Moisture Removal Mechanism of Sludge Conditioning by Freeze-Thaw Process

The purpose of this study is to investigate the effects of sludge floc size and solids content on freeze-thaw conditioning and to enhance our understanding of the freeze-thaw conditioning mechanism. Digested activated sludge and an unidirectional freezing apparatus were used to perform the experiment. The results indicated that if sludges of small floc size was frozen at low speed, i.e, 10 mm/h, floc migration and aggregation occurred with the formation of ice front causing dehydration. This resulted in increasing floc compactness and narrowing the particle size distribution (PSD). For sludges composed of large floc, the floc were easily destroyed by the migrating ice front resulting in a smaller mean floc diameter and PSD than sludges composed of small floc. At low freezing speed, development of ice front for sludge with high solid concentration is slower than that of sludge with low solid concentration. When high freezing speed was employed to freeze sludge, floc destruction by ice front was more obvious, and the compactness of particle layer was lower than that caused by low freezing speed. Furthermore, under high freezing speed sludge with high solid concentration was easily entrapped by the ice crystal, and floc diameter was increased. However, gross migration of sludge was not found under this condition. Based on the experimental results, a modified model for the sludge freeze conditioning is proposed with the consideration of the effect of floc size and solids concentration. In addition, pretreatment to increase the solid concentration and decrease floc size before freeze-thaw conditioning are recommended to enhance the efficiency of this technique.