Comprehensive insights into the inorganic coagulants on sludge dewatering: comparing aluminium and iron salts

BACKGROUND: Extracellular polymeric substances (EPS) play an important role in sludge dewatering. Inorganic coagulants treatment has been widely used to remove the EPS and improve sludge dewaterability. This research evaluates whether there are advantages of selecting one of two chemical coagulation pretreatments, conventional aluminium or iron salts, over the other to enhance sludge dewaterability. The study traced variations in EPS characteristics to understand the mechanisms of sludge dewaterability. RESULTS: The results showed that iron coagulants improved dewatering more than aluminium coagulants, by approximately 28.66%. The improved sludge dewaterability was further confirmed in pilot‐scale experiments. Compared with aluminium salts conditioning, iron salts were better able to neutralize stronger charge, formed smaller and more compact flocs, and had a stronger acidification effect. Sludge dewaterability was significantly related to EPS concentrations (i.e. the protein and polysaccharides), EPS composition (i.e. tyrosine/tryptophan amino acid, tyrosine/tryptophan protein, and polysaccharide), and functional groups (i.e. random coil, β‐sheet, and Nₚᵣ) in the loosely bound EPS (LB‐EPS). A two‐step mechanism was proposed for the iron conditioning process. Moreover, the economic analysis shows iron treatment was more economical than that of aluminium. CONCLUSION: Iron salts conditioning removed more LB‐EPS than aluminium salts, thus more significantly improving sludge dewaterability. © 2018 Society of Chemical Industry