Operation and optimization of a two-stage, vertical flow constructed wetland system at Bridger Bowl Ski Area

Abstract

Treatment wetlands are an internationally accepted technology for treatment of domestic wastewater and modern designs have become the preferred option for small communities in several European countries for their ability to produce a high quality effluent. To evaluate performance of modern treatment wetland designs with respect to carbon and nitrogen removal in Montana and other challenging contexts, a two-stage, vertical flow system with recycle capabilities has been constructed and tested at a ski area near Bozeman, Montana. Site climatic and operational conditions provide a 'worst-case' scenario to test the efficacy of treatment wetlands in Montana. Intensive sampling of influent and effluent concentrations of chemical oxygen demand (COD) and nitrogen containing compounds after the second season of plant growth is used to optimize and correlate performance as influenced by loading rate, dose volume, and recycle ratio. COD removal was greater than 90% and increased linearly with loading rate even when loading rate exceeded European design guidelines by nearly a factor of 10, with effluent concentrations approximately 100 mg·L -1. The system was also able to nitrify and denitrify. With the use of water recycling, effluent could be optimized for complete removal of ammonium and total nitrogen removal around 50%, even though influent concentrations were approximately 4 times greater than typical domestic wastewater. Mass removal rates were as high as 20 g-N·m-2·day -1, higher than expected based on European guidelines. These results indicate that treatment wetlands are capable of high nitrogen and organic carbon removal, even when applied at a high concentrations, low temperatures, and variable flow situations.