A novel simultaneous partial nitrification and denitratation (SPND) process in single micro-aerobic sequencing batch reactor for stable nitrite accumulation under ambient temperature

[Display omitted] •Simultaneous NH4+-N oxidation and NO3–-N reduction was achieved in a SPND system.•AOB activity was greatly enhanced by prolonging aeration without NOB overgrowth.•DNB influenced negligible AOB activity and retained high NTR of 96.5% under 13.1 ℃.•A high SPND efficiency of 95.5% was the cause of significant NO2–-N accumulation.•NOB was stably inhibited at low-DO (0.18 mg/L) and high FNA (>0.01 mg/L) in SPND. For reducing the risk of wastewater containing nitrate to natural ecosystem, transforming nitrate to nitrite then removed by energy-efficient anammox was considered as a promising method. Here, this work developed an innovative simultaneous partial nitrification and denitratation (SPND) process in a single micro-aerobic sequencing batch reactor treating domestic and nitrate wastewaters to supply stable nitrite for mainstream anammox. The new SPND biosystem was operated for 195 days under ambient temperature conditions (12.6–31.4 ℃). Results showed that, under low-dissolved oxygen of 0.18 mg/L, low-COD/N of 1.5, and aeration duration of 23 h, the final achieved high SPND efficiency was 95.52% and ideal nitrite production was 49.53 mg/L. Nitrogen banlance estimated that 95% of total nitrogen was transformed to nitrite via SPND process. Batch tests revealed that nitrate-to-nitrite transformation ratio of denitratation stabilized at 96.5% and activity ratio of ammonia oxidizing bacteria (AOB) to total nitrifying bacteria reached 82.4% at the steady-state despite low temperature (13.1 ℃ on average). The average AOB activity was significantly improved to 8.02 mgN/gVSS/h with the prolonged micro-aeration duration, while nitrite oxidizing bacteria was stably inhibited at DO-limited (0.18 mg O2/L) and strong free nitrous acid (>0.01 mg/L) conditions. Metagenomic analysis revealed that the discrepant expression levels for functional genes (amoA, nxrA, nxrB, napA, narG, nirK, nirS, cnorB and nosZ), and the labor metabolism cooperation between AOB and heterotrophic denitrifying microorganisms potentially facilitated SPND high nitrite accumulation performance.