Cyclic regeneration and up-concentration for nitrate recovery from wastewater via in situ NaNO3 generation under saline, fully autotrophic nitrification conditions.

Nitrogen recovery from wastewater is essential for resource-oriented and circular wastewater treatment but remains challenging. This study developed a coupled process integrating ion exchange, cyclic up-concentration via regeneration and side-stream nitrification, and electrodialysis to recover high-purity NaNO3 from municipal wastewater. NaNO3 was an effective and process-compatible regenerant for NH4+-loaded exchanger, producing a concentrated ammonium stream. The side-stream nitrification reactor (SNR) maintained stable and complete nitrification under high-salinity, autotrophic conditions at salinity ≤ 8.6 g Na+/L and ammonium nitrogen loading rates ≤ 7.5 mg N/(L·h). Although the acclimated SNR sludge exhibited lower maximum autotrophic growth rates (0.70 - 0.86 d-1), it achieved a higher peak specific ammonium utilization rate of 11.2 mg N/(g SS·h) at 6.0 g Na+/L. The high-salinity, nitrogen-rich autotrophic environment selectively enriched salt-tolerant nitrifiers and stress-resilient microbial consortia, characterized by increased Nitrosomonas abundance, a shift in dominant nitrite-oxidizing bacteria from Nitrospira to Nitrobacter, and the enrichment of oxidative stress-resistant and osmoprotectant-producing heterotrophs. This was accompanied by a 26.2 - 65.2 % enrichment of nitrification genes (amoA, nxrA/B), while increased relative abundance of genes involved in assimilatory nitrate reduction to ammonium enhanced osmotic and oxidative stress tolerance under high-NaNO3 autotrophic conditions. Under optimal operating conditions (SNR1), in situ-generated NaNO3 was cyclically up-concentrated to ∼ 40 g/L, further concentrated to 120 g/L by electrodialysis, and recovered via evaporative crystallization with an average purity of 99.1 % (SD = 4.5 %). Overall, this study demonstrates a viable route for integrating nitrogen separation and recovery with carbon redirection, advancing closed-loop and resource-oriented nitrogen management in municipal wastewater treatment.