Valorization of ammonia concentrates from treated urban wastewater using liquid–liquid membrane contactors

Abstract

The removal of ammonium from tertiary effluents by zeolites generates basic ammonia concentrates (up to 1–3 gNH3/L in 1–2 g NaOH/L). This study evaluates the use of hollow fibre liquid–liquid membrane contactors (HFMCs) as a concentration and purification step for ammonia effluents to produce NH4NO3 and (NH4)2(HPO4) solutions for potential use as liquid fertilizers. The influence of various operational parameters (i.e., flow rate, initial ammonia concentration and stripping acid concentration) was investigated using a closed-loop setup. Due to the high basicity of the ammonia feed streams (pH > 12), the mass transport process was primarily controlled by the free acid concentration in the stripping phase (e.g., HNO3 and H3PO4). A mass transport algorithm to predict the pH of the stripping stream was developed to describe the contactor performance, predict the requirements of the free acid concentration in the stripping phase and optimize the ammonia recovery. Therefore, the closed-loop configuration allowed for ammonia recovery ratios higher than 98% when the required free acid concentration of the stripping phase was maintained. The exhausted NH3/NaOH streams after NH3 removal can be re-used for regeneration of the ammonium-exhausted zeolite filters.