Implementation of an experimental vertical flow treatment wetland for combined sewer overflow in the south of Chile

Abstract

This work aims to evaluate the performance of Vertical flow (VF) treatment wetlands (TWs) operated in parallel and sequentially for the treatment of combined sewer overflow (CSO) in Chile during the first operative period. An experimental plant was operated at three outflow rates: 0.01 L/(s*m²), 0.03 L/(s*m²) and 0.05 L/(s*m²). Influent and effluent samples from VF TWs were taken every 24 h for a period of 72 h. The water quality parameters that were evaluated included pH, Electrical Conductivity (EC), turbidity (TU), total suspended solids (TSS), 5-day biological oxygen demand (BOD₅), chemical oxygen demand (COD, total and dissolved), forms of nitrogen (NH₄⁺-N, NO₃⁻-N, TN), total phosphorus (TP), and E. coli. Reductions in pH value below 1.0 and no significant reductions (p > 0.05) on EC were observed in effluents. Furthermore, removals above 75 % for TSS, TU, BOD₅, and CODt, were stable during the 4-day operational time and for the three outflow rates. In terms of nutrients, similar behavior was observed for the three outflow rates. NH₄⁺-N was effectively removed, without complete transformation into NO₃⁻-N, and TN removal was enhanced after 2-day of operation. TP removal was above 60 % and related to Ca-content in sand used in the wetland bed. Significantly improved results were achieved at 0.03 L/(s*m²). However, E. coli showed low removals of 1 log-unit across the three outflow rates, necessitating an effluent disinfection system. Hence, the system based on VF TWs with parallel operation and sequential feeding, is a promising technology for CSO treatment in the Chilean context.