Salinity effect on freeze/thaw conditioning of activated sludge with and without chemical addition

The effects of salinity on freeze/thaw conditioning of sludge with and without chemical addition were investigated in terms of specific resistance in filtration (SRF) and final solid content of mechanical dewatering. The biological sludge was collected from a local sewage treatment plant. Three levels of salinity, 5000, 10 000, and 20 000 ppm were tested. Typical chemical coagulants such as alum, ferrous chloride, and polyelectrolytes were employed for some of the experiments at the optimal dosage. It was found that increasing salinity resulted an increase in SRF. For the range of the salinity tested, the SRF increased from 4E14 to about 8E12 m/kg. The chemical coagulants show insignificant effect on SRF. For the final solid content of sludge, increase salinity from 5000 to 20 000 ppm can bring 3-4% increase in sludge final solid content due to salting out effect. With chemical addition, the final solid content of freeze/thawed sludge can only be increased slightly. Alum was found to be the best coagulant in increasing the final solid content. The nonionic polymer does not show any effect. It is interesting to note that at 20 000 ppm salinity, the final solid content of freeze/thawed sludge is 34% after adding chemicals.