Effect of increased carbon load on denitrification efficiency and nitrate isotope enrichment factors in subsurface...

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 -8.7‰ to -10.6‰ and -5.9‰ to -8.2‰, respectively, as the C load increased from 18 to 36 g/(m²  d). The contribution of denitrification to nitrate removal was 62%, 71%, and 77% when C loads were 18, 27, and 36 g/(m²  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/(m²  d) is recommended in SWIS to improve the N removal performance and denitrification efficiency.