Enhanced coagulation by two-stage alum addition: the role ofsolution pH, floc breakage and assistant ofnon-ionic polyacrylamide.

Coagulation processis 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), wascompared to their one-stage addition with the same total dosage.Thedoseratio of primary and secondaryadditionwasoptimized. The effects of solution pH, floc breakage and dosing timing of coagulant aid of non-ionic polyacrylamide(NPAM)on coagulation efficiency and floc propertywere investigated. The results showed that two-stage addition ofPAClshowedless sensitivitywith the variation ofpH.Compared to PACl, two-stage addition of Al2(SO4)3was 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 removalof colloidal particles. The coagulation mechanism ofprimarycoagulantwas charge neutralization, while other mechanism was involved with the secondary addition ofcoagulant.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 coagulationstrategy.