High-rate partial nitritation of high ammonia permeate from anaerobic membrane reactor treating food waste: Kinetics, enriched Nitrosomonas and enhanced tolerance to free ammonia and free nitrous acid

Partial nitritation (PN) is commonly identified as the rate-determining step in the two-stage partial nitritation/anammox (PN/A) process. This study explored the potential of applying PN to undiluted permeate from an anaerobic membrane bioreactor (AnMBR) digesting food waste. A high nitrogen loading rate (NLR) of 9 gN/L/d was realized while maintaining approximately 54% ammonium conversion. With influent ammonium concentration of ca. 4500 mgN/L, strong tolerance to free ammonia and free nitrous acid was established with specific ammonia oxidation rates reaching 2.3 gN/gMLVSS/d. Selective nitrite accumulation was achieved via Ammonia Versus Nitrite (AvN) control, enabling manageable response to ammonium fluctuations under oxygen-limited conditions. High sludge concentrations and good settleability were maintained, and microbial analysis confirmed the dominance of Nitrosomonas with relative abundance over 30% without detectable nitrite-oxidizing bacteria (NOB) . These findings demonstrate that PN can serve as a robust and high-rate pretreatment approach for the direct treatment of high-strength AnMBR effluents, while simultaneously providing a stable and nitrite-rich influent that is well suited for downstream anammox processes. • Undiluted AnMBR permeate from food waste digestion was effectively nitritated. • Nitrogen loading rate reached 9 gN/L/d with ca. 54% ammonium conversion. • Ammonia oxidation activity reached 2.3 gN/gMLVSS/d with no signs of inhibition. • High sludge concentration and settleability supported long-term process stability. • Nitrosomonas was highly enriched without detectable nitrite-oxidizing bacteria .