Change in membrane performance due to organic fouling in nanofiltration (NF)/reverse osmosis (RO) applications

The effect of fouling by organics on rejection was observed for the removal of 36 neutral trace plastic additives, endocrine disruptors and other organic contaminants. Two membranes (a reverse osmosis membrane and a nanofiltration membrane) with very different volume fluxes and chloride rejections characteristics were used. Membranes were fouled by activated sludge and landfill leachate to simulate long-term membrane condition in field applications. Performances of unfouled (virgin) membranes and fouled membranes with foulant precipitation not resulting in a cake-layer formation were determined. The adsorbed particulates (foulants) changed the membrane surface characteristics which resulted in severe flux decline and affected the rejection pattern of the targets. Fouling led to increased adsorption of targets, produced narrower membrane pores and decreased mass transport by the imposed hindrances of the fouling partitions causing a higher diffusion of solutes across the membrane.Two obvious cases were distinguished due to the presence of foulants: firstly, a lowering of the rejection of high MW solutes attributed to the attachment of foulants on membrane polymer and subsequent diffusion of the large solute molecules through the membrane. Thus, facilitating the transport of high MW contaminants producing higher permeate concentrations of these solutes. Secondly, an increased rejection of low MW solutes attributed to the effect of the narrower pores resulting in the more dominant size exclusion. Thus, hindering the transport of low MW solutes producing lower permeate concentrations of these solutes.Overall, the dominant removal mechanism is dependent on solute type but largely involving sieving, diffusion and other membrane solute interaction by polymer. Findings indicate that fouling significantly impacts on membrane performance and varies the removal pattern of neutral trace organic contaminants.