Revealing the Underlying Mechanisms of How Initial pH Affects Waste Activated Sludge Solubilization and Dewaterability in Freezing and Thawing Process

In this study, the effect of initial pH ranging from 3 to 11 on waste activated sludge solubilization and dewaterability in the freezing and thawing (FIT) process was investigated. Experimental results showed that alkaline conditions enhanced the solubilization of sludge in F/T treatment, whereas acidic conditions improved the dewaterability of sludge. Optimum solubilization with organic substances being 189.7 mg COD/g VSS occurred at initial pH 10, which was 12.9 times higher and more biodegradable than the control. Optimal dewaterability performance was achieved at initial pH 4, with capillary suction time and specific resistance to filterability reduction being reached to 85.4% and 87.8%, respectively. It was found that F/T treatment at initial alkaline condition also obtained good dewatering performance, and F/T treatment at initial acidic condition acquired fine solubilization too. Mechanism explorations exhibited that the OH and freezing had synergetic effects on the degradation of extracellular polymeric substances (EPS) to enhance sludge solubilization, and OH- can be concentrated to a much higher level in the liquid-like boundary region upon freezing, which further strengthened this effects. The H+ and freezing also showed synergetic effects on the protonation of functional groups of EPS and flocculation of the colloidal sludge to improve sludge dewaterability. This study clearly reveals the role and mechanisms of initial pH on the F/T process aiming at solubilization and dewaterability of sludge, and might provide supports for the application of F/T-based strategy in field situations in the future.