Study on the effects of multiple factors on reverse osmosis (RO) and nanofiltration (NF) membranes' performance and rejection efficiency

Membrane filtration is a promising technology to treat secondary effluents for meeting various reuse water quality regulations. However, membrane fouling is the most serious obstacle to the wide application of membranes. Fouling of membranes is affected by many factors, and it is not clearly understood.; In this research, several important factors affecting membrane fouling and rejection efficiency were investigated simultaneously using a factorial design. Both qualitative and quantitative analyses were performed for flux decline, biotic and abiotic fouling, and rejection rates of several important water quality parameters. A series of statistical models were developed and validated to relate membrane performance, fouling and rejection efficiency to membrane properties, feed water characteristics and their interactions. In addition, membrane flux decline rate and membrane initial fouling were also studied using multivariable regression analysis.; The results showed that the initial fouling and fouling rate of dense membranes were strongly related to the strengths of initial permeate drag and electrostatic repulsion function, while relative loose and smooth membranes were fouled very quickly, and their initial fouling was mainly associated with the amount of foulants in the feed water. The established models indicated that the flux decline of membrane processes was most significantly influenced by membrane properties. Hydrophobic interactions, electrostatic interactions and permeate drag were identified as the significant mechanisms affecting membrane flux decline. Biofouling was found to be strongly controlled by membrane characteristics with lower surface roughness and higher surface charge (more negative) contributing to lower biofilm formation. In addition, high monovalent cation concentrations in the feed water were observed to help biofouling control. It was also found that the abiotic fouling of membranes resulted mainly from the humic acid component in the feed water, and that membrane surface charge and permeate drag were identified as the important factors influencing abiotic fouling formation. Finally, the developed models for rejection evaluation revealed that both mechanical sieving and electrostatic interactions were the main mechanisms involved in membrane rejection. Since under the high ionic strength environments, electrostatic repulsion was repressed, size exclusion became almost the sole rejection mechanism.