Efficient and simultaneous recovery of ammonia and carbon from municipal wastewater by the combination of direct membrane filtration (DMF) and the use of a membrane contactor.

Ammonia in municipal wastewater, which is commonly treated by energy-intensive biological nitrification/denitrification systems, should be recovered in terms of energy efficiency, resource utilization and mitigation of global warming (prevention of emission of NO). The use of a membrane contactor (MC) seems to be effective for recovery of ammonia in municipal wastewater. However, fouling of the membrane in the MC is a potential problem. The effectiveness of direct membrane filtration (DMF) as a pretreatment for controlling the fouling in an MC was investigated in this study. Although DMF can efficiently capture the carbon in municipal wastewater, it cannot remove or capture ammonia in municipal wastewater. Therefore, it was thought that the combination of the use of an MC and DMF would compensate each other. Experiments were carried out by using both synthetic wastewater and municipal wastewater under a high alkaline condition (pH of 11) and a moderate alkaline condition (pH of 9.2). For municipal wastewater, the effluent from primary sedimentation and the permeate from DMF using microfiltration (pore size: 0.1 µm) were examined. It was shown that pretreatment by DMF significantly mitigated the fouling in the MC, leading to maintenance of efficient mass transfer of ammonia in the MC. Ammonia in municipal wastewater could almost completely be recovered by the MC within 6 h regardless of pH. Thus, it was shown that the integration of DMF and an MC could recover 70-80 % of carbon and almost 100 % of ammonia from municipal wastewater. Long-term operations of the proposed system with multiple replacements of the feed wastewater demonstrated that concentration of ammonia nitrogen in the recovery solution (sulfuric acid) could be increased to >350 mg/L. In the long-term operations, due to strict removal of particles/colloids by DMF, the performance of an MC operated at pH of 9.2 was comparable to that operated at pH of 11. This justifies the operation of an MC under a moderate alkaline condition rather than a high alkaline condition, for enhancing the feasibility of the use of an MC for recovery of ammonia from municipal wastewater.