Water Journal : Water Journal April 2011
refereed paper wastewater treatment water APRIL 2011 147 to be beneficial to reduce the return sludge rate to a low value (RS ≅0.25) at peak wet weather flows as a means of allowing the sludge blanket to compact, reducing the volume needed for separation and increasing the volume available for sludge storage and thickening. This provides temporary relief from potential solids overloading (Derek Wilson, Kellogg Brown & Root Pty Ltd, personnel communication, 2010). This feature is included in the design of the Caddadup, Halls Head, Gordon Road and Kwinana plants. Discussion of Possible Variations to the ATV (2000) Recommendations The ATV (2000) guidelines have been shown by more than 40 years of practice in Germany to produce robust and reliable outcomes, but it is also recognised that the recommendations with regard to overflow rates and the sludge collector performance parameter ηeff may be conservative under some conditions. The following discussion is provided on some possible variations to the ATV (2000) approach. There is anecdotal evidence that suction type sludge collectors of the "Tow-Bro" type may perform better than "organ-pipe" sludge collectors and conventional scrapers with regard to lower sludge blanket heights and higher return sludge concentrations (Ekama et al., 1997 and Kinnear, 2002). With the "Tow-Bro" type sludge collector, there is no direct short circuiting between the inlet and the return sludge draw off, and the sludge collector may be designed to remove only a small depth of sludge, close to the tank floor, with each revolution. As a result, sludge collector efficiency and ηeff of up to 1.0 may be achieved with "Tow- Bro" type sludge collectors. For large diameter settling tanks where the area determined from the ATV (2000) guidelines is governed by the maximum recommended value of qA = 1.6m/h (DSV ≤ 312L/m3 for qsv = 500L/m2h), consideration may be given to the use of flocculator- clarifiers to achieve higher overflow rates. These settling tanks have energy dissipating inlets, large flocculator centre wells, deep side-water depths (5.5-6.0m), suction sludge collectors of the "Tow-Bro" type and inboard or baffled peripheral weirs (Parker et al., 1996). Values of qA up to 2.4m/h have been shown to produce low effluent suspended solids (< 20mg/L), provided that the settling tank is well optimised hydraulically and operated with a low sludge blanket and relatively low return sludge rate (Ekama et al., 1997). Settled sludge volume analysis indicates that it is feasible to operate with a value of RS of 0.5 or less at values of DSV ≤ 333L/m3, and not result in significant sludge accumulation in the settling tank with ηeff = 1.0 and a thickening time of 1.0 hour. The solids loading must also be considered, and a capacity factor of up to 1.0 at peak wet weather flows may be appropriate where the settled sludge has a low potential for flotation due to denitrification. For heavily loaded horizontal flow settling tanks (400L/m3 < DSV ≤ 600L/m3), settled sludge volume analysis indicates that sludge will accumulate in the settling tank due to sludge removal limitations for RS = 0.75, ηeff = 0.7 and a maximum thickening time of 2.5h recommended by ATV (2000). Under these conditions, the sludge storage volume is the most important consideration in sizing settling tanks. Where peak flows are relatively infrequent, and of short duration, it may be practical to store some sludge in the settling tank during peak flows. The settled sludge must have a low potential for flotation due to denitrification, and in warm climates this may best be achieved by long sludge age biological nutrient removal (BNR) plants achieving high levels of nitrogen removal. There is anecdotal evidence that short- circuiting of influent to the underflow in settling tanks may be minimised with well-designed spiral sludge scrapers (Albertson and Okey, 1992), and sludge collector efficiencies and η eff greater than 0.9 may be achieved under favourable conditions (Grau, 2010). For long sludge age BNR plants equipped with well- designed spiral sludge collectors and settling tank volumes similar to ATV (2000) recommendations, operating experience in Australia indicates that these plants may be successfully operated with values of DSV of 600L/m3 or more, with peak overflow rates of 0.8-1.2m/h and values of RS up to 0.8 (de Haas, 2005). Summary The ATV (2000) guidelines result in broadly similar settling tank areas as commonly used in contemporary Australian practice. The benefit for designers of using the ATV (2000) guidelines is that they provide an integrated approach to determining the area and depth of the settling tank and the return sludge pumping capacity. Discussion is provided on some possible variations to the ATV (2000) recommendations with regard to peak overflow rates and sludge collector performance which may in some circumstances achieve more cost effective secondary settling tank designs than ATV (2000). Beenyup Wastewater Treatment Plant Stage IV Secondary Settling Tank Internal Features.
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