Identification of foulants, fouling mechanisms and cleaning efficiency for NF and RO treatment of produced water

This study investigated the fouling tendency and chemical cleaning efficiency of hydrophilic nanofiltration (NF) and reverse osmosis (RO) membranes during treatment of produced water from the petroleum industry for reuse as a potential water resource. Foulants in produced water were identified and characterized, fouled membranes were analyzed and the influences of foulant size, pressure and pH on fouling mechanisms studied. The produced water was found to contain 975 mg/L total dissolved solids, 26.9 mg/L total organic carbon, 42,000 cfu/mL bacteria and 38 NTU turbidity. Foulant diameters ranged from 462.8 to 653 nm and zeta potentials ranged from −21.0 ± 4.76 to −26.6 ± 8.22 mV. Both biofouling and organic fouling caused decreased flux, and extracellular polysaccharides were formed in the biofilms. Foulant size had a greater effect on RO membranes than NF membranes, while the latter were more heavily influenced by pressure variations. High pH mitigated fouling of both membranes, while chemical cleaning with alkali, metal chelation or anionic surfactants was effective. Multiple mechanisms of fouling were observed and their roles in structuring fouling layers were proposed in detail. Finally, a correlation between the results of this study and proposed measures to control fouling caused by produced water is described.