Ammonia harvesting via membrane gas extraction at moderately alkaline pH: A step toward net-profitable nitrogen recovery from domestic wastewater
Wooram Lee, Seonyoung An, Yongju Choi
Abstract
Membrane gas extraction technique enables recovery of high-purity ammonia from waste streams through gas-phase mass transfer across hydrophobic membranes. This study demonstrates the potential of ammonia recovery with gas permeable membrane, operated at moderately alkaline feed pH, as a substitute for the current approach of biological nitrogen removal in domestic wastewater treatment process. The measured apparent ammonia mass transfer coefficient in 3-h operation at a feed pH of 9.2 was 0.0713 cm/min, which, despite being 1.53-fold smaller than that at a feed pH of 11, was excellent for application to domestic wastewater. Application of lower feed pH prevented inorganic fouling of membrane. A notable advantage of the moderately alkaline feed pH condition is that the consumption of alkaline agents added for deprotonating bicarbonate instead of ammonium can be minimized. Calculation shows an opportunity that the monetary value of recovered ammonia exceeds the cost of the base consumed when moderately alkaline feed pH is applied. This study substantially extends current knowledge of the membrane gas extraction technique for ammonia recovery by investigating the process kinetics, fouling propensity, and chemical requirements as functions of pH, thereby verifying the potential of this technique to contribute to improving the sustainability of the global nitrogen cycle.