Resource recovery from industrial wastewaters by hydrophobic membrane contactors: A review

Hydrophobic membrane contactors (HMCs) are emerging as a promising alternative to conventional mass transfer and adsorption technologies for various wastewater treatment applications. The key advantages of this technology include large mass transfer area-to-volume ratios, no phase dispersion, ease of operation, and low energy consumption. More importantly, removed species have the potential to be concentrated in the stripping phase, allowing for recovery and recycled use. In this review, major applications of HMCs for resource recoveries from industrial wastewaters are summarized. We first start with the discussion on recoveries of ammonia nitrogen, dissolved methane, and cyanide from wastewaters via gas transfer HMCs. Next, we highlight the use of HMCs as a tool for liquid-liquid extraction for the recovery of phenol and heavy metal ions. The different process principles, mass transfer phenomena, and challenges facing each application are outlined and examined. We then focus in detail the Achilles heels of HMCs, namely membrane wetting and fouling, discussing their impacts on membrane and process performances, as well as the current mitigation methods. Last but not least, we share our insights and present the outlook for the HMCs, addressing critical gaps and recommendations for future research in areas such as fouling and wetting, long-term and scaled-up operations, economic feasibility as well as other ideas to reduce mass transfer resistance.