Freezing of Water and Wastewater Sludges

Abstract

When sludge is allowed to freeze slowly and then thaw, its dewatering properties are usually greatly improved. The mechanisms by which this improvement occurs are not well understood. This paper proposes a conceptual model for how sludge freezes. The validity of this model is discussed using past research findings as well as experimental evidence. The effectiveness of freeze/thaw is measured using the capillary suction time apparatus. The results indicate that colder temperatures, longer freezing times and slower freezing rates all have a beneficial effect. These results suggest that if freeze/thaw is to be used commercially, thin layers of sludge must be frozen slowly for long periods of time, thereby promoting the growth of crystals that will exclude the solid particles and promote particle aggregation.