Most of the extracellular enzymes (except a-amylase) in sludge flocs were present in the pellet and tightly bound EPS (TB-EPS) layers, with minor quantities detected in the loosely bound extracellular polymeric substances (LB-EPS) and slime layers, and almost none detected in bulk solution. As for α- amylase in sludge flocs, most of it (52.6%) was also bound with the pellet layer; however, the rest of it was dispersed nearly uniformly throughout the sludge flocs.
Linking the higher ratios of SCOD/COD to the higher contents of proteins and polysaccharides and the higher activities of extracellular enzymes in the slime layer, the optimum ultrasonic pretreatment conditions lasted 10 min and had a density of 3 kW Là1 with a frequency of 20 kHz. After ultrasonic pretreatment under the optimum conditions, the sludge reduction for TSS and VSS in aerobic digestion was 30.9% and 35.0%, respectively, compared with 20.9% and 20.9% for control, after a digestion time of 10.5 d. Improvement of aerobic digestion can be attributed to the enhancement of enzymatic activities by ultrasonic pretreatment, and the shifts of extracellular proteins, polysaccharides and enzymes from pellet and TB-EPS to slime. This led to increased contact among extracellular proteins, polysaccharides and enzymes that were originally embedded in the sludge flocs. As for both control and ultrasonic pretreatment, the enzymatic activities of sludge flocs during digestion were lower than those of raw sludge.
