Coupling mechanism and parameter optimization of sewage sludge dewatering jointly assisted by electric field and mechanical pressure

Pressurized electro-dewatering technology is considered to be one of the most effective methods for improving dewatering performance of sewage sludge. In this paper, four dewatering protocols were developed to further explore the coupling mechanism of sludge dewatering through mechanical pressure, electric field and their joint. The results showed that the dewatering performance of the four dewatering protocols were as follows: pressurized electro-dewatering with constant voltage gradient mode (G-PEDW) > first mechanical dewatering then electro-dewatering > mechanical dewatering > electro-dewatering. The coupling mechanism was revealed from the perspectives of pore structure and moisture distribution of sludge cakes. The pore structure was discussed in detail from the whole pore size distribution and fractal dimension. The fractal dimension could quantitatively describe the change of pore structure. The moisture distribution was analyzed by the thermogravimetric differential scanning calorimetry test. The results showed that the electric field could reduce bound strength of moisture and produce new micro-pores to build more water flow paths, the water was discharged through interior capillary channels, and mechanical pressure could accelerate the water removing process. Response surface method was used to establish an empirical prediction model of G-PEDW, and discussed the selection of optimal parameters. The R-square values of mathematical models of moisture content and energy consumption were as high as 0.9863 and 0.9838, respectively, which was more advanced than other mathematical models. Experiments showed that G-PEDW could reduce the time to 20 min or reduce the energy consumption to 7.1 Wh/kg •H2O when the target moisture content was set to 41.7%. • Microscopic model of sludge is developed based on detected pore and moisture. • Moisture distribution is fully studied to discuss the role of electric field. • Coupling mechanisms of pressurized electro-dewatering are revealed by fractal. • Relatively optimal parameters for a new dewatering protocol