â&#x20AC;¢ DECEMBER 1988 JOURNAL OF THE AUSTRALIAN WATER AND WASTEWATER ASSOCIATION
EXECUTIV E DIRECTOR P. Hu ghes P.O. Box A232 Sydney South 2000 (02) 269 6814 Fax (02) 264 3668
Offici al Journal
AUSTRALIAN WATER AND WASTEWATER ASSOCIATION
Vol. 15, No. 5, December 1988
T. Smythe G. H. & D. P.O. Box 39 Railway Sq uare Sydney 2000 (02) 690 7070
FEDERAL SECRETARY G. Cawston Box A232 P.O. Sydney Sth ., 2000. (02) 522 1148
FEDERAL TREASURER J . D. Molloy , Cf· M.M.B.W. G.P.O . Box 4342, Melbourne 3001 , (03) 615 5991
BR ANCH SECRETARIES Canberra, A.C .T. · M. Sharpin , Wi l ling & Part. , P.O. Box 170, Curtin , A.C.T. 2605. (062) 815 811
My Point of View-Martin East . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Association and Industry News, Views and Comments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IAWPRC News . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Water Management Strategies for Mines in the Alligator Rivers Region -Tony McGill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Book R9view . . ..... .. .. . ... .. .. ............ . . ... .. . ... ...... .
The Limnology of Water Bodies in the Monsoonal Region of the N. T. - An Overview -K. T. Boland ........... .......... . ..... .. . ... .. . .... .. .
Haz Waste Update .................. .. .......... . ....... . . ... .
Control of Legionella in Cooling Towers and Warm Water Systems -L. J. Hedges .. ....... . .... .. . . ...... . . . . ....... .' ...... .
Development of Aquaculture and Irrigation in the Darwin Region '1 - Ian L. Smith . . .... ... . . ... .. . .... .... . . . ........... ... .
Chlorine Dioxide and Related Ions in Water Treatment (1987) Summary of Toxicity Data -Dr. W. J. Masschelein .. . ..... ...... .. .. ...... . . .. . ..... .
Monitoring of Sewage Flows in the Darwin Region -L. Monteith .... .. .. ... ... ..... .. ...... . ... .. . ... . .. ... .
Watercomp '89 - A Timely First -Peter Manuel ... .. .. .. ........... ... . .. .. ... . .. .. ..... .
Mosquito Breeding and Sewage Treatment in the Northern Territory -Peter I. Whelan .... ....... .... .. ... . . . ... ... . ... . . . . ... .
Commercialization in the Water Industry . ...... . ................ .
Calendar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Products• Plant• Equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
New South Wales Mrs S. Tonkin-Hill , Sinclai r Knight & Part. 1 Chandos St., St. Leonards, 2065, (02) 436 7166
Victoria J . Park , Water Train ing Centre ,
P.O. Box 409, Werribee , 3030. (03) 74 1 58 44
Queens land D. Mac kay, P.O. Box 412, West End 4102 . (07) 844 3766)
Sout h Australia R. Townsend , State Water Labora to ries , E. & W.S. Priva te Mai l Bag , Sal isb ury, 5108. (08) 259 024 4
We stern Austra lia A. Gate , Binnie & Part Pfl , P.O. Box 7050, Cloisters Square, Perth 6000 (09) 322 7700
Tasmani a A. B. Denne P.O. Box 78A , Hobart 7001 (002) 30 5562 Northern Territory
P. Abbey, P.O . Bo x 37283 Winne l lie, N.T. 5789. (089) 89 7290
ED ITORI AL & SUBSCRIPTION CORRESPONDEN CE E. A. Sw inton 4 Pleasant Vi ew Gres . Glen Waverley 3150 (03) 560 9306 (Previo usly G. Goll in)
ADVERTISING Ann Syke s Appita 191 Royal Parade, Parkville 3052 03 347 2377
COVER PICTURE At the Ranger Uranium Mine, Jabiru, storm run-off from ore stockpiles can contain elevated levels of uranium . A drain around the stockpile area collects the run-off and diverts it to a Retention Pond from where it is used as process water in the production of yellow cake . See the Water Management Strategy paper in th is issue. Photo co urte sy of Northern Territory Departm ent of Mines and Energy
The statements made or opinions expressed in 'Water ' do not necessa rily reflec t th e views of the Australian Water and Wastewater Associatio n, its Co uncil or committees .
WATER December, /988
Water Management Strategies for Mines in the Alligator Rivers Region Tony McGill INTRODUCTIO N Th e possibility th a t co nta min a ted water from min e sites in th e Alligato r Ri vers Region mi g ht ca u se e nvi ronm e n ta l da mage d ownstream has been th e m ain argument ad vanced against new projec ts in th e a rea . The a rgum ent generall y ta kes th e form that th ere is a lack o f kn owled ge and nothing should be do ne o r a t least grea t caution should be exercised until that kno wledge has been o bta ined . T he proponents of this ' no risk ' type of ar~ument do not see th e need to provide evidence of detrim ent in order to asse rt its likelihood . The scientific meth o d a pplied to uranium mining a nd water man age ment in th e Alligator Ri vers Region ca n be generalised into two basic problem solving strategies, th e first bei ng to di vide th e problem into its comp onent pa rts a nd ana lyse each component in depth a nd th e second to adopt a n holistic a pproach and increase th e sco pe of the problem so as to include such m a tters as the arms race a nd social concern s. Neither of th ese stra tegies has produ ced complete solutio ns to ma ny o f the problems still rem a ining nine years after mining commenced. The dilemm a of solution to problems with onl y in compl e t e in fo rm a ti o n available confron ts enginee rs daily a nd solution strategies have evolved to ena ble proj ects to proceed . T ypically, translation of data from elsewh ere is used , or a stimulus is increased until a response is obse rved , a nd then a fac tor of safety is applied . This methodology is a lways coupled with a ' mo nitoring of performan ce' phase. It has yet to be approve~ in th e Alligator Rivers Region for assessing such subj ec ts as th e environm enta l impact o f wa ter releases a nd has bee n strongly challenged in th e area of water treatment . . by la nd a pplication. A ll mines in th e Alligator Rivers Region , opera tin g a nd proposed, are prohibited from releasing an y water fr om th e Restricted Rel ease Zones, regardless of quantity a nd qu ality, excep t in obscurely defin ed extr eme circum sta nces , a nd regardl ess of th e cost o f the consequences of this prohibition . It is to be hoped that this situation will soon change . This paper describ es how minin g compa nies have adapted to the very specia l regula tory envi ronm ent of th e A lligator Ri ve rs Region and m a kes some suggestions as to how imp rovements to water ma nagement can be effected .
IMP ACT OF CLIMATE ON WATER MA NAGEMENT STRUCTURES While the reliability o f the rain fa ll in the Alliga tor Ri vers Region is high on a n annu a l basis, th e ra infall varia bilit_y during both th e dry a nd we t season s 1s quite significant a nd is equ a lly importa nt in th e design of water ma nagement
systems. T he in tensity of rain fa ll can also be high , tax ing spillways, di versio n cha nnels a nd tra nsfer structu res. In addition , water retai ning stru ctures must be designed to withstand waves ge nerated by cyclo ni c winds. Average a nnual rain fa ll is aro und 1500 mm a nd evaporatio n generally exceeds average rain fall by so me 600 mm per year. M onthly pa n evapo ration fo llo ws an extreme a nnu a l cycle, being highes t in the wet seaso n a nd lowest in the d ry (Fig. I) . The excess of evaporation over a nnu al ra in fa ll su gges t s t h e po ss ibility of designing water ma nagement systems which use evaporation as the m ain o r only so urce o f water disposal.
Tony McGill is th e Principal Water Engineer with th e N orthern Territory Department of M ines and Energy. He has worked fo r both the Common wealth an_d the No rthern Territory Governments in the en vironmental control of uranium mining in the A lligator Rivers Region since 1979.
LEGISLATIVE FRAMEWORK Northern Territory Legislation
In t h e Nort h e rn Ter rit o r y, t h e Common wealth gove rnm ent sets the generalised gro und rules a nd the T erritory go vernm ent regul a tes the detail. It is this mix of legislat io n whi ch sets th e bounda ry condition s for m inin g a nd wa t e r ma nagement in th e A lligator Rivers Regio n . W ith 39 separate Commo n wealth a nd Northern T errito ry Acts re la ting to environmental protec tion th ere is some sco pe for confu sio n , a nd wo rkin g arra ngements have been adopted fo r cla rity . T he powers of gove rnment in the Northern Territo ry are no t necessarily applicable anyw here else in A ustralia a nd th e followin g a re examples of legislation w hi c h h ave a n eff ec t o n wa t e r m a nagement.
Uranium M ining (En vironment Control) Act T his is the principal Ac t relatin g to th e day- to-d ay regulation of th e uranium min in g i ndu s tr y in th e No rth e rn Territory. The act sugges ts m atters with r es p ec t to whi c h co nditi_o n s _of authorisation may be imposed, m cl udmg matters rela tin g to ' the design a nd operati o n of water m a nagement sys tems. W hile the Act contains a number of general enviro;iment control conditions, m an y of which pa raph ras: C omm?nwealth legisla tio n , the key to its o perat10n is th e requirement of section 8 th a t ' the owner or m a nager of a mine shall not constru ct o r use a ny works, processes or equipment with respect to mining except with a nd in accordance with conditions of an a uthorisation '. T he principa l environmental conditions. fo r water management are: • C on trol o f Water T he Act specifies that th ere sha ll be designa ted res tricted release zones from whi ch rad ioac tive ma terials should not be remo ved or allowed to esca pe except under the conditions of a government a pproval.
Commonwealth Legislation - Environmental Protection (Impact of Proposals) Act 1974 - Atomic Energy Act - En vironmental Protection (A lligator Rivers R egion) Act 1978 - E n vironm ent Pro tection (N uclear Codes) Act 1978 - A boriginal L and Rights (N orthern Territory) Act 1976 450 R 400 a 350 i · n 300
f a I I
2 50 200 150 1 00 50 0
-1~ ·,..__, / t
_,, . f ')( ) "~, •/
0 . 80
0 .60 0.40
.,.._ NABARLEK -A-
·><- PINE CREEK 0.20 •
·•- '2-/AP. FACTOR
SEP a:;r N'.JJ CE JAN FEB MAR APR MAY JUN JUL PlG MO'-ITH
Figure 1. C ompa rison of regional monthl y rainfall. W A T ER December, 1988
â&#x20AC;˘ Technology In a ll cases the 'best practicab le' technology must be used. This clause was the source of a major study into the application of best practicable tec hnology (BPT) to the water management system at the Ranger Urani um Mine and whic h concluded that controlled release to the Magela C reek was part of BPT for that mine . The finding was politicall y controversial has not been acted upon. â&#x20AC;˘ Monitoring The monitoring program is to be designed to continue witho ut interruption, and include measurements in relation to biota, water, sediments and air within the project area and include both a personnel and environmental monitoring program. â&#x20AC;˘ Research T he company is to provide a research program as directed by the government. All of the above controls place a considerable burden on the companies and . place a heavy onus of responsibility on the supervising authorities to see that the requ irements are being met and correctly interpreted. To understand how these environmental controls are put into effect it is instructive to look at the design approaches to water management being adopted by the mining companies.
ADOPTED DESIGN STRATEGIES As a result of a political lack of suppo rt for testing the safety of releasing excess mine water to the environment, all existing and proposed mines in the Alligator Rivers Region have adopted 'no release' water management systems, each with its own site-specific characteristics.
Koongarra Proposal In summa ry, the proposal is to: (i) Mi ne and stockpile the major ore body over a period of between 18 months and two years. (ii) Construct a mill and associated evaporation ponds over a period of three years. (iii) Treat a ll ore with a grade of greater than 0.1 CJ/o UJOs . (iv) Import sulphu ric acid for leaching of ore. (v) Generate electricity on site. (vi) Operate a 'no intentional release' water management system. (vii) Progressively rehabilitate the mine, tailings ponds and evaporation ponds, returning the land to as near as possible to pre-mining conditions. The advantages of this proposal are: (i) The quantity of water to be evaporated is less than with a longer-term mining period, thus minimising the size of evaporation ponds. This in turn reduces the amount of environmental disturbance and further shortens the operating life of the project. (ii) In the revised design, the mill and evaporation ponds are confined to a single compac t location. (iii) Less risk of release of sulphu r dioxide fumes, since sulph uric acid wo uld not be manufactured on site. 14
WATER December, 1988
(iv) A small wo rkforce during the treatment phase, since mining would have been completed before milling started. (v) As the pit is required to remain stable only during the mining phase it can be constructed with steeper batter angles, thus reducing the volume of material mined which in turn reduces rehabilitation costs. (vi) T he shorter operating life should lead to lower radiation expos ures for the workfo rce . (vii) Rehabilitation is designed to commence almost immediately and continue progressively. The dams have been designed to impound a I in 10 000 year high ra in fa ll with provision for overflow from one pond to the next. Any overflow is then directed to the pit. A perimeter seepage drain will be installed to intercept and recycle seepage. The water management proposal is conceptually neat with a low ratio of catchment to evaporation area.
Queensland Mines Limited Nabarlek The shape and location of the orebody at Nabarlek made an open cut approach the most economic solution. The highmining method vo lume, cont inu ou emplo yed by the operator is usuall y only see n in much larger m ines. The remoteness a nd difficult working conditions initially caused high staff turnover and one of the more unusual problems was that since the mine is on Aboriginal land, road access is restricted, and workers are only allowed to enter or leave the lease by air. The mining at Nabarlek was performed by contract over a period of 128 days, simultaneously with earthworks for dams, and construction of the plant. As wit h the Koongarra design the rapid mining concept presented many advantages: (i) efficient pit design, with a high ore-towaste ratio owing to the short mining period . (ii) mining was performed at present cost wit hout the need for capital outlay for mining equipment and ancillary services. (iii) the excavated pit became an immediate receptacle for below-grade tailings disposal as well as an emergency flood control sump . (iv) Waste rock was immediately available for construction purposes as it was mined . With the imminent completion of mill ing of the Nabarlek ore stock pile, attention was given to reducing the water inventory being held in the evaporation ponds to reduce the time required for re habilitation. The procedure finally adopted was a combination of spray irrigation and flood irrigation on areas adjacent to the evaporation ponds. While there have been some environmental effects arising from the use of this water disposal method, it has meant that the ponds will be empty and available for immediate rehabilitation at the time of cessation of milling. In this case the environmental tradeoff was between a small increase in the impact area and a reduction in the duration of the impact.
Ranger Ura nium Mines -
The water management system at Ranger has been the subj ect of numerous investigations and enq uiries since the production of the company's first Environmental Impact Statement in 1974. The conceptual principle being adopted by the company involves containing and concentrating the worst-quality effluents from the process in the tailings dam, and releasing impounded runoff water from the least-contaminated catchments at times of high stream flow in the adjacent creek. The waste-rock dump runoff is diverted to settling ponds which overflow to downstream billabongs. Some control of th is overflow is possible but if the release of this water were to be prohibited, a complete redesign of the present water retaining ar.d evaporatio n stru ctures on the mine wo uld be required. In the long term the ex isting wa ter management system is well balanced and fine tun ing is avai lab le to cover prolonged wet or dry spells. Land appl ication of cleaner water during the dry season can remove short term excesses and the mill is ab le to run directly from borefields during drier periods. Seepage into the pit is controlled by dewatering bores and the water from these is flood irrigated on an adjacent area of land . So fa r, the quantity of seepage into the pit has been small. However a wetter mine operating in this area could have water management difficul ties. The tailings dam, being the most contaminated impoundment, is required to hold a I in 10 000 year wet season wit hout failure. It is presently operated using a semi-dry tailings management system which redqces the impact of seepage through the floor and walls of the dam. This method a lso allows the tailings to achieve a higher density of deposition than the previously used sub-aqueous method, hence the stored tailings volume is reduced and final in situ rehabilitation is made easier. T he impact of wave action on the inner batters of the dam is also eliminated. On the debit side, subaer ial tai lings deposition increases the radon exhalation rate and the possibility of dusting from the surface. It also removes a proportion of the evaporation a rea from the water management system, but the impact of this is not yet known. The huge size of the Ranger orebodies has precluded the optio n of pre-mining. However mining in advance of the milling rate is taking place and the pit should become available for tailings storage in 1991 . The company proposes to sequentially develop the orebodies in the project area and return fut ure tailings to the present pit rather than constructing add itional tailings dams. The fina l placement of the tailings in the existi ng tai lings dam has yet to be determined.
Pancontinental Mining Limited Jabiluka The Jabiluka ore body is huge compared with Nabarlek, Koongarra and Ranger and it is located at such depth that
an open cut operation would be inappropriate. Access to the orebody will be by a decline with an underground crusher located approximately 400 m from the surface. From the conveyor portal, a surface conveyor will transfer the ore 6 km to the ore treatment plant. From a water management point of view, the underground operation has the advantage that the catchment area for rainfall is reduced . In addition, the company proposes to return approximately half of its tailings underground to the open stopes after stabilization with cement. The water management system follows a 'no-release' principle with the following characteristics: (i) contaminated catchments are isolated by topograp hy or cut-off drains and embankments from clean catchments. (ii) all runoff from contaminated catchments is to be diverted into sumps for subsequent evaporation or use as make-up water in the process plant. (iii) by-pass pumping systems are provided to transfer water to the tailings pond in the event of extreme climatic conditions. (iv) the ore-treatment plant is surrounded by bunds, and spills are redirected to the mill from containment ponds. (v) all containment structures are lined either with impervious clay, membrane liners, or concrete, to prevent seepage. (vi) the tailings pond is the final repository area for all contaminated water. (vii) the storage requirements were determined using predicted rainfall and evaporation data for the 1 in 1000 three year wet period and sufficient operating freeboard to contain a 1 in 10 000 year extreme rainfall. (vii) The company proposes to rehabilitate the tailings dam in situ. When complete, it will contain approximately 26 million tonnes of tailings and 10 million tonnes of waste rock and cover an area of 176 ha. The milling process is different from that of the other three mines in that it will also recover gold using cyanide leaching.
GENERAL COMMENTS ON WATER MANAGEMENT FOR MINES IN THE REGION The design climate To date there has not been any consistent approach to determining rainfall return periods, evaporation or runoff. The Alligator Rivers Region covers an area of approximately 25 000 sq. km with Jabiru close to its centroid . Long term meteorological records are available for Oenpelli (1910 to present), Pine Creek (1874 to present), Katherine (1872) to present) and Darwin ( 1869 to present). While there is good agreement between the rainfall records at Jabiru with Oenpelli, Pine Creek and Katherine, such agreement does not hold between Jabiru and Darwin. Synthetic data for Jabiru created from the complete Katherine, Oenpelli and Pine Creek records yields much the same result. Data from the Ranger operation show a st rong linear relationship between monthly rainfall over a wide range of
\GOO,- - - - - - - - - - - - - - - - - - - - - - ----..--~--,.......... , , 0 1400
362 .946, A-squared : .923 0
.,. ,, GOO
o,,,,, "".,.,, "O , , , ,
.,..,.--,,,"'.,.,."o o 0
0 0 Âˇ200
300 400 MONTHLY RAINFALL (mm)
Figure 2. Relationship between monthly rainfall and volume change at Ranger.
return periods which could be used as a guide for water management design (Figure 2). It should be reasonably safe to translate the meteorological data throughout the region with minor adjustment for latitude. Orographic effects were noted at Koongarra but are likely to be minor in a regional context.
segregating water of different qualities within the restricted release zones. This may give rise to areas adjacent to the mine where limited and controllable impact occurs. Examples would be land application areas and the mixing zones at water release sites . Fairly intense monitoring needs to be carried out in these limited impact zones both for changes in chemistry and behaviour of biota.
Impact of project area size As a general rule, the smaller the area of the project the smaller the environmental impact is likely to be. In practice the shape and location of the orebody and its overlying topography are likely to be the major influences on the size of the project and the design of its water management system. Neglecting the impact of seepage, the ratio of the catchment to the evaporation area gives some indication of the need to release from a given water management system (Table 1). In this context, Ranger is most susceptible as it has the highest catchment to evaporation ratio. Another way to look at this concept is that an additional 160 ha of evaporation surface would be required to bring the Catchment/ Evaporation area ratio at Ranger down to 1: 5 and this would create a major environmental impact in itself.
Control of 'Restricted Release' Water Not all water which falls on a mining project area is likely to pick up chemical contaminants. Much of the area is merely exposed waste rock stockpiles which may or may not give rise to acid waters, depending upon its sulphide content. This is not a problem with existing mines. Other areas may be exposed to periodic spills and overflows and the water quality can be quite variable. A common strategy employed in mine design is to contain and concentrate the most contaminated water while diluting and dispersing (perhaps after treatment) water of better quality. This requires
Lining of ponds The requirement to line ponds is related to the quality of water they may contain and hence the.,need to restrict seepage. In general, containment bunds around process tanks are made of concrete or have synthetic liners. In the case of the Ranger tailings dam it was calculated that the attenuation of seepage which would be attained by the use of synthetic liners would not be significantly less than that which would occur naturally because the tailingsâ&#x20AC;˘ themselves act as a low permeability blanket. A cutoff trench to weathered rock combined with upstream blankets and grouting in zones of suspected high permeability appears to have provided a satisfactory solution for the tailings dam as there has been only minor contaminant migration from this structure. At Nabarlek, one metre of very low permeability compacted clay was used on the pond floors and this also appears to have been a satisfactory solution. TABLE I. RATIO OF CATCHMENT TO EVAPORATION AREAS FOR OPERATIONAL AND PROPOSED WATER MANAGEMENT SYSTEMS IN THE ALLIGATOR RIVERS REGION
Nabarlek Koongarra Jabiluka Ran ger
Catch / Evap
66 116 275 280
45 108 200 133
WATER December, 1988
Seepage return A fea ture common to a ll th e mine designs has bee n the provision of seepage collectors around the containment structures which collect see page in a ce ntral sump and return it to the water management sys tem for subsequ ent evaporation. However, the quality of the wa ter being returned from the seepage collector sys tem at Ra nge r would indicate that most of the water being returned is coming from outside th e restricted release zo ne which is in effect increasing the catchmen; area . The environmental benefi ts assumed from seepage return may not outw.eigh the cost of its reco very at Range r as the relati vely small vo lume of seepage has not been found to create undesirable effects whereas the impact of addit ional water to th e water management system can ha ve serious consequences.
Failsafe design The use of spillways a nd diversion structures rather than pumps a nd pipes is to be encouraged. Where th e terrain allows, the pit should be utilised as an emerge ncy sump in the event of equipment breakdown or a structural fai lure.
Construction material§ In a minin g operation these are usuall y won ?n sne. J-:Iowever, timing and geological contraints may mean that suitable rock may not be availab le when required . In the above mines a co mpromise was adopted. At Ra nge r the tailings_ dam 1s co nst ructed in stages . At Jab1luka th e construction material wi ll come from a carefull y sited quarry. The waste rock at Nabarlek, being sc hist , was prone to weathering and large 'sacrificial ' boulders were used for erosion protection.
BOOK REVIEWS FUELWOOD" PRODUCTION POTENTIAL ON DEGRADED AND UNDER-UTILIZED SITES E. B. Allender, Land Energy Pt y Ltd 1988. ' 85 pages, $10.
Allender's report documents resea rch into eucalypt plantation sil viculture on sites "of und er- utili zed lands whi ch a re: •. unprodu ctive due to induced degradatton a nd salinisation level toxic to co nventional horti cultural and agr icultural crops. .• effected by rising wa ter tab les (e.g. adjace nt or down st ream of inten sive irrigation areas), are subj ec t to periodic inundat ion th erefo re rendering them un sui table for co nventional horti cultural an d agricul tural crops. •. suitabl y located and on appropriate sites to a llow th e re-use of industri a l wastewater a nd muni cipa l effl uent to enhan ce growt h rates". Trial sites were located a t Loxton Ba rmera, Waikerie, Boli va r , Keyneton: Bern and Winkie. The tria ls a nd performance of va riou s species of eucalypt planta tion at eac h of the sites is described. It was found that sustaina ble yields of over 20 torm es/ ha/ yea r co uld be ac hi eved using irrigated effluent, compared to a round 0.4 tonnes/ ha / year sustaina ble yield und er natural conditions. Euca/ypIu_s occ1den1a/is 1s performing well in so me tri a ls, and o_n free drainin g so ils eu ca /yplus grand1s is th e most success ful species of those assessed . The report provides useful data on es ta blishment of eucalypts on deg raded lands, and hi ghli ghts the potential for the return of such la nds to productive use.
SUMMARY Despite legislati ve fru strations, all min e proposals for the Alligator Rivers Region to date have introduced innovative water management sys tems. With the benefit of nine years of hindsig ht we are a ble to co nclude that the water manage ment systems being adopted by the minin g proponents do not lead to detrimen t in the wider environment over a wide ran ge of climatic condition s. It is clear that uranium mines can quite happily exist without detrim ent to other land users in World Heritage • areas. 16
WATER December, 1988
P. NADEBAUM Scott & Furphy
C.R. WEEKS G.H. &D.
Provision for progressive rehabilitation One method of reducing the catchment area of a mine is to progressivel y rehabilitate areas which are no longe r bein g used. The Koon ga rr a proposal removes the pit area as a catchm ent immediately after mining and uses progressive rehabilitation of the ta ilings pits as a feature of its design . Progressive rehabilitation has the advantage of reducing the time of the environmental impact and allows rapid access for a lternative la nd use .
electroplatin g rinse waters; recovery o·f heavy metals fro.rfl wastewaters: recycling of co nd ensate co nstitu ents: reuse of sewage effluents; recove ry of organic c he mical s; sta bilit y of ion -exc ha nge resins; co ntacting equipm ent for co nt!nuous processes, optimisation of operations: a nd finall y, a n exte nsive catalogue of so me 45 pages outlining technical data of co mmercially avai lable resin s. Each chapter includ es a co mprehensive list of references, and th e book will pro ve a va lua ble reference so urce to worke rs in the fi eld . The diffi cult y of releva nt literat ure today in a particular technical field must not be und erestima ted , a nd it is of interest to note that there are so me 1700 papers per a nnum dealing with ionexc ha nge in ge neral, so me 200 papers per a nnum dea ling with wastewater treatment by ion -exchan ge, a nd so me 170 papers per a nnum dealing with the recove ry of water a nd chemicals by ion -excha nge. The authors no te that ion-exchange is often co nsidered as being too specia lised a technique for use in wastewater treatment , and it is their intent ion that the book should highlight the adva ntages of ion-exchan ge so th a t its benefits ca n be more wide ly appreciated. The book do es this admirably, a nd will be appreciated for its lucid style and the thought which has gone into co nd ens in g th e extens ive li te rat ure into a n accessible and usefu l text. The book is stro ngly recommended for th ose practising in th e fi eld of industrial wastewater treatment and fo r those who wish to bette r understand the science an d engin eeri ng of io n-,exc ha nge .
RETI CULATION SEWER/REHABILITATION Anderson et al WASTEWATER TREATMENT BY ION EXCHANGE B. A . Bolio, Division of Chemical and Wood Technology, CSIRO, Melbourne. L. Pawlowski, Department of Environmental Chemistry and Technology, Polytechnic of Lublin, Poland. 1987. E. & F.N. Spon, London. 262 pages , 24.50 pounds.
This publication is a we lco me addition to the li terature on wastewater treatment. Th e book is wr itten for the researcher a nd engineer, a nd presents a com prehensive review of th e literature in the field, a nd a lso draws on th e co nsidera ble ex perience of the authors over many yea rs of in novative resea rch and development in the field. Th e book is di vided into 15 chapters dea ling with ion-exchan ge materials; ion ~ exc hange reac tions applied to water and wastewater tre a tment ; reno va tion of exhau sted process liqu ors; recove ry of mineral acids a nd their salts by siteshar ing a nd ion-retardation tec hniques; recovery of water an d chemi cals fro m
In Figure 4 of this paper, p. 44 of rhe October iss ue, graph la belling is reversed , 'Co lla pses' and ' lengt h ' should be reversed .
REVIEWS AND CORRESPONDENCE The columns of 'Water' are open to Members and nonmembers for comment on any . pertinent industry topic as a 'Letter to the Editor' or on the contents of this Journal. Also, books, publications and articles relevant to the AWWA sphere of interests may be submitted for review. Address: The Editor, 'Water' 4 Pleasant View Cres. Glen Waverley 3150 Ph. (03) 560 9306
The Limnology of Water Bodies in tke Monsoonal Region of the N. T. An Overview K. T. Boland ABSTRACT Increasing exploitation of lake systems in Australia's monsoonal north is generating considerable change in th e morphometry and water qu ality of many systems. The paucity of research on the effects of exploitation should be addressed by way of investiga tions into the capacity of th ese lakes to wi thstand the impact of increased use.
Kevin Boland, BSc(App) is th e Senior Limnologist for the Northern Territory Po wer and Water Authoriry. He is in charge of the Authority's Microbiology and ln vesrigations Laborarory (Water) and has extensive experience in many aspects of monsoonal limnology.
INTRODUCTION Kevin Boland
~ackground While many studies have been und ertaken in th e monsoonal north a broad characterization of its water bodies rem ains conspicuously absent. Certainly the variations in water body morphometry and the seasonality of billabongs and remnant lagoons makes the task difficult, but with the increased use and exploitation of these reso urces it is imperative that we develop an understandin g of th e processes affecting water quality within these systems.
The two most prominent influences on water qualit y processes in the ' Top-end ' are water temperature and the annual rainfall regime . Elevated water temperatures mean that significant stratification can occur throu gh a much lower temperature differential than in cooler more temperate regions. For example the increase in water density from 32 C to 31 C is more than twice th at which occurs from 16 C to 15 C. This refle cts on the stability of stratification in the region as well as its inherent energy. The monsoonal rainfall regime is such that while the annual rainfall may not approach levels found in rain-shadow areas of North Queensland, it fa lls in a relatively short period - A sort of annual flood, followed by an annual drou ght. The cycle results in a rapid recharge of water bodies followed by a long period of evaporative concentration. This has been interpreted by many as a yearly 'flush' of the regions lakes and billabongs. However , the term 'flush' is misleading especially when app lied to we tland billabongs wh ich undergo yea rly inundation . Because of the vast areas of flood plain in these regions the ac tual water velocity through man y of these systems is not as great as may be imagined and certainly not enough to remove all sediments, aquatic macrophytes (etc .) from the water bodies, thus returnin g them to pristine conditions.
DISCUSSION Stratification Stratification occurs in all Top-end lakes. Hart and MacGregor (1978) compiled some information on billabongs in the Magela Creek region. This indicated that stratification in th ese small water bodies may not vary greatly with diurnal temperature fluctuations and may only be significantly broken down with the wet season flow. This study was however, conducted in a short period of th e wet season (December 1977 and January 1978) and takes no account of dry season effects. Boland (1986) , in four investigations of Jabiru Lake from October 1985 to August 1986, found that the lake exh ibited strongest stratification in February 1986, but that some degree of stratification existed in all four study periods. The most comprehensive stud y of top-end stratification is reported by Townsend and Boland (1987). They concluded that stratification and mi xing in the ' top-end' is most influenced by seasonal rather than diurnal effects. The main mixing period is in th e middle of 18
WATER December, 1988
the ' dry' season , May-Ju ly, and a less predictable mi xing period may occur in co njunction with wind effects in the wet seaso n (January/ February). It is likely, excepting annua l flow conditions in rive rine systems that 'top-end ' standin g wa ter bodies, even small billabongs, will follow a similar pattern with diurnal effects having littl e influence.
Fish-ki lls in th e mon soonal region are a relati vely fr equent occurrence. They generall y coincide with localized pre-wet seaso n rains a nd may be related to the stratification regime present in the system effected. While little research has been conducted, va rious cases in ves ti gated have shown a severe depl etion in dissolved oxygen. This is caused by low temperature run-off water carrying organic materials with high instantaneo us/ post instantaneous oxyge n demand into a water body -causing dissolved oxygen sag throu ghout the wa ter co lumn and hence fis h mortality. This phenomenon may be accompanied by inorganic or orga nic ichthyocides in the run -o ff water. Metals, for example iron aluminium a nd manga nese, may be mobilized by low pH run-off water. On the other hand Tremolieres (1988) has shown that some Canadian trees produce chem icals which deox yge nate water at a rapid rate and / or produce materials with direct toxic effects. The relationship between Australia's northern flora and water quality is only sparsely understood and considerab le research is required to clarify this link .
Light Kirk (1977) described four components of the aquatic sys tem which contribute to the light absorption coe ffici ent for any given water body . Firstly there is the water itself which absorbs light throu ghout th e visible spectrum with absorption being strongest at the red end. T he second component is 'yellow substance', a complex mi xture of breakdown products, mainl y polymerized oxidised ph enolics. This factor is lik ely to be relevant in the top-end in systems where seasonal growth and subsequent decomposition of algae and plants is a yea rly occurrence. Evaporative concentration of billabongs and lakes will tend to enhance the importance of this fa cto r as th e dry season progresses. Kirk's third co mponent is the absorption capacity of phytoplankton in th e water column. This aspect of the absorption coefficient is dependent not only on gross algal load but also on species dominance. As with 'yellow substance ' thi s will vary in the ' top-end ' environment with seasonal factors. H ence primary production or the trophi c state of any water body may vary considerably through the seasons. These fluctuation s in trophic status have an obvious bearin g on the capacity of a system to support fish populations and may in some cases be linked to annual fi shkills previously discussed .
The final component which co ntributes to the total absorption coefficient is non -li ving suspended partic ulate matter , often term ed suspended solids and related closely to turbidit y (in the absence of strong co lo ur a nd / or hi gh colloid. levels). This facto r is th e most obvious variable, between sys tem s a nd seaso nall y within a give n sys tem , in the monsoo na l region. Wh ile ambient variat ion is significant the man -made increase on base levels of suspe nd ed solids is criticall y relevant to th e maintenance of water qualit y. It is clea r that so me regions of the 'to p-end' are undergoing radi cal changes as a res ult of di sturbances which effect th e level of suspended so lids avai la ble for mobili zation with a nnual run- off. The clearest examples are popular fi shin g areas on the Mary Ri ver sys tem where a proliferation of access tracks has caused severe di sturbance of surface soils and grasses in the seaso na lly littora l area of the flood plain. Another aspect of suspend ed solids is th e tendency of new artificial lakes in the top-e nd to ex hibi t hi gh turbidity levels in th eir early stages . This has been obse rved in the Marlow Lagoon development in Palmersto n and in the early stages of Jabiru Lake and Darwin Ri ve r Rese rvoir. This turbidity delays the es tablishment of rooted macroph ytes which stabilize sediments and preve nt th eir resuspension through periodic na tural turbulence .
HAZWASTf: UPDATE In all future issues of the journal it is intended to include an update on news items relevant to the man age ment and di sposal of hazardous waste . Federal Co uncillor Errol Samuel , acting in the role of an assoc iate editor, will coordinate this activ it y and should you have an y material for inclusion please do not hesitate to contact him on (02) 412 1388 or c/ o Metropolitan Waste Disposal A uthorit y, P.O . Box 699, C hatswood 2057, Sydney, Australia .
PRE-TREATMENT OF METAL-CONTAINING WASTEWATERS BY CONTINUOUS ION EXCHANGE CSIRO's Di visio n of Chemi cals a nd Pol ymers has co mpl eted a stud y of treatm ent of metal fini shing wastewaters containing toxic metal ca tions by cont inu ous moving-bed ion exchange. The novel ion exc hange process, wh ich uses a movi ng bed of magn eti c resin , was developed origina ll y for treatment of potable water, and its app licat ion to desalina ti on and dealkali za ti o n was d esc ribed at th e Assoc ia tion 's 198 1 an d I 983 Federal Co nvent ion s. The diagra m represents the 40 litre per minute pilot pla nt used in th e work on metal finishing wastewaters.
Macrophytes Levels of aquatic macro ph ytes have a signifi ca nt influence on wate r quality. In the top-end the level of mac rophyte grow th is go ve rned by ava ila ble nutri en ts a nd th e a mount of sedim ent ava ila ble (there are few native flo a tin g macroph ytes in the monsoo nal region). It is possible that increased sediment in run-off co uld have the effect of chan ging the profile shape of so me waterbodies from a more or less ri ght angle section to a shallow hem iellipti c one. This would th en favour increased growth of macrop hytes o ut into the wa terbody. McKenna (1984) co nt ends th at lac ust rin e sys tems can be affected by mobili zati o n of nutrients from plant tiss ue degradation and act ive macroph yte growt h . This implies that macro ph ytes contribute to th e availab le nutrient pool that fertilizes lake water and sediments, sub sequently promoting even more grow th .
___.- - REGE NE RATOR _,....,..-h 300 cm f 15cm
~ h 200cm , 33 cm
FEED LOADED RESIN RECYCLED CONCENTRATE ( LUBE WATER)
Moving bed ion exchange pilot plant.
There is extreme seaso nal va riability in water quality of mon soo nal regio n lakes. Nat ural facto rs affectin g this variabilit y will be exacerbated by increased access ibilit y to pop ula r areas a nd conseq uent di sturbance of the nat u ra l environmen t. The ab ilit y of monsoo nal lake systems to acco mmodate th is change remains largely a matter of co njecture . Howeve r, th e quantification of common features is a n esse nti a l base to understand and predict the significance of inevitable impac ts.
REFERENCES BOLAND , K. T . (1986). An nu al Report - J ab iru Lake Investiga ti on, October 1985 to August 1986. N.T. Department of Min es a nd Energy, Water Resources Divisio n . (In terna l re port) HART, B. T . a nd MACGREGOR, R. J . ( 1978). Limno log ica l Survey of Eight Billabongs on the Magela Creek Catchment. Cau lfield In st itute of Technology, Water Studies Centre, Report o. 9. KIRK, T. O. (1977). Atten uat ion of Light in Nat ura l Waters. Aust. J. Mar. Freshwater Res. 1977, 28. McKENNA, K. A. ( 1984). Re la ti o nships between Aquatic Macrophytes a nd the Chemical a nd Physical Compos ition o f the Substrate in Ka hle Lake, C la ri o n Venango Co unti es, Penn sylvan ia. Ecological Assessment of Macroph ytes Collect ion, Use a nd Measuring Da ta. ASTM STP 843, W . H. Dennis and B. G. Ison , Eds, American Soc iety for Test ing and Materia ls. TOW SEND, S. A. and BOLAND, K. T . ( 1987). Reservoir Strat ificat ion a nd Mixi ng in the Monsoo nal Region of the Nort hern Territory in Techn ical Papers, 12th Federa l Convent ion, Au stra li an Wa ter a nd Was tewater Assoc iat ion. TREMO LI E RES, M. ( 1988). Deoxygenating Effect and Toxicit y of Grou nd-U p Dried Conife rous Need le and Deciduous Leaves of Canad ian Trees in Water A Preliminary Study in Comparison with Litter of European Trees. War. Res.
Vol: 22, No. I, Pergamon.
The stud y, funded by th e MMBW's Industrial Waste Grants Scheme , showed that the level of heavy metals such as nickel in wastewa ters destined for sewe r di sc harge co uld be lowered from about 50 mg/ L to well u nder 10 mg/ L whi le the concentration of the regeneration effl uent was above I g/ L. After polishing steps such as carbo n so rption of orga ni cs a nd sa nd filtration th e trea ted wa ter co uld be rec ycled to rinsing and in favo urabl e cases th e metal va lues co uld a lso be recovered . Operating cos ts a re low and overall the process appears co mpetit ive with precipitation a nd landfi lling of low solids-co ntent sludges . A repo rt on the investigation, titl ed 'Treatm ent of Industrial Wastewaters with Magnetic Ad so rbent s for H eavy Metal Ion Recove ry ' is ava ila ble from th e MMBW Industri a l Waste Grants Co mmittee, PO Box 4342 Me lbourne . BOB ELRIDGE CSIRO
Commencing with the issue for February 1989, this Journal has a new Editor with a new address for correspondence: E. A. Swinton, Editor, 'W ater' 4 Pleasant View Cres. Glen Waverley 3150 Phone (03) 560 9306
â&#x20AC;˘ WATER December, 1988
Control of Legionella in Cooling Towers and Warm Water Systems L. J. Hedges ABSTRACT Methods for the control of Legionella populations in warm water systems and air conditioning cooling tower water are described and compared. One hundred and eleven samples from seven warm water systems and 62 samples from four cooling towers in NSW were tested for the presence of Legionella. Factors found to reduce the incidence of Legionella in warm water systems included high water temperatures (>60°C), the use of mixing valves with short distances to the outlet, chlorination of make-up water and high flow rates. Results of testing water from cooling towers suggest that biocides are required in cooling towers, although Legionella specific biocides may not be essential, and that the interval between regular draining of the cooling tower can be three months rather than the commonly used period of one month. Even less frequent draining of the cooling tower . may be acceptable, however further trials need to be carri ed out to examine this possibility.
INTRODUCTION During 1987, Australia experienced its largest recorded outbreak of Legionnaires' disease in Wollongong, NSW which resulted in 122 suspected cases, 60 confirmed cases and 11 deaths. Legionnaires' disease is a rare but serious form of pneumonia which was first recognised in Philadelphia 1976 when a large number of delegates attending an American Legion conference were affected by the disease. (McDade 1977). The organism responsible for this outbreak was Legionella pneumophila. However, many of the other 22 species of Legionella have also been implicated in outbreaks of the disease. These organisms are ubiquitous in aquatic environments and grow best under warm conditions. The disease can occur when a person inhales air contaminated with aerosols containing certain species of Legionella. Cooling towers and warm water systems provide ideal conditions for both the growth of Legionella and the formation of aerosols containing Legionella. In the Wollongong incident, the cooling tower implicated as the most likely source of the outbreak, was difficult of access and it probably was not well maintained. The water collected from the tower, described as brown and containing sludge, was found to contain 2000 colon y forming units (CFU) L. pneumophila / mL (Christopher 1987). The Standards Association of Australia has since investigated the problem of controlling the contamination of air-handling and water systems with the aim of producing an Australian Standard which will contain guidelines for proper maintenance of cooling towers and warm water systems . This preliminary study of cooling towers and warm water systems examines some different maintenance schedu les for effectiveness in the control of Legionella.
METHODS Sampling One hundred and eleven samples were collected from seven warm water systems and 62 samples were taken from four cooling towers located in Sydney's western suburbs, and throughout the central west, south coast and south eastern regions of NSW. These samples were collected by health surveyors of the NSW Department of Health and submitted to the Division of Analytical Laboratories in insulated containers packed with cooler bricks, within 24 hours of collection. Laboratory testing was usually commenced within eight hours of receipt.
Legionella Analysis Samples, 100 mL each of cooling tower water were filtered through a 0.45 µm low-protein binding membrane (Gelman 'Tuffryn HT') and washed with 20 mL of sterile distilled water to 22
WATER December, 1988
Len Hedges, BSc, MASM is a microbiologist with the Division of A nalytical Laboratories, Department of Health, NS W. Since graduating from the University of NSW he has worked in many fields of microbiology, branching into public health when he joined DAL in 1985. He established the Legionella testing laboratory in 1987 and is continuing to work in this and other areas of public health microbiology.
L. J. Hedges
remove any residual biocide. The membrane was then cut into pieces, placed into 10 mL of sterile distilled water and vortexed for 20 seconds; 2 mL of this concentrate and 2 mL of unfiltered sample were heat treated in a waterbath at 50°C for 30 minutes to reduce background levels of other bacteria likely to interfere with Legionella growth (Dennis, 1984) . Samples from warm water systems, spas, reticulation systems etc. were treated in the same manner as the unfiltered cooling tower samples. Half mL quantities of the neat and 1: 10 dilution of these heat treated samples were spread-plated onto Modified Wadowsky and Yee Agar (MWYA-Oxoid, Wadowsky 1981 , Vickers 1981, Edelstein 1982). These plates were then incubated aerobically at 37 °C for five days and examined under a stereomicroscope for the presence of colonies possessing a characteristic 'ground-glass' or 'snowstorm' appearance. Typical colonies were streaked onto nutrient agar, lysed sheep blood agar and buffered charcoal yeast extract agar (BCYE), which has the same composition as MWY A but lacks selective antibiotics. All plates were then incubated in a humid environment for three days at 37°C. Presumptive Legionella were considered to be those with a characteristic colony morphology which were ab le to grow on BCYE but not on nutrient or blood agar. Isolates possessing these growth characteristics were confirmed as Legionella spp. by an indirect fluorescent antibody method. Our method allows a minimum detection level of 2 Legionella CFU / mL, which is well below the suggested maximum level in a satisfactorily maintained cooling tower (Broadbent 1987).
Indirect Immunofluorescent Identification of Isolates Monovalent rabbit antisera to most known Legionella species and serotypes were obtained from the Central Public Health Laboratory, London. Goat anti-rabbit IgG conj ugated with fluorescein isothiocyanate was obtained from TAGO Inc. Immunodiagnostic Reagents. To reduce the workload required for characterizing the Legionella isolates, the strain specific antisera were pooled to make four lots of polyvalent antisera. The maximum number of pooled sera was limited to 8 so as to avoid crossreactions. The polyvalent antisera were prepared by mixing 5-8 ten µL aliquots of each specific antiserum together and diluting with phosphate buffered saline supplemented with 0.1 C1/o sodium azide to a final volume of 2 mL. The reactions of these polyvalent sera were then checked against type cultures to confirm the reliability of the identification procedure. Isolates were stained with fluor escent antibodies as follows: A loopful of growth from a BCYE Plate was aseptically transferred to 0.5 mL of phosphate buffered saline (pH 7) supplemented with 3.5% formalin and resuspended. After 20 minutes incubation at room temperature, 13 µl quantities of each culture were transferred to four shallow 6 mm diameter wells of Wellcome 'PH 17' slides. Excess liquid was aspirated off and the slides were air dried and lightly heat fixed. Ten µI quantities of each of the 4 polyvalent antisera were added to each well. The slides were then incubated for 30 min at 37 °C, washed for 10 min in filter sterilised phosphate buffered saline, rinsed with filter-sterilised deionized water and air dried. Slides were then counterstained with a 1:20
dilution of the FITC goa t-anti rabbit conjugate using the same procedure. After drying, the slides were mounted in buffered glycerol under a coversli p and examined with a Leitz Dialux epifluorescence microscope equipped wit h a 'No. 3' filter assembly sui tably for blue light fluor escence.
DESCRIPTION AND MAINTENANCE PROCEDURES OF SAMPLING SITES Warm/Hot Water Systems Maintenance details for all systems described below summarized in Table 1. Site A - had two cold water storage tanks which supplied the who le building. These were monitored for total co lony count by maintenance contractors . When bacterial levels became high (eg. > l04 CFU/mL), the cold water tanks, hot water tanks and all tap fi xt ures were drained . The empty tanks were scrubbed clean and refilled with approx imately 12 ppm chlorine soluti on. After four hours the whole system was drained and refilled with fres h makeup water. Chlorine tablets were added daily to the cold storage tank to maintain a slight residual chlori ne level of 0.6 to 3 ppm. ·Hot water tanks kept at high temperat ures were coupled to mixing valves which reduced water temperature before use.
Site B - contained a number of cottages, each supplied with its own 180 L warm water tank. The tanks (thermostaticall y co ntrolled at 42 °C) were roof mounted and inaccessable to regular maintenance . Being of low volume, the tank s were emptied every day by the user . Site C - had a 580 L tepid water tank (42 °C) fed by a cold water make- up tank . The tepid tank was heated by hot water running thro ugh a coil immersed in the tank. It did not receive regular cleaning and the cold water make-up tank was not treated with chlorine. 580 L was sufficient to sup ply warm water for 2-3 days . Site D - here a tep id water tank set at 42°C was emptied every two months via a drain at the base of the sump and refilled with water from a creek. The make-up water was chlorinated but did not contain free residual chlorine since the creek water was contaminated with high levels of organ ic matter. Site E - used hot water tanks coupled to mixing valves, tanks and mi xing valves did not receive regular cleaning. Site F - tepid tanks (42°C) and the cold water make-up tanks were cleaned every six months . Initially, 100 ppm chlorine solution was flu shed through the systems, and allowed to stand for one hour. Th e systems were drained, spray-cleaned, scru bbed and refilled . A lower dose of 10 ppm chlorine was added and allowed to stand for fo ur hours. The tanks were rinsed and refi lled wit h clean fr esh water. Site G - had hot water tanks held at 70°C. Mixing valves added cold water to reduce the temperature to 38°C prior to use. Mixing valves were cleaned monthly. The distance from the mixing valve to the shower outlet was generally long, the longest being 20
TABLE I. MAINTENANCE DETAILS OF THE SEVEN WARM / HOT WATER SYSTEMS INVESTIGATED De1ails
Warm Water Tanks (42 °C) Regu lar Draini ng P eriod between dra ining Chlorin a tio n Water usage
Maintenance details for cooling towe rs'are given in Table 2. Site H - had cooling towers which were inspected every two weeks and drained every three months. The sum p was scrubbed but the fill material was onl y hosed clean. Towers were re-filled and treated with corrosion/ scale inhibitors and a general purpose biocide. Site I - at this site all towers were drained and cleaned month ly. Each tower was fi lled with 70-90 ppm chlorine in water and allowed to stand for two ho urs. T his water was drained and the tower cleaned of any slime and dirt dislodged by the chlorine. It was filled again and treated wit h chemicals for pH and corrosion control. On the second day a Legionet/a specific biocide was added while a general purpose biocide was added on the third day. Site J - once a month, 100 ppm chlorine solution was circu lated through the tower for one hour . It was allowed to stand for four hours. The tower was drained , spray-cleaned and refilled with water containing anti-corrosion /s cale chemicals and a ge neral purpose biocide. Site K - this site contained many towers which were drained mon thly and spray-cleaned with high pressure water jets. No chemicals were used to control bacteria, corrosion or scale. TABLE 2. MAINTENANCE DETAILS OF THE FOUR COOLING TOWERS INVESTIGATED D e1ails
Reg ularly Drained P eriod between draining (months) C leanin g: hosed Chlorinati on*
yes 3 yes no
yes I yes yes
yes I yes yes
yes I yes no
Chemical treat ment : General purpose biocid e Legionella-spec ifi c biocide Corrosio n/ Sca le Inhibitors
yes no yes
yes yes yes
yes no yes
no no no
* See text for deta ils of ch lorination .
RESULTS Warm/Hot Water Systems Of the 111 samples tested , Legionella was detec ted in 25 samples spread over six of the seven sampling sites (Table 3). While Legionella was not fo und in any of the 50 samples from site A, a population of 160 CFU/ mL was fo und in one particular system at site B, which we later found to be used infrequently . All samples from site C were fo und to contain Legionella with counts reaching 2000 CFU/ mL. Site D yielded 4 of 5 samples positive for Legionella with counts rangin g from 20 to 54 CFU/ mL. Six of-20 samples from site E were positive for Legionella, with two samples having relatively high counts of 470 and 1200 CFU/ mL. The distances from the mixing valves to the shower outlets for these two units were considerably longer than those of other units and the showers were used less frequent ly. Only 1 of 9 samples from site F contained Legionella, while all six samples from site G contained Legionella at levels of < 99 CFU / mL.
yes yes 2 mth s no
yes yes 6 mt hs yes•
Legionella was not detected in any of the 18 samples from site H while only one of the 20 samples from site I contained a low level of Legionella (10 CFU / mL - Table 3). Legionella was not detected in any sample from site J, while 10 of 18 samples from site K contained Legionella, 5 of which were in the range of 300 to 400 CFU/ mL.
DISCUSSION Warm/Hot Water Systems
H ot Water Tank s (over 60°C) with Mixi ng Valves
Co ld Water Make-Up Tank s Chl orin a ted Regu lar Drain ing Ch lorin a tion
yes yes yes yes•
* See text for details of chlorination.
yes cree k no pa rti a ll y no no
Legionella was not detected in any of the 50 samples from site A, whereas it was detected to a varying degree from the remaining six sites (B to G). Th is indicates that warm water systems at site A were properly maintained for the pu rpose of controlling Legionella contamination . T he rema ining systems can now be WATER December, 1988
compared to es ta blish the fea tures of warm water system maintena nce important in th e co nt ro l of Legionella. The followin g features appear to inhibit the grow th of Legionella:
High flow rates Sites B an d C were simi lar in th a t both had tep id ta nks set at 42Â°C and nei th er received regular m a intena nce. The major difference betw ee n th em was th e flo w rate of water from the tanks . Wa ter usage at site B was heavy, with each of the ta nks hav ing its vo lume emptied eac h day. Site C had a slower flo w rate of o ne tank volume per two or three days . This slo w flow rate has allowed growth of Legionella. Hence hi gh flow rates seem to inhibit th e growt h of Legionel/a by washin g the cells o ut of the tank befo re they ca n mu ltip ly to large numbers.
in th e ran ge of 300-400 LegionMa CFU / mL. C lea rl y th en , th e mont hl y drainin g and cleanin g of a tower is.i nsufficient if it is not suppl ement ed by an o n-going biocide treat ment program . Legionella sp ecific biocides may not be necessa1y. Sites H a nd J used a ge neral purpose biocide whi ch was successful as no Legionella was detected in eit her tower. The use of a general purpose biocide a nd a L egionella-s pecific biocide at Si te I do es not appear warranted as sites H a nd J ha ve successfu ll y used ge neral biocides on ly, for the control of Legionella spp. Month ly draining of towers is unnecessa1y . Th e prac ti ce of dra inin g towers every mo nth at Site J seems ove rly ca uti o us compared with th e eq ually success ful practice of draining towe rs eve ry three month s at Site H. lncFeasing the interva l betwee n drain ing saves on expe nsive chemicals a nd manpower a nd redu ces the chemi cal load released to the environment. It is therefo re des ira ble to find the max imum interval between drainin gs that can safel y co n tro l Legionella.
Mixin g valves with short distances to th e outlet
Mixing va lve allow hot water to be cooled to a temperature that will not scald use rs, by mixing co ld water immedi a tely prior to use. In site A they were effective in preve ntin g the growth of Legionella by allo wing tanks to be kept at a temp erat ure inhospitable to Legionella growt h . Howe ver, a problem ca n occur if the di stance fro m th e mi xi ng va lve to the sho we r o utlet is too long or if the system is rarely used . These situations are favo ura ble for Legionella growth because water temperat ure in th e lin es from the mi xing va lve, are lower than that in the hot wa ter tank. This situ ation existed at sites E and G where Legionel/a was fo und at levels up to 1200 CFU / mL. A self dr a in ing line may a lleviate this problem .
Good Quality incoming make-up water
Quality of inco min g make- up wate r is also an important factor. Make-up water at site D was pumped from a creek which has in troduced Legionel/a into the sys tem in spite o f bi-monthl y d ra ining of the sys tem . Site Fon the ot her hand, used tre ated town ta p water. Tepid samples were mostly nega tive , sugges tin g that good quality make-up ware r allows less freq uent draining . Site A represents the idea l set-up with chlorine trea ted make- up wate r , high temperatures in hot water ta nk s, mixing valves with short lines and hi gh flow-rat es.
TABLE 3. ISOLATIO N OF LEGIONELLA FROM WARM / HOT WATER SUPPLIES AND COOLI NG TOWERS Sampling Site
A B C D E F G
No. of Samples tested
No . of samples containing Legionella: ?.2 / mL
Wa rm / H ot Water Systems 50 0 14 l 7 4 20 6 9 I 6 6
?. 100/ m L 0 I
5 0 I 0
Cooling Towers H
18 20 6 18
There is a ge neral lack of data on the maintenance of warm wa ter sys tems and coo ling towers for th e co ntrol of Leg ionella . A la rge r a nd more ex tensive survey is needed to defin itely reso lve m aintena nce iss ues. Such a survey is a lread y being und ertaken at the Di vision of Analy ti cal Laboratories. This prelim inary sur vey has found th a t the use of high water tempera tu res coupled to mi xi ng va lves wit h short dista nces to the out let, chl orinatio n of mak e- up water a nd hi gh flo w rates red uce the incidence of high pop ul a ti o ns of L egionella in warm water systems . This sur vey has a lso found that biocides are req uired in coo ling towers, al th o ugh Legionella-specific biocides ma y not be esse ntia l a nd that monthl y drainin g of towe rs is unn ecessa ry.
0 I 0 10
The a utho r th a nk s Glenda A rnold for review of the ma nusc ript a nd A nj a li Boronowsky for tec hni cal assistance.
REFERENCES BROADB ENT, C. R., BU BB , C. T. J. (1987). ' Meas ures 10 contro l Legionn ai res' Disease H aza rds in Buildings'. Com mo nwea lth Depan mem of Ho using a nd Construct io n . Techn ical lnfo rmaii on / Meooa ni ca l Branc h Apri l 1987 T l 178 ME . C HRI STOP H ER, P. J. , 00 , L. M . , C HI EW, R. ( 1987). ' Epidemic of Legion naires' Disease in Wollo ngo ng'. Prelimin ary Co mmunicat ion in Th e Medical Joumal of Australia Vol. 147 August 3. DEN IS, P . J ., BART H EYY, C. L . R. and WR IG HT, A. E. (1984). 'Comparison of iso lation method s for Legionella spp .' In ' Legionella: Proceedin gs o f the 2nd Int ern ationa l Sympos ium: Washington, DC, pp. 294-6. Edit ed by Thornaberry, C ., Barlows, A., Feeley, J .C . , and J ak ub owski , W. A merica n Society for Microbio logy, Was hingto n . EDELSTE I , P . H . ( 1982). 'Co mparative st ud y of selective media for isolation of Legionella pneu mophila from potable water'. J. Clin. Microbial 16:697-9. Mc DADE , J . E., S H E PARD , C. C., FRASER , D. W. , TSA I, T . R., RE DUS , M.A ., DOUDLE , W . R. La bo rato ry In ves ti ga tio n tea m ( 1977). ' Legion na ires' disease: isolat io n of a bacte rium and de monstrat io n of it s ro le in o th er resp irato ry disease'. N.Engl.J.Med. 297: 11 97- 1203. VICKERS, R. M. , BROW , A. , GARR ITY , G. M . ( 198 1). ' Dye-co ntainin g buffered c harcoa l-yeast ex tract mediu m for different iation of members of the fami ly Legio nellaceae. J. Clin . Microbial, 13:380-2. WADOWSKY , R. M ., YEE , R. B. ( 198 1). ' Gl yc ine-co nt a inin g se lec ti ve med ium fo r isolation of Legio nellaceae from env ironment al specime ns' . Appl. Environ .
Microbial. 42: 768-722.
Cooling towers Althoug h only fo ur sites with cooiing to wers were monito red , some implications concerning their proper ma intena nce are ap parent. Biocides are usually needed. Of the water samples collected from towers at site K, 56% co nta ined Legionella . When this result is compared with the Legionel/a isolat io n rates from sites H , I and J, (0%, 5% and 0 %, res pecti ve ly) it is obviou s that the cooling towers at site K we re improp erly m ain tained . Towers at site K did not receive a ny chemi cals whi le those at sites H , I a nd J were treated with biocides. Legio nella co unts were lo w at these three sites while towe rs at Site K ge nerall y had counts 24
WATER December, 1988
AWWA DATABASE For information retrieval in the water and wastewater field computer data base of A WWA technical articles is now available . See the item under this heading on page 7 of this issue .
CONJUNCTIVE DEVELOPMENT OF AQUACULTURE AND IRRIGATION IN THE DARWIN REGION Ian L. Smith
ABSTRACT This reconnaissa nce pa per presents a strategic overview of water manage ment iss ues for th e proposed deve lopment of fre shwater aquacu lture and irriga tion in the Darwin Region. It compares th e effecti ve ness of flu shin g and trea tment to maintain water quality in aquacu ltu re ponds and co nsiders the possible co njunctive use of pond effluents for irriga ted agricu lture. A simple mod el of water qua lity behaviour in aquaculture po nds estimates the rate of production of wastewater effl uent for varying degrees of trea tm ent, which is th en converted to equi valent areas of irriga ted crop s. Both fre shwater aquaculture and irrigated agriculture compete for a share of available water reso urces within th e region. Full-scale deve lopment of either the aquaculture or irriga tion sector , but not both, is possible within a 50 yea r region a l water resource strategy. The conjun cti ve dev elopment of both sectors to 50 0Jo of th e stand-alone potential for eac h ma y be possible by treati ng aquaculture pond s and reusing pond effluent s for irrigation.
INTRODUCTION The Darwin Regio n is th e land area bounded by th e Adelaide Ri ver a nd th e Finniss Ri ver . Preliminary strategic pla nning has tentatively allocated th e region ' s water reso urces to three demand sectors for the nex t 50 years and beyond . The three sectors, in order of priority, are: urb a n potable water supply; irriga ted agriculture; and aquacu lture. Uncertainty in pre dicting growth in both aquaculture and field croppin g support co nsideration being given to the conjun cti ve use of water resources by these lower priority demand sectors. The possi bility of co njunctive use arises from the likely effluent production rate associated with th e operation of freshwater aquac ultu re ponds which could be app lied to irrigation of cro ps. A typical freshwater aquaculture farm proposed for the region wo uld consist of three metre dee p ponds with farm uni ts of between 50 and I 00 hectares. A total of 500 ha has bee n sugges ted .
FRESHWATER AQUACULTURE WATER QUALITY CONTROL-BASICS The management of wa ter quality is a critical constraint on th e yield of both fi sh and crustacean species being farmed in aquacu lture ponds. While other factors are important , toxic loading of the water column is one of the fund a mental water quality m a nage ment iss ues. Toxic loading arises throu gh wastes ge nerated by biological production in the ponds. A simple method of limiting toxic load buildup, which has bee n found satisfactory in other parts of Australia, is to discard and replenish one third of pond water vo lume each day. T he simple a pproac h to water qualit y control is desc rib ed in the fo llowing model : L assuming: (a) daily in situ load increase c!c c (b) daily changeover proportion r = (1 -C) (c) daily retention proportion (d) no load in troduced by changeove r Load a t end of day I Lr Load at end of day 2 (L + Lr).r = Lr + Lr' Load at end of day n , Ln Lr + Lr' + Lr 3 + __ + Lr" Lr (l - r")/ (1 - r) Simplifying gives L., 0 since r < I , for large n ; r" thus: L Lr/ (1 -r) for one third changeover; c = ½ , r = 2/2 and : L, = L ½/(! - ½ ) i.e. L, = 2L Thus the daily chan geover of one third of the pond vo lume (ass uming no load int ro du ced in the changeover water), ca uses 26
WATER December, /988
fan Smith is the Source and Receiving Waters Engineer based in the Strategic Planning Directorate, of the No rth ern Territory Po wer and Waler Authority. He has had 18 years experience in water resources p lanning in the Nort hern Territory. Ian Smith th e long-term pond load to reach a stead y state equi valent to dou ble th e da ily load increment. It is assumed, therefore, for th e following disc ussion that wate r qu alit y man age ment in fr es hwat er aquaculture should be aimed a t this criterion, i.e. th e stead y state load to be no more th a n d o ubl e th e da ily load increment.
EFFECT OF DAILY CHANGEOVER ON DARWIN'S WATER RESOURCES For three metre deep ponds; I hectare co ntains 30 ML so onethird changeover co nstitu tes 10 ML per day per hectare. This changeover rate represents both the prima ry wa ter supply required for aquacu ltu re and approximately the rate of efflu ent production from operating ponds. Lar'ge-scale fre shwa ter aquacu lture deve lopment curr entl y being co nsidered in the Da rwin Region is based on farm units of between 50 and 100 hectare area, with a total projected sce nario for th e regio n of 500 hectares. These projections transla te int6 th e following demands for water suppl y: 50 hec tare of ponds require 500 ML/day; 100 hectare of ponds require 1000 ML/day; 500 hectare of ponds require 5000 ML/day. The significanc e of these demands is apparent when placed in th e perspect ive of the region 's capacity for water resource supply. Current developed public water supply so urc es total 140 ML/ da y; A ll major regional groundwater resources to tal 200 ML/ day; A ll major water reso urces in th e region tota l 1600 ML/ da y. This co mpariso n indicates that for any size of aquaculture indu stry th e simple expedient of disca rdin g a nd replacing one third of th e pond vo lume daily is not feasible in th e Darwin Region. As an a lternative, there is th e possibility th at treatment in-situ could decrease the demand for water suppl y. Disc ussion on the specific types of treatment which co uld be app lied (e .g. aeration) is beyo nd th e sco pe of this paper, but would be worthy of detailed co nsideration in the future. The criterion for water qua lity manage men t wou ld still be to limit load build -up in the ponds to no more than doubl e th e dail y increment , as es tablished earlier. Table I tabu la tes the effects of va rying degrees of treament on water demand.
EFFECT OF IN SITU TREATMENT ON WATER DEMAND The simple mod el develo ped previously can be refin ed to acco unt for any treatment whi ch wou ld redu ce the d a il y increment of toxic load. Adding th e co nce pts of: proportion of load removed by treatment = y proportio n of load rema ining before chan geove r x 1-y Load at end of d ay I L. L.r. x Load a t end of d ay n , L., = L.r.x . I - r':x" I - r.x
since r and / or x is less than 1, r"x" - 0 thus L L rx 1- r X and for L 2L
2 3X I -
water resource to freshwater aq uacu lture and field irrigatio n . Thus, 48 ML/da y may be availab le primary suppl y to aq uaculture, with an associated effl uent flow avai lab le for reuse by ir rigation. The res ults of a nal yses conducted above can now be applied to a development scenario with 48 ML/ day primary supply to freshwater aquaculture: Water demand from each hectare of freshwater aquac ulture pond; w 30(1 - _1_ ) + 0.1 ML/day 3x Aq uacu lture pond area; 48 / w Hectares A Efflue nt available for irrigation; (w - 0.l)A ML/day F (48 - A/ 10) ML/day Supplementary irrigation area matchi ng effl uent flow rate Sia = F/ (0.02) Hectares (2400 - A/ 0.2) Hectares Full irriga tion area Fia = Sia/2 Hectares The potential for conjunctive development of treated freshwater aquacu ltu re and field irrigation from a primary water supp ly of 48 ML/day is as follow s.
Note that if the degree of treatment is such that one third of the load is removed by in situ treatment, in theory no chan ge-over water is required. The effect of lesse r degrees of treatment on change-over volume wi ll be intermediate between zero and onethird of pond vo lume. In practice, there will a lso be evaporation and seepage which will vary during the Dry Season. For this discussion, a co nstant combined amount of 10 mm per da y is ass um ed. Considering one hectare of a 3 m deep pond, co ntaining 30 ML, evaporation a nd see page would consume 0.1 ML/ d. Water demand, w = 30. c + 0.1 ML per da y per .hectare . Effluent discharge, f = 30 . c ML per day per hectare.
-TABLE 1. EFFECT OF TREATMENT ON WATER DEMAND PER H ECTA RE Proportion of load removed by
Propor1ion of load remaining
Proportion of volume 10 be changed
Water demand M L/d
TABLE 3. POTENTIAL CONJUNCTIVE USE OF 48 ML/ d Propor1ion of load removed by
load rem aining X
0.0 0. 1 0.2 0.3 0.32 0.33
1.0 0.9 0.8 0.7 0.68 0.67
0.33 0.26 0. 17 0.05 0.02 0.0
IO. I 7. 9 5.2 1.6 0.7 0. 1
Th e importance of in si1 u trea 1men1 is see n in the rapid decl ine in wate r demand. For exa mple. if
the treatment process can remove more than 300/o of the daily increment, then the water dema nd for the projected 500 ha of aquac uhure is reduced from 5000 M L / d 10 vo lumes whic h are within
th e range of the region's water resources. At th e same time, 1hc efn uent from th e ponds ca n be
0 0.2 0.3 0.32 0.331 0.333
1.0 0.8 0.7 0.68 0.669 0.666
Possible areas Aquaculture Irrigation ha Supplement Full ha ha 4.8 9.2 30 70 240· 480t
2380 2354 2250 2050 1200 nil
I 190 11 77 11 25 1025 600 nil
• half the 48 ML/ d used for eva poration and seepage, half as crnuent for use in agr iculture. ta ll th e 48 ML/ d used for evaporation and seepage, no efnuc nt availa bl e fo r use in irriga1ion.
consi dered for conjunctive development of irrigation.
IRRIGATION DEVELOPMENT USING AQUACULTURE EFFLUENT
CONCLUSION AND SUMMARY
The dail y demand patterns of horticultural irrigation are probably too var iable to be suited to reuse of the relatively uniform effluent flows from aquaculture. The less stringent irrigation needs for field cropping, howe ver, offer the possibility of reuse . Two types of field cropping may be irrigated in conjunctive sequen ce over the year . Supplementary irrigation may extend over one month after the end of the wet season, with a water demand of 0.02 ML / hectare. This co uld th en be fo llowed over the next 6 to 9 months by full irrigation of other crops, with a water demand of 0.04 ML/hectare . The potential for co njunctive deve lopment of field irrigation with th e effluent production rates for a range of treatment performances in aquaculture ponds is tabulated in Table 2, which further highlights the significance of the degree of treatment. TABLE 2. IRRIGATION AREA PER HECTARE OF AQUACULTURE: EFFECT OF TREATMENT Propor1ion of load removed by
Proportion of load remaining
Effluent volume M L/d
1.0 0.8 0. 7 0.68 0.67
10 5.1 1.5 0.6 0
Area of irrigation Supp/emenl Full ha ha
y 0 0.2 0.3 0.32 0.33
500 255 75 30 0
250 127 37 15 0
A TENTATIVE STRATEGY FOR THE DARWIN REGION Current planning nominall y allocates I 8 ML/ day from an existing surface reservoir plus 30 ML/ day from a potential gro und -
It is apparent that freshwater aquaculture can require large water supplies to co ntrol toxic loading in ponds . A satisfactory management criterion for water quality seems to be an ultimate pond loading equivalent to twice the daily toxic load increment. Simply discarding and re plac ing one-third of the pond vo lume daily can achieve the criterion but only at great expense in terms of wa ter supply and this makes freshwater aquacu lture not feasible in the Darwin Region, since only 48 M L/d is scheduled to be available for both irrigation and aquaculture. In situ treatment to reduce pond loading will reduce th e water supply requirement. Treatment removing one-third of total pond load daily, then the only make-up water is that required to cover losses due to evaporation and seepage. Even at this level of efficiency, the water demand is still very high in the regional context but 500 ha co uld conceivably just be sustained. This could only occur, howeve r, at the cost of no field irrigation since no further primary water supply would be available and there wou ld be no effluent flow from the aquaculture sector. Modelling shows that the water supply requirement is sensitive to the level of treatment efficiency. With pond treatment efficiency reduced only approximately 2% from the one-third criterion, the water demand is doubled - then only 50% of potential aquaculture could proceed . This would, however, allow 50% of potential field irrigation development to proceed through reuse of effluent flows from aquaculture ponds. The argument for conjunctive developmen t of both aquacu ltu re and field irriga tion cou ld be based on equitab le sharing of limited water resources. Further justification might also be provided in considering of th e crucial importance of water quality in aquaculture ponds and the apparent high sensitivity of water demand and effluent production to level of treatment. It may well prove prudent to encourage in situ treatment together with some effluent discharge from a reduced area of the aquaculture sector as a safeguard for water quality maintenance, with the added return of beneficial reuse of pond effluents by the irrigation sector. •
WATER December, 1988
Chlorine Dioxide and Related Ions in Water Treatment (1987) Summary of Toxicity Data. The tabulation reproduced below forms part of a paper with the above title and is reproduced by courtesy of the A mtror, Dr W. J. Masschelein, Director of Laboratories and Technical Services, Chaussee de Waterloo 764, 1180 Brussels, Belgium. In 1987 and at the in vitation of the major Authorities, Dr Masschelein delivered his paper in all A ustralian maJo:· centres. EFFECT & PARAMETER UNIT
Lowes t effect level (wate r mg L- ') NOAEL m g kg-• d -' LD,. mg kg- • Chronic toxici ty, mg ct-• mg L-'
12 I, (rat s) 140 (rat s) none at 5 no ne at JO positive at 30 100
0. 7-1, (rat s) 140-200 (rat s) none at 5
200 (human s) none at 5
Methe m oglobin aemia thro ugh water, mg L-' Hemo lyti c effect s tlTrough water, mg L-'
non 2, positive
15 pigeon 100 ch icken 30 monk ey 50
G lu tathion loss , m g V ' Effect on kidne y' s, mg L-' E m bryotoxicit y, mg kg-• ct-•
100 (rat s) none up to 70 stress at 50 50
10- 100 (rat s, c hicke n) 6-300 (monke y)
5 as sum of the three paramete rs (humans) All reversib le up to 1000 Distincti(?n between met hemoglobin aemia and hemolysis is arguable
Tests in vitro , no effect s observed on hu m ans .
none at 100 (m ice) none at 4 ,
Monali ty pregnant rat s, mg kg- • ct-• Tera toge nic effects. mg kg-•
50 (no ne at I 0)
REMARKS Acu te effec t: 1000- 1500 for CIO;
NOAEL: I (rat s) 10 (rat s)
positive at 14 (rat s) no ne a t 20, positive at 100 OAEL: I (rat s) 10 (rat s)
Carcinogenic effec ts, mg L-' Increa sed intestina l turnover, mg L- 1
Body we ight e ffec t , mg kg- • ct -• Thres ho ld waterbod ies, m g L- ' Organo lep ti c threshold water, mg L-'
5 (rat s)
2 ra ts
NOAE L: I (rat s) 10 (rat s) none at 10 not observed 10
AWWA 13th FEDERAL CONVENTION LAKESIDE HOTEL, CANBERRA -
MARCH 6-10, 1989
REGISTER NOW! THEME: INVESTING IN WATER FUTURES - THE AUSTRALIAN WATER INDUSTRY IN THE 1990s Topics will include Management and Administration, Distribution Technologies, Treatment Technologies, Science and Technology and Public Health. FEATURES: Robyn Williams on The Future; David Beal , UK, on lnformat,ion Management; 130 Technical Papers; Theme and Overview papers, 14 Workshop Sessions; Specialized Seminars. GENERAL: Technical and Social Tours , Entertainment, Soc ial Gatherings. Registration papers have been mailed to all Members. If further information required contact: AWWA Convention Organiser, IE Aust, 11 National Circuit, Barton 2600. Phone (062) 733 633, Fax (062) 731 488.
WATER December, /988
MONITORING OF SEWAGE FLOWS IN THE DARWIN REGION L. Monteith ABSTRACT A number of sewage flow monitoring projects have recently been carried out in the City of Darw in and the Town of Katherine 320 km south of Darw in . The obj ect of these studies has been to provide data for re-assessing the design criteria for sewerage systems in this tropical area and to evaluate the performance of the va rious components of the syste ms. This paper presents some of the resu lts of these studies and examines their significance with respect to the condition of the system and the adopted design criteria.
PROJECTS UNDERTAKEN Darwin North Zone This study was aimed at identifying existing and potential odour and corrosion problems in the Northern Zone System of the Darwin sewerage scheme. This catchment includes the majority of the Darwin residential area with development ranging in age from new to around 20 years. The flow monitoring was carried out to assess the general performance of the system and allow estimation of detention times in wet wells and rising mains where odour generation was likely, however the results Obtained proved to be one of the more valuab le products of the study.
Katherine Flow Monitoring This proj ect was undertaken for the specific purpose of deriving sewer design criteria appropriate for use in the Katherine area. This was to provide a realistic bas is for the planning and design of new works necessary to handle the rapid growth of Katherine brought on by the constru ction of the nearby Tindal Air Base. Six catchments were monitored covering the entire Katherine township.
dressing in more detai l a number of problems identified including an evaluation of the present and futu re flo ws in the rehabilitated system . The suburb of Rapid Creek was one of those ha rd est hi t by Cyclone Tracy. The extent of cyclone damage to property and subsequent damage to shallow house connections and manho le covers by heavy earthmoving plant used in the clean up operation is believed to be one of the causes of high infiltration flows in the area .
WATER December, 1988
The Rapid Creek catchment is one of the oldest residential areas in Darwin 's Northern Zone system and as a known problem area for stormwater infiltration has been the subject of a 'Reveal and Seal' sewer rehabilitation scheme undertaken by the Power and Water Authority. The monitoring was a follow on from the North Zone stud y and was aimed at ad-
allowed an accurate measure of the depth and intensity of rainfall associated with measured wet weather flows . For the City Zone project it was necessary to measure flows in a number of catchments not served by pumps. For this purpose a combination of in-sewer depth / velocity probes, data loggers and specifically developed processing software was also used. This system requires some costly equipm ~nt and a lot of manhours to ensure the reliability of the results however the advantages of the system are an ability to provide accurate flow measurements even when the system is surcharged and a continuous record of
Darwin City Zone Flow Monitoring
Rapid Creek Flow Monitoring
Lindsay Monteith is a Principal Civil Engineer with Gutteridge Haskins & Davey Ply Ltd. He has 15 years experience, mainly in the water and wastewater field, and has spent the last Jive years as Senior Water/ Wastewater Engineer in GHD 's Darwin office. Lindsay has been responsible for carry ing out the field monitoring, data processing and analysis for the projects dealt with in the paper.
For the Nort h Zone stud y the monitoring was done principally by exam ination of pump hours-run records a nd flo wmeter records however mo re detailed dail y flo w records were also obtained for the principal pump stations by installation of data loggers to continuously record the pump 's status. These records were downloaded directl y to a compute r and when combined with the tested pumping rates using programs developed by the author gave an accurate picture of the average inflows during each pumping cycle and the resulting dail y flow hydrograp hs . This development in monitoring techniques gives good results so lo ng as the flows remain sufficiently within the capaci ty of the pumps to keep pump run times of moderate duration. The technique was refin ed with furth er development of the softwar e a nd was used exclusively for the Kat herine project and in combination with other methods on the other proj ects . The use of tipping bucket rain gauges connected to the data loggers
The ÂˇDarwin City Zone system includes most of the CBD and some adjoining residential and special purpose areas. This proj ect sought to obtain sufficient data on the flow conditions in the area to verify the future augmentation requirements of the system.
I/ ,,, ~ rf \\ \\ I I
l~ ~ ' - , r--......
\ \ \1
TIME OF DAY (hrs) 0 WEEKDAY + WEEKEND
Figure I. Dry weather dail y flow hydrograph for a typical Darwin residential catchment showing weekday/ weekend variation.
been adopted f~r flow prediction and sewer design. The formu la used for calcu lating design flows is as follows: Peak Dry Weather Flow Qd = EP/270 r (I) where the peak factor r = ( I. 74 + 3 30/ EP 0 55 ) 0 5 (2) Peak Wet Weat her Flow Qw = D X Qd (3) The dilution factor D determines the I/I flow as a multiple of the peak dry weather flow and for normal residential densit ies in the NT is taken as 4. The value EP/270 gives ADWF at the rate of 320 L/EP / day as traditionall y specified for use in Darwin and man y other parts of the NT. The concept of evaluating the I/I as a multiple of the dry weather flow is at best a rough approxi mation and quickly loses relevance when dispersion factors are introduced whic h reduce the dry weather flow peak to take into account non -sync hronou s discharges and end up reducing the estimated wet weather flow without a stro ng logical basis.
15 14 13 12 U)
0 u.. ...J
5 4 3 2
TIME OF DAY (hrs)
The No rth Zone system co ntains sewers
Figure 2. Dry weather daily flow hydrograph for the Darwin central business district.
flows at close intervals. Other measurement techniqu es such as flumes or weirs fa il when flooded making them unsuitable for the highly loaded systems most likely to require mon itoring. The detailed hydrograph is in valuab le for the identification of the various flow components and, when examined in combin atio n with the simultaneous rainfa ll intensity record from the ra in ga uges, allows an accurate assessment of the type and location of inflow so urces. For the Rapid Creek study, the principal flows were measured by pump monitoring. In order to proportion the total flow between the various contribu ting sewers a rep resentative length of each sewer was calibrated usi ng a portable velocity meter and flow measurements made by manual measurement of flow depth s and hydrau lic grades. T his was adequate for the purposes of the study but wou ld not no rmall y be suitab le fo r detailed flow a nalysis.
FLOW COMPONENTS To fully interpret the measured flows it is necessary to iden tify the various components. Sanitary Flow (SF) - What the system is th ere to collect, the wastes from domestic and commercial fixtures. Th is component var ies throughout the day according to the activity of the population, normall y peaking aro un d 8.00 am and for residential catchments fa lling to nearly ze ro between 4.00 am and 6.00 am. Commercial areas on the other hand often have a basefl ow due to autoflushing fixtures and other 24 hour process flows. Groundwater Infiltration (G WI) - The leakage of ground water into sewers laid below the permanent water table. This is a co nstant flow und erlying the SF a nd can be assessed from the ea rly morning flow when the SF is small.
Rainfall Dependent Inflow and Infiltration (l/I) - The direct inflow of surface runoff in to sewers and infiltration of rainfa ll ind uced excess gro undwater. Direct inflow is characterised by a rapid rise in flow fo llowing rainfall and a n equall y rapid dropoff as runoff ceases. Infiltration flow develops more slowly and can take man y days to return to zero. The I/I flow is evaluated by subtracting the dry weather flow hydro graph from the measured wet weat her flows. One wou ld like to believe that (SF) wou ld predominate however it is seldom more than 25-30% of the design flow. In the Northern Territory the methods given in 'Design of Separate Sewerage Systems' by the Metropoli tan Water Sewerage & Drainage Board Sydney, have in the past
ranging from 20 year old concrete ma ins to new and near new AC a nd PVC mains. The Average Dry Weather Flow (ADWF) was found to be consistently of th e order of 240 L/EP / day. Wet weat her flows however indicated dilution factors ranging from less than two for th e newer suburbs to over eight for the older suburbs despite considerable rehabilitation . Values over 20 were found for some local unrehabilitated catchments. The 'dry weather flow' was also found to vary by some 200Jo between the 'wet season ' and the 'dry season ' probably due to seaso nal variation iit the permanent gro undwater levels. This being a dormitory area for the CBD/ commercial area there was also a substantial vari atio n in flow s between weekdays and weekends. Figure I shows dry weat her flows from a resident ial area for typical midweek and weekend days. The difference is substantial and needs to
V ~ i'--
TIME OF DAY (hrs)
Figure 3. Dry weather daily flow hydrograph for a residential area with high groundwater infiltration . WATER December, 1988
6Q-r----- - -- - - - - - - - - - - - - - - - - - - - - - -
50-+---- - + - - - - - - - t - - - - + - -----t-+ - - - - - - + - - - - - l - - - - - - - l
40 - j - - - - - - t - - - - - - - t - - -- +----lf--+-----+------t-------1
- - - - - + - - - - - + - - - - - + - - -# - - + - - - - - - + - - - - - + - - - -~
20 - j - - - - - - t - - - - - - - t - - - - + - - ----++--+-----+--- - - - t - ------1
inflows. The apparent dilution factor was around 8.4. Figure 5 is for th e same catchment as F igure 3 and sho ws the extent of the problem s in this catchment. The peak flow shown is only part of the total with another 40 to 50 L/ s going o ut overflowing manholes upstream of the monitored pump station. The main culprit for the short term flow was a small subcatchment on a military base where the poor condition of the sewers allo wed inflows 40 times the peak ctr/wea ther flow . This was exacerbated by high infiltration flow derived from the residential area in the lower catchment. Th e groundwater le els are highl y variable in the sand y foreshore area and once ra ised are slow to dissipate. The sewers in the area are obviously fairly porous!
DAY OF THE WEEK (from 15/3/87)
Figure 4. Weekly flow hydrograph showing high inflows resulting from a single 20 minute storm.
be taken into account when evaluati ng flow results . The flows shown are otherwise typical of residential catchments with a distinct morning peak , moderate midday flows, and a dispersed evening dropping to near zero in the earl y hours of the morning the detailed study of the Rapid Creek catchment found that a large portion of th e excess flow responsible for surcharging of manholes in the lower areas deri ved from an adjoining catchment via a shared overflow connection. The unrehabilitated areas identified as high flow contrib utors have since been rehabilitated and will be re-assessed during the coming wet season. The adjoining catchment has also been recommended for more detailed study and modifications to the overflow system proposed to more equably distribute the flows. The Katherine system again indicated ADWFs of the order of 240 L/EP/ day with very low GWI due to the deep groundwater level. The sewerage system is less than 15 years old and dilution factors (D) were consistently below 3. A couple of anomalous high results were characteristic of direct inflows to flooded gully grates or similar isolated faults which can have a major impact on the total flows . The system was generally found to be performing well within capacity despite theoretical predictions of overloading based on the prevailing design criteria. The City Zone system is mostly 20 to 30 years old and includes a variety of land uses and topography. The main area including the CBD is elevated flat land however a portion of the system includes medium and lo w density residential development in hillside and lo w lying beachfront areas with high groundwater leve ls . N in e s ub -catchm ents were monitored covering the entire City Zone system. The ADWFs reflected the variety 32
WATER December, 1988
of conditions with typical daily flow hydrograp hs shown in Figures 2 a nd 3. Figure 2 is for the CBD and shows a typical commercial area pattern of highly dispersed daytime peak flow s. Night-time flows remain fairl y high due to restaurant and hotel contributions and auto-flushing fixtures . Figure 3 is for a lo w lying residential area with obvious GWI problems. Wet weather flows provided some startling results and some flows resulting from a 1 in 1 year 15 minute storm are shown in Figures 4 and 5. Figure 4 is for a small high ground residential catchment which exhibited extremely high sho rt term
The value of these projects has been twofold . The results for Katherine have shown the previously adopted design criteria to be unduel y conservative and a 250Jo reduction in flows has been allowed for future designs. Th e cost saving will be significant. For the other areas studied the main value of the monitoring has been to identify a number of catchments with substantial problems of stormwater inflow and infiltration . These areas, such as the military base in the Darwin City Zone, contribute flows out of all proportion to their population and place an extraordinary load on the sy-stem. This one catchment with just 188 EP contributed flows in excess of 80 Lis or 300Jo of the wet weather flow from th e entire system serving 8600 EP , Identification of these areas has allowed the localization of rehabilitation efforts in the areas where the benefits are greatest. The saving in wasted time and effort on rehabilitation of less critical areas and the potential saving in augmentation of the systems are both significant. CONTINUED ON PAGE 39
DAY OF THE WEEK (fro m 15/3/87)
~igure S. Weekly flow hydrograph showing high inflow and continued infiltration from a 20 minute storm.
W ATERCOMP '89 -
A TIMELY FIRST
30 May-1 June 1989~ Regent Hotel, Melbourne Peter Manoel , Chairman of Organising Committee For the first time in A ustralasia, an enti re conference wi ll be devoted specificall y to the effective use of computers in the water sciences, engineering and management. For three days, water scientists, engineers and information techno logists wi ll ga ther to talk computing . The strong ar ray of case stud y papers submi tted for presentation at WATERCOMP '89 indicates that th ere is plenty to ta lk a bout. The increasing emergence of a wider array of choice in computer hardware and so ftware makes the opportunity a fford ed by W ATERCOMP '89 for Water Industry professionals very timel y indeed. The more establi shed areas of app lication such as water reso urces assessment, flood protection and hydraulic design are now being joined by app lication s in environmental water quality ma nage ment and asset in format ion systems for maintenance management and asset life cycle manageme nt. As computer grap hics become more affordab le, CAD is being adopted by more design offices. User fr iendl y interfaces offer users of these and ot her applications faster learnin g curves and improved productivity . These trends, and the increasing flexibili ty of communications and porta bility of microcompu ter devices, ensure that the time is near when some form of computer device wi ll be standard equipment for virtually all of the Water Industry's wh ite colla r personnel. Significant use can a lso be predi cted for increasing numbers of th e blue collar supervisors and workfo rce.
GUEST LIST Professor D . P. Loucks, of Corn ell University USA, is an auth ority on the applicat io n of economic th eory and systems analys is methods to the solu tion of environmental and regional water resource prob lems. For W ATERCOMP '89 he is preparin g a perspective on th e impac t of computat io na l too ls in th e work of public regulatory planning and development age ncies; utilities, private consul ting firms and uni versities and research institutes . He promises to ' look ahead into the future a li tt le to see what might be, or should be' . Professor R. A. Deininge r , from the Un iversity of Mic higa n USA, has been invited to share his expertise in the a pplication of microcomputers to a ran ge of water suppl y problems including water quality monitoring and modelling. His views on the futur e of microcomputer hardware are eagerly awaited . To present a perspecti ve on the importance of CAD as a Water Industry tool, WATERCOMP '89 wi ll be addressed by Harold T. Glaser , Vice President and Ma nager of Computer Aided Design , with J ames M . Montgomery, Co nsulting Engineers of Pasedena , Cali fornia, USA. He wi ll describe the process of water or wastewater treatment plant design in th e environment of a highly integrated electronic CAD office. Mr Glaser is being brought to Australia for WATERCOMP' 89 by Sinclair Knight and Partners. In Australia, as internationally, pipe network analysis remains one of th e most used computer too ls of the water industr y. As a contrast to th e range of local developments in this fie ld, WATERCOMP '89 will have the benefit of the expertise of Alvin G. Fowler of Municipal Hydraulic Ltd, Va nco uver, Canada. Mr Fowler commenced writing network ana lysis programs in 1964 and hence is uniquely able to present a first hand perspective of the first 25 years of development of these methods. H e, too, has some exciting views on what th e future should hold .
WHICH SOFTWARE TO BUY? Another feature of WATERCOMP '89 will be th e presentation of the res ul ts of some es pecially commissioned comparative software reviews and benchmark tests.
Pipe Networks Mr T. Carpenter, of the Melbourne & Metropolitan Board of Works, carried out an evaluati on of programs used in analys ing pipe networks in 1980. This resulted in the ACADS Publication
M 13 . His current work will be an upd ate of th is. Only a few of the programs initiall y evaluated are still in use, man y of the new o nes being reviewed are proprietary packages. His review has been expanded to include dynamic as well as 'static' heads, ease of use and presentation of output.
Databases for Asset Management in the Water Indu stry Mr N. Aposto lidis, of Gutteridge, Haskins & Davey, will present an examinat io n of some of the databases currently used in asset management in the wa ter industry and provide some guidelines for th e needed attributes of such programs if they are to be sui table for present an d future use in th is area.
Water Quality Models Dr G. Codner, of Monas h University, will concent rate his evaluation on those programs which model wate r qu ality.i n catchment areas. He wi ll give a brief overview of those concern ed with Rivers; Reservoirs & Lakes; and Est ua ri es.
Dynamic Flow Models Mr N. Collins, from th e Brisbane office of McDonald Wagner , will be extending the work o n comparisons of dynam ic flow models which he commenced as a post-gradu ate research project at the Universit y of Q ueensland. Models wh ich are in widespread use are being compared through application to an actual flood record wit h tidal influence in a creek-estuary system . In each case, th ese studies are being aarried out with the ge nerous support of the researcher's employer. With the add itional ass istan ce of ACA DS, atten dees at WATERCOMP '89 will be provided with a current a nd valuable set of Water Industry software reviews. Comprehensive feports will be available at WAT E RCOMP '89 to complement th e presentat ion of stud y res ults.
TRADE EXHIBITION A comprehensive Trade Ex hibition is being planned . The organisers are planning to team computer ve ndors, showing the latest hardware and ge neral purpose software, with specialist water so ftw a re developers. This will enab le as much of the Industry's application software as possible to be availab le fo r live demonstration at th e conference venue. Of particular interest are some of the app licatio ns developed in -house by th e major water authorities and research institutions which are not (yet?) commerciall y distribu ted . CAD will also be strongly featured, at this, the only Melbourne based ACADS trade ex hibi tion next year. While a number of approaches have been made already in this regard , the orga nising committee wo uld li ke to hear from anyone wit h app lications to demonstrate or hardw are and systems software to ex hibi t. Space is limited , so make contact soon - ring Ian Beckett (03) 6 15 4022.
DIVERSITY OR DUPLICATION? T his iss ue will be the subj ect of a wo rkshop session on the final day of WATERCOMP '89. This promises to provide a li vely debate . Water autho ri ties large a nd small , research laboratories, consultants and vend ors can each be expected to have their own perspective on whether Industry co-o peration or a fr ee mark et for competing products a nd services wo uld best serve future advancement. WATERCOMP '89 offers an opportuni ty for Water Industry computer users to influence the programs of ACADS and other Industry Associat ions in promoting software development and availability with either or both approaches. For further information co ntact: Conference Ma nager, The Institution of E ngineers, A ustralia, 11 Natio nal Circuit , Barto n , ACT 2600 . Tel: (062) 70 6549. Fax: (062) 73 1488. WATER December, 1988
Mosquito Breeding and Sewage Treatment in the Northern Territory Peter I. Whelan ABSTRACT Sewage treatment facilities are major so urces of pest an d vector mosquitoes in the Northern Territory. Mosquito breeding is usually associated with inadequate design, operation, maintenance and final disposal practices. This paper outlines the problems, gives some examp les from existing installations and suggests design and operational practices that can reduce mosquito breeding .
INTRODUCTION Sewage and treated sewage effluent have been major man-made sources of mosquitoes in the Northern Territory (Whelan, 1981, 1984) . Nutrient rich sewage has the capacity to produce enormous Âˇnumbers of mosquitoes, and as treatment facilities are relatively close to communities, they usuall y give rise to large and continuous problems from pest and vector mosquitoes. Mosquito breeding is usually associated with inadequate design, operation and maintenance or faulty methods of effluent disposal or dispersal. Some of these practices can be relatively easily rectified but there is a need for increased awareness of the nature of potential breeding places among designers and operators of sewage treatment faci li ties. The object of this paper is to emphasise this need and to outline some design considerations and operational practices that can reduce mosquito breeding.
MOSQUITO SPECIES Cu/ex quinquefasciatus: ' The Brown House Mosquito.' This species usually breeds in organically polluted water near human communiti es. It is frequently found breeding in high numbers in unsealed septic tanks and primary sewage ponds, although it will sometimes be found in organically overloaded secondary sewage ponds. This is a very sign ificant pest species wherever favourab le breeding sites exist. The fema les rarely travel more than two kilometres from their breeding sites. Due to the nature of these sites, this species can be present throughout the year. Cu/ex quinquefasciatus is a suspected vector of Australian encephalitis and also a vector of heartworm in dogs. Cu/ex annulirostris: 'The Common Banded Mosquito', is the most common mosquito in the Territory. It breeds in a variety of natural sites and is common ly found in open, shallow, vegetated freshwater swamps, stream s and lagoons. The most prolific artificial breeding places are in secondary sewage treatment and evaporation ponds and sewage pond effluent (Whelan, 1984). The larvae are most frequently found in still, sheltered areas where vegetation offers protection from disruptive wave action and aquatic predators. This species can disperse up to IO kilometers from the breeding area (Russell, 1986). It is abundant in the 'Top End' of the Northern Territory from January to August and in the so uthern region from October to April. Cu /ex annulirostris is the most significant pest species in the Northern Territory and is a confirmed vector of a number of arboviruses (Doherty, 1972). The most important disease carried by this species is the potentially fata l Australian encephalitis. There have been four confi rmed cases of Australian encep halitis in the Northern Territory in 1987/ 88 (C.D.I., 1988) . It is also a vector of Ross River virus, the cause of ep idemic polyarthritis. This disease is relatively common and widespread in the Northern Territory. The most effective long term protection from these viruses lies in eliminating the breeding sites within flight range of urb an areas. Anopheles annulipes s. l. : 'The Freshwater Malaria Mosquito' breeds in open, sunlit , temporary and permanent freshwater ground pools, streams or swamps. It is very rarely found in sewage ponds but it is frequently found in the lower organically loaded sewage effluent, particularly where the effluent flows into shallow , open, grassed areas . 34
WATER December, 1988
Mr Peter Whelan is th e senior medical entomologist, in charge of the Medical Entomology Branch of th e North ern Territory Department of Health and Community Services. Peter has been responsible for the co-ordination and implementation of all aspects of vector monitoring and control operations in the No rth ern Territory since 1975. He has been a regular contributor to national courses on vector control, with a particular in terest in the physical control of mosquitoes.
The fema les can disperse up to two ki lometres from their breeding places. The species is a suspected vector of malaria and is a significant pest species in inland areas where suitable breeding sites exist.
DESIGN CON SID ERA TIONS Site Selection and Design Disposal of Effluent Appropriate planning for the final disposal is an important part of site selection. Disposal near the coast is relatively easy, but in inland areas can lead to major problems if appropriate techniques are not employed. These are discussed in a later sectio n of this paper. Wind Ideally ponds should be located in open windy areas since wind and the associated waves play an important part in preventing breeding by disrupting the larvae aod pupae at the water surface. Also, wind action can prevent tne growth of algae, aquatic floating ferns (Azo//a sp.) and duck weed (Lemna sp.). These floating plants shelter the larvae from both wave action and aquatic predators. An examp le of bad siting is at Snake Bay, Melville Island, where small ponds with steep banks were protected from the wind by both a hill and a close, tall eucalypt forest. Colonisation of the surfaces by Lemna sp. enab led Cu/ex to breed in significant numbers.
Drainage The choice of a site should consider the necessity to drain the ponds for maintenance without thereby creating swamps or pools of stagnant water. Effluent release from the final pond is usually suitably arranged, but provision for emptying the intermediate ponds into suitable areas is overlooked. Site design should ensure that there is no disruption to normal site drainage pathways caused by any of the works. New diversionary drains should be constructed in concrete, with inverts for low flows and should discharge to su itab le endpoints. Groundwater seepage must also be considered. With ponds constructed near tidal areas, particular consideration must be given to preserving existing tidal drainage patterns. If these are blocked, problems with salt water species of mosquitoes can be expected. An example occurred at the Leanyer sewage ponds in Darwin, where considerable engineering work was required to reduce the mosquito problems.
Access Site design should a llow for all-weather access all around the installation . Weed growt h , tree growth, erosion , si ltation must all be controlled, and fire prevention must not be forgotten. There have been instances of seepage from large pond systems causing swampy conditions and preventing vehicle access.
Pond Dimensions Pond Size Sewage pond size is primarily 'determined by engineering parameters related to design flowrates, pollution loadings, and the required effluent quality and frequently there is little consideration given the effect of pond size on mosquito breeding. Adoption of oversized ponds, either from inaccurate predictions of sewage volume or a desire to provide for future capacity, can lead to the ponds becoming shallow thickly-vegetated swamps, capable of breeding large numbers of mosquitoes. This situation occurred at Batchelor , where the final evaporation pond has become such a swamp. Consideration should be given to stagin g of pond construction, or the use of multiple ponds, although, as discussed earlier, the use of smaller multiple pond s may inhibit wind and wave action. In most cases, it is the margins of the ponds which provide the breeding conditions. Multiple small ponds obviously suffer in this respect also. Attempts to reduce short-circuit ing through a large pond by installing peninsular barriers can markedly increase the mosquito breeding areas. Pond Depth Selection of pond depth is usually dictated by the function of the pond, i.e. primary, secondary, or evaporation. Adequate allowance must be made for solids deposition, particularly in primary ponds, otherwise excessive local deposits wi ll lead to co lonisation by vegetation creating breeding locations. Profiling the pond base, with th e deepest side at the pond entry, can help, particularly if there is a seasonal variation in input. For evaporation ponds, particularly those with earth sides or which may operate on a 12 month cycle, a depth of two metres or more is recommended, to prevent the continued growth of semi aquatic weeds such as Eleocharis sp. and Typha sp.
Construction Details Vertical Concrete Margins These have proven to be the most satisfactory means of contro lling mosquito breeding by promoting wave action, and maintaining margins free of vegetation and debris. Concrete walling can be precast for remote locations or constructed in situ, and are cost-effective. They should be deep enough to allow for wide variation in water level and should have a concrete-lined horizontal bench just above the bottom level of the pond, to prevent establishment of vegetation along the margin in the initial stages of operation and silt accumulation in the longer term. Sealed verges around the top of the banks are desirable to facilitate maintenance and to prevent entry of soil into the pond. Walled ponds may still have problems with floatables and wind-blown debris in the corners and cut-off pools near entry and exit points. Truncated corners and multiple or underwater entry and exit points can help to eliminate these latter problems.
Unlined earth banks Sewage ponds with unlined earth banks have the greatest capacity for mosquito breeding, particularly those wit h gentle slopes where marginal vegetation can grow. They are accordingly not recommended , except as temporary or emergency measures. The banks should be constructed using low porosity materials such as compacted clay. If neglected, unlined earth banks can become either eroded, or overgrown with grass, shrubs and even trees such as Acacia, Mimosa and Melaleuca sp. Corrective measures can be a major problem. Other Linings Various systems have been used to line earth banks as a temporary measure, to reduce growth of vegetation, but they have not been entirely satisfactory. Stone pitching of the margins is not satisfactory as it does not offer sufficient deterrent to growth and mechanical maintenance is difficult. Overlapping cement sheets have been used, but have
prob lems with breakage and subseq uent weed growth. Various types of bituminous or plastic sheeting have also been tried, and have shown promise as short to medium term solu tions. Problems encountered include inadequate anchoring, weed growth, ultraviolet deterioration, and human interference. The more modern ultra-violet-resistant heavy duty plastics, anchored with earth mound s back from the rim of the ponds, have been more successful. Sloping concrete margins have been tried in a number of locations. While better than unlined ponds they have the drawback that wave action is damped by the slope. Dust and organic matter can build up and vegetation establish. It is important that they have a slight rim, and sealed verges to prevent accumulation .
Maintenance Before commissioning sewage pond systems, a general survey of the whole site shou ld be conducted to ensure that mosquito breed ing places have not been created inadvertantly such as borrow pits formed during pond construction, the pools of water in site drainage works or pooling caused by road access. Any problems should be rectified before the ponds are commissioned . Pond maintenance is a vital part of pond management. The highest levels of maintenance wi ll be required for earth lined ponds with low and seasonall y-variable effluent flow rates. Some form of maintenance will be required even for ponds with vertical concrete margins and sealed verges. Even those ponds in favourable locations, with ideal effluent characteristics, must have adequate provision for people and resources to carry out a regu lar and defined maintenance program. Aspects of maintenance frequently overlooked are the regular control and removal of vegetation on the margins or the pond verges, the regular removal of floatables and other flotsam from accumu lation points, and the repair of cracks and other fai lures that can allow increased soil moisture levels on the banks and subsequent vegetation growth . For some ponds, a program of water level management may be adopted which a lternately floods and strands marginal vegetation or floatables. The form of maintenance will depend heavily on the pond design, effl uent parameters, and staff,experience. Regre ttably , it has been the experience in the Northern Territory that regular and adequate maintenance is not common in sewerage systems to prevent mosquito breeding. If there is any anticipation that proper maintenance ~II not be carried out regu larly, a maintenance-free design should be chosen.
EFFLUENT DISPOSAL OR DISPERSAL Problems In many of the early sewage treatment facilit ies in the Territory insufficient consideration was given to disposal of the effluent. It was assumed that having received 'adequate treatment' it was n0 longer a problem. It could therefore be left to run down the nearest flow line. In fact, the effluent often formed flooded, overgrown, stagnant pools that created breeding gro unds for Cu /ex. Examples were provided by the early plants at Coonawarra Naval Base and Nhulunbuy South by the lack of provision for proper disposal. In some instances, effluent from secondary sewage treatment ponds was channelled or piped just beyond the perimeter fence or to the nearest available low lying area. In some situations, as at Nhulunbuy and Yirrkala, the effluent was directed into sand dunes in the belief that infiltration would provide a satisfactory disposal method. This proved totally inadequate because the high organic loads of the effl uent and algae in variab ly sealed against infiltration and resulted in pooling of effluent throughout the dunes. Even after tertiary treatment in evaporation ponds, the resulting 'treated' effluent still retains a great capacity to breed mosquitoes. An example of this, the uncontrolled release of treated effluent into the II Parpa Swamp area near A lice Springs. The continued release of effluent has created a large permanent reed swamp (see Fig. I) and very high numbers of Cx. annulirostris and An. annulipes.
Large Evaporation Ponds In the Northern Territory evaporation ponds either of intentional or 'ad hoc' design have commonly been used as an effluent WATER December, 1988
downstream effects of the disposal. When the flow in the rivers or creeks is small or subj ect to wide seaS1Jnable variation, this method is unsuitable as eut roph ication or ecological and vegatation changes will lead to mosquito breeding .
Disposal to Land Sprinkler dispersal
SEWAGE EFFLUENT IRRIGATED AREA
MOSQU I TO AREA
Figure 1. Mosquito Breeding Areas near Alice Springs.
disposal method. Large evaporation ponds are rarely full to capacity for the entire year, and in many instances are just bunded areas containing effluent against escape to other areas. Because of -their large area, the variable inflow and the variation in climate, large evaporation ponds can exist as shallow , flooded, swamps with dense weed and reed vegetation for at least part of the year. Also, evaporation ponds that dry up and are then seasonally inundated during rainy periods can become breeding grounds for floodwater mosquitoes such as Aedes normanensis. The numerous aspects to be considered in designing large ponds to reduce the amount of mosquito breeding , include: • initial and regular removal of all emergent vegetation within the evaporation area; • levelling of the floor of the evaporation area; • division of the evaporation area into a number of smaller areas; • constructing a sloping floor to concentrate the water in a 'sink' area at the effluent entry point; • concrete lining of the 'sink' area on the floor of the evaporation area and the lining of embankments. However, incorporating some of these aspects into the design can be prohibitively expensive. The alternatives are to have a regular maintenance program, which could be more expensive in the longer term, or to choose a more suitable method of effluent disposal.
This method has been re latively successful in areas where there have been particular problem s with other disposal methods. It is most successful onto are;is wit h well-developed stands of trees on soils of good permeability. Jabiru provides an example of successful sprinkler dispersal. The final effl uent is automatically and periodically dispersed via a system of overhead sprink ler heads, onto a fenced area of native eucalypt forest. Initial problems from fire damage and algal blocking of spray heads were rectified by the construction of an underground pipe system. Ideally, sites should be relatively flat but with good drainage · during rainy periods. Feeder ,lines to sprayheads should be laid out along contours, rather than at right angles, so that water retained in the lines after the finish of spraying will drain out of all sprinkler heads rather than the lowest. This will avoid creating permanent pools of effluent. The area required will depend upon the volume of effluent to be disposed, and the long term absorption capacity of the soil and the vegetation. In monsoonal areas, extra care is required to ensure that effluent-contaminated runoff cannot pool in nearby flow lines or creeks. Sprink ler dispersal can be used for tree and pasture growing or landscape watering, but the National Health and Medical Research Council Guidelines for the Reuse of Waste Water must be ad hered to (NH & MRC, I 979). Tertiary chlorination has been used to provide a high-quality effluent for drip irrigation and recreational area watering at Yu lara. Sprinkler disposal using large spray heads has been successfully used on Blatherskite Park in Alice Springs, but potential problems exist in this area because of the high salinity of the water and rising water tables .
Small Furrow Irrigation This method is useful for relatively small volumes of effluent on sandy soil in low rainfall areas. A feeder channel is used to deliver effluent to a ploughed area of small furrows slo ping gently away from the feeder channel. Disposal is by infiltration into the sandy soil. When infiltration becomes less efficient, the flo w is directed to an adjacent ploughed area, and the original area is allowed to dry out and is reploughed. T his system requires a considerable amount of attention and maintenance, but has been used successfully at Perth Airport.
Small Evaporation Ponds The use of small concrete-lined evaporation ponds can be a very effective method of effluent disposal. The best designs incorporate a series of relatively small ponds that can progressively fill by gravity overflow. Such a system may be expensive to construct, particularly if the whole evaporation area required is relatively large. However, the method has the advantage of being relatively maintenance free and it can cope with variations in effluent volume .
Disposal to the Sea Disposal direct to the sea or to a daily-flushed tidal area is one of the most suitable methods for effluent disposal. However, dispersal at the end of a relatively long, narrow or tortuous tidal creek can result in effluent build up in the creek which can be pushed higher up the creek line by incoming tides into areas where mosquito breeding sites can develop . Disposal onto large flat inadequately flushed tidal areas can create breeding sites not only for freshwater species of mosquitoes, but also for brackish water species such as An. farauti, and salt water species such as Cx. sitiens, An. hi/Ii, and Ae. vigilax.
Disposal to Rivers The suitability of discharge to rivers depends upon the volume of flow in · the river, the seasonal variability of flow, and the 36
WATER December, 1988
Channel Infiltration In this system, permanent infiltration channels are constructed and effluent flow is directed down a number of groups of channels which are alternatively spelled and maintained. The method can be used on less porous soils than is possible for furrow irrigation . If this method is to be used for the irrigation of tree or bush crops, intensive monitoring is required to ensure viability of the crop in the long term. Small scale use of this method has been tried at Batchelor and at the Katherine Ab batoirs, but proved to be relatively labo ur intensive. A variation of the method has been used on a larger scale at II Parpa in Alice Springs, to grow eucalypt trees. Problems on the larger scale have included high labour input, weed control in the channels, rising salinity levels and rising water tables. Flood Bay Irrigation The degree of land preparation for flood bay irrigation is usually considerable, as a system of correctly-graded flood bays is necessary to allow for efficient flooding and to prevent pooling at the end of the flood bays . The bays are periodically flooded and the effluent is allowed to evaporate or infiltrate in the bays over a period of four to five days. T his method has been used successfully to grow irrigated pasture in Mildura. Problems with flood bay irrigation arise during rainy periods, when extended flooding of the bays can occur and can result in mosquito breeding . Recently a more sophisticated delivery system for a flood disposal method
has been designed for Gapuwiyak. It incorporates an automatic siphon a nd a distribu tion drain designed to release effluent evenly over a very large flood bay but it has not ye t been tested und er operational conditions.
BIOLOGICAL CONTROL Biological control, though no t a pplicable to primary ponds, can be a very efficient means of controll ing mosquito larvae in secondary and evaporation ponds. The major biological contro l age nts are fish, aquatic beetles and aquatic bugs. Fish can control mosq uito larva l numbers directl y by eating the larvae, or indirectly by eating algae or aq uatic weeds which provide protection from other predators or wave actio n. Fish are usually onl y suitable for the higher oxygenated waters. Several species have shown promise in th e Northern Territory, including the bony herring, which has been successful in reducing sur face algae in the former Ludmilla sewage ponds in Darwin , and Melanotaenia sp. , which is a ve ry efficient larval predator in the final evaporation ponds for th e Ranger sewage treatment. It is essen tial that marginal vegetation such as couch grass and Âˇ reeds be eliminated or kept to a minimum , so that fish can have physical access to th e mosq ui to la rvae. Actively growing Eleocharis sp . and Typha sp., wit h upright stems, may not restrict access. However, when these weed species die or lodge over, they prevent physical access for the fi sh and enable mosquito breeding. Aq uat ic beet le larvae (Carabidae) and aquatic bugs (Belostomatidae) are the most effi cient mosq uito larvae predators in secondary and evaporation pond s. The aquatic bugs are able to li ve in higher organic water than the aquatic beetle larvae, a nd can be present in enormous numbers. Again , ph ysical impedance by thick vegetation at the margins will redu ce the effecti veness of these predators. These insect predators can achieve almost total control of mosq uito larvae in sewage ponds of suitable water qu ality, and vegetation-free margins.
SUMMARY In the past , the design of sewage treatment and efflu ent disposa l fac ili ties was usually dictated solely by engineering and microbiological principles. Little attention was paid to the possibility of breeding mosquito populations close to habitat ions, with resultant danger to public health. Appreciation of this potential risk fo llowed by application of simple design principles and adeq uate mai ntenance can reduce this danger. Bio logical controls can then operate effectively. Chemical controls can be used, but only for start-up or emerge ncy situations.
ACKNOWLEDGEMENTS Wendy Kelton, Geoff Dav is and Kevin Hodder of the Medical Entomology Branch, have assisted with the man y years of observations and discussions that have enabled these guidelines and recommendations to be built up . Their contributions is grate full y acknow ledged. Particu lar thanks are given to Wendy Kelton for, typin g th e manuscript and assisting wi th the organization.
REFERENCES C D I (1988). Au stra li an encephalitis in the Nort hern T erritor y. Four case repo rts . Co mmunicab le Diseases In te llige nce 88/ 2 1: 2-7. (Commonwealth Department of Commu ni ty Services a nd Health , Ca nberra) . DOHERTY , R. L. ( 1972). Arboviruses in Austr a lia. A ust. Vet. J. 48: 172- 179. NH & MRC ( 1979). 'G uidelines fo r Re- use of Wastewater', Australian W a ter Reso urces Coun cil , Na ti onal Health and Medi cal Researc h Coun cil , Depa rtment of Na tio na l Development, Canberra. RUSSELL, R. C. ( 1986) . Dispersal of the arbovirus vec tor Cu /ex annulirostris Skuse (Diptera : C ulicidae) in the Mu rray Va lley of Vict oria Australia. Gen. Appl. Ent. 18: 5-9 . WHELAN, P. I. (1981). The vuln erabilit y and rece ptivit y of the Northern Territory to mosquito borne disease . Tra nsactions of the Menzies Foundation 2: 165- 17 1. WHELAN , P. I. (1984) . ' Mosquitoes of Pub lic Heal th Importan ce in th e Northern Territo ry and their Cont rol '. (Northern Terri tory Department of Health, Darwi n).
CHEMICAL CONTROL The aim of chemical control of mosquito larvae should be to apply th e minimum amount of insecti cide to prevent the production of adu lt mosqu itoes. Chem ical control should not be used as a long term strategy in sewage treatment areas, in order to avoid insecticide resistan ce and un wanted effects on non -targe t organisms. However, it may be necessary to app ly insecti cides during the initial operational period or when proper maintenance has not been carried out. The insecticides of choice to control mosquito larvae in sewage ponds a nd effl uents are either Temephos or (Bacillus thuringiensis var israelensis) (Bti) .. Correct rates for Temephos must be strictl y adhered to, as overtreatment can kill fish and other aquatic insects.
MOSQUITO SAMPLING Regular inspections for mosq uito larvae should be carried out in sewage pond s and their effluents to determine whether breeding is occurring and to determine the necessity for weed contro l or chem ical control. Chemical control with Temephos or Bti may be necessary at weekly or longer in tervals; the presence of pupae indicates that control should have been carried out at shorter intervals. If only first and second instar larvae are present, th en either biological control is quite efficient, or the mosquitoes have just started to breed in that area, and continued monitoring is necessary. Mosquito larval or pupal samples can be collected by dipping into sheltered vegetated zones with a soup ladle. Any larvae collected should be stored in small vials with 700Jo alcohol or meth ylated spirits, together with in fo rmation on collection locality, site , da te and collector. Larval or pupal identifications should be checked by an entomologist. Adult specimens collected by biting or harbourage collections can be sent for identification, packed loosely in tiss ue paper in a small box, together wit h all the details of collection. Chironomid midge pupae or adults are often mistaken for mosqu itoes and their presence has often resu lted in control programs being instituted where none has been necessary.
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WATER December, 1988
Commercialisation in the Water Industry EDITOR'S N OTE The followin g paper was prepared as a syndicate exercise during the recent A WWA Water Industry Ma nagement Development Program held at th e A ustralian Co llege of Management, Mount Eliza. It was written within severe time constraints and therefore confined itself mainly to concepts. The paper is reprinted here to promote th ought and discussion. The members of th e syndicate are: Owen Karsen (Sydney Board), Graham Rawnsley (E. & W.S.), Mike Keegan (Tubemakers), Ro be rt Seear (M.M. B . W.), Da vid McRae (Hunter Board), John Stevens (Sydney Board), John Mills (W.A . W.A.)
1. BACKGROU ND The current economic situation in Australia requires that service indu stries beco me more commercial in their approach . This paper explores how the water industry should develop and maintain such an approach , characterised by a business orientation, doin g more with less, accountability for performance, rates of return on investment whilst at the same time meeting long term Asset Management , environmental responsibilities a nd socio-political requirements. Key issues are: • Customer Service • Economic Management • Regulation • Unity of Purpose and Industry Voice • Strategic Research and Development Diagrammaticall y this is represented by a 'Strategic Diamond ', showing the current situation, a desirable futur e and the paths by which that future can be reached. From an appreciation of the strategic diamond approach, the authors came to its opinion of what ' commercial success' means to the water industry . To be successful in the future, the industry needs to perform three essential roles . These roles are: - commercial approach to services (getting a real rate of return for services provided) - social equity (providing essential services to disadvantaged groups) - stewardship (providing for the long term protection of water industry assets and the environment) . The strategic objective of the water industry is to effectively manage the balance of all three roles. If the industry fails in any one then it cannot be success ful. The mission statement for th e industry would therefore be: The Water Industry is committed to providing water and related services on a commercial basis, whilst operating in a socially responsible manner and providing long term stewardship of its assets and the surrounding enPironment.
2. FUTURE SCENARIOS From the key issues and Mission, the authors developed future possible 38
WATER December, 1988
STRAT EGIC DIAMOND
NOW LACK OF DIR ECT IO N IND USTRY FRAGMENTED R & D NEE DS ORGANISATION POLITI CA L INFLUENCE TOO FAT DEBT TOO HI G H MANY INFLUENCE LEVELS NOT BUSINESS OR IENTE D
BUS INESS CO RPOR ATIO N MARKETING DEBT REDUCTION GOVE R MENT AP PR EC IAT IO N H UMAN RESOURCES
R& D ASSET MANAGEMENT TOTAL QUA LITY ASSU RANCE REGULATION INFO RM AT IO N SYSTEMS
ENV IRO NMENT DISTRIBUTION OF WEALTH
FUTURE COMME RC IA LISAT ION VIABLE WATER INDU STRY CONSUMER APP REC IATIO N & SAT ISFACTI ON BUSINESS OR IENTE D PROGR ESS IV E MANAGEMENT ST RATEG IES UNION PARTI C IPATION ACCOUNTABLE RESPON SIB LE IMAGE
scenarios and came up with a list of outcomes. The starting point was 'If nothing is done to influence current trends, what are possible futures?' These have been segre gated into gro ups, so that strategies can be developed to address the outcomes. The gro upings used were: • Probable a nd Desirable • Improbable and Desirable • Probable and Undesirable • Improbable and Undesirable Setting out the outcomes in this way focuses strategy development on making undesirable scenarios improbable and probable scenarios desirable. Scenarios can be grouped as in Tabulation 'A'. Obviously, some of the gro upings can be debated . Some would see privatisation
as desirable and severe pollution as less than improbable . The authors welcome such debate . Strategies need to be developed to move the issues from the Undesirable/ Probable and Desirable/ Improbable areas into the Desirable/ Probable area. If these strategies are not effective then undesirable scenarios may eventuate or desirable scenarios may not be brought about. The team did not develop lists of strategies for each key issue. Instead, we decided that such strategies could only be use full y de ve loped if a unifyi ng framework was first developed . It was thought that such a 'strategic overview' would serve as a useful guide in developing strategies.
TABULATION A DESIRABLE
• • • • •
Indu strial Co-operatio n Debt Reduction Better Marketing Management Development Acco untability
• Strategic R & D IMPR OBA BLE • Effective rela ting to community/ c ustomer • Sufficient Corporatisation • Pricing flexibi lit y
UNDESIRABLE • Fragmentation • Lack of visio n • Lack of know ledge and orientation to customers • Lack of industry voice • Connic t from wealth polarisati on • Severe Po llu tio n o f Austra lia n environment • Priva tisat ion • 'Federalisat ion ' (i. e. placing responsibi li ty for water manage ment wit h federa l age ncies)
TABULATION B PROD UC T
CUS TOM ER
VAL UE A DDED
( -ve) (or) ( + ve)
Pri va te: e .g. ra tepayer pensio ners WATER
• Commercial : Co mpan y I Compan y 2 etc. • Industry Compa ny 3 Compa ny 4 etc.
water industry and...generates a number of advantages: (a) It prov.ides a framework for identifying and collecting strategic information relating to trends/ opportunities/ threats in products, customers and value added (as a measure of productivity) as well as relating to the markets where customers and product intersect. (b) Once these trends, opportunities and threats have been identified, managers and employees are in a position to make decisions on where maximum value can be added by investment in technology, research and development, human resource development, etc . (c) It is perhaps a more logical and appropriate application of program budgeting to the water industry to relate costs to individual markets in this way. (d) Because in many cases the ' value added' figure for products such as ' welfare' and 'stewardship' (e.g. asset management) will actually be negative, the framework provides a powerful approach to highlighting the subsidies entailed in the provision of the product/service in question. (e) The framework also provides a mechanism for improving pricing effectiveness by identifying mismatches between demand and value added. It must be emphasised that the framework is proposed as one example of how the use of a commercial, marketing approach integrates the three major roles (commercial, social and steward) . We believe this example gives an indication of how a marketing api;,roach can be used to provide a basis on which the water industry can move into the 1990s with a mechanism which: • Provides a clear message to employees: adding valu e"' in the provision of products or services to customers. • Demonstrates and accounts for performance in the provision of those products/ services. • Focuses performance measurement on two issues: customer satisfaction and value added. • Assures government and the community that the industry is capable of providing services in a socially and environmentally • responsible manner.
URBAN DEVELOPMENT ' WELFAR E'
• Governm ent • disad vantaged
STEWARDSHIP (e.g. asset manage ment , public policy, int egra ted catchment management etc.) (a,b ,c,x,y,z et c. being ' va lu e added ' a nd ' return' a mounts .)
One possible overview is to adopt a commercial, marketing approach and to view all activities in the light of that approach. The team was unable to identify a framework for the application of such an approach at the industry-wide level. We were able to develop a framework, at the enterprise level. The framework is based on the following simple premises: • Water industry enterprises incur costs to process raw materials to • Provide Products/ Services to • Customers for a • Financial Return • The difference between cost and return can be said to be the VALUE ADDED* for that product. *The authors used the value added concept (i.e. the difference between resource costs and the price realised in the market). ft may be more appropriate to use Return on Investment instead. In order to compute value added, costs and returns should therefore be recorded by product or service . That is,
the programs in the enterprises ' Program Budget should each have as their outputs one of the enterprises' products or services. This can be represented schematically (Tabulation B) . The actual products/services nominated are of course open to discussion and will vary from enterprise to enterprise (e .g. irrigation) . Similarly, categorisation of customers can also be varied - e.g. they can be viewed in terms of the distribution systems which service them (so called 'supply side' categorisation) or in terms of the forces which influence their purchasing decisions ('demand side' categorisation) . But however we divide them up, the important things here are: • firstly, that we identify what our products>are; who are the customers for these products and the-value added for each product, and • secondly, that we recognise that wherever a product and a customer get together we have a market. This simple construct allows the application of marketing principles to the
L. MONTEITH Continued from page 32 The presence of just one substantial inflow source such as a dislodged or broken manhole cover can contribute flows equivalent to the sanitary flow from over a thousand people and cost many thousands of dollars to accomodate. Monitoring schemes using computerized techniques can provide a quick test of the state of the system. When properly followed up with an economic evaluation of possible rehabilitation and a soundly based source detection and repair program in the economically viable areas, significant saving in augmentation costs can be achieved.
The extreme variability of the wet weather flows recorded during these studies has highlighted the degree of approximation involved in the estimation of design flows and the futility in becoming overly preoccupied with the precision of computations . The wet weather flow allowances are merely a reasonable upper limit of flows beyond which 1/1 becomes a source of concern and the system becomes a possible candidate for rehabilitation. The excellent performance of the newer systems built with modern materials gives promise of reduced design flows however, it remains to be seen what the aging process will do. There is a case for the staging
of pump capacities to take advantage of the low early flows from new works, this deserves serious consideration.
REFERENCES 'D es ig n of Se p a rate Sew e ra ge S ys tem ' Metropolitan Water Sewerage & Drainage Board, Sydney, 1979. ' Report on Odour Generation and Corrosion in th e Darwin North Zone Sewerage System ' - Gutteridge Haskins & Davey, 1987. 'Report on Monitoring of Wet and Ory Season Sewage Flows at Kath erine' - Gutteridge Haskins & Davey, 1987 . ' Report on Darwin City Zone Sewerage Flow Monitoring - Gutteridge Haskin s & Davey, 1988. WATER December, 1988