Thermal properties of treatment wetlands operated under freezing conditions

Abstract

The use of treatment wetlands (TWs) presents particular challenges in regions with sub-zero winter temperatures, due to reduced biological activity and risk of pipe breakage or clogging due to freezing. We studied the vertical temperature distribution in four pilot-scale TWs exposed to winter temperatures in order to determine the impact of operational system parameters and the role of insulation on heat conservation inside the filtering bed. The overall temperature pattern was similar in all wetlands, with a trend of increasing temperature from the surface toward the bottom during the cold season. No freezing was detected in the wetlands despite average daily temperatures as low as −20 °C. Influent water temperature and hydraulic loading had a stronger influence on TW temperatures in winter than air temperature. The vertical distribution of temperatures in TWs is more sensitive to hydraulic loading variation in the percolating operating condition than in the saturated flow with forced aeration configuration. Our results suggest that TW systems can remain operational under cold winter conditions provided the surface is properly insulated by vegetation, mulch and/or snow. HIGHLIGHTS In winter, temperature increases from the surface towards the bottom in vertical flow treatment wetlands.; The vertical distribution of temperatures is more sensitive to hydraulic loading variation in the percolating operating condition than in saturated flow with forced aeration.; Given proper surface insulation, influent water temperature and hydraulic loading have a stronger influence on treatment wetland temperatures in winter than air temperature.; Vertical flow treatment wetlands can remain operational in winter, if the surface is properly insulated by vegetation, mulch and/or snow.;

Keywords

• cold climate
• insulation
• thermal distribution
• treatment wetland
• vertical flow