Pathogen Reduction Capabilities of Freeze/Thaw Sludge Conditioning

Freeze/thaw conditioning of sludges has been shown to be an effective and economical method of sludge dewatering especially for areas where natural freezing is possible. Freeze treatment dehydrates the sludge particles by freezing the water associated to the floc and upon thawing an easily drainable granular type of material is obtained. The solid content of the freeze/thawed sludge from specially designed freezing beds ranges from 30% to as high as 60%. Pathogen reduction capacity of freeze/thaw conditioning has not been evaluated. The purpose of this study is to determine the variables affecting the pathogen reduction in freeze/thaw sludge conditioning. Previous research has shown that freezing rate, time in the frozen state, and temperature all influence the condition of the freeze/thawed sludge, and these are chosen to be the independent variables of the research. The three microorganisms- salmonella, fecal coliforms and plaque forming units for viruses-are chosen to be the dependent variables, since they best represent the other pathogenic microorganisms. Three different rates, four different temperatures and three different times of freezing are tested for their effects on the reduction of the three selected microorganisms. Results indicate that all of the parameters, time, rate and temperature of freezing have significant effects on the reduction of the indicator microorganisms.