Waste activated sludge conditioning by direct slurry freezing

Abstract

It has been widely observed and noted that waste sludges subjected to freezing undergo a conditioning effect that greatly enhances subsequent dewatering. However, wide-spread application of freezing for conditioning purposes has never been practiced because of the high operating costs associated with conventional indirect freezing systems. Furthermore, all reported efforts involved the solid freezing of the experimental sludge. The purpose of this research was to evaluate the technical and economic feasibility of waste activated sludge conditioning for dewatering using a direct slurry freezing system. Liquid butane was used as the refrigerant and it was bubbled through the liquid sludge to form an ice-sludge slurry. Economic investigation included butane recovery techniques. The results of the investigation showed that the dewaterability of waste activated sludge can be greatly improved by slurry freezing with liquid butane. Slurry freezing results in better conditioning and a better quality supernatant than does solid freezing because fewer cellular solids are destroyed by the freezing pressure. The conditioning effect is also comparable to or better than conditioning by widely used processes such as heat treatment. Furthermore, recovery of butane for subsequent reuse reduces the cost of slurry freezing to less than the cost of nearly all conditioning processes in current use.