Hollow fibre membrane contactors for ammonia recovery: current status and future developments
Hydrophobic membrane contactors represent a credible solution to the problem of recycling ammoniacal nitrogen from waste, water or wastewater resources. This study critically evaluated existing literature in terms of process principles, membrane types and functionality, membrane contactor application, technology status, and future research required. The key operational parameter was the presence of ammonia gas and thus pH should be above 9. Hollow fibre membranes are usually employed, composed of primarily polypropylene, polyvinylidene fluoride, or polytetrafluoroethylene. The stripping solution is normally sulphuric acid which reacts with ammonia to create ammonium sulphate. The acid is best circulated inside the lumen with any suitable velocity, and kept in excess concentration. In terms of operational parameters: feed fluid velocity is important in open loop configurations due to the effect upon ammonia residence time at the membrane surface; and, ammonia concentration did not notably impact the mass transfer coefficient which was ca. 1 x 10-5 m/s until in excess of 2000 mg/L wherein the transport process diminished. The greatest quantity of ammonia was recovered in the initial membrane stages where the driving force is greatest. Bench and pilot plant studies concerned wastewater treatment plants, anaerobic digesters, manure management, industrial manufacturing, and animal rearing operations. It is recommended to focus upon challenges such as development of new membrane types customised for ammonia removal, a greater understanding of the process engineering and economics involved, consideration of the impact of osmotic distillation, integration of membrane contactors with other water treatment technologies and development of cleaning in place procedures.