The interrelationships between activated sludge plant operation and floc characteristics were examined in order to determine process control mechanisms for the effective treatment of domestic wastewaters and surplus activated sludge. A quantitative indicator of the physiological floc characteristics was developed and termed the morphological index. This may be evaluated by plant operators using a simple assessment form and matching photographic guide of various floc structures. The morphological index and floc size distribution were found to be useful tools for activated sludge process control and monitoring the dewaterability of surplus activated sludge. It was found that floc size, morphology and in turn the dewaterability of surplus activated sludge varied with different operating conditions. Hydraulic retention times (HRTs) of < 10 hours and sludge ages of ≤8 days, resulted in the presence of small diffuse floc structures in the mixed liquor. The effluent quality was poor and the activated sludge had unfavourable filtering and dewatering properties. Switching plant operation to sludge ages in the region of ≥8 days and HRTs of ≥10 hours, was found to improve effluent quality and the dewaterability of surplus activated sludge. Here the flocs were large compact structures with short filaments protruding from the floc body.

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