A feasibility study of ammonia recovery from coking wastewater by coupled operation of a membrane contactor and membrane distillation

More than 80% of ammonia (NH₃) in the steel manufacturing process wastewater is contributed from the coking wastewater, which is usually treated by biological processes. However, the NH₃ in the coking wastewater is typically too high for biological treatment due to its inhibitory concentration. Therefore, a two-stage process including a hollow fiber membrane contactor (HFMC) and a modified membrane distillation (MD) system was developed and applied to reduce and recover NH₃ from coking wastewater. The objectives of this paper are to evaluate different membrane materials, receiving solutions, and operation parameters for the system, remove NH₃ from the coking wastewater to less than 300 mg N/L, which is amenable to the biological process, and recover ammonia solution for reuse. As a result, the polytetrafluoroethylene (PTFE) HFMC using sulfuric acid as a receiving solution can achieve a maximum NH₃-N transmembrane flux of 1.67 g N/m²·h at pH of 11.5 and reduce NH₃ in the coking wastewater to less than 300 mg N/L. The NH₃ in the converted ammonium sulfate ((NH₄)₂SO₄) was then recovered by the modified MD using ice water as the receiving solution to produce ≥3% of ammonia solution for reuse.