Effect of freezing temperature on freeze/thaw sludge organic matter transformation

To research the effect of freezing temperature on organic matter transformation during freeze/thaw treatment of excess sludge, and to seek the best way of freeze/thaw process, the organic matter transformation during excess sludge disintegration, frozen at -20, -10 and -5℃ and thawed at room temperature (20℃), respectively, were investigated. The characteristics of organic matter changes of freeze/thaw sludge were also examined using infrared spectrum analysis. The results indicated that the dissolution of solid matter in sludge mainly occurred in the freezing stage, reaching stable after prolonged freezing time. The floc matrix structure was disrupted for the completely frozen sludge (-20℃, after 2.5 h; -10℃, after 3-4 h and -5℃, after 4-6 h), and organic matter sharply released after thawing for 12 h. Under the condition of -5/20℃ (freeze/thaw) treatment, the proportion of hydrophilic fraction (HPI) component was increased to 59.3% and the proteins of hydrophobic components in extracellular biological organic matter (EBOM) were disintegrated thoroughly. In addition, hydrocarbon and carbohydrate absorption peak were increased under -5℃ freezing. Compared with the -20/20℃ (freeze/thaw) treatment, the intracellular organic matter could be released more thoroughly and the sludge biodegradability was improved via -5/20℃ (freeze/thaw) treatment. To obtain the better performance of freeze/thaw treatment, sludge could be firstly frozen at -5℃. When sludge was frozen completely (frozen 4-6 h), it could be transferred into the lower freezing temperature for subsequent curing and finally thawed at room temperature.