Subsurface flow constructed wetland: treatment of domestic wastewater by gravel and tire chip media and ultraviolet disinfection of effluent

Abstract

Subsurface flow constructed wetlands (SFCWs) are becoming increasingly common in on-site treatment of wastewater. Gravel is the most popular form of wetland fill medium, but tire chips provide more porosity, are less dense, and cheaper. Before spray application, wetland effluent must be disinfected (traditionally by chlorine). This study determines the treatment efficiency of SFCWs filled with gravel or tire chip media to treat domestic wastewater and the effectiveness of ultraviolet (UV) light in effluent disinfection. The influent and effluent of six SFCWs filled with tire chip medium and six SFCWs filled with gravel were monitored for six to twelve months. Parameters measured included pH, biochemical oxygen demand (BOD₅), total and volatile suspended solids, NH₄, P, and fecal and total coliforms. Differences between medium types in wetland performance were found for the parameters of BOD₅ and P, in which tire chip wetlands outperformed gravel wetlands. The average percent reduction for BOD₅ and P in the tire-chip filled wetlands was 86 and 65%, respectively, compared to 72 and 32% in the gravel-filled wetlands. The higher reduction in BOD₅ in the tire chip wetlands may be due to the prolonged detention time of the wastewater caused by higher pore volume. Soluble P was reduced more efficiently in the tire chip wetlands, most likely because iron in the wires of steel belted tires complexed with P to create an insoluble compound. There was no correlation between temperature and BOD₅ reduction. Because chlorination generates toxic byproducts, UV disinfection may be a better alternative. Two low pressure (254 nm) and two medium pressure UV bulbs (190 to 400 nm) were used to determine disinfection efficiency. Upon installation, all units disinfected effluent successfully. After two weeks in operation, three of the four units were failing due to decreased light intensity from films that developed on bulbs. One unit consistently disinfected water for one year without maintenance. Ultraviolet disinfection, as utilized, was not suitable for use with SFCWs because of inconsistency in disinfection. Tire chips may be a better fill medium for SFCWs than gravel because of higher porosity, lower cost, and greater reduction of P in effluent.