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Abstract
The effect of freezing speed on dewaterability of waste activated sludge thickened by flotation was investigated. The average dewatering rate of the sludge after freezing/thawing treatment was remarkably increased, and was found to be larger in the order: slow-frozen (−10°C, −20°C) > fast-frozen (−80°C) > unfrozen sludge. This order was consistent with those of the sludge settling, elution of intracellular water and the numbers of the viable bacteria in the sludges after freezing/thawing. The expression characteristics and the final moisture contents of unfrozen and frozen sludge were evaluated from expession experiments at constant pressure. The wet-basis moisture content of final cake of frozen sludge was about 10% lower than those of unfrozen sludge, and even the cake obtained under additional 2 kgf/cm2 pressure may burn without auxiliary fuel. In addition, the mechanism responsible for the sludge dewatering was also examined.