The performance of the activated sludge process is limited by the ability of the secondary sedimentation basin to separate and concentrate the activated sludge from the treated effluent. This limitation stems from physical properties of the activated sludge floc that contribute to hindering its settleability and compactabilty.
Unified Theory of Filamentous Activated Sludge
Authors: Denny S. Parker, Sezgin Mesut, David Jenkins
1978 Journal Water Pollution Control Federation