Sulfur autotrophic denitrification filter with mixed filling materials for wastewater secondary effluent treatment

In this study, a sulfur autotrophic denitrification (SAD) filter with a mixture of fine (Type I) and coarse sized (Type II) sulfur element-based filling materials was developed to enhance nitrogen removal from wastewater secondary effluent, with a heterotrophic denitrification (HD) filter packed with quartz sands for a comparison. During long term operation, the SAD with an optimal composition of filling materials (Type I: Type II = 1:2) displayed ~13% higher TN removal with ~50% lower external carbon consumption, and extended backwash duration (28 days vs. 16 days) compared to the HD filter, although both reactors had comparable solids and organic removal ratios. The impacts of HRT and NO 2 − -N accumulation on both SAD and HD performances were examined. It was found that variable HRTs and nitrogen sources in both reactors did not compromise TN removal efficiency ( e.g. , > 26% TN removals at HRT > 3 h). Microbial analysis showed that the dominant autotrophic denitrifies on the surface of type I and type II fillers were Thiovirga and Sulfuricurvum , but their abundances and microbial metabolic functions were influenced by filling material and HRT. Furthermore, the biofilm on the filling materials I had higher abundances of the functional genes relevant to dissimilatory and assimilatory nitrate reduction and denitrification, nitrite reduction pathway, as well as nitrification pathway than those on the filling material II. While the biofilm on filling material II made more efforts in denitrification of NO 2 − to nitrogen gas. Accordingly, the composite fillers facilitated completion of denitrification pathways to enhance nitrogen removal. The cost estimation also highlighted that the SAD yielded ~24% lower total cost per treated wastewater and ~ 76% lower total cost per removed nitrate-N than those of the HD, highlighting the economic feasibility of the composite filler-packed SAD.