Tests were run on samples of four chemical sludges including two water treatment sludges resulting from coagulation by alum and ferric sulfate, respectively, and two advanced wastewater treatment sludges, resulting from coagulation with ferric sulfate and lime, respectively. The purpose of the testing was to determine the effect of freeze conditioning on the drainability of the chemical sludges and to evaluate operational parameters affecting optimum drainability by freeze conditioning. Specific resistance testing and gravity sand filtration and refiltration studies were used to determine the drainability of unconditioned and freeze conditioned samples. Unconditioned sludge was characterized by testing for total and total suspended solids, total volatile and total volatile suspended solids, and pH.

Results indicated that freeze conditioning dramatically improved drainability of all chemical sludges investigated. Conditioned samples exhibited a granular consistency which settled rapidly. Specific resistance testing indicated that values obtained varied inversely with solids content for freeze conditioned samples. A direct relationship existed between filtration and refiltration rates and solids content for freeze conditioned samples, It was also determined that with increased storage time in the frozen state, drainability of the chemical sludges increased, as measured by specific resistance testing and gravity sand drainage studies. Filtrate quality of freeze conditioned samples was comparable to the quality of wastewater secondary effluent and some surface waters.