The fate of nitrogen and microplastics as wastewater contaminants in treatment wetlands under cold climate conditions

Proper wastewater treatment is necessary to prevent environmental pollution and far-reaching consequences for aquatic ecosystems and water resource availability. Historically, wastewater treatment has focused on the removal of solid matter, organics, pathogens and nutrients (nitrogen and phosphorus), but now confronts an increasing array of human-made pollutants such as pharmaceutical residues and microplastics (MPs). Concurrently the competition for energy and the need for climate change adaption measures are raising interest in nature-based solutions for wastewater treatment. This is particularly relevant in areas where high-tech, maintenance-intensive solutions are impractical, such as the sparsely populated subarctic regions. Treatment wetlands have been used around the world including cold climate regions. However, freezing temperatures and ice formation are known to have substantial negative effects on these systems' functioning.
Year-round field observations in Northern Finland were combined with laboratory freezing experiments to understand the effects of cold conditions on the environmental fate and removal processes of nitrogen and MPs. We examined two wetland types that are common in cold climate regions: peat-based horizontal subsurface flow wetlands and pond-type surface flow wetlands.
The wetland type essentially determines thermal and hydraulic properties, as well as the response of environmental parameters such as oxygen availability, under freezing conditions. The main findings show that nitrogen removal depends on the inflow water quality (what is the dominant nitrogen fraction, ammonium or nitrate?) in combination with the wetland type. While high removal of nitrogen when in form of ammonium was demonstrated year-round for the peat-based wetland, this was low in the pond-type wetland and even turned into leaching of ammonium-nitrogen after extended frozen conditions. Regarding MPs, the pond-type wetland displayed no retention, and atmospheric deposition appeared to convey significant amounts of MPs to the water. Laboratory freezing tests demonstrated that MPs contained in wetland water are incorporated into ice. They also indicated an influence of MP particle properties and particle surface charge in the water matrix on ice entrapment.
Overall, this study confirms that treatment wetlands can function even in cold climate, but also highlights some important limitations that need to be considered when making design choices regarding different treatment goals.