Assessing resilience of natural treatment system to variable contaminant loads in an urbanizing catchment

Abstract

Constructed wetlands (CWs) offer solution to increasing wastewater concerns. Long term monitoring data (six years) provides information on robustness and life time of such systems. The CW system examined in this study was deployed to serve as a polishing stage for the sewage treatment plant (STP) effluent, however, it also receives raw sewage through stormwater drains. The hydraulic loading rate (HLR) of system increased from 215.5 ± 6.7 mm/day to 466.9 ± 21.6 mm/day in 6 years thereby reducing the hydraulic retention time (HRT) of the system. The effluent quality complied with the discharge standards except for ortho-P and fecal coliforms. Corresponding to HLR, the organic and nutrient loading rate also displayed significant variation during the period 2015 to 2020. Despite this, the organic load removal efficiency was comparable with only small variation. The nitrogen removal efficiency displayed significant fluctuation with occasional decline followed by improvement in the year 2020. The system released ortho-P into the water column increasing its load at the outlet, the ortho-P removal efficiency reduced from 19 to −44 % during the study period. Desilting, regular macrophyte harvesting, aeration and pre-treatment of raw sewage from open storm water drain can further enhance the system performance and robustness.