Microfiltration of humic-rich water coagulated with cationic polymer: The effects of particle characteristics on the membrane performance

• Cat-polymer was very effective for removal of NOM and for improving MF performance.
• Under-dosing to −20 mV was successful but over-dosing to +20 mV was catastrophic.
• Characteristics of flocs produced were strongly associated with the MF performance.
• Flocs produced by charge neutralization coagulation were most favorable for MF run.
• Charge neutralization treatment was also good for TOC removal by the subsequent MF.

The main issue explored was the effects of different coagulation conditions on the particle characteristics that would also significantly affect the performance of membranes when filtering coagulated humic-rich water. The size distribution and morphological properties of flocs formed through the coagulation of natural organic matter (NOM) were characterized and the impact of Ca2+ on these characteristics and on the performance of the MF membrane was determined. The multi-cycle MF experiments with hydraulic wash between cycles were conducted for raw and coagulated humic-rich water, and the performance was evaluated by measuring the permeate water quality, resistance to filtration, and permeability recovery with cleaning. Coagulant (polydiallyldimethyl-ammonium chloride) additions from 50% to 100% of the charge neutralization dose substantially decreased fouling compared to when filtering raw humic-rich water. Short-term fouling was increased when 1 mM Ca2+ was added, but the charge neutralization coagulation removed almost all of the fouling tendency that had occurred when filtering Ca-NOM and resulted in the highest permeability recovery. The median diameter and the two-dimensional fractal dimension of flocs produced were increased as the zeta potential reached close to zero, which resulted in the formation of a cake layer that was easily removed from the surface of the membrane.