High-rate nitrogen removal from carbon limited wastewater using sulfur-based constructed wetland: Impact of sulfur...

This study aims to explore the application of sulfur-based constructed wetlands (CWs) for effective nitrogen (N) removal from wastewater. Two solid sulfur sources namely elemental sulfur (S⁰) and pyrite (FeS₂) were used as substrates in two CWs, i.e. S-CW and P-CW, respectively. The CWs were vegetated with a common wetland plant Iris pseudacorus, and were operated to investigate the effects of hydraulic retention time (HRT) and temperature on N removal. The use of S⁰ resulted in the highest denitrification rate (19.0 ± 7.5 g m^-2 d^-1), whereas up to 20 times slower total inorganic nitrogen (TIN) removal was observed with FeS₂. Different sulfur sources had negligible effects on the growth of I. pseudacorus, but the element contents (e.g., N, S, and P) within the plant tissues were different. Iris roots in S-CW had higher S content compared with those in P-CW, which resulted in the difference in shoots colors. The characteristics of rhizospheric microbial communities were closely related to the sulfur and nitrogen sources. Briefly, denitrifying and sulfur-oxidizing genera (e.g., Denitratisoma, Sulfurimonas, Thiobacillus) were dominating in the S-CW, suggesting the occurrence of both autotrophic and heterotrophic denitrification processes in the wetland. On the other hand, nitrifying bacteria were more abundant (e.g. Nitrospira, Piscinibacter) in the P-CW. S⁰ layer and rhizosphere accounted for 99.3% of nitrogen removal and the former part most likely played important roles with a decrease in HRT. Low temperature strongly affected the rate and efficiency of denitrification but recovered to 49.2 ± 25.8% when added with 30 mg L^-1 sodium acetate. This study broadens the applications of sulfur-based CWs and provides a promising management strategy for denitrification at low temperatures.