Evaluating the performance of horizontal sub-surface flow constructed wetlands: A case study from southern India

Abstract

Constructed wetlands are a nature-based engineering solution enabling polishing of septic tank effluents at low-cost. However to date, the influence of planting on treatment efficiency remains little understood. Here we report a case study evaluating the performance of two near-identical Horizontal Sub-Surface Flow Constructed Wetlands (HSSF-CW) deployed at a school in southern India. The HSSF-CWs were of similar size and construction with the exception that one system was planted (Canna indica) whilst the other was operated without plants. Both systems were operated at similar hydraulic loading rate (HLR) and hydraulic retention time (HRT) of 84 mm day−1 and 3.7 days, respectively to treat the effluent from septic tanks. The systems were monitored fortnightly for one year and the performance kinetics, nutrient and organics removal efficiencies were evaluated. Significant reduction in biochemical oxygen demand (BOD5) and chemical oxygen demand (COD) (p < 0.05) were observed in both systems with BOD5 removal efficiency of 67% and 61% in the planted and unplanted systems, respectively. Whilst the effluent from both systems met the environmental discharge standards set by Central Pollution Control Board (CPCB), India, the total phosphorus (TP) and total suspended solids (TSS) removal in the unplanted system were significantly greater than in the planted system. The first-order decay rate constants (K) obtained for TN (KTN) and BOD5 (KBOD5) in the planted system (0.16 day−1 and 0.30 day−1) were higher than in the unplanted system (0.09 day−1 and 0.27 day−1). Greater R2 values obtained for the planted system (~ 0.90) suggests applicability of a first-order decay model to assess contaminant degradation. Plants contributed to 7% (0.3 g/m2/day) BOD5, 18% (1.9 g/m2/day) COD and 5% (0.09 g/m2/day) TN load removal. Our data demonstrates that planting is effective in improving treatment efficiency in constructed wetlands, and whilst the improvement is marginal here, it is noted that a rust infection could have limited effectiveness of the plants in this case.