In this work, we employ unidirectional freezing of waste-activated sludge to investigate the effects of freezing speed on the sludge performance including filterability, zone settling, and floc density versus size relationships. With a high freezing speed, no global particle migration occurs, but the sludge filterability is markedly improved. However, the floc density and the floc morphology are only slightly changed; therefore, the zone settling characteristics are those of the original sludge. With a low freezing speed, however, global particle migration occurs. Consequently, both the sludge filterability and settleability are enhanced. The floc density and the morphology are changed significantly. With an intermediate freezing speed, a gradual transition of sludge characteristics occurs between the two extremes, resulting from an average behavior of the constituting flocs. No precise definition for the critical freezing speed can thereby be defined. If improvement of sludge filterability is the only concern, a high freezing speed is acceptable. Furthermore, a low freezing speed is necessary if both sludge settleability and bound-water content must be changed.

No abstract

This paper experimentally addressed the discrepancies in sludge freezing literature regarding the effect of electrolyte (sodium chloride) on the subsequent freeze/thawed sludge dewaterability. Waste-activated sludge is vertically frozen at fixed freezing speeds with sodium chloride concentration as a parameter. Results obtained herein demonstrate that if the sludge is completely frozen, regardless of the electrolyte, the sludge filterability can be markedly improved. However, the associated sludge settleability and the final sediment height do not alter much except when flocs gross migration occurs. Adding NaCl retards the flocs gross migration, thereby reducing the corresponding critical freezing speed. We speculate that the transition-layer freezing point suppression, the double-layer compression, and the initiation of wavy ice front are possible factors for the electrolyte effects. However, whether the double-layer compression is an influential mechanism in freeze/thaw treatment still remains unclear.

No abstract

No abstract