Mitigating membrane wetting in the treatment of unconventional oil and gas wastewater by membrane distillation: A comparison of pretreatment with omniphobic membrane

The treatment of unconventional oil and gas (UOG) wastewater by membrane distillation (MD) is a promising alternative to disposal via deep-well injection for UOG wastewater management. However, MD has been constrained by membrane pore wetting, which could be induced by surfactants that are commonly used in hydraulic fracturing fluids. Herein, we investigated and compared two strategies, namely pretreatment via coagulation-adsorption and the use of omniphobic membranes, for wetting mitigation in MD treatment of UOG wastewater from the Denver-Julesburg Basin, Colorado. Both strategies were able to mitigate membrane wetting, but pretreatment exhibited better treatment efficiency than the use of omniphobic membranes that was subject to significant membrane fouling. Membrane characterization using scanning electron microscopy, energy dispersive X-ray spectroscopy, and attenuated total reflectance-Fourier transform infrared spectroscopy demonstrated the effective reduction of membrane fouling and scaling after pretreatment. Further, ultrahigh pressure liquid chromatography (UHPLC) coupled with quadrupole time-of-flight mass spectrometry (LC/QToF/MS) was performed to investigate the efficacy of pretreatment in removing surfactants. Pretreatment was found to be highly efficient at removing polyethylene glycol (PEG) and polypropylene glycol (PPG) surfactant classes, which were likely responsible for membrane wetting. Our findings suggest that pretreatment could have comparable or even better effectiveness than omniphobic membranes for improving MD treatment of UOG wastewater with high wetting potential. Comparative investigations on these two wetting mitigation strategies are valuable for rational selection of appropriate strategies to promote practical applications of MD to industrial wastewater treatment.