Enhanced coagulation by two-stage alum addition: the role of solution pH, floc breakage and assistant of non-ionic polyacrylamide

Coagulation process is highly influenced by solution pH, floc breakage, and coagulant aid. In this work, two-stage addition of alum-based coagulants, polyaluminum chloride (PACl) and aluminum sulfate (Al2(SO4)3), was compared to their one-stage addition with the same total dosage. The dose ratio of primary and secondary addition was optimised. The effects of solution pH, floc breakage and dosing timing of coagulant aid of non-ionic polyacrylamide (NPAM) on coagulation efficiency and floc property were investigated. The results showed that two-stage addition of PACl showed less sensitivity with the variation of pH. Compared to PACl, two-stage addition of Al2(SO4)3 was more sensitive to the solution pH, especially when the pH ranged from 7 to 10. For all cases, the presence of floc breakage improved the removal of colloidal particles. The coagulation mechanism of primary coagulant was charge neutralisation, while other mechanism was involved with the secondary addition of coagulant. Compared to one-stage addition, the settling performance of aggregates and particulates removal in two-stage addition was more effective with the variation of dosage or dosing timing of NPAM. The present work provides detailed operating parameters for this coagulation strategy.