This research investigated the feasibility of enhancing ammonia recovery from wastewater using a negatively charged poly(tetrafluoroethylene) (PTFE) membrane in a direct contact membrane distillation (DCMD) system. The influences of phosphate solution types (as the permeate solutions) and feed pH on ammonia recovery were analyzed. Three types of permeate solutions-DI water and two types of phosphate solutions (H₃PO₄ and KH₂PO₄)-were investigated for recovery of ammonia gas on the permeate side. From the obtained results, the H₃PO₄ solution was found to be the most suitable permeate solution to recover ammonia gas in the DCMD operation with the highest overall ammonia mass transfer coefficient of 7.4 × 10⁻⁵ m/s, compared to values of 1.2 × 10⁻⁵ and 2.4 × 10⁻⁵ m/s for DI water and KH₂PO₄ solution, respectively. Moreover, an increase in the H₃PO₄ concentration from 0.3 to 0.5 M in the permeate solution also could significantly enhance ammonia recovery. With an increase in the feed pH from 10.0 to 11.8, the ammonia recovery could be enhanced to 92.98% at a pH of 11.8. Liquid ammonium phosphate fertilizer could be produced by the DCMD system with the use of 0.5 M H₃PO₄ solution. Therefore, the DCMD process using a negatively charged PTFE membrane with an appropriate permeate solution is one of the challenging processes for ammonia recovery from wastewater to promote the circular economy concept.
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Kywe et al. (2022) studied this question.
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