Evaluating the performance of gravity-driven membrane filtration as desalination pretreatment of shale gas flowback and produced water

The shale gas extraction industry generates a large quantity of highly contaminated flowback and produced water (FPW), with great impacts on human health and the environment. In this study, gravity-driven membrane (GDM) filtration was evaluated over a 612-day period as a pre-treatment of FPW for its subsequent desalination. The various investigated GDM systems showed similar contaminant removals, and their steady-state fluxes (i.e., 0.65–0.82 L/(m²·h)) were not significantly correlated to membrane configurations or to the hydrostatic pressures. The flux decline was primarily due to a reversible resistance, which accounted for a large proportion (>89%) of the total hydraulic resistance. Compared to traditional ultrafiltration, the GDM pretreatment resulted in better desalination performance for the subsequent nanofiltration or reverse osmosis step, which were characterized by higher organic removal and generally higher permeate fluxes. More than 60 bacterial genera and 8 eukaryotic genera were detected in the shale gas FPW, with the kingdoms Alveolata and Stramenopiles (within the eukaryote domains) reported for the first time. The biofouling layer of GDMs had a lower bacterial diversity but a higher eukaryotic diversity than the FPW feed water. The eukaryotic community, including Alveolata, Fungi, Stramenopiles and Metazoa, played a major role in the flux behavior.