Removal performance of high hydraulic loading rate subsurface wastewater infiltration systems with added Si -Al porous clay material and scrap iron filings

To enhance pollutant removal performance in the treatment of decentralized domestic sewage, a subsurface wastewater infiltration system (SWIS) with turfy soil was improved by adding Si-Al porous clay material W (PCMW) and scrap iron filings. The permeability at different depths of the SWIS was improved. Under the conditions of 186.18 +/- 34.82 mg/L COD, 25.84 +/- 2.57 mg/L NH4+-N, 3.02 +/- 0.91 mg/L NO3--N, 33.87 +/- 3.78 mg/L TN, 6.93 +/- 1.21 mg/L TP, a 0.28 m(3)/m(2).d hydraulic loading rate and 716 d operation time, the average effluent concentrations of COD, NH4+-N, NO3--N, TN and TP were 31.28 +/- 5.87 mg/L, 3.39 +/- 1.42 mg/L, 3.71 +/- 0.87 mg/L, 7.11 +/- 0.97 mg/L and 0.24 +/- 0.07 mg/L, respectively, and the TN removal efficiency increased by 20.7% compared with that of the control SWIS. According to the pollutant variations with depth, 76.79% COD, 80.96% NH4+-N and 95.35% TP were removed at a depth of 110 cm, after which their concentrations remained stable; the TN concentration decreased with depth and was quite different from that of control SWIS, NH4+-N in TN was removed at a depth of 0-90 cm, and NO3--N in TN was removed at 90-170 cm depth. Moreover, both the number of nitrifying/denitrifying bacteria and nitrate/nitrite reductase activity reached their maximum at 90 cm depth. Adding PCMW and scrap iron filings to SWISs could be an ideal method to improve the performance and service life of SWISs.