Thermal Aspects

The temperature conditions in a wetland affect both the physical and biological activities in the system. In the extreme case, sustained low-temperature conditions and the resulting ice formation could result in physical failure of the wetland. The biological reactions responsible for BOD removal, nitrification, and denitri-fication are known to be temperature dependent (Benefield and Randall, 1980; Gearheart et al., 1989); however, in many cases the BOD removal performance of existing wetland systems in cold climates has not demonstrated clear temperature dependence. This is because the long hydraulic residence time provided by these systems tends to compensate for the lower reaction rates during the winter months. Several systems in Canada and the United States do demonstrate a decrease in nitrogen removal capability during the winter months. This is believed to be caused by a combination of temperature influence on the biological reactions and to a lack of oxygen when an ice cover forms on the water surface.

Temperature-dependent rate constants for the BOD and nitrogen removal models are presented elsewhere in this chapter. It is necessary here, then, to provide a reliable method for estimating the water temperature in the wetland for the proper and effective use of the biological design models. This section presents calculation techniques for the determination of the water temperature in SSF and FWS wetlands and for predicting the thickness of ice that might form on the FWS wetland.

Because the water surface is exposed to the atmosphere in a FWS wetland, some ice formation, at least on a temporary basis, is likely in northern locations that experience periods of subfreezing air temperatures. The presence of some ice can be a benefit in that the ice layer acts as a thermal barrier and slows the cooling rate of the water beneath. In ponds, lakes, and most rivers, the ice layer floats freely and can increase in thickness without significantly reducing the volume available for flow beneath the ice cover. In the case of the FWS wetland, the ice may be held in place by the numerous stems and leaves of the vegetation so the volume available for flow can be significantly reduced as the ice layer thickens. In the extreme case, the ice layer may thicken to the point where flow is constricted, the resulting stresses induced cause cracks in the ice, and flow may commence on top of the ice layer. Freezing of that surface flow will occur, and the wetland is then in a failure mode until warm weather returns. The biological treatment activity in the wetland will also cease at that point. This situation must be prevented or avoided if a constructed wetland is to be considered. In some locations that experience very long periods of very low air temperatures (<-20°C; <0°F), the solution may be to utilize a seasonal wetland component with waste-water stored in a lagoon during the extreme winter months. A number of systems in South Dakota and northwestern Canada operate in this mode (Bull, 1994; Dornbush, 1993). FWS constructed wetlands have, on the other hand, performed successfully throughout the winter months in Ontario, Canada, and in several communities in Iowa where extreme winter temperatures are also experienced. It is essential for each project in northern climates to conduct a thermal analysis, as described in this section, to ensure that the wetland will be physically stable during the winter months and can sustain water temperatures that allow the biological reactions to proceed.

The calculation procedure presented in this section was derived from Ashton (1986) with the assistance of Darryl Calkins (USA CRREL; Hanover, NH). The procedure has three parts:

1. Calculate water temperatures in the wetland until conditions that allow ice formation (3°C water temperature) commence. Separate calculations are required for densely vegetated wetland segments and for large area open-water zones.

2. Water temperature calculations are then continued for the ice-covered case.

3. An estimate is made of the total depth of ice that may form over the period of concern.

The temperatures determined during steps 1 and 2 are also used to determine the basic feasibility of a FWS wetland in the location under consideration and to verify the temperature assumptions made when sizing the wetland with either the BOD or nitrogen removal models. These BOD and nitrogen models are the first step in the design process because their results are necessary for determining the wetland size, HRT, and flow velocity to be used in the subsequent thermal calculations. The total depth of ice estimated in the third step above also provides an indication of the feasibility of a wetland in the location under consideration and is used to determine the necessary operating water depth during the winter months.

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