Pilot-Scale Investigation of Floating Treatment Wetlands as Retrofits to Waste-Stabilization Ponds for Efficient Domestic Wastewater Treatment

Abstract

This study investigates the efficacy of floating treatment wetlands (FTWs) for retrofitting waste-stabilization ponds (WSPs) operating in cold continental climates, such that in Alberta, Canada. The objective was to determine whether WSPs augmented with FTWs could achieve equal or superior treatment efficiency at shorter hydraulic retention times (HRTs of 25–45 days) than those required in the conventional WSPs (60–365 days). A field study was carried out on treatment of domestic wastewater in a two-stage pilot-scale FTW mesocosm (i.e., FTW treatment train composed of Stage 1 and Stage 2 FTW cells (S1-FTW→S2-FTW), with an overall volume of 84 m3 and 41.85 m3 per cell). An identical system without FTW served as a control (i.e., control treatment train of S1-C→S2-C). Overall, the FTW-augmented WSP system achieved greater treatment of 5-day biochemical oxygen demand (BOD5), total suspended solids (TSS), total nitrogen (TN), ammonium nitrogen (NH4+-N), dissolved reactive phosphorus (DRP), and total phosphorus (TP) than the control. The corresponding removal efficiencies were 91.5% versus 85.0%, 91.3% versus 87.7%, 85.0% versus 75.6%, 93.9% versus 86.1%, 77.6% versus 57.7%, and 75.8% versus 58.7%, with a statistically significant difference (p<0.05) for all except TSS. This difference in performance resulted from the better performance of the Stage 1 FTW cell for BOD5, TN, and NH4+-N removal, and the Stage 2 FTW cell for DRP and TP removal. Simultaneous nitrification-denitrification (SND) occurred in the FTW treatment train, whereas only nitrification occurred in the control treatment train, which resulted in statistically higher TN removal efficiency in the former. Contribution of plant uptake to phosphorus removal was marginal (6.76%) and adsorption-desorption on solid particles was assumed to be the major removal pathway. The FTWs generally decreased the physicochemical parameters, with stronger effects on diurnal variations than seasonal fluctuations. The effect of FTW on daily physicochemical conditions played a key role in superior performance of FTW treatment.