PHOSPHORUS REDUCTION IN EFFLUENT FROM SUBSURFACE FLOW CONSTRUCTED WETLANDS FILLED WITH TIRE CHIPS

Subsurface flow constructed wetlands (SFCWs) are an increasingly common method for on-site treatment of wastewater. Gravel is the most common type of wetland fill medium, but the overabundance of scrap tires has initiated alternate uses. Chipped rubber tires are less dense and less expensive than gravel medium. This study determines the treatment efficiency of SFCWs filled with gravel or tire chip media to reduce P content in domestic wastewater, both in field practices and in experimental columns. The P content in the influent and effluent of six SFCWs filled with tire chip medium and six SFCWs filled with gravel were monitored for 5 to 16 months. Additionally, five columns filled with tire chips and five filled with gravel were evaluated for 3 months for their ability to reduce P. There was a clear difference between medium types in reduction of P. Soluble P in the effluent averaged 1.6 ± 1.0 mg l-1 in the tire chip-filled wetlands and 4.8 ± 3.2 mg l-1 in the gravel-filled wetlands. Phosphorus reduction was 60 ± 13% in columns filled with tire chips and only 17 ± 15% in columns filled with gravel. Greater P reduction most likely occurred in the tire chip-filled wetlands and columns because the Fe from exposed wires in shredded steel-belted tires complexed with P to create an insoluble P compound. 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.