Characteristics of free cells and aggregated flocs for the flocculation and sedimentation of activated sludge

Laboratory experiments were conducted to investigate the correlation of the free cells/aggregated flocs characteristics and the sludge flocculation and separation behavior during the activated sludge process. Activated sludge was cultured in three laboratory-scale batch reactors with same carbon sources of glucose but different sludge retention times (SRTs) of 5, 10 and 20 days. The variation in the operation condition produced sludge with different flocculation and separation characteristics. The reactor performance in terms of bioflocculation as measured by the amount of suspended solids in the effluent, sludge sedimentation and compression as measured by the sludge value index improved considerably as the SRT lengthened. The higher SRT was related to less negatively charged surface of free cells and more hydrophobic of aggregated flocs. The negative zeta potential of aggregated flocs (11-13 mV) was smaller than that of free cells (15-23 mV), and free cells carrying lesser negative surface charges resulted in effluent clarification. Hydrophobicity of aggregated flocs (10-17 %) in each reactor was higher than that of free cells (5-8 %). The increase in hydrophobicity of aggregated flocs had positive effect on settleability. Floc strength measured by breakage/erosion index was closely related to hydrophobicity of aggregated flocs, but no correlation could be established with zeta potential of aggregated flocs. In addition, different bacterial species existed in free cells and aggregated flocs. Microbial similarity between free cells and aggregated flocs increased with shorter SRT and was closely related to effluent clarification.