Behavior of Electro-osmotic Dewatering of Biological Sludge with Salinity

Abstract

The salinity effect on electro-osmosis dewatering was investigated by measuring the solid content of sludge after dewatering. Three levels of salinity were studied, 5,000, 10,000, and 12,500 ppm. Coagulant chemicals such as alum, ferrous sulfate, and organic polyelectrolytes were used for some tests. Increasing the salinity from 5,000 to 10,000 ppm and 10,000 to 12,500 ppm was found to increase the sludge solid content about 88 and 28%, respectively. A 2 V/cm increase in voltage intensity can also increase the solid content about 46%. The effect of chemicals is evident at low salinity. At 5,000 and 10,000 ppm, addition of chemicals in average can enhance the sludge solid content about 44 and 16%, respectively. Because of the electrolysis of salt solution and the release of chlorine gas, the pH of the sludge increased. Consequently the zeta potential of the sludge increased, which in turn results in an increase in dewatering rate. Therefore an S-shape solid content–time curve was observed for most of the operating conditions, contrary to the conventional shape of a linear increase followed with a decrease of rate before reaching zero rate.

Acknowledgments

This research work was supported in part by ECWW 98/99, EG09 from Environment and Conservation Fund, Hong Kong. The authors also thank Mr. Chen Xue Ming and Mr. S. T. Lui for their discussion and technical supports and Ms. Frances Chuen for her permission in using the centrifuge in Biochemistry Department of the Hong Kong University of Science and Technology.