Influences of K+, Ca2+, and Al3+ on electrokinetic process and conductive mechanism of sludge electro-dewatering

In the process of sludge electro-dewatering (EDW), ions migration accompanied has significant influences on the dewatering efficiency. However, the effects of ions on sludge electrochemical properties and the roles in EDW are still not well understood. In this paper, influences of K+, Ca2+, and Al3+ on the mechanical dewatering and EDW process were investigated, and the mechanisms of EDW were analyzed from electrokinetic phenomena and electric conduction model. The results showed that the increase of cationic valence improved sludge mechanical dewatering but deteriorated EDW performance. The spatially distributed voltage used for ionic electromigration indicated that the lagged migration of divalent cation Ca2+ consumed more energy and reduced the dewatering rate in the later stage of EDW compared to that of monovalent cation K+. And trivalent cation Al3+ migrated less and took less water out, while it made the electrochemical reaction of EDW system easier to carry out. The decrease of the absolute zeta potential value of K+, Ca2+, and Al3+ also revealed the degeneration of dewaterability limit. An electric conduction model of partially water-saturated state porous media related to geometric factors was used and clarified the weight variation of continuous phase conductivity and surface conductivity during EDW. The results showed that the conductivity of Al-sludge was more affected by geometric factors and resulted in worse dewatering performance. This study provided a theoretical guidance for understanding the ions behavior affecting the dewatering efficiency and energy consumption of EDW. •The valent increase of cations improved mechanical dewatering but deteriorated EDW.•Electrode reaction kinetics of EDW system was analyzed.•Electromigration was analyzed from activation energy and ionic migration potential.•The action of different valent cations on conductive model of EDW was analyzed.