Performance of a surface flow constructed wetland system used to treat secondary effluent and filter backwash water

The performance of a surface flow wetland used to treat activated sludge effluent and filter backwash water from a tertiary treatment facility was evaluated. Samples were collected before and after vegetation removal from the wetland system, which consisted of two densely vegetated settling basins (0.35 ha), an artificial stream and a 3-ha surface flow wetland. Bulrush (Scripus spp.) and cattail (Typha domingensis) were the dominant plant species. The average inflow of chlorinated secondary effluent during the first two months of the actual study was 1.84 m³ min^-1, while the inflow for backwash water treatment ranged from 0.21 to 0.42 m³ min^-1. The system was able to reduce TSS and BOD₅ to tertiary effluent standards; however, monitoring of chloride concentrations revealed that wetland evapotranspiration is probably enriching pollutant concentrations in the wetland outflow. Coliphage removal from the filter backwash was 97 and 35% during 1999 and 2000, respectively. However, when secondary effluent entered the system, coliphage removal averaged 65%. After vegetation removal, pH and coliphage density increased significantly (p<0.05) at the outlet of the wetland. This study showed that surface flow wetlands are an alternative technology for TSS, BOD₅ and turbidity removal from both secondary or backwash water. However, growth of bacterial populations or recovery of injured bacteria may occur.