological growths in pipelines and undertook a comprehensive study of the whole problem. A paper covering this study was presented by Gold Coast City Council officers to the June 1985 meeting of the Queensland Branch of the

AWWA.

The Pine Rivers shire water supply scheme has also been subject in recent years to increasing complaints of dirty water and staining of laundry. Pine Rivers Shire is the Local Authority area just to the north of Brisbane and includes the areas of Petrie, Strathpine, Kallangur and Ferny Hills. The scheme serving these 28

WATER March, 1987

Terry Loos, B.Eng. (1970), D. Q.I. T., M.I.E.Aust., is an Executive Engineer in the Department of Local Government, Queensland. Manganese problems in water supplies was one of the projects he was involved with when in the Water Management Section . He is now in the Investigation and Review Section, which works on water supply, se werage, drainage and flood mitigation schemes .

E.T. Loos areas also supplies water to the Redcliffe City Council. The Pine Rivers Shire Council had been following a practice that included swabbing of mains to overcome these complaints. They had obtained advice in this regard from the Gold Coast City Council, as other Local Authorities had done, on the procedures developed by them. _ Seeing a need to seek a more permanent solution and realising that they were not experiencing these problems in isolation, the Pine Rivers Shire Council convened a 'Dirty Water Summit' symposium in late June 1985. 9 In order that the one-day symposium provide the best opportunity for communication between all levels of Authorities and people working in the water supply field, a select audience of representatives, including chairmen and members of Local Authorities, Local Authority engineers and chemists, consulting engineers and analysts, university researchers and Departmental officers, was invited . Representation was limited to the south-east Queensland region.

Not surprisingly, it emerged duribg the discussion that there was a considerable amount of information to exchange and it became apparent that some Authorities were duplicating experimentation and minor research. The conclusion was also reached that, as indicated by the speakers, there were a number of aspects of the problem that were only poorly understood and further research was necessary in these matters.

The 'Summit' resolved that a Steering Committee comprising representatives from each of nine Local Authorities in the southeast Queensland area should be organised to co-ordinate research, and the dissemination of information, on the subject. The Department of Local Government was asked to organise, and chair, the committee. (Membership of the committee was subsequently extended to several other Councils.)

FIRST COMMITTEE MEETING The major conclusions and resolutions arising from the Steering Committee's initial meeting in August 1985 included the following.

• Dirty water problems could be caused by a number of factors, the common ones being corrosion products, post-flocculation of alum, untreated water turbidity, and iron and manganese compounds. It was agreed that there was adequate knowledge on how to deal with the first four factors but less than complete knowledge on how to deal with some of the problems involving manganese.

• There was a need to first survey and assemble all information available on the extent of dirty water problems in water supplies in Queensland. Based on the information assembled, this survey report was to recommend on the aspects of the problem where further research was needed. The Department agreed to carry out this survey and to prepare the report. The findings of this survey are discussed in the fallowing section.

• As part of its research on dirty water problems, the Gold Coast City Council had devised a procedure for ready analysis of sediments from dirty water samples for percentages of iron,

manganese, aluminium, calcium and volatile substances. A ll Local Authorities were invited to take advantage of an offer .by the Gold Coast City Council to analyse samples of dirty water and associated sludge. The information from such analyses would enable Councils to determine to what extent their problems were manganese-related.

The Gold Coast City Council had also discovered, through microscopic analysis, that manganese-related precipitates taken from its system generally contained significant numbers of hyphitype bacteria. Based on a study of the literature, in particular the work of Tyler, 11 it was presumed that these bacteria were of the genus Hyphomicrobium. A microscopic analysis was included in the overall analysis offered.

It was expected that a number of dirty water problems would occur during the fort hcoming summer (1985 / 86), and therefore that a fu ll p icture would be obtained, in parallel with the survey, of the extent of manganese-related problems in Queensland.

THE INFORMATION SURVEY Forty-four water supply schemes throughout Queensland were · surveyed during the period November-December 1985. 5 • Information held by the Department had shown that all these schemes had either experienced dirty water problems or had recorded the presence of manganese in their supplies. The schemes surveyed were variously served by the four common types of sources namely, large dams, small weirs, regulated river flow, and ground water. All schemes used full treatment, some with pre-chlorination, some with permanganate. Summarising, five large schemes still experienced major problems due to manganese, eight had occasional problems in the summer months, the remainder reported either no problems or that such had been corrected by the treatment system.

• The survey sought also to gather data on the numbers and types of dirty water complaints which proved difficult because few Authorities have kept records of complaints, and fewer still have distinguished between the types of complaints. Of the complaint records available, in terms of the maximum number per month per thousand persons, the figures recorded were: Gold Coast 13; Hervey Bay 10; Maryborough 5; Pine Rivers 1; and Brisbane 1.

The survey also revealed the following: • It was not possible to ascertain whether providing treatment facilities some time after the construction of the basic scheme had contributed to the present problems by allowing the mains to become contaminated.

• Manganese is known to be present in certain of the rock formations which occur in the catchment areas of the Gold Coast, Brisbane and Cairns water supplies, and in the Mary River Valley .

In all of these cases manganese is found in significant concentrations in surface waters. There was insufficient information available to determine if clearing of catchments had lead to increased leaching of manganese. However, manganese is ubiquitous in the environment in concentrations sufficient to cause problems at some time for all surface water supplies, particularly supplies drawn from a storage where manganese compounds can accumulate in the sediments.

Clearly, although some correlation exists between the geology of the catchment and the magnitude of the manganese concentrations in the water it was agreed that any furt her investigation along this line should have low priority.

• It is well known that manganese concentrations vary with depth in a storage, and yet few Councils practised inlet level management to any sophisticated degree. One of the reasons for this was that inlet towers are, in many cases of a simple design having only a very limited choice of only two or three inlet levels. Where scope existed for inlet level control, management was often faced with compromise in water quality, for example, between a low manganese-high algae surface water or vice versa for a deeper water.

• The need was apparent for more data on storage water quality variations with depth and over time, for the range of storage types involved. The merits of artificial destratification as a means of primary water quality control could also be assessed for each storage, based on this data.

• The chemistry of the oxidation of manganese was not fully understood. Oxidising agents used to oxidise dissolved manganese

also oxidised iron and organic substances. Alkalinity, pH and time were recognised to be the factors important to the reaction.

The form in which manganese was present, that is, either truly dissolved or as oxidised colloids, was another factor that impacted on the dosing required. Most Councils which added an oxidising agent (usually potassium permanganate) had, through trial and error, devised an effective dosing practice. It was desirable that these various practices be documented in order that they might be better understood .

• Excluding the large Councils, there was little expertise on the intricacies of the sampling and analysis of manganese. There was no general appreciation of the importance of acid-washing sample bottles, acidifying samples for transport to analysis, filtering (through a 0.45µm filter) for dissolved manganese determination, and of having laboratory equipment ca_pable of accurately measuring manganese down to low concentration. Much of the available data on manganese concentrations in water came from analyses of the dissolved manganese content of poorly preserved samples.

• Few Councils carried out in-plant monitoring of manganese.

Such monitoring is of particular interest with conventional treatment plants where there are several processes involved. It is important to know the extent to which manganese is oxidised and removed by the various plant processes in order that the efficiency of the overall removal process can be maximised .

• Clearly, there was a need for better understanding of the factors involved in the growth and sloughing-off of biofilms in distribution and reticulation systems. Initial investigatory work by the Gold Coast City Council had found, among other things, that manganese-dominated biofilm contains hyphi-type bacteria, also that sloughing-off of biofilm occurs during the summer when manganese levels and water temperatures are higher and when flow rates are higher as a result of higher demands.

SECOND COMMITTEE MEETING The matters addressed at the second meeting of the Steering Committee in January 1986 included: • The announcement by the Gold Coast City Council that it had commissioned research into thefactr;.rs involved in the formation and sloughing-off of biofilm in its distribution system. The research to be carried out by the Microbiology Department of the University of Queensland.

• The Department of Local Government agreed to seek financial support for this proj ect from other Local Authorities experiencing similar problems. Th is led to an amount of $25 000 being made available to aid this research. The total cost of the project was expected to be of the order of $125 000. A brief description of the project fallows later in this paper.

• It was observed that there are similarities in the water supply schemes serving Brisbane and the Gold Coast, inasmuch as they are stored surface water supplies known to contain significant amounts of manganese, and all supplies are fu lly treated. The question arising was why Brisbane does not experience manganese-related dirty water problems to the same extent as does the Gold Coast. It was decided that a sub-group of Brisbane and Gold Coast Council representatives and Departmental officers should meet to compare the water supply quality management practices of the two Councils in detail with a report on these discussions to be prepared by the Department. See the following section of this paper.

• It was also agreed that the special matter of the complexities of potassium permanganate dosing (to oxidise dissolved manganese) should be addressed by this sub-group in its report to the Committee.

• It was thought likely that water supply authorities in other States would have documented examples of their experience with dirty water problems. The Department agreed to seek this information. A report on this is being prepared. 8 • It was noted that artificial destratification is widely recognised as an effective means of controlling the seasonal f luctuation of iron and manganese concentrations in surface water storages. The process has been applied with considerable success to a number of storages in Australia. The Department agreed to conduct a review of the applications throughout Australia and to prepare a report, with particular emphasis on the design of these applications.

WATER March, 1987 29

COMPARISON OF THE BRISBANE AND GOLD COAST SCHEMES A recent report by the Department• provides the following brief description of the four source-treatment systems (two each) serving both Councils. All four plants provide full treatment.

Brisbane 1. Wivenhoe Dam to Mt Crosby Treatment Plant - Regulated releases from Wivenhoe Dam to the Brisbane River are withdrawn 60 km downstream at Mt Crosby. The water released from Wivenhoe is known to be rich in dissolved manganese, up to 3.3 mg/ L, but by the time it reaches Mt Crosby natural oxidation in the river has typically reduced this to around 0.3 mg/L. The BCC has found evidence that this oxidation of manganese is largely microbially mediated. Manganese oxidising bacteria have been found in the biofilm growths on rocks in the bed of the river .

Most of the manganese in the water drawn from the pumping pool at Mt Crosby is of the particulate, oxidised form . This is readily removed by the coagulation-flocculation-sedimentation process. Potassium permanganate dosing is practiced only on the rate occasions when the dissolved manganese concentration of the water is high - perhaps as a result of a turnover of the 2200 mL pumping pool.

The filters at Mt Crosby have been found to be able to reduce a dissolved manganese concentration of around 0.05 mg/ L to around 0.005 mg/ L. There is evidence that this action, too, is microbially mediatated. The filter sand particles have, with age, developed fine microbial coatings which through oxidation and adsorption remove dissolved manganese .

The output of manganese from the Mt Crosby plant is consistently below 0.04 mg/ L. 2. North Pine Dam and Treatment Plant - Water from North Pine Dam is drawn by pipeline to the nearby treatment plant. The water is often high in dissolved manganese, and potassium permanganate dosing is adopted on a fairly regular basis. The manganese content of the raw water can vary significantly over a matter of a few hours . This makes the control of the manganese oxidation process quite difficult.

It was found initially that the simultaneous dosing of permanganate and alum was less than fully effective due to difficulties with the pH. A compromise pH between the low pH, which favours coagulation with alum, and the high pH, which favours manganese oxidation, had to be found. One advantage with dosing permanganate at the flash mixer was that it allowed for the visual control of permanganate dosing, by observation of the decay of the pink colouration of the water. A residual colouration at a chosen point in the flow path through the sedimentation tank was taken to be indicative of an overdosing of permanganate .

Gold Coast 1. Hinze Dam to Molendinar Treatment Plant - Water from Hinze Dam on the Nerang River is pumped through 13 km of 1440 mm rising main to the Molendinar plant. The GCCC has experimented with artificial mixing of the Hinze Dam' storage with some success in early 1986. This is described in the Department's report on Aeration/Destratification. The performance of the process is being evaluated through the summer of 1986/ 87. (Unfortunately the low water levels currently prevailing may prejudice .a proper evaluation of the process .)

Over the last few years the manganese content of the water entering and leaving the plant has at times been significant and this has contributed to the dirty water problems experienced.

The GCCC has carried out a considerable amount of experimentation with the dosing of agents to xidise manganese.

Dosing trials in 1984 with potassium permanganate were less than successful although trials throughout I 986 with chlorine dioxide have proved far more promising .

Chlorine dioxide is currently dosed prior to the filters. The practice of dosing the oxidising agent just prior to the filters has the advantage that most of the other compounds in the raw water that would otherwise react with the chlorine dioxide have been removed, at this stage, by the sedimentation process . It has the disadvantage that it precludes the opportunity for man ganese removing microbial coatings to develop on the filter sand particles . The evidence prior to the introduction of ch lorine dioxide dosing was, however, that as at the North Pine plant, this sand particle coating process was not taking place .

2. Little Nerang Dam to Mudgeeraba Treatment Plant - Supply from Little Nerang Dam gravitates to the Mudgeeraba plant.

Supply is, as required, also pumped from an upper intake in the Hinze Dam storage. The GCCC has experimented considerably with potassium permanganate dosing, chlorine dioxide dosing and filter operation at this plant. Experiences here are generally similar to those at Molendinar.

In conclusion, an examination of the record of manganese output from the two treatment plants at the Gold Coast over recent summers, shows that the main reason the scheme has experienced manganese-related dirty water problems is the occurrence of high manganese levels (above 0.06 mg/ L). Furthermore, there is evidence that the distribution system at the Gold Coast is subject to extensive biofilm growth which inoorporates manganese and it is th e sloughing-off of this biofilm that causes the dirty water problems. The research project currently in progress at the Gold Coast is aimed at identifying the factors involved in the growth and sloughing-off of biofilm in the distribution system.

DOSING WITH POTASSIUM PERMANGANATE

Effective dosing with potassium permanganate is somewhat of an art and this is clearly demonstrated by the experiences at the The permanganate dosing point has now been relocated to well North Pine Dam Treatment Plant and at some of the other plants in advance of the alum dosing point. Visual control is no longer surveyed. It was found that factors such as the variability of the possible to the same extent although the emergence of pinkish raw water dissolved manganese, the presence of other compounds water from the raw water pipeline is evidence of an overdose. that may use up the permanganate (for example, iron and Control of the permanganate dosing is primarily based on fre- organics), the pH and alkalinity at reaction, the degree of mixing, and the time available for reaction are of considerable imporquent monitoring of the raw and treated waters.

Virtually all of the oxidised manganese is removed in the tance.

Some of the basic guidelines found to apply to permanganate horizontal flow sedimentation tanks. The effectiveness of the sedimentation process in removing manganese, and other dosing are as follows. substances, may account for the fact that the filter sands at North • Where the dosage reqired is being assessed in terms of the Pine have not developed manganese oxidising microbial coatings. manganese content of the raw water , it should be the dissolved, BCC staff are currently investigating the factors involved in the and not the total manganese content that is monitored . This may development of microbial coatings on filter media particles and sometimes be complicated by the occurrence of oxidised manganese in a less than 0.45 J.lm particulate form which is thus will publish their results in due course. defined as ' dissolved' .

Occasionally, about once a year, a slug of dissolved manganese • The oxidation reaction is favoured by a high pH (above 7.5). It may inadvertently pass through the plant. Complaints that laun- is therefore important to ensure that the permanganate dose is addry detergents and bleaches are producing dirty water soon ded well ahead of any alum dose. follow, particularly if the dissolved manganese concentration of • There is some conjecture as to the reaction time necessary the water exceeds 0.05 mg/L. before the pH can be allowed to fa ll. Some references say the In conclusion, it is apparent that the fundamental reason Bris- reaction is instantaneous, others say it can take up to five bane does not experience major dirty water problems is that the minutes. At North Pine a reaction time of about ninety seconds is product water from the two major treatment plants serving the adopted and this is regarded as more than adequate. city contains only very small quantities of iron and manganese. • Where the dissolved manganese content of the raw water is Also, the Brisbane distribution system has been maintained in a known to vary significantly over a short period, continuous fairly biofilm-free condition by the effective disinfection practice monitoring of the raw water is desirable in order that the dosage could be adjusted accordingly. (chloramination) followed for over the last fifty years.

30

WATER March, 1987

• Where continuous, or even regular monitoring of the raw water manganese is not practicable, the use of a layer of manganese greensand in the filter would be desirable in order to remove any surplus of either potassium permanganate or dissolved manganese.

It is most important that any proposal to effect oxidation by potassium permanganate dosing first be pilot-tested to determine the likely optimum dosing procedure.

THE AERATION/DESTRATIFICATION REVIEW The seasonal thermal stratification of surface water storages can have a profound effect on the quality of water over the depth of the storage. It provides the mechanism for the curtailment of the supply of oxygen to the bottom waters and the subsequent release of iron and manganese from the sediments to the lower portion of the water body. When such water is used for urban supply, suitable forms of treatment must be applied to remove this iron and manganese if dirty water problems are to be avoided.

By artificially preventing (or destroying) stratification, the supply of oxygen to the bottom waters of a storage can be maintained (or restored) and this prevents the release of iron and manganese · from the sediments. Properly used, the technique affords a high degree of control over iron and manganese, maintaining low concentrations of each in the water body throughout the summer months when dirty water problems are frequently encountered .

Low concentrations of manganese and iron remaining after aeration/ destratification can frequently be removed by conventional coagulation/ filtration processes.

Although widely used internationally, the technique was not applied in Australia until the mid 1960s . Since that time the technique has been applied to some 50 storages ranging in size from 160 mL to 4 000 000 mL. The technique has been used to address a wide range of problems, not only those relating to the presence of iron and manganese. Very few Australian applications could be considered to have failed completely, although, in examining case histories, it was evident that a degree of confusion exists as to the nature of the process and the way in which the technique should be used to achieve the best effects.

As all systems used in Australian storages are essentially of the same type, the review' provided an ideal opportunity to examine, in detail, the physical and performance characteristics of the systems and also to look at some of the underlying design concepts.

Notable differences in system characteristics were observed when comparisons were made with international systems. Only one part of the design published by the Water Research Centre, UK, was found to give consistent results when used with the range of conditions encountered in Australian storages. Constraints on the application of the full design method were discussed in the review and a way of avoiding such constraints was proposed .

Overall, it was found that aeration/ destratification would address many of the adverse effects associated with seasonal thermal and chemical stratification, including the accumulation of iron and manganese compounds in the bottom waters of such storages. To be fully effective, the systems must be properly designed and operated. In such circumstances a high degree of primary quality control could be achieved, often at a cost less than that of conventional forms of treatment required to achieve the same degree of quality control.

FOURTH COMMITTEE MEETING (Proceedings at the third meeting of the committee were not relevant to the context of this paper.)

A fourth meeting of the committee was held in September 1986.

The meeting was held at the Gold Coast so that members could inspect the research facilities installed for the project being conducted by the University of Queensland and learn of progress to date. The meeting also discussed the various reports in preparation or recently completed.

The Gold Coast City Council reported briefly on a recent Churchill Fellowship Study undertaken in the USA by one of its officers. 3 The study focussed on biofilm growth in pipe systems.

The GCCC also confirmed an earlier announcement that it had organised a Consultative Committee comprising representatives of the University of Queensland, the CSIRO, the Atomic Energy Commission and the Department of Local Government to advise on the problems peculiar to the Gold Coast scheme.

RESEARCH AT THE UNIVERSITY OF QUEENSLAND Research on Biofilm Growth

A research project being conducted by the Microbiology Department of the University of Queensland for the GCCC is studying the factors influencing the formation and the sloughing-off of biofilm in pipes in the Gold Coast distribution system. A report on the first stage of the project has recently been completed. 10 This report covers the winter period - when manganese levels in the water are generally low.

One of the major conclusions reached in this work is that, at least as far as the Gold Coast system is concerned, a situation of no complaints can only be achieved if the total manganese content of the treated water is maintained below 0.01 mg/L. A concentration of 0.03 mg/ L is likely to lead to an unacceptable level of complaints. The project is continuing through the summer of 1986/ 87.

Research on Manganese Speciation

Over the last few years, the Chemistry Department of the University of Queensland has been conducting research on the identification of the species of manganese present in raw waters.

This work has been partly supported by the Department of Local Government. 1 •2 The bulk of the research to date has been on the raw waters of Hinze Dam, which serves the Gold Coast, and North Pine, which serves Brisbane. This research will provide a better understanding of the nature of manganese in surface waters, which in turn will assist in the understanding of the manganese oxidation and removal processes necessary at the treatment stage.

There has been some speculation that dissolved manganese occasionally occurs in a complexed, perhaps organic form, particularly in surface storage waters. It is thought that this form of manganese may be difficult to oxidise and so may pass through the treatment plant. The evidence of this, based on the experiences of the water supply schemes surveyed is, however, less than conclusive. This matter is being addressed by the University's research work.

CONCLUSIONS The subject of manganese-related dirty water problems has been one of significant interest in Queensland in recent months. It is expected that this interest will continue. Certainly, it is proposed that the Steering Committee continue to function in the foreseeable future.

A variety of investigations, studies and research projects on the subject, some initiated by the committee, have commenced. Some of these projects have been completed and reported on, others are the subject of progress reports . The Steering Committee has recently produced a Position Paper on the subject.

It is not the author's intention in this paper to pre-empt the findings of any of the reports prepared outside of the Department.

The purpose of this paper is simply to advise briefly on the nature of the projects undertaken so that any interested reader can pursue particular matters with the individual authors, listed in the References.

The experience gained from the functioning of the committee over the last 12 to 15 months has demonstrated the value of organising such co-ordinating committees of membership drawn from the Local Authorities experiencing the same problem. The Department of Local Government is in a key position to coordinate such activities .

ACKNOWLEDGEMENTS The permission of the Director, Engineering and Technical Services Division of the Department of Local Government to prepare this paper is acknowledged.

The opinions expressed herein are those of the author and not necessarily those of the Department.

The contributions by Departmental officers Ian Brown and Don Gardiner to the sections on Aeration/ Destratification, and the Introduction, respectively, are acknowledged.

CONTINUED ON PAGE 37 WATER March, 1987 31

PHOSPHORUS PRECIPITATION WITH PICKLE LIQUOR AT GLENFIELD WPCP I. Lim and T. Nguyen ABSTRACT A plant scale trial of phosphorus removal using pickle liquor at a conventional activated sludge was instituted at Glenfield WPCP. Objectives of the trial were to determine the optimum dosing point and dosing rate to produce an effluent containing less than 1 mg/ L of phosphorus. The trial also served to identify operational problems and their solutions . Experience gained from the trial will be incorporated in the design and construction of future phosphorus removal facilities in other WPCPs of the Metropolitan Water Sewerage and Drainage Board .

I. Lim

T. Nguyen

1. INTRODUCTION A water quality modelling study of the Georges River carried out on behalf of the Board by Consulting Engineers Brown and Caldwell, indicated that increasing waste loads on the river from Board 's Water Pollution Control Plants (point sources) and urban run-off (diffuse sources) had resulted in a decline in water quality and amenity of the river, in particular the creation of eutrophic conditions in the impounded fresh water section .

Originally, four Water Pollution Control Plants (WPCP) discharged effluent to the Georges River , viz, Campbelltown, Fairfield, Glenfield and Liverpool. The Board has taken measures to reduce the waste loads from these plants to the river . The Campbelltown WPCP has been decommissioned and its catchment amalgamated with the Glenfield WPCP catchments. The Fairfield WPCP has been replaced by a storm sewage treatment facility and only operates and discharges effluent to the Georges River in wet weather. Dry weather sewage flow from the catchment is diverted to the Southern and Western Surburbs Ocean Outfall Sewer (SWSOOS) System. The Glenfield and Liverpool WPCPs will also discharge effluent to the river in wet weather only, on completion of an effluent transfer scheme, expected in late 1987 . At that time tertiary treated dry weather effluent will be pumped to the SWSOOS system draining to the Malabar Ocean Outfall.

The philosophy of these measures is that the effluent from water pollution control plants needs to be diverted from the river only in dry weather when it contributes a major percentage of the waste load on the river. The waste loads from WPCP effluent in wet weather, on the other hand, are significantly less when compared with the waste loads from urban run-off. Therefore diversion of WPCP effluent from the river in wet weather would not appreciably improve the water quality of the river.

This total strategy will serve to remove all dry weather effluent discharges from the Georges River by 1987 and will significantly improve water quality in the river.

However, it was realised that an interim measure was desirable to reduce eutrophication in the impounded fresh water section of the river which received effluent dicharges from the Glenfield water pollution control plant.

To achieve this a chemical process was selected to precipitate phosphorus from sewage entering Glenfield WPCP . This process has the fo llowing advantages: (i) Alagae and macrophytes require both phosphorus and nitrogen to grow. A significant reduction of phosphorus level in the river would result in a significant reduction of algal and macrophyte growth. (ii) The process can be implemented quickly without any modification of the existing structures or processes. (iii) The capital cost involved is low.

A phosphorus precipitation facility was commissioned at Glenfield WPCP in March, 1983 . This facility served as an on-line operating process as well as a plant scale trial facility for the Board's investigation into phosphorus removal facilities for all its inland water pollution control plants. The State Pollution Control Commission (SPCC) in a report on Water Quality in the Hawkesbury-Nepean River, has forshadowed the imposing of license limit of 1 mg/ L of phosphorus for effluents discharge to the river under dry weather conditions.

Ivan Lim is a Chemical Engineer in the Sewage Treatment Planning and Investigation Sub-Branch of the Metropolitan Water, Sewerage and Drainage Board, Sydney.

Tung Nguyen is a Chemical Engineer in the Pollution Control Branch of the Metropolitan Water, Sewerage and Drainage Board, Sydney and the Operation Engineer for Glenfield Sewage Treatment Works.

This paper documents the results obtained from this investigation in terms of planning the installation of similar facilities at other plants .

2. DESCRIPTION OF THE PROCESS 2.1 General

The phosphorus in sewage can be removed by chemical precipitation processes using metal ions such as aluminium, iron, etc. The phosphorus reacts with these ions to form insoluble compounds which are removed as primary or secondary sludge.

2.2 Properties of Pickle Liquor

Pickle liquor is a waste from the acid descaling or etching of iron products . The properties of pickle liquor vary depending on the pickling operation . The pickle liquor used in this study is supplied by ICI and has the following average properties: lOOJo w/ w as Fe • Iron Content: • Free Acid: 0.50Jo w/ w as HCl • Specific gravity: 1.18 2.3 Chemical Reactions

The chemical reactions between ferrous / ferric ions and phosphorus are complex. The phosphorus content of raw sewage is in various forms (orthophosphates , polyphosphates and organic phosphates) . However, during the biological process the polyphosphates and organic phosphates are converted by biochemical reactions to orthophosphates . Therefore it is assumed that in domestic sewage, phosphorus reacts as H,PO;; and HPO 2 ;; (which are predominant orthophosphates) at a pH range of between 6.8-7.5. This assumption may not be correct, but it gives an indication of the chemical requirements .

The chemical reactions are listed in Table 1.

TABLE 1. CHEMICAL REACTIONS BETWEEN FERROUS/ FERRIC IONS AND ORTHOPHOSPHATES Reactions of ferrou s ions 3Fe" + 2HPO.'3Fe'' + 2H, Po,Fe'· + HPo;- + ¼ 0, Fe" + H, Po,- + ¼ 0, Fe'' + ¼ O, + 5/2 H,O Reactions with ferric ions Fe'' + HPOlFe'' + H,PO, + 2H" Fe'' + 3H,O

= = = = =

Fe,(PO,), + 2H' Fe,(PO,l, + 4H' FePO, + ½ H,O FePO, + ½ H,O + H' Fe(OH), + 2H'

=

FePO, + H' + 2H' Fe(OH), + 3H'

= FePO, =

WATER March, 1987 33

2.4 Stoichiometry

From the equations in Table 1 the molar ratio of Fe:P for iron/ phosphates reactions is 1 for ferric ions, and 1.5 for ferrous ions when the reaction does not involve the oxidation of ferrous ion to ferric ion . The corresponding weight ratios are 1:8 and 2:6 respectively.

Some of the ferric or ferrous ions would also react with hydroxyl ions to form insoluble ferric hydroxide. Therefore the stoichiometric requirement of iron would be higher than 1:8 (or 2:6) .

2.5 Description of Glenfield WPCP

Glenfield WPCP is a conventional activated sludge plant providing the following treatment processes: (i) Preliminary treatment: screening and grit removal (ii) Primary treatment: primary sedimentation (iii) Secondary treatment: aeration and secondary clarification (iv) Tertiary treatment: effluent polishing pond The plant is designed to serve an ultimate equivalent population · (EP) of 200 000. The present capacity of the plant is 100 000 EP, and the current load on the plant is approximately 86 000 EP.

(equivalent to 20 mg of Fe per litre) over a period of at least 3 weeks. Daily sampling and analyses for phosphate, pH, Suspended Solids (SS) in raw sewage, settled sewage, clarifier effluent and plant effluent were carried out. Data on sludge production was also obtained. One 48-hour continuous test for each dosing point was also carried out to obtain the phosphate profiles and the overall performance of the plant.

3.5 Determination of the Optimum Dosage

The dosing point giving the best phosphorus removal was then selected to further study the effects of different pickle liquor dosing rates . For vary_ing dosage rates, each of which was maintained for at least 3 weeks, the same sampling and analysis programme described in section 3.4 was carried out.

3.6 Split Dosing

Pickle liquor application from multiple dosing points simultaneously, or split dosing, using the optimum dosage was then studied to determine whether the advantages associated with each dosing point could be fully realised.

4. TEST RESULTS 2.6 Plant Performance

4.1 Laboratory Test Results

The average performance of the Glenfield WPCP without pickle liquor dosing is shown in Table 2. It can be seen from the results that the existing processes do not remove either ammonia or phosphorus effectively.

TABLE 2. PERFORMANCE OF GLENFIELD WPCP BEFORE PICKLE LIQUOR DOSING (Average over 1982) Parameters Suspended solids B.0.0.

Ammonia-nitrogen Total phosphorus

Influent (mg / L)

Effluent (mg / L)

% removal

210 180 38 12

10 7 30 9

95 96 21 25

3. TRIAL PROGRAMME 3.1 Objectives of the Trial It was expected that the trial would achieve the following objectives: - Determination of the optimum dosing point for pickle liquor application.

- Determination of the optimum dosing rate of pickle liquor to reduce effluent phosphorus level to less than 1 mg/ L. - Determination of the effect of pickle liquor dosage on sludge production and sludge characteristics.

- Identification of operational problems and their solutions.

3.2 Pickle Liquor Dosing Points

Three dosing points were selected for this study. Pipework was arranged so that pickle liquor could be dosed to three locations within the process flow either independently or simultaneously.

These three locations were: • The screened sewage channel immediately downstream of the gauging flume .

• The settled sewage channel immediately upstream of the aeration tanks .

• The mixed liquor channel.

3.3 Control Data

Before the commissioning of the pickle liquor dosing facilities one 48-hour continuous test was carried out to obtain control data on the overall performance of the plant and the phosphorus profiles in raw sewage and clarifier effluent. Data on sludge production was also recorded. This data was used to assess the effectiveness of pickle liquor in removing phosphorus.

3.4 Determination of the Optimum Dosing Point

For each of the dosing points described in section 3.2, pickle liquor was applied at a rate of 200 mL per litre of incoming sewage 34

WATER March, 1987

Prior to the commencement of the field programme, laboratory tests were conducted to simulate operational conditions and to predict possible outcomes of the trial. Conclusions drawn from the results of the laboratory tests were: - Mixing of raw sewage with pickle liquor alone was not effective in reducing the phosphorus level in sewage.

- Aeration of the mixture of raw sewage/pickle liquor for at least 1 hour, at a dosage of 200 to 300 mg/ L of pickle liquor to sewage, was effective in reducing the phosphorus concentration to less than 2 mg/ L. - Alkalinity depletion in the sewage due to pickle liquor addition at the above rates was small and pH adjustment was not required.

- The rate of reaction increased ten-fold using ferric instead of ferrous compounds.

4.2 Effects of Pickle Liquor Dosin,: Point

(a) Phosphorus Removal Efficiency The effect of the pickle liquor dosing point on phosphorus removal efficiency was studied over a period of 5 months (from 3.3.83 to 1.8.83). The results are summarised in Table 3.

It can be seen that the dosing pickle liquor substantially reduced the phosphorus level in the effluent. However at the pickle liquor dosage of 200 mg/ L the target average effluent phosphorus concentration of 1 mg/ L was not achieved .

The dosage point at the settled sewage channel gave the highest removal efficiency of 81 OJo . This is probably due to the higher oxidation efficiency of ferrous to ferric ions in the aeration tanks.

(b) Sludge Production Prior to pickle liquor dosing, the quantity of raw sludge, including waste activated sludge, withdrawn from primary sedimentation tanks was 130 kL/d. This quantity was increased to 140 kL/ d towards the end of test 1, which involved the dosing of pickle liquor into the mixed liquor channel, and maintained at 140 kL/ d throughout tests 2 and 3. This represents a volumetric increase of 80Jo. The solids content of raw sludge also increased from 3.20Jo, before pickle liquor dosing, to 4.0-4.30Jo during the tests. This corresponds to an increase of approximately 35-400Jo in sludge solids production. This increase is probably due to the iron precipitates as indicated by the increase of inorganic components of the sludge solids from a pre-test of 180Jo to 300Jo during tests, and the insignificant increase in gas production. Table 4 summarises the above results.

(c) Effluent Quality Dosing of pickle liquor significantly improved the settleability of the activated sludge. The sludge density index (S.D.I.) increased from a pre-test of 0.64 to 1.5 during the tests. Significant reduction of scum was also recorded. However the effluent BOD,

TABLE 3. SU MMARY OF THE EFFECT OF PICKLE LIQUOR DOSING POINT ON PHOSPHATE REMOVAL EFFICIENCY (Pickle Liquor Dosage: 200 mg/ L) PRE-TEST

TEST I

TEST2

TESTJ

N il

Mixed liquor channel

Seuled sew. channel

Screened sew. channel

Dosing point lnOuent Total phosph orus

Ra nge (mg/ L) Mean (mg/ L)

10.2- 17.0 12.3

8.3-20.1 12. 1

3.4-19 .9 9.9

3.4- 17. 9 9. 7

Clarifier efnuent Total phophorus

Range (m g/ L) Mean (mg/ L) 0/o remova l

7. 1- 13 .6 8.9 28

1.1 -4 .7 2.7 78

0.4-5.3 1.9 81

0.7-3.8 2.5 74

Pond efnuent Total phos ph o ru s

Ran ge (mg/ L) Mean (m g/ L) 0/o removal

7.8- 12.1 9.3 24

1.3-6.5 3.4

0.4-3.9 1.8 81

0 .5-4 .0 2.4 75

72

TABLE 4. SUMMARY OF THE EFFECT OF PICKLE LIQUOR DOSING POINT ON SLUDGE PRODUCTION (Pickle Liquor Dosage: 200 mg/ L) PR E-TEST

TEST I

TEST2

TESTJ

N il

Mixed liquor channel

Sellled sew. channel

Screened sew. channel

Dosing point Average ra w sludge T.S.R. (0/o )

3.2

4.3

4.0

4.2

Average raw slud ge FR/ T .S. R .

0 . 18

0.29

0 .30

0 .28

Volumetri c raw sludge productio n (k L/d)

Up to 130

130-140

140

140

4160

5800 ave.

5600

5880

Raw sludge solids prod ucti on (kg/ d) Vo lumetric increment of raw sludge (0/o)

0

4 ave .

8

8

Mass increment of raw sludge solids (0/o)

0

40 ave.

35

41

and SS did not change much from pre-test levels as shown in Table 5.

It should be noted that the effect of pickle liquor on Nocardia scum was the topic of another study carried out by Heath and Chan (1985) . Pickle liquor dosing was also effective in the control of filamentous bulking caused by high strength paper wastes in a trial at Liverpool WPCP by Lim and P sang (1985) .

4.3 Effects of Pickle Liquor Dosage Rate

(a) Phosphorus Removal Efficiency The effect of picl< e liquor dosage rate on phosphorus removal efficiency was studied over a period of 3 months (9 .8.83 to 5. 11.83) . The results are summarised in Table 6.

Increase in pickle liquor dosage improved the phosphorus removal efficiency . The target of average effluent phosphorus concentration of 1 mg/L was achieved with a pickle liquor dosage of 300 mg/ L. The lower phosphoru s removal efficiency of 770'/o recorded for pickle liquor dosage of 250 mg/L is probably not reliable because of the small body of data. (The duration for this test was only 7 days).

(b) Sludge Production TABLE 5. SUMMARY OF THE EFFECT OF PICKLE LIQUOR DOSING POINT ON THE PERFORMANCE OF THE ACTIVATED SLUDGE PROCESS PRE-TEST

TEST I

TEST2

TEST 3

Nil

Mixed liquor channel

Sellled sew. channel

Screened sew. channel

10 7 0 .64 Plenti fu l Plentiful 156

14 9 1.5 1 Reduced Negligible 66

11 7 1.36 Red uced Negligible 73

13 7 1. 29 Reduced Negligible 78

Dosing poin t E fnu ent suspended so lids (mg/ L) Efn uent BOD, (mg/ L) Ac ti vated slu dge den sity index Scum Nocardia Activated sludge volume index

TABLE 6. SUMMARY OF THE EFFECT OF PICKLE LIQUOR DOSAGE ON PHOSPHATE REMOVAL EFFICIENCY (Pickle Liquor Dosing Point. Settled Sewage Channel) PRE-TEST

TEST I

TEST2

TEST 3

Nil

200

250

300

Pickle liquor dosage (mg / L) InOuent Total phosphorus

Range (mg/ L) Mean (mg/ L)

10.2- 17. 0 12.3

3.4- 19.9 9.9

7.2- 13 .9 9.8

4.0- 16.9 9.0

C larifier efnuent Total phophorus

Range (mg/ L) Mean (mg/ L) 0/o rem oval

7 .1-13.6 8.9 28

0.4-5 .3 1.9 81

0.6-2.2 1.6 84

0 . 1-4 .4 1.0 89

Pond efnu ent Total phos pho rus

Ra nge (mg/ L) Mean (m g/ L) 0/o remova l

7. 8- 12. 1 9 .3 24

0 .4-3 .9 1.8 81

1.4-3.2 2.3 77

0.1-3.4 1.1 88

The volume of raw sludge produced increased from 80Jo for a pickle liquor dosage of 200 mg/ L to 150'/o for a dosage of 300 mg/ L. The corresponding sludge solids production b'y weight increased from 35 to 550'/o. These results are summarised in Table 7.

(c) Effluent ~uality The sludge density index (SDI) of the activated sludge reduced to approximately 1.0 which is still significantly better than pre-test SDI. Scum was still absent and no significant recurrence occurred .

BOD, and Suspended Solids (SS) of the effluent remained unaffected by pickle liquor.

4.4 Effect of Split Dosing A pickle liquor dosage of 300 mg/ L was split between dosing points in the settled sewage channel and mixed liquor channel in a ratio of 2: 1. No significant change in phosphoru s removal efficiency was observed, as shown in Table 8. Sludge production and effluent quality were also not affected.

4.5 Fluctuation of Phosphorus Level

TABLE 7. SUMMARY OF THE EFFECT OF PICKLE LIQUOR DOSING ON SLUDGE PRODUCTION (Pickle Liquor Dosing Point: Settled Sewage Channel) PRE-TEST

TEST 2

TEST4

TEST 5

Pickle liquor dosage (mg / L}

N il

200

250

300

Average ra,v sludge T.S.R. (0/o)

3.2

4.0

4.7

4.3

Ave rage raw sludge FR/ T.S.R .

0 . 18

0 .30

0.2

0.35

Volumetric raw sludge production (kL/d) Raw sludge production {kg/ d)

Up to 130

140

140

150

4160

5600

6580

6450

Volumetric increment of raw sludge (0/o)

0

8

8

15

Mass increment of raw sludge (0/o)

0

35

58

55

Results of the 48-hour continuous tests are shown graphically in Figure 1. The phosphorus level in the raw sewage seems to follow the same diurnal fluctuation pattern as the sewage flow, but these fluctuations have been dampened . The phosphorus profile in the clarifier effluent indicated that the precipitation process had also coped with these fluctuations quite well . Phosphorus removal efficiency calculated from these 48-hour continuous tests, summarised in Table 9, confirmed that the settled sewage channel provides the optimum dosing point.

WATER March, 1987

35

TABLE 8. SUMMARY OF THE EFFECT OF PICKLE LIQUOR SPLIT DOSING ON PHOSPHATE REMOVAL EFFICIENCY TEST5

TEST6

To selfled sew. chan.

To mixed Liq. Chan.

300 N il

200 JOO

Influent Total p hosphorous

Range (mg/ L) Mean (mg/ L)

4.0- 16.9 9.0

4.9-12.2 8.5

Clarifier efflu ent Total phosphorous

Range (mg/ L) Mean (mg/ L) "lo removal

0.1 -4.4 1.0 89

0.3-4.2 1. 2 86

Pond effluent Total phosphorus

Range (mg/ L) Mean (mg/ L) "lo removal

0.1-3.2

0.1 -3.2 I.I. 87

Pickle liquor dosage (mg / L)

A

I.I 88

5.2 Corrosive Action of Pickle Liquor

After a few months of pickle liquor dosing, steel walkways and handrails in the vicinity of the dosing points were coated with rust. It was initially thought that these surfaces were corroded by acid in pickle liquor splashed onto them. However, further analysis showed that the rust was actually insoluble ferric salts originating from the pickle liquor and not from corrosion of the steel surfaces. This problem was solved by submerging the pickle liquor outlets so that splashing could not occur.

5.3 Health Aspects

No P,ck le Li quor

15

10

mg/L

0 h-.~-,...,.~-,-+-~...,.-,,...,....,...,...,.-+-,-~....+~c-T-t-,...,.~-h--~ 60m 11am I.pm 9pm 2am 7am 12pm 5pm

8.12.82

B

rt---................-~.,.,...,.-

10pm

9.12.82

3am

5.4 Quality of Pickle Liquor

The pickle liquor was supposed to contain (10 Âą 1)% of iron by weight. However random sampling showed a much wider variation with some samples having an iron content as low as 5.3%.

Such variation is not acceptable if a consistent phosphorus removal efficiency is to be achieved. Assurance was then obtained from the supplier to improve the quality of pickle liquor. Subsequent regular sampling showed a more consistent iron content.

Pickle Liquo r Dos age :

mg / L

5.5 Overall Operation of the Plant sam tlam 7.6 83

C

4pm

9am

2am 8.6.83

7am

12pm

5pm

10pm

3am 9.6.83

8am

3am 2710.83

8am

Pick l e Liquor Dos age : 300 mg / L into Set t led Sewage Channel

15

10

mg /L

0

The same insoluble ferric salts were also found on steel guard rails surrounding aeration tanks. This prompted the Water and Sewerage Employees Union to place a ban on the handling of pickle liquor because of suspected health risks associated with its use in aeration tanks. Subsequent tests carried out by the N.S.W .

Department of Health showed that there was not any significant quantity of total particulate with respect to iron in the aerosol around the aeration tanks. The health risk associated with the use of pickle liquor , therefore, is insignificant.

8am

10.12.82

15

0

the socks in the period prior to pickle liquor dosing could also have contributed to this problem.

6cm 11am 25 -10.83

4pm

9pm

2am 26.10 83

7am

12pm

5p m

10pm

Figure 1. Effect of pickle liquor on phosphorus concentration in clarifier effluent.

The obvious benefits of pickle liquor dosing to the operation of the plant were seen in the reduction of scum and the improvement of sludge settleability. Prior to the dosing of pickle liquor , control of Nocardia scum to a manageable level was expensive and time consuming. During periods of bad infestation , aeration tanks and mixed liquor channels were complettly covered with scum. Dosing of pickle liquor had resulted in the reduction of Nocardia to insignificant levels thus improving the appearance and performance of the plant. The improvement of sludge settleability relieved the need for extra secondary sedimentation capacity.

The increase in sludge production did not affect the operation of the plant because of the availability of spare sludge digestion capacity . However, if chemical precipitation of phosphorus is to be implemented permanently, additional capacity may have to be provided to handle the increase in sludge production.

TABLE 9. PHOSPHORUS REMOVAL EFFICIENCY FROM 48-HOUR CONTINUOUS TEST RESULTS Test No.

Date

Dosing point

Pickle liquor dosage (mg / L)

Removal efficiency @ clarifier effluent

Nil 200 200 200 300

18

% A B C D

E

8,9, 10.12.82 6,7,8.4.83 7, 8,9 .6.83 26,27 ,28 . 7 .83 25,26,27 .10 .83

Nil M ixed liquor Settled sewage Screened sewage Settled sewage

59 81 66 81

4.6 Settling Characteristics of the Activated Sludge

The settling characteristics of the activated sludge were significantly improved by the dosing of pickle liquor. The sludge volume index (SVI) was reduced from a pretest of 156 to 66 during the test as shown in Table 5.

5. OPERATIONAL EXPERIENCE 5.1 Blockage of Sock Aerators

More frequent cleaning of the stock aerators was required during the first year of the test. Although there was some evidence of iron precipitate on the socks, the lack of regular maintenance of 36

WATER March, 1987

6. COSTS 6.1 Capital Costs

The total capital costs of the temporary phosphorus precipitation facilit y at Glenfield WPCP was $50 000. This cost, which is dissected in Table 10, does not include overhead charges.

TABLE 10. CAPITAL COSTS OF THE TEMPORARY PHOSPHORUS PRECIPITATION AT GLENFIELD WPCP Item

Capital costs ($@1983)

Sto rage ta nk Dosing pumps Fittings Base slab Safety equipment Electrical installation Mechanical installati on

23000 7000 1000 5000 1000 8000 5000

To ta l

50000

6.2 Operating Costs

The operating cost of the phosphorus precipitation facility at Glenfield WPCP is approximately $100 000 per year (equivalent to $12/ML of sewage) . This cost is solely chemical cost and does not include wages or sludge disposal charges.

7. CONCLUSIONS The following conclusions were derived from the results of this investigation: • Pickle liquor can be used to reduce substantially the phosphorus concentration in the plant effluent of a conventional activated sludge plant.

• The optimum pickle liquor dosing point is the settled sewage channel.

• An average phosphorus concentration of I mg/L in plant effluent can be achieved with a pickle liquor dosage of 300 mg/ L. • Performance of the activated sludge process (BOD and SS removal) is not significantly affected by pickle liquor.

• Sludge production will increase 150Jo by volume, and 550Jo by dry weight with a pickle liquor dosage of 300 mg/ L. • Split dosing does not substantially affect the efficiency of the phosphorus removal process.

• Added benefits of pickle liquor dosing are reduction of scum, and improvement of sludge settleability.

• More frequent cleaning of sock aerators would be required when dosing with pickle liquor.

• Health risks due to aerosols associated with use of pickle liquor are insignificant.

• Quality control of pickle liquor supply should be rigidly maintained.

AWWA MEMBERS WANTED

REFERENCES BROWN and CALDWELL (1971). Water quality study of the Georges Ri ver System. Prepared by M.W.S. & D . Board, Sydney.

HEATH, C. W . and CHAN, J. (1985). Laboratory and plant scale trial using pick le liquor for Nocardia control in activated slud ge plants. Proceedings Inter-

national Convention of the A WWA (1985).

LIM , I and PSANG, P . Effect of pickle liquor dosing on filamentou s bulking.

Unpublished report.

A. J. MOSS Continued from Page 14 POSTMA, H. (1967). Sediment Transport and Sedimentation in the Est uarine Environment. In Esruaries, Pub/. No. 83, Amer. Assoc. for the Advancement of

Science.

SIMMONS, G. L. and CHENG, D. M. H. ( 1985). Rates and pathways of phosphorus assimilation in the Nepean River at Camden, New South Wales.

Water Res. 19(9), 1089-1095.

WOFSY, S. C . (l 983) . A simple model to predict exti nction coefficients and phytoplankton biomass in eutrophic waters. Limnol. Oceanogr. 28(6), l 144- 1155.

E.T.LOOS Continued from page 31

THE POSTER ENCLOSED WITH THIS ISSUE IS THE INITIAL MOVE IN THE DRIVE FOR MEMBERS CALLED FOR BY THE AWWA STRATEGIC PLAN.

Display it - push the message - the Association has much to offer - make· it known.

REFERENCES I. CHISWELL, B. and MOKHTAR , M. B. (1986). The Speciation of Manganese in Freshwaters . Talanta Vol. 33, No. 8: pp 669-667, 1986.

2. C HISWELL , B. and RAUC HLE, G. (1986) . Manganese in Water Storage Dams Proc.R.Soc.Qd 97: pp 53-67, 1986.

3. Churchill Fellowship Trust (1987). Hamilton, G. Report on the Growth of Biofilm in Water Supply Pipelines. Yet to be published .

4. Gold Coast City Council (198 5). Gold Coast Dirty Water Problem .

5. Queensland Department o f Local Government (1986). Survey of Water Supply Schemes in Queensland in regard to Manganese-related Dirty Water Problems.

May 1986.

6. Queensland Departme nt of Loca l Government (1987). Manganese Control Practices in the Brisbane City and Gold Coast City Water Suppl y Schemes. Yet to be published .

7. Q ueensland Department of Local Government ( 1986). The Application of Aeration / Destratification T echniques in Austra lian Surface Water Storages.

September 1986.

8. Queensland Department of Local Government (1987). Manganese Experiences in Water Supply Schemes Interstate. Yet to be published.

9. Shire of Pine Rivers (1985). Proceedings of ' Dirty Water Summit'.

IO. SLY, L. (l 986). Report on Investigation into Biological Manganese Oxidation and Deposition in the Gold Coast Water Distribution System. Uniquest Ltd. 1 l. TYLER a nd MARSHALL (l 967). Hyphomicrobia - A significant factor in manganese problems. Journal of the American Water Works A ssociation.

August 1967.

C. K. HERTLE and M. L. LEVER Continued from Page 20 24. U.S. EPA Process Design Manual for Sludge Treatment a nd Disposal. EPA 625/ 1-79-011, Washington D.C. ( 1979).

25. WALKER, J . D. Successful digestion - a review (unpublished paper).

Peabody-Wells Co ., 11linois (1978).

26 . CARROLL, W. D. and ROSS, R. D. A full scale compari son of confined and unconfined gas lift mi xing systems in a naerobic digesters. In : Sewage Sludge Stabilization and Disinfection. (Ed) Bruce, A. M . WRC Ellis Moorwood Ltd, Chichester (1984).

27. DEPARTMENT OF LOCAL GOVERNMENT QUEENSLAND. Gu idelines for Planning and Design of Sewerage Schemes. Prepared by: Engineering and Technical Services Division (1984) .

28. BAUMANN , P. G. and HUIBREGTSE, G. L. Evaluation of comparison of digester gas mixing systems. Journal Water Pollurion Control Federation. Vol. 54, No . 8 (1982).

29. O'CONNOR, J. (formerly Principal Engineer , T echnical Services Branch, Engineering and Technical Services Division , Queensland Department of Local Government) Personal Communication (1987).

30. WALKER, J . D. Anaerobic fermentat ion sludge digestion (unpublished paper) .

Peabody-Wells Co. 11linois (1975) .

31. DEGREMONT. Water Treatment Handbook. John Wiley and Sons, New York ( 1979).

WATER March, 1987

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