Autotrophic nitrogen removal characteristics of PN-anammox process enhanced by sulfur autotrophic denitrification under mainstream conditions

[Display omitted] •The SADN-PN-anammox process was successfully coupled under mainstream conditions.•SADN alleviates strict conditions for PN stability under mainstream conditions.•S2− oxidation process have competition between O2 and NO3− in low DO environment. Stabilization of nitrification process and reduction of NO3−-N concentration in effluent are the keys to realize mainstream application of partial nitrification-anaerobic ammonia oxidation (PN-anammox) process. The sulfur-based autotrophic denitrification (SADN) process was coupled with the PN-anammox in a single reactor to enhance and stabilize the nitrogen removal performance, and the feasibility and reaction characteristics of the coupling system under mainstream conditions were investigated. The results showed that the NO3− of PN-anammox effluent dropped from 22 to 24 mg/L to 5 mg/L after the SADN process coupled, and the total nitrogen removal efficiency and total nitrogen removal rate reached 83.5% and 0.15 kg/(m3·d), respectively. This coupling system doesn’t need to over-strengthen PN control. Batch experiments showed that sulfur autotrophic oxidizing bacteria used O2 to oxidize S2− in the coupling system, which competed with SADN to remove NO3−. Moreover, Nitrosomonas, Candidatus Brocadia and Thiobacillus were the main genera for nitrogen and sulfur conversion.