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

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.