Review of ammonia recovery and removal from wastewater using hydrophobic membrane distillation and membrane contactor

Membrane distillation (MD) and membrane contactor (MC) have received a lot of attention in ammonia removal and recovery due to the use of hydrophobic membranes that allow the volatile ammonia gas to pass through. This comprehensive review summarizes practical applications and mass transfer models of ammonia recovery from wastewater using MD/MC technologies. First, a bibliometric analysis of published literature was performed from 2001 to 2021, and the ammonia recovery process and various types of membrane devices are becoming research hotspots recently. Second, the most widely investigated influencing factors including feed and permeate characteristics, membrane characteristics and operating conditions on ammonia removal are critically reviewed. The ammonia removal was greatly dependent on operating conditions rather than the feed and permeate solution characteristics, while membrane and devices characteristics are of tremendous research interest. Then, theoretical and empirical models of ammonia gas transfer in the feed, permeate and membrane are analyzed to predict the fate of ammonia gas in MC/MD. Finally, the challenges including membrane wetting and fouling, lack of long-term experience, development of accurate ammonia transfer models, and nutrient co-recovery process, are discussed in detail, and corresponding research directions are suggested.