Comparative evaluation of membrane stripping and air stripping for ammonia recovery from reject water

Summary

This study presents a direct experimental comparison of liquid-to-liquid membrane stripping (LLMS) and conventional air stripping for nitrogen (N) recovery from real reject water under controlled and comparable laboratory-scale conditions with reference to a full scale air stripping plant. By integrating laboratory experiments with real wastewater, model-based analysis and full-scale operational considerations, this study enables a critical assessment of LLMS as a viable alternative to the energy-intensive air stripping process. Although maximum N recovery mass transfer (kREC) values achieved in the air stripping system were lower than those observed for LLMS (12.9 d−1 vs 21.6 d−1), air stripping exhibited higher overall average performance, with mean kREC values of 8.1 ± 3.7 d−1 compared to 4.5 ± 6.7 d−1 for LLMS, but the observed values were highly variable depending on operational pH and temperature. The study demonstrated that differences in N recovery mass transfer between LLMS and air stripping arise from distinct transport mechanisms governing each process. A multi-criteria assessment focused on process performance, implementation feasibility, and environmental sustainability revealed that, while air stripping remains a robust and mature technology, LLMS offers greater implementation feasibility, process scalability, resource efficiency, emissions control and benefit per ton of recovered product. Although these findings position LLMS as a promising next-generation alternative for sustainable N recovery, future work should focus on long-term process stability, and developing robust predictive models applicable under realistic wastewater conditions.