Optimal hydraulic shear strength and mechanism of activated sludge floc re-growth after breakage

In order to evaluate the effects of hydraulic shearing action on activated sludge floc aggregation, floc aggregation, breakage and re-growth performances under different shear strengths were systematically examined. Performances were evaluated by measuring the floc size variations using a laser particle size analyser on-line monitoring technique. The flocs after breakage were characterised by investigating the composition of extracellular polymer substances (EPS), floc size, substances released due to surface erosion and chemical structures using Fourier transform infrared spectroscopy (FTIR) to clarify the breakage model and re-growth mechanism. The results showed that activated sludge flocs broken at a hydraulic shear strength GT value (the product of the velocity gradient and time) of 56,280 exhibited enhanced re-growth performance compared with the control. The substances released from sludge increased with shear strength, indicating that the floc breakage mode gradually transitioned from fragmentation to surface erosion. FTIR revealed that, after breakage, the spectrum of loosely bound EPS in sludge tended to be similar to that of tightly bound EPS. The results suggest that the breakage of activated sludge flocs under moderate hydraulic shear strength can fragment flocs into smaller particles without surface erosion and promote the exposure of inner tightly bound EPS, thereby improving re-growth performance.