Combining MBR and NF/RO membrane filtration for the removal of trace organics in indirect potable water reuse applications

The aim of this study was to demonstrate the complementarities of combining membrane bioreactor (MBR) treatment with nanofiltration (NF) or reverse osmosis (RO) membrane filtration for the removal of trace organic contaminants for potential indirect potable water recycling applications. Four commercially available NF/RO membranes, namely NF270, NF90, BW30 and ESPA2, were selected for this investigation. Challenge tests were conducted with 40 trace organic compounds at concentrations of approximately 2 μg/L in initial wastewater solutions using a laboratory scale MBR system and a cross-flow NF/RO rig. The results suggest that the MBR system effectively removes hydrophobic and biodegradable trace organic compounds. The adsorption of these hydrophobic compounds onto the MBR mixed liquor suspended solids (MLSS) leads to an extended retention time in the biological reactor, thereby enhancing their removal. The remaining, mostly hydrophilic, trace organic compounds were shown to be effectively removed by NF/RO membranes. The combination of MBR and a low pressure RO membrane resulted in more than 95% removal or removal to below the analytical detection limit of all 40 trace organic compounds investigated in this study. The results also suggest that NF/RO membrane fouling could be mitigated by appropriate membrane selection. Despite a relatively high initial permeate flux, negligible flow rate decline was observed with an NF membrane and a low pressure RO membrane over more than 25 h of filtration, possibly due to the smooth surfaces of these two particular membranes.