Water Journal : Water Journal April 2011
wastewater treatment refereed paper technical features 146 APRIL 2011 water SSVI are taken to be global averages, Equation (5) is considered to be valid for the type of comparison undertaken in this paper. A comparison of the ratio of the applied solids loading rate recommended by ATV (2000), to the maximum permissible solids loading rate from solids flux theory ( ) and the modified Water Research Centre procedure [ ] (Ekama and Marais, 1986) are shown in Table 1 for different values of DSV and DSVI. For DSV values less than or equal to 400L/m3 (the maximum value recommended by ATV (2000) to prevent the accumulation of solids for RS = 0.75, ηeff = 0.7 and tTH = 2.5 hours), the ratio SLRapplied / SLRmax is less than or equal to 0.80 for 70mL/g < DSVI < 160mL/g. For other values of DSVI and DSV, the ratio SLRapplied / SLRmax is greater than 0.80, but less than 1.0. For higher values of DSV, (400L/m3 < DSV ≤ 600L/m3) where sludge removal rather than solids flux limitations may determine settling tank capacity, the ratio SLRapplied / SLRmax is less than or equal to 1.0 for all combinations of DSV and DSVI recommended by ATV (2000). This analysis predicts that solids may accumulate at peak loading rates in some settling tanks sized in accordance with ATV (2000) recommendations, highlighting the important role of the sludge storage zone in German practice. Examples of Contemporary Australian Practice The ratios of actual tank diameter, depth and volume compared to the recommendations from ATV (2000) are shown in Figure 1, for examples of contemporary Australian practice in the design of secondary settling tanks. It may be seen that for most of these plants, the tank diameters and tank volumes are slightly smaller and the depths are generally deeper than determined from ATV (2000) recommendations with one settling tank out of service (N-1). The main deviations from ATV (2000) are at Halls Head, where the tanks are shallower than desired due to high groundwater levels, and to compensate, all tanks are normally in service, a selector is incorporated in the activated sludge process, and an innovative return sludge control strategy is used; Luggage Point where the actual sludge settleability is better than the design value; Eastern Treatment Plant where the impact of step-feeding has not been incorporated into this analysis; Karana Downs where the ratios shown are for one settling tank in service, whereas both settling tanks are normally in service (i.e. no redundancy); Western Treatment Plants, where the settling tank diameters are similar to ATV (2000) recommendations but the depths and volumes are significantly greater than ATV (2000) recommendations due to the low peaking factors. Overall, the comparison shows a good degree of consistency between contemporary Australian design practice and the recommendations from ATV (2000). At the Subiaco and Beenyup Plants, the tank diameters are similar to the ATV (2000) recommendations, although the settling tank depths are lower than ATV (2000) recommendations at both Subiaco and Beenyup Stages I and II. This limits the capacity to store sludge during peak wet weather flows, which is manageable due to the low values DSVI (60mL/g) that are achieved by continuous return sludge chlorination and the use of "Tow-Bro" type sludge collectors. At Beenyup, the evolution of settling tank design from the early 1970s to Stage IV in 2008 has seen the introduction of higher return sludge flow rates, lower overflow rates, deeper tanks, twin-arm "Tow-Bro" type sludge collectors and the introduction of larger centre feedwells and in-board peripheral weirs. Stages III and IV at Beenyup are almost immune to solids washout at high flow rates as the 5.9m deep settling tanks can hold the entire solids inventory without the sludge blanket rising to a level that results in the excessive loss of solids. Step-feeding of the aeration tanks and 6.1m deep hydraulically efficient secondary settling tanks enable the Eastern Treatment Plant in Melbourne to cope with very high peak weather flows of 5.3 times average daily flow at overflow rates of up to 2.8m/h. Where peak wet weather flows are of relatively short duration, it has been found 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Halls Head (WA) Gordon Road (WA) Kwinana (WA) Caddadup (WA) Subiaco (WA) Beenyup (Stage I & II) (WA) Beenyup (Stage III & IV) (WA) Alkimos (WA) Oxley Creek (QLD) Luggage Point (QLD) Gibson Island (QLD) Sandgate (QLD) Wynnum (QLD) Wacol (QLD) Karan Downs (QLD) Eastern Treatment Plant (Vic) Western Treatment Plant 55E (VIC) Western Treatment Plant 25W (VIC) Ratio of Actual Value/ATV Rec ommend ations Diameter Actual:ATV Depth Actual:A TV Volume A ctual:ATV Figure 1: Comparison between contemporary Australian practice and recommendations from ATV (2000). (qsv = 500L/m2h, RS = 0.75, tTH = 2.5h, N-1 and no allowance for inlet disturbance zone).
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