Cold-climate removal of emerging contaminants in a two-stage vertical flow wetland at Bridger Bowl ski area

Abstract

Contaminants of emerging concern (CECs), including pharmaceuticals, are found in wastewater (WW) at trace levels and can resist removal by conventional wastewater treatment plants (WWTPs). Consequently, CECs are discharged to the environment through WWTP effluent, posing ecological and public health risks. Meanwhile, the U.S. WW sector faces aging infrastructure, tightening effluent standards, and limited budgets, making targeted CEC treatment technologies inaccessible for most municipalities and U.S. households that rely on decentralized treatment. These limitations, combined with rising public awareness and expectations, demonstrate a growing need for low-cost systems capable of both CEC removal and conventional treatment. Treatment wetlands (TWs), particularly vertical flow treatment wetlands (VFTWs), where water passes vertically through planted sand or gravel media, offer nature-based solutions to municipal WW polishing and decentralized treatment. TWs combine physical, chemical, and biological processes to remove CECs and conventional pollutants. Despite widespread adoption elsewhere, their use in the U.S. is limited by outdated design guidelines and a lack of demonstrated long-term performance. A knowledge gap in the use of VFTWs for CEC treatment in cold climates was addressed with a pilot-scale study investigating the presence and removal of CECs at the Bridger Bowl two-stage VFTW, which treats ski resort wastewater in winter (Bozeman, MT). The Bridger VFTW has been consistently monitored over its December-April operating season since 2015, providing valuable long-term data. Influent, midpoint, and effluent WW samples were analyzed for chemical oxygen demand (COD), nitrogen species, and target CECs. Solid phase samples were analyzed to identify CECs sorbed to media or biomass. Despite low WW temperatures (2-4 °C), the VFTW achieved high removal of COD (96%), ammonia (98%), and total nitrogen (71%). Of the 38 CECs detected, half demonstrated removal greater than 80%, primarily in the aerobic zone of the VFTW. Performance equaled or exceeded that of VFTWs and WWTPs in literature studies. By filling the cold-climate data gaps for CEC removal and demonstrating robust treatment of both CECs and conventional pollutants, this work establishes VFTWs as cost-effective, nature-based solutions capable of advanced WW treatment in cold regions.