Agitation and Filterability of Freeze/Thawed Sludge

Abstract

Particle size has opposing effects on the effectiveness of freeze/thaw sludge conditioning. Small particles migrate easily and coagulate into larger particles during freezing, but small particles also cause poor sludge dewaterability. Since agitation can change particles size, the effects of agitation on sludge de‐watering before and after freeze/thaw conditioning is of practical significance. The capillary suction time (CST) device is used to measure the dewaterability of several water and wastewater treatment sludges before and after freeze/thaw. The results show that preagitation may have a detrimental effect on the dewaterability of some sludges after freeze/thaw conditioning. For other sludges such as raw primary sludges that have large initial particle size distributions, agitation prior to freeze/thaw will not be detrimental to subsequent dewatering. Postagitation clearly has a substantial deleterious effect on dewaterability, leading to the conclusion that if freeze/thaw is used for conditioning, the thawed sludge should undergo minimum agitation before dewatering.