Sewage sludge electro-dewatering

Original sludge from wastewater treatment plants (WWTPs) usually has a poor dewaterability. Conventionally, mechanical dewatering methods are used to increase the dry solids' (DS) content of the sludge. However, sludge dewatering is an important economic factor in the operation of WWTPs; high water content in the final sludge cake is commonly related to an increase in transport and disposal costs. Electro-dewatering could be a potential technique to reduce the water content of the final sludge cake, but the parameters affecting the performance of electro-dewatering and the quality of the resulting sludge cake, as well as removed water, are not sufficiently well known. In this research, nonpressure and pressure-driven experiments were set up to investigate the effect of various parameters and experimental strategies on electro-dewatering. Migration behavior of organic compounds and metals was also studied.Application of electrical field significantly improved the dewatering performance in comparison to experiments without electric field. Electro-dewatering increased the DS content of the sludge from 15% to 40% in nonpressure applications and from 8% to 41% in pressure-driven applications. DS contents were significantly higher than typically obtained with mechanical dewatering techniques in WWTP. The better performance of the pressure-driven dewatering was associated with a higher current density at the beginning and higher electric field strength later on in the experiments. The applied voltage was one of the major parameters affecting dewatering time, water removal rate, and DS content of the sludge cake. By decreasing the sludge loading rate, higher electrical field strength was established between the electrodes, which has a positive effect on an increase in DS content of the final sludge cake. However, interrupted voltage application had a negative impact on dewatering in this study, probably because the off-times were too long.Other factors affecting dewatering performance were associated with the original sludge characteristics and sludge conditioning. Anaerobic digestion of the sludge with high pH buffering capacity, polymer addition, and freeze/thaw conditioning had a positive impact on dewatering. The impact of pH on electro-dewatering was related to the surface charge of the particles measured as zeta-potential.One of the differences between electro-dewatering and mechanical dewatering technologies is that electro-dewatering actively removes ionic compounds from the sludge. In this study, dissolution and migration of organic compounds (such as short-chain fatty acids), macrometals (Na, K, Ca, Mg, Fe), and trace metals (Ni, Mn, Zn, Cr) was investigated. The migration of the metals depended on the fractionation and electrical field strength. These compounds may have both negative and positive impacts on the reuse and recycling of the sludge and removed water.Based on the experimental results of this study, electro-dewatering process can be optimized in terms of dewatering time, desired DS content, power consumption, and chemical usage.