Waste activated sludge hydrolysis during ultrasonication: Two-step disintegration

► 2-step disintegration process was introduced for understanding of WAS hydrolysis. ► Bound EPS and RNA were suggested as new criteria for 2-step disintegration process. ► Different decrease pattern of Bound EPS and RNA supported the 2-step process. ► Floc disintegration (FD) and cell lysis (CL) appeared to take simultaneously. ► After 10 min, the dominant hydrolytic process was shifted from FD to CL.