One-stage hollow-fiber membrane bioreactor for wastewater treatment and resource recovery through hydrogen-driven mixotrophic nitrogen assimilation.

The growing need to recover nutrients from wastewater pushes towards the development of new biological processes enabling the removal and upcycling of nitrogen (N) through microbial assimilation. In this study, a one-stage H₂-fed aerobic hollow-fiber membrane bioreactor, seeded with a mixed culture of hydrogen-oxidizing bacteria (HOB), was developed to treat a synthetic urban wastewater below discharge limits in terms of chemical oxygen demand (COD) and ammonium nitrogen (N-NH₄⁺). The removal and recovery of the latter as microbial protein (MP) through mixotrophic N-assimilation, together with the avoidance of greenhouse gas emissions (CO₂, N₂O) during wastewater treatment, were the main objectives of this study. The performance of the H₂-driven mixotrophic N-assimilation process was evaluated in continuous mode by studying the influence of the hydraulic retention time (HRT), COD_H2:COD_acetate and H₂:O₂ ratios in the mixotrophic system. Under mixotrophic conditions, higher HRT and COD_H2:COD_acetate ratios ensured stable reactor performance with 484.4 mg VSS∙L^-1 biomass, 90 % nitrogen assimilation and up to 64.7 % protein content, whereas low HRT yielded a higher biomass concentration (604.9 mg VSS∙L^-1) but unstable performance. Conversely, under heterotrophic conditions, both nitrogen assimilation (40 %) and protein content (40 %) were significantly lower, while the residual nitrate concentration (16.1 mg∙L^-1 of N-NO₃^-) prevented compliance with total nitrogen discharge limits (<10 mg∙L^-1). Importantly, both CO₂ and N₂O emissions were minimized under mixotrophic conditions, indicating a high greenhouse gas mitigation potential. The obtained results indicate that one-stage H₂-driven mixotrophic N-assimilation process in hollow-fiber membrane bioreactors offers a viable solution for simultaneous wastewater treatment and resource recovery.