How Does Hydraulic Loading Rate Affect Fates of ¹⁵NH₄⁺-N Tracer and Pollutants Removal in Subsurface Wastewater Infiltration System?

Subsurface wastewater infiltration systems (SWIS) have been considered an efficient and economical technology for sewage treatment in recent years, especially in rural and remote areas. However, the migration and transformation of NH4+-N in SWIS remain unclear. This study investigated the fates of 15NH4+-N tracer and pollutants removal performance in SWIS under different hydraulic loading rates (HLRs). The results indicated that an increase in HLR negatively affected the performance of SWIS in removing N and organic pollutants from the sewage. Considering the shortages of land resources and the pollutant removal efficiency, we recommend an HLR of 10 cm/day as an optimal rate. After 15N labeling, 22.3% of 15NH4+-N remained in the system in the form of 15NO3−-N, 12.8% remained in 15N-Organic nitrogen, and 4.6% remained in the original 15NH4+-N form at HLR of 10 cm/day. Furthermore, within the tested loads of 6, 10, and 14 cm/day, 15N retention in the middle layer was dominant in SWIS, and the relative contribution to the total 15N retention ranged from 59.4% to 76.7%. Overall, our results are valuable for comprehending the N migration and transformation mechanism, and it helps improve the pollutant removal efficiency in SWIS.