performance is to be obtained . There have been manufacturers who claimed that no sludge was produced from their plants , but such an approach can only lead to poor effluent quality. In Perth it was found that best results could be obtained with M.L.S.S . somewhat higher than those normally associated with extended aeration . The level normally adopted was 5000-6000 mg/I. Despite these high mixed liquor solids level, excess sludge production was also reasonably high at 0.4 to 0.5 kg/kg of B.O .D. applied. Effluent disposal was effective at all sand sites as in most cases flows were low enough to allow rotation of disposal areas throughout the site. Al l but the smallest and most temporary sites were ringed with monitoring bores to check on the groundwater condition . No problems have been detected at these disposal areas. With the exception of one plant which became heavily overloaded, no odour problems have occurred, even though some plants were adjacent to ·houses . Noise was a prob lem in some early installations even when houses were some distance from the plants . The Board eventual ly specified low-speed blowers and in small instal lations when blower buildings were not provided, the blowers were often moved from the top of the tank to the ground on the opposite side to the subdivision . Operation In order to ensure good performance and prevent odour complaints a high level of operator attendance has always been maintained . If skimmings and foam are not controlled regularly and disposal areas rotated, both odours and f ly-breeding can occur in addition to decreased effluent quality . Attend ance was one to two hours per day on every weekday (up to three hours at Gosnells) and in addition a quick check was made of all plants on Sunday mornings . Excess sludge was regularly removed by tanker and at the larger plants when on full load. This was an expensive aspect of operation. It was noted that a fresh water supply was essential for cleaning purposes as the use of effluent alone eventually led to unhygienic conditions . This meant that' a water main was often laid over considerab le distances from the nearest subdivision . The operators carried out sludge sett leability (S .V.1.) tests daily in most plants when on full load . Thus was an "grab" or "bulked " samples to measure plant performance. This testing was occasionally supplemented with longer sampl ing programs bulked over a working day. Performance samp ling was original ly done by chemists but their tours were frequently delayed by periods of no flow, and eventually operators took over this task, leaving the samples on ice at the plants for later collection by the chemists.
18
without some interim step. ,·l\ ' was Maintenance decided that the best metnod of Comminutors failed regu larly on providing ,early -relief . for the small most plants and required considerable temporary plants wa~ to construct the attention. It appeared that most units aerati9n taDkS and sec'?n-d·ary'..,c larifiers . provided were under-powered for the of the _per,rnanent p.lants and operate job required. Blowers were generally these under -extendep aerati.PIJ. unti l the reliable and required minimum mainprimary and sludge treatmelft+-facilities tenance but some required shading in cou Id be .added. summer time to prevent cut-outs from Fig . 4 shows average results of two overheating. of the plants. Fitters and electricians visited plants Table 4 shows the details of the five once per week for regular maintenance plants including the results achieved and checking . None of these plants was and the method of effluent disposal. equipped with alarms to the Board's The Beenyup plant which has just been central control room and this was the converted to conventional treatment main reason for the operator check will be connected to an ocean outlet once on each weekend . within a few weeks, but the other three Site . Maintenance-The disposal plants wil l continue to use sand sites areas also required limited but regular for eff luent disposal. maintenance to prevent wet cond itions Kwinana will be converted to convenpersisting . Solids were carried over the weirs from time to time and thus areas tional treatment within a few months and Pt. Peron wi ll follow in about three had to be dried out and either raked, rotary -mown or ploughed. Occasional years. The convent ional capacities of cutt ing of weeds was also necessary these five plants (in the same order as Table 4) are 3636, 4730, 9460, 27270 and although on some sites this was effectively carried out by grazing horses 5450 m3/d . It should be noted that all from nearby riding schools. the conventional facilities are only initial stages and each plant wil l Although most of these plants were undergo further expansion from time to near housing subdivisions the actual time . sites were often in isolated bush It can be seen from Table 4 and the locations and despite the normal detailed results from two plants (figure security fence and patrols, vandalism 4) that effluent quality was generally was a problem in some areas. This very high. This was to be expected as common ly took the form of objects such large extended aeration systems being thrown into tanks and causing blockages . were less susceptible to the problems TABLE 3 UNIT OPERATING COSTS Costs are for operation and maintenance only and do not Include capital debt servicing PLANT SIZE
FINANCIAL YEAR
OPERATING COST C,ENTS/m3
SMALL (BULLCREEK)
1971 /72 1972/73 1973/74
25 44 23
MEDIUM (ROCK. PARK)
1972/73 1973/74
23 13
LARGE (EDEN HILL)
1974 / 75 1975/76 1976/77
12 14 16
(GOSNELLS)
1974/75 1975/76 1976/77
10 11 13
Operation Costs-typical costs are set out in Table 3. Although these seem high they are not unusual for smal l plants and certain ly don't detract from the convenience and flexibility of small extended aeration plants . INTERIM STEPS FOR PERMANENT PLANTS Installation and Results Planning for sewerage in Perth's expanding urban corridors required the construction of five new permanent treatment plants but there was neither time nor funds to achieve this objective
of high pumping rates and excessive upflow velocities in clarifiers. However Canning Vale and Westfield were both eventual ly overloaded hydrau lica lly due to increased pumping rates and their performance did deteriorate . Beenyup was the only plant served by gravity and this, coupled with the largest units resulted in good results even when the plant became 250% overloaded. It should be noted however that the plant was extreme ly difficult to operate when overloaded and was particularly sensitive to slight variations in air or M .L.S.S. levels. Bulking was a regu lar occurrence in the last