Membrane fouling and reusability in membrane distillation of shale oil and gas produced water: Effects of membrane surface wettability

Extensive efforts have been made to tailor membrane surface wettability in order to mitigate fouling and wetting in membrane distillation (MD), but the developed membranes have rarely been challenged with real industrial wastewater. This study compared three membranes - a hydrophobic PVDF membrane, a superhydrophobic PVDF membrane, and a composite PVDF membrane with hydrophilic coating - in MD desalination of shale oil and gas produced water from the Wattenberg field in northeast Colorado. Two produced water samples with varied chemical compositions were collected and used as the feedwater. In a single treatment cycle, the composite membrane showed the best fouling resistance for the first sample, while all the tested membranes experienced similar flux decline with the second sample. Thus, the relationship between membrane surface wettability and fouling propensity in MD treatment of real produced water was influenced by feedwater composition. This effect was reflected by distinct features of fouling layers resulting from the two produced water samples, revealed by detailed microscopic and spectroscopic characterization. In three treatment cycles with physical membrane cleaning, the hydrophobic and composite membranes suffered from accelerated membrane fouling after each cycle, whereas a decelerated flux decline was observed for the superhydrophobic membrane. The better reusability of the superhydrophobic membrane, however, was achieved at the expense of initial water vapor flux. Our study suggests that one should comprehensively consider fouling/wetting resistance, water productivity, and reusability in the design and selection of appropriate membranes for MD treatment, and that long-term testing with multiple treatment cycles should be performed to assess MD membrane performance more accurately.