Resource-Efficient Nitrogen Removal from Source-Separated Urine with Partial Nitritation/Anammox in a Membrane-Aerated Biofilm Reactor

Source separation and decentralized urine treatment can cut costs in centralized wastewater treatment by diverting 80% of the nitrogen load in sewage. One promising approach for nitrogen removal from source-separated urine is partial nitritation/anammox (PN/A), reducing the aeration demand by 67% and organic dosage by 100% compared to nitrification/denitrification. While previous studies with suspended biomass have encountered stability issues during PN/A treatment of urine, a PN/A biofilm was hypothesized to be more resilient. Its use for urine treatment has been pioneered here for maximum rates and efficiencies in an energy-efficient membrane-aerated biofilm reactor (MABR). Nitrogen removal rates of 1.0 g N L^-1 d^-1 and removal efficiencies of 80-95% were achieved during a 335-day operational period at 28 °C on stabilized (pH > 11.5), diluted urine (10%). A balance between N₂ and NO₃^- formation was observed while optimizing the supply of O₂ through intermittent aeration and was rate limiting for the conversion toward N₂. A short-term operation on less- and undiluted urine yielded N removal rates of 0.6-0.8 g N L^-1 d^-1 and removal efficiencies of 93% on 66% urine and 85% on undiluted urine. Metataxonomic analysis and fluorescence in situ hybridization confirmed the presence in the biofilm of nitrifiers (Nitrosomonas, Nitrospira) at the membrane side and anammox bacteria ("Candidatus Brocadia") at the anoxic bulk side. The findings suggest that a biofilm approach to PN/A treatment of urine overcomes stability issues and that a PN/A-MABR has significant potential for resource-efficient decentralized urine treatment. In human long-duration deep-space missions, this gravity-independent technology could produce N₂ to compensate artificial atmosphere losses while facilitating water recovery from urine.