Characterization of coagulation behavior of titanium tetrachloride coagulant for high and low molecule weight natural organic matter removal: The effect of second dosing

Normalized UF permeate flux profiles with different TiCl4 dosing modes for pre-coagulated (a) HA suspensions; (b) FA suspensions under different shear rates and breakage periods conditions. •A small second TiCl4 dose could improve both turbidity and organic matter removal.•Second TiCl4 dose could increase floc recoverability and compactness.•For same total TiCl4 dose, two-stage dosing increases floc size and recoverability.•Two-stage dosing could reduce membrane fouling under certain shear conditions.•Flux reduction displayed small variations with different shear rates in case of FA. A two-stage titanium tetrachloride (TiCl4) dosing strategy is applied for investigating the coagulation effect on simulated water containing humic acid (HA) and fulvic acid (FA) and these represent the dissolved organic matter (DOM). The influence of second TiCl4 dose on coagulation efficiency, flock re-growth and the fractal dimension properties is studied for the beneficial effect of the second coagulant dose. Moreover, the impact of increased shear on the ultra filtration permeate flux in coagulation–ultra filtration (C–UF) hybrid process is explored. A small second dose essentially decreases the residual turbidity and improves the organic matter removal. Also, flocks with larger size and better recoverability result and having more compact structure than those obtained without the second TiCl4 coagulant dose. At the same total TiCl4 coagulant concentrations, the two-stage dosing mode reveals significant advantage over the single coagulant dosing in terms of turbidity removal, flock size and recoverability although organic matter removal and flock compactness degree are more or less deteriorated during the two-stage dosing process. The second TiCl4 dose contributes to larger re-generated flocks with better re-growth ability than those obtained without the second dose within a wide range of high shear rate and even with prolonged breaking period. Two-stage dosing mode could improve the ultra filtration permeate flux under certain shear conditions. An extension in breakage time leads to much severer flux decline than short breakage periods. In case of FA simulated water, the ultra filtration permeate flux was less sensitive to increasing shear rate than in case of HA simulated water.