THE EFFECT OF FREEZING AND THAWING PROCESS ON THE EXPRESSION CHARACTERISTICS AND FINAL MOISTURE-CONTENT OF EXCESS ACTIVATED-SLUDGE

The expression characteristics and the final moisture content of unfrozen and frozen excess activated sludges produced at a municipal wastewater treatment plant were evaluated from constant-pressure expession experiments at 206∼2950 kPa. The modified consolidation coefficient Ce and creep constant η were approximately constant over these experimental pressures, but both values were increased 20∼25 % by the freezing and thawing process. Consequently, it is concluded that the freezing and thawing process increases the rate of both primary and secondary consolidation. The values of η of unfrozen and frozen activated sludges were nearly equal to those for typical inorganic sludge, but the values of Ce were 1/1000 of those of typical inorganic sludge. Regardless of expression pressure or the freezing and thawing process, the values of B, which is the ratio of secondary consolidation to the total consolidation, were about 0.9, which was much larger than those of inorganic sludge. Therefore, the rate of secondary consolidation is a controlling factor in the consolidation of excess activated sludge. The wet-basis moisture content of final cake of frozen excess activated sludge were 0.1 lower than those of unfrozen sludge. Thus, it is suggested that even the cake obtained under low pressure (200kPa) may burn without auxiliary fuel. © 1990, The Society of Chemical Engineers, Japan. All rights reserved.