Evaluating denitrification performance and microbial com-munities in sorghum stalk-based systems: Optimal conditions for nitrate removal

Biological denitrification has promise for the removal of nitrate (NO3−-N) from water. In this study, the effects of different temperatures, hydraulic retention times (HRTs), and influent nitrate concentrations (INCs) on denitrification performance were determined in a reverse nitrification system with sorghum stalk (SS) in a flow field environment. The optimum temperature for denitrification in constructed wetlands is 30 °C, under which the NO3--N and total nitrogen removal rates were 75.46 ± 6.77 % and 69.85 ± 6.06 %, respectively. The optimal HRT and INC for the sorghum stalk-denitrification reactor (SS-DR) were 32 h and 50 mg/L, respectively. Under these conditions, NO3--N and total nitrogen removal rates of SS-DR were 95.71 ± 1.75 % and 93.46 ± 1.36 %, respectively. High-throughput sequencing analysis revealed that the dominant phyla in the constructed wetlands were Proteobacteria, Chloroflexi, Firmicutes, Acidobacteriota, Desulfobacterota, Nitrospirota, Actinobacteriota, and Bacteroidota. Most Proteobacteria were facultative and obligate anaerobic denitrifying bacteria ubiquitous in sludge. Bacteroidota plays an important role in nitrogen cycling and energy conversion in ecosystems and decomposes macromolecular organic matter such as protein, cellulose, and lipids. Firmicutes were involved in the denitrification and degradation of cellulose, which has hydrolysis and acidification effects on agricultural waste. These findings provide a reference for the use of SS as an additional carbon source to enhance denitrification in the treatment of aquaculture wastewater.