A THERMAL-ANALYSIS OF A SUBSURFACE, VERTICAL FLOW CONSTRUCTED WETLAND

Since November of 1991, an experimental constructed wetland has succes sfully treated municipal sewage effluent on a year round basis in a co ol climate. The sub-surface, vertically pulsed flow system is located at a latitude of approximately 43 degrees, 15 minutes north latitude i n south-central Canada. The 5 metre long by 5 metre wide by 1.2 metre deep constructed wetland cells were designed to operate through extend ed freezing periods via a number of specific features. The most import ant features being the allowance of thatch accumulation atop the syste m, ice accretion within the upper cell strata both acting as insulatin g layers, and the transfer of thermal energy to the system from warmer deep soils. The cells were hydraulically loaded below this frozen lay er of granular matrix six times a day. A dense three dimensional array of thermocouples was planted within the first of the three constructe d wetland cells in a series to allow for the assessment of thermal dat a at a high level of temporal and spatial resolution. Thermal data wer e 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 wint er thermal data was made to provide parameters for the use of the HEAT FLOW density-dependent ground water flow and thermal energy transport numerical model (Molson and Frind, 1995). The use of this coupled Darc y flux, thermal transport model has allowed for a better understanding of the importance of various thermal design considerations, and has a llowed for the undertaking of sensitivity analyses for design assessme nt and optimization. The sensitivity analyses indicate that the retent ion of deep soil heat and top insulation from plant thatch are the mos t important thermal features. It is likely that this technology can be used in areas colder than Niagara-On-The-Lake. (C) 1997 IAWQ.