Fouling of nanofiltration and reverse osmosis membranes by organic macromolecules and their mixtures

The performance of nanofiltration (NF) and reverse osmosis (RO) membranes can be significantly restricted due to fouling by organic macromolecules during water and wastewater treatment. Fouling causes a reduction of the membrane permeation flux, demands more frequent cleaning, and shortens the membrane life span. Proteins are among the main macromolecules that foul NF and RO membranes. Protein fouling (single, binary mixture and protein-polysaccharide mixture) of NF and RO membranes was systematically investigated in the current work. In the study of single protein fouling, the effect of hydrodynamic conditions, feed solution chemistry and membrane properties on fouling by bovine serum albumin (BSA) was evaluated under crossflow conditions over a 4-day fouling period. The initial flux behavior was highly dependent on membrane properties, as membranes with a smoother, more hydrophilic and more charged (favorable electrostatic repulsion with BSA) surface experienced a slower fouling rate. Interestingly, the flux at the end of the 4-day tests (J96hr) showed little dependence on membrane properties, with RO, NF and UF membrane fluxes all converging into a nearly identical value. This suggests that the long-term flux was primarily controlled by the foulant-fouled-membrane surface interaction. Membranes tested at different initial fluxes had a strong tendency to approach a surface-interaction-limited value (limiting flux), although a slightly lower J96hr was observed at higher applied pressures, likely due to foulant layer compaction. BSA fouling was more severe at pH values close to its isoelectric point (IEP), at high ionic strength and in the presence of Ca2+ and Mg2+ as a result of reduced electrostatic repulsion or the promotion of specific ion interactions under these conditions. A linear correlation was observed between J96hr and the square of the zeta potential of BSA, suggesting that this parameter can be potentially a good indicator for predicting the long term fouling behavior.Doctor of Philosophy (CEE