Strength and regrowth properties of polyferric-polymer dual-coagulant flocs in surface water treatment.

The floc strength and regrowth properties of three polyferric-cationic polymer dual-coagulants were comparatively evaluated using surface water sample. The first dual-coagulant PFC-PD was prepared by premixing of polyferric chloride (PFC) and polydiallyldimethylammonium (PDADMAC) before dosing. The other two, PFCF (PFC dosed firstly) and PDF (PDADMAC dosed firstly), were achieved by dosing PFC and PDADMAC in different order. Floc strength properties were measured in response to increasing shear levels in the long period and high shear level in the short or long period. For the given optimum dose (3.0 mg L(-1)) and water pH (6.5) condition, the order of floc strength was PFCF>PDF>PFC-PD. The dual-coagulant which gave stronger flocs also gave a lower absolute value of zeta potential. The floc regrowth properties of all three dual-coagulants after short and long period high shear level were also investigated. The floc recoverability was in the following order: PFCF>PFC-PD>PDF.