Operational parameters for mechanical freezing of alum sludge

Freezing tests were conducted with alum sludge that had been dewatered to a solids content typically produced by a gravity thickener, vacuum filter, and belt press. These sludges were then frozen at various rates in thin layers to simulate a horizontal belt freezer. The tests indicate that a low freezing rate and a high initial solids content produce larger alum sludge particles. Curing time has no effect on grain size. The maximum freezing rates for the gravity-thickened, vacuum-filtered, and belt-pressed sludges were 6.6, 15.5, and 19.8 kg h−1m−2, respectively. The electrical cost of freezing sludge with this device was estimated to be $0.004 m−3. These tests show that dewatering prior to freezing not only saves energy because it reduces the amount of sludge to be frozen, but it improves the final product in terms of a larger effective grain size.