Comparative study of iron and aluminium coagulants in conditioning sludge: Sludge dewatering performance, physicochemical properties, and risk of heavy metal migration

Inorganic coagulants are essential agents that can facilitate the sludge deep dewatering, but comprehensive comparative research of commonly used coagulants is lacking. The systematic comparison of six coagulants, including Ferric Chloride (FeCl 3 ), Polymeric Ferrous Sulfate (PFS), Ferrous Sulfate (Fe 2 (SO 4 ) 3 ), Aluminium Chloride (AlCl 3 ), Aluminium Sulfate (Al 2 (SO 4 ) 3 ), and Polymeric Aluminium Chloride (PAC) regarding their performance in conditioning sludge dewatering , physicochemical properties, the risk of heavy metals migration in sludge cake and the mechanism of sludge dewatering, was investigated. Within a dosage range of 20% dry solids (DS), the PFS and FeCl 3 showed a better dewatering effect among the six coagulants, and the water content (Wc) of the sludge cake decreased from 81.2%±1.1% to 57.7%±1.0% and 59.2%±1.6%, respectively. There was a correlation between the pH, zeta potential , viscosity, and bound water content of the sludge and the Wc of the sludge cake during dewatering conditioned by six coagulants. However, there was no correlation between sludge particle size and Wc of the sludge cake. The PFS and FeCl 3 reduced the protein content of tightly bound extracellular polymeric substances (TB-EPS) by 66.1% and 62.6%, respectively. Moreover, the iron coagulants significantly reduced the content of As, Cu, Ni, and Zn in the sludge cake, where the risk levels of As and Ni were largely weakened. The economic analysis shows that iron coagulants were more valuable than aluminium coagulants for engineering applications. Meanwhile, the dewatering mechanisms of the two types of coagulants were revealed. This study provides significant insights into the application of coagulants in engineering practice.