Investigations into excess activated sludge accumulation at low temperatures

Abstract

Low temperature operation has been reported to increase the yield of volatile suspended solids (VSS) but little quantitative data are available and an explanation for the reported phenomena is lacking. A laboratory study was initiated using two parallel 41. sequencing batch reactors operating at a sludge retention time (SRT) of approx. 10 days, and a hydraulic retention time (HRT) of approx. 1.25 days. An experimental design with two levels of temperatures (8 and 20°C) and two levels of influent volatile solid concentrations (approx. 15 and 70 mg/l) was selected. The experimental results verified the hypothesis that the observed yield (gΔ VSS/g Δ COD) increased at lower temperatures for both levels of influent suspended solids. As, experimentally, it was not possible to determine the relative composition of the accumulated volatile solids, the IAWPRC activated sludge model was adapted to estimate the fractions of: viable organisms, accumulated influent particulates and endogenous decay products. The simulations indicated that the actual concentration of viable organisms apparently decreased from approx. 52–63% of the MLVSS at 20°C to 29–47% at 8°C. Simulations were utilized to estimate the additional waste sludge production during cold weather at typical treatment plants. Increases of approx. 12–20% could result.