Effect of increased carbon load on denitrification efficiency and nitrate isotope enrichment factors in subsurface wastewater infiltration system

Abstract

Denitrification plays a dominant role in nitrate removal in subsurface wastewater infiltration system (SWIS). However, the effect of increased carbon (C) load on denitrification efficiency in the SWIS remain unclear. In this study, we used analyses of stable isotopes of nitrogen (N) and oxygen (O) in nitrate to investigate the N and O isotope enrichment factors ( 15 ε and 18 ε) and quantified N losses via denitrification in SWIS. The results demonstrated that an increase in C loads positively affected the pollutant removal performance of SWIS. The natural abundance of 15 N and 18 O increased with decreasing nitrate concentration from 12.5 to 7.3 mg/L, accompanied by increased 15 ε and 18 ε from À8.7‰ to À10.6‰ and À5.9‰ to À8.2‰, respectively, as the C load increased from 18 to 36 g/(m 2 d). The contribution of denitrification to nitrate removal was 62%, 71%, and 77% when C loads were 18, 27, and 36 g/(m 2 d), respectively, indicating that increased C loads could improve the nitrate removal through denitrification in SWIS. Practitioner Points• Increasing C loads positively affected the nitrate removal performance of SWIS.• N and O isotope enrichment factors of nitrate increased with the enhancement of influent C load.• A C load of 36 g/(m 2 d) is recommended in SWIS to improve the N removal performance and denitrification efficiency.

Nitrogen removal performance of improved subsurface wastewater infiltration system under various influent carbon–nitrogen ratios

“…and N 2 ) is a heterotrophic microbial reaction that has long been recognized as the primary process for removing N pollutants, which is affected by soil pH (Zhou et al, 2023). In this study, different soil depth and C-N ratios had little impact on soil pH (Table S2).…”

“…Multiple functional genes of microbes regulate or co‐regulate denitrification in SWIS (Zhou et al, 2023). The reduction of NO 3 − ‐N to NO 2 − ‐N is the first reaction process in denitrification, which is catalyzed by the nitrate reductase (narG and napA) genes (Pan et al, 2017).…”

“…The level of DO determines the processes that dominate N transformation processes at different soil depths (Su et al, 2023). Generally, the concentration of DO in the soil decreases significantly with increasing depth, mainly because of the limited diffusion capacity of oxygen (O 2 ) from air into the matrix (Zhou et al, 2023). DO concentrations range from 0.2 to 0.5 mg/L at soil depths from 60 to 90 cm, suggesting an anoxic environment (Jiang et al, 2017).…”

“…Results showed that the NO 3 − ‐N removal performance increased with the enhancement of C–N ratio. Denitrification (the stepwise reduction of NO 3 − ‐N to NO 2 − ‐N, nitric oxide, N 2 O, and N 2 ) is a heterotrophic microbial reaction that has long been recognized as the primary process for removing N pollutants, which is affected by soil pH (Zhou et al, 2023). In this study, different soil depth and C–N ratios had little impact on soil pH (Table S2).…”

“…Annually, approximately 359 × 10 9 m 3 of wastewater is generated worldwide, with nearly half of which is discharged into the environment without being purified, posing considerable risks to environmental safety and human health (Zhang & Liu, 2022). Traditional wastewater treatment processes, such as membrane separation and activated sludge technology, have been widely used worldwide for water pollution control (Zhou et al, 2023). However, such wastewater treatment systems are impractical for small towns and rural areas because of cost constraints and dispersed settlements (Li et al, 2017).…”