Evaluating the impact of pretreatment processes on fouling of reverse osmosis membrane by secondary wastewater

Pretreatment of secondary effluents, containing relatively high concentrations of colloidal particles, suspended solids, dissolved organics and high level of biological activity, is required to reduce fouling of reverse osmosis (RO) membranes treating these effluents. Various indices are used to evaluate pretreatment efficacy. The effect of two different pretreatments for secondary effluent (BCMF - biofiltration, coagulation and microfiber filtration) vs. ultrafiltration (UF) were evaluated for their effect in reducing the fouling potential of the effluent as expressed by the indices of BOD (biofouling), TOC (organic and biofouling), and MFI0.45 (colloidal fouling). Actual RO fouling tests were conducted using identical RO flow cells at constant flux conditions. Fouled membrane autopsies were subsequently carried out at the end of each test run. The actual fouling based on real-time performance and post-operation autopsies were then correlated to the fouling indices. The results showed that BCMF pretreatment significantly reduced the fouling potential as reflected in all three indices, but UF pretreatment process reduced BOD and MFI0.45 to a significantly greater extent. RO membranes were in general fouled more severely by BCMF treated effluents as reflected in increased transmembrane pressure (TMP). This correlated with BOD and MFI0.45 analyses of the treated effluents and with membrane autopsies at the end of the run. For both treatments, the calculated biofilm porosity dropped as the experiments were carried out for longer periods. As expected, the hydraulic resistance of the fouling layer was found to contribute significantly more to deterioration of RO performance than biofilm-enhanced osmotic pressure for both BCMF-RO and UF-RO.