A thermal analysis of a sub-surface, vertical flow constructed wetland

Abstract

Since November of 1991, an experimental constructed wetland has successfully treated municipal sewage effluent on a year round basis in a cool climate. The sub-surface, vertically pulsed flow system is located at a latitude of approximately 43 degrees, 15 minutes north latitude in south-central Canada. The 5 metre long by 5 metre wide by 1.2 metre deep constructed wetland cells were designed to operate through extended freezing periods via a number of specific features. The most important features being the allowance of thatch accumulation atop the system, ice accretion within the upper cell strata both acting as insulating layers, and the transfer of thermal energy to the system from warmer deep soils. The cells were hydraulically loaded below this frozen layer of granular matrix six times a day. A dense three dimensional array of thermocouples was planted within the first of the three constructed wetland cells in a series to allow for the assessment of thermal data at a high level of temporal and spatial resolution. Thermal data were sampled every five minutes and averaged and stored every hour over a two year period (1994 and 1995). The data were reviewed statistically to determine the operating envelope experienced at the Niagara-on-The-Lake experimental constructed wetland site. A detailed review of winter thermal data was made to provide parameters for the use of the HEATFLOW density-dependent ground water flow and thermal energy transport numerical model (Molson and Frind, 1995). The use of this coupled Darcy flux, thermal transport model has allowed for a better understanding of the importance of various thermal design considerations, and has allowed for the undertaking of sensitivity analyses for design assessment and optimization. The sensitivity analyses indicate that the retention of deep soil heat and top insulation from plant thatch are the most important thermal features. It is likely that this technology can be used in areas colder than Niagara-On-The-Lake.