Theoretical and experimental study of organic fouling of loose nanofiltration membrane

Loose nanofiltration (NF) membranes are an attractive avenue in effective separation of organic matters and salts for resource recovery from highly-loaded wastewater. However, membrane fouling remains an unclear and complex factor in practical applications. In this work, the flux of a loose NF membrane fouled by humic acid at various solution compositions was systematically investigated. The strong hydrophilicity of the loose NF membrane allows for slight deposition of humic acid on the membrane surface, yielding an outstanding antifouling performance. However, a moderate flux decline was observed at low pH and high ionic strength, due to reduction in charge density of membrane surface for formation of a porous foulant layer. At higher ionic strength, cake-enhanced concentration polarization was the fouling mechanism that dominates the membrane flux. The presence of calcium ions induced bridging between humic acid molecules to generate a compact foulant layer, tremendously deteriorating the membrane flux. Based on COMSOL simulation for the membrane module, the hydrodynamics near the membrane surface had a more significant effect on membrane fouling than the solution chemistry, which is consistent with scanning electronic microscopy observation. This indicates benign hydrodynamic condition can be an effective strategy to fouling control for loose NF membranes.