The Effect of Sludge Age on the Operation Cost of Activated Sludge Systems

Abstract

An effect of sludge age (SRT) on the operation cost of a 10 MGD activated sludge plant was evaluated using an activated sludge computer model. For a completely-mixed system an increase in SRT generally reduced operating cost, while for a system with an anaerobic selector the reverse was generally observed. Nitrification significantly increased operating cost and, if not required, can be avoided by lowering SRT. The effect of SRT on the operating costs of BNR systems cannot be predicted theoretically; field studies were required to determine this effect. This paper presents the results of such a field study. Deviation of the SRT from its optimum value may carry substantial financial penalties in addition to operating problems. Precise maintenance of SRT using an automatic control system is recommended.

Influence of Temperature on the Performance of a Full-Scale Activated Sludge Process Operated at Varying Solids Retention Times Whilst Treating Municipal Sewage

“…Microbial activity and population is significantly affected by variations in SRT that in turn is influenced by temperature [14]. SRT also has a significant impact on the operational costs of running a wastewater treatment plant (WWTP) [15]. It is generally recommended that a SRT for a conventional activated sludge process should be between 4 and 9 days in warmer climate (15-25 °C) [13,[16][17][18] and 10 days or more in temperate zones (>10 °C) [11,19] where in addition to BOD removal, ammonium removal by nitrification and nitrate removal by denitrification will often also be required in order to meet more stringent effluent discharge standards.…”