Contaminant removal efficiency depending on primary treatment and operational strategy in horizontal subsurface flow treatment wetlands

This study aimed to evaluate the contaminant removal efficiency of shallow horizontal subsurface flow treatment wetlands (SSF TWs) as a function of (1) primary treatment (hydrolytic upflow sludge blanket (HUSB) reactor vs. conventional settling) and (2) operation strategy (alternation of saturated/unsaturated phases vs. permanently saturated). An experimental plant was constructed, operated and surveyed for the main water quality parameters over a period of 2.5 years. The plant had 3 treatment lines: a control line (settler-wetland permanently saturated), a batch line (settler-wetland operated with saturated/unsaturated phases) and an anaerobic line (HUSB reactor-wetland permanently saturated). In each line wetlands had a surface area of 2.80 m², a water depth of 25 cm and a granular medium D₆₀ = 7.3 mm, and were planted with common reed. During the study period the wetlands were operated at a hydraulic and organic load of 28.5 mm/d and about 4.7 g BOD/m² d, respectively. Effluent average redox potential was lower for the anaerobic line (−45 ± 78 mV) than for the other two lines (3 ± 92.7 and −5 ± 71 mV for control and batch, respectively). Overall, chemical oxygen demand (COD), biochemical oxygen demand (BOD₅) and ammonium mass removal efficiencies were slightly greater for the batch line (88%, 96% and 87%, respectively) than for the control line (83%, 94% and 80%) and the anaerobic line (80%, 87% and 73%). During cold seasons, COD and ammonium removal in the batch line was around 30% and 50% higher than in the control line, respectively. The results of this study indicate that the implementation of a HUSB reactor as primary treatment did not enhance the treatment capacity of the system (in comparison with a conventional settler). The efficiency of treatment wetland systems with horizontal subsurface flow can be improved using a batch operation strategy.