Breakage and regrowth of flocs formed by sweep coagulation using additional coagulant of poly aluminium chloride and non-ionic polyacrylamide

Abstract

The breakage and regrowth of flocs formed by sweep flocculation were investigated on different flocculation mechanisms using additional dosage coagulant of poly aluminium chloride (PACl) and non-ionic polyacrylamide (PAM) to explore the reversibility after floc breakage. The optimal dosage of PACl was 0.15 mM (as alum), and zeta potential exceeding 1 mV meant that sweep flocculation was dominant in the pre-flocculated process. Re-coagulation efficiency increased with additional coagulants dosing, and sedimentation rates of flocs re-formed by small additional dosage of non-ionic PAM are faster than that of flocs re-formed by additional PACl. For additional inorganic coagulant (PACl) during regrowth processes, few negatively charged particles that existed in water sample restricted the effect of charge neutralization. An amorphous aluminum hydroxide precipitation could re-activate the weaker points on the broken floc surface, but regrown flocs have loose structure indicating worse settleability. For additional non-ionic PAM dosing, lower dosage showed large values of fractal dimension and average size, probably due to that unfolded curly molecular chain and exposed amide groups of non-ionic PAM which provide superb conditions for amide group interacting with particles. The use of non-ionic PAM in flocculation has advantage of being more effective than the cationic PACl, probably because it may avoid the re-stabilization of broken flocs by polymer adsorption driven by electrostatic attraction. Hence, appropriate dosing of PAM after breakage could improve the flocs characteristics with large size and compact structure.

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