Comparative analysis of membrane fouling mechanisms induced by colloidal polymer: Effects of sodium and calcium ions

Polymer (anionic polyacrylamide, APAM) flooding produced wastewater has a relatively high degree of mineralization and abundant ionic species. A comprehensive and systematic investigation of the influence of ion identity on APAM-induced membrane fouling is extremely necessary but has not been conducted to date. A comparative investigation was performed herein to reveal the underlying mechanisms of the influence of Na+ and Ca2+ (1000 mg/L) on APAM-induced membrane fouling in the adsorption and microfiltration (MF) processes. Na+ and Ca2+ exhibited contrasting influences on the filtration efficiency, cleaning efficiency, and fouling resistance. Compared to Na+, Ca2+ promoted reversible fouling and the formation of a loose cake layer; moreover, a higher removal rate and flux recovery were achieved. Additionally, simulations based on adsorption kinetic and membrane fouling models, and a series of microscopic analyses were performed to validate the contradictory influences. During the APAM-based MF process, the membrane fouling was effectively mitigated at the applied ionic strength because of the stronger hydration repulsive force generated by hydrated Ca2+ compared to that by Na+. This study provides vital guidance for membrane fouling control in the microfiltration of polymer flooding produced wastewater.