Sulfammox forwarding thiosulfate-driven denitrification and anammox process for nitrogen removal

The coupled process of thiosulfate-driven denitrification (NO₃^-→NO₂^-) and Anammox (TDDA) was a promising process for the treatment of wastewater containing NH₄⁺-N and NO₃^--N. However, the high concentration of SO₄^2- production limited its application, which needs to be alleviated by an economical and effective way to promote the application of TDDA process. In this study, TDDA process was started in a relatively short time by stepwise replacing nitrite with nitrate and operated continuously for 146 days. Results presented that the average total nitrogen removal efficiency of 82.18% can be acquired at a high loading rate of 1.98 kg N/(m³·d) with maximum nitrogen removal efficiency up to 87.04%. It was observed that the increase of S/N ratio improved the denitrification efficiency and slightly inhibit the Anammox process. Batch tests showed that Sulfammox process appeared in TDDA process under certain conditions, further contributing 2.59% nitrogen removal and 10.46% sulfur removal (14.42 mg/L NH₄⁺-N and 37.68 mg/L SO₄^2--S were removed). This finding was mainly attributed to the reduction of sulfate in TDDA system to elemental S⁰ or HS^-, which subsequently was used as an electron donor to realize the recycling of sulfate (SO₄^2--S) pollutants and promote the sulfur-nitrogen (S-N) cycle. High-throughput analysis displayed that Anammox bacteria (Candidatus_Kuenenia), Sulfur-oxidizing bacteria (Thiobacillus) with relatively high abundance of 5.37%, 7.74%, respectively, guaranteeing the excellent nitrogen and sulfate removal performance in the reactor. The enrichment of phyla Chloroflexi (31.79%), Proteobacteria (31.82%), class Ignavibacteriales (10.55%), genus Planctomycetes (13.57%) further verified the exitence of Sulfammox process in the TDDA reactor. This study provides a new perspective for the practical application of TDDA in terms of reducing the production of high concentration SO₄^2- and saving operational cost and strengthening deeply nitrogen removal.