UNIDIRECTIONAL FREEZING OF WASTE-ACTIVATED SLUDGE - THE PRESENCE OF SODIUM-CHLORIDE

This paper experimentally addressed the discrepancies in sludge freezi ng literature regarding the effect of electrolyte (sodium chloride) on the subsequent freeze/thawed sludge dewaterability. Waste-activated s ludge is vertically frozen at fixed freezing speeds with sodium chlori de concentration as a parameter. Results obtained herein demonstrate t hat if the sludge is completely frozen, regardless of the electrolyte, the sludge filterability can be markedly improved. However, the assoc iated 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 fr eezing speed. We speculate that the transition-layer freezing point su ppression, the double-layer compression, and the initiation of wavy ic e front are possible factors for the electrolyte effects. However, whe ther the double-layer compression is an influential mechanism in freez e/thaw treatment still remains unclear.