How Does Hydraulic Loading Rate Affect Fates of (NH4+)-N-15-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 (NH4+)-N-15-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 10cm/day as an optimal rate. After N-15 labeling, 22.3% of (NH4+)-N-15-N remained in the system in the form of (NO3-)-N-15-N, 12.8% remained in N-15-Organic nitrogen, and 4.6% remained in the original (NH4+)-N-15-N form at HLR of 10cm/day. Furthermore, within the tested loads of 6, 10, and 14cm/day, N-15 retention in the middle layer was dominant in SWIS, and the relative contribution to the total N-15 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.