What are the causes & effects of activated sludge settling problems

Most activated sludge treatment units still rely on secondary clarifiers to settle biological solids for recycle and wasting. A slow settling rate caused by bulking results in loss of biomass over the clarifier weir - causing effluent TSS compliance issues and a loss of MLSS concentration in the aeration basin. The following is a short list of the most common settling problems with both cause & effect (From Jenkins et al. (1984).