Effect of long-term operation of a subsurface wastewater infiltration system (SWIS) based on the limiting value of environmental carrying capacity

To evaluate the environmental carrying capacity of a subsurface wastewater infiltration system (SWIS) and the removal effect of nitrogen, the simulated soil column experiments were performed. After the SWIS was operated for three years with a hydraulic loading of 5 mm d−1, the results indicated that the total nitrogen (TN) removal rate and the ammonia nitrogen (NH4±-N) removal rate were 74.22%, 99.69%, respectively, for the high pollution load treatment system and 80.45%, 99.72%, respectively, for the low pollution load treatment system (TN concentrations of 80 mg L−1 and 40 mg L−1for the two treatment systems, respectively). The ratios of the nitrate nitrogen (NO3−-N) concentration in the effluent, which satisfied grade III of the Chinese groundwater standard and the maximum international contaminant level of drinking water were 13.33%, 0.00% and 87.87%, 37.43%, respectively. Adopting the 20 mg L−1 of the NO3−-N concentration in the soil leachate as the evaluation criterion, the environmental carrying capacity of nitrogen in the SWIS was in a range of 600 kg N ha−1 y−1 to 1200 kg N ha−1 y−1.