Evaluation of the removal performance in long-term operation of bioaugmented subsurface wastewater infiltration systems under a high hydraulic loading rate

Abstract

To enhance the pollutant removal performance of a subsurface wastewater infiltration system (SWIS) used in decentralized treatment of domestic sewage, two nitrogen-removing strains were isolated from a SWIS with high removal efficiency and inoculated into another SWIS. Three SWISs were analyzed: a sterilized SWIS (system 1), a control SWIS (system 2), and a bioaugmented SWIS (system 3). Under the conditions of 0.28 m3/m2 ⋅ d hydraulic loading rate and 716 d running time, the removal capacities of COD, NH4+-N, TN and TP of system 3 were 167.19 ± 14.21 mg/L ⋅ d, 23.85 ± 0.74 mg/L ⋅ d, 25.21 ± 0.96 mg/L ⋅ d and 6.46 ± 0.06 mg/L ⋅ d, respectively. Correspondingly, the removal efficiencies of COD, NH4+-N and TN were 89.63%, 92.69%, and 86.95%, respectively, which were 6.41%, 6.45%, and 11.48% higher than those of system 2 and 63.01%, 55.15%, and 51.27% higher than those of system 1. The concentrations of COD, NH4+-N, TN and TP of system 3 stabilized at depths of 130 cm, 110 cm, 150 cm, and 130 cm, respectively. Additionally, both the number of nitrifying/denitrifying bacteria and the nitrate/nitrite reductase activity (NRA/NIRA) showed that the nitrogen removal zone was 90–130 cm deep. The coefficients of determination (R2) between NO3−-N concentrations and the number of denitrifying bacteria/NR activities in system 3 showed the greatest correlation, which were 0.97 at 150 cm and 0.92 at 90 cm. Bioaugmentation by the re-inoculation of strains isolated from a SWIS into new SWIS could be an ideal method to improve the performance and service life of SWISs.