How do aeration mode and influent carbon/nitrogen ratio affect pollutant removal, gas emission, functional genes and bacterial community in subsurface wastewater infiltration systems?

This study investigated the influences of aeration mode and influent carbon/nitrogen ratio on matrix oxygen concentration, pollutant removal, greenhouse gas emission, functional gene abundances and bacterial community in subsurface wastewater infiltration systems (SWISs). Intermittent or continuous aeration enhanced oxygen supply at 0.6 m depth in the matrix, which improved organics removal, nitrogen removal, the abundances of bacterial 16S rRNA, amoA, nxrA, narG, napA, nirK, nirS, norB, nosZ genes, bacterial community Alpha diversity, the relative abundances of Actinobacteria at 0.6 m depth, the relative abundances of Chloroflexi, Gemmatimonadetes, Bacteroidetes and Firmicutes at 0.9 and 1.2 m depth and reduced CH₄ and N₂O conversion efficiencies, the abundance of mcrA gene with carbon/nitrogen ratio of 12 and 16 compared with non-aeration. Increased carbon/nitrogen ratio resulted in higher TN removal efficiencies and lower CH₄ and N₂O conversion efficiencies in aeration SWISs than those in non-aeration SWIS. Intermittent aeration SWIS obtained high removal efficiencies of 83.2, 85.4 and 90.8% for TN, NH₄⁺ -N and COD and low conversion efficiency of 0.21 and 0.65% for N₂O and CH₄ with optimal carbon/nitrogen ratio of 12. However, high TN (82.6%), NH₄⁺ -N (84.9%) and COD (92.2%) removal efficiencies and low CH₄ (0.67%) and N₂O (0.23%) conversion efficiencies were achieved in continuous aeration SWIS with carbon/nitrogen ratio of 16.