If the thermal models for FWS wetlands predict sustained internal water temperature of less than 33.8°F (1°C), a wetland may not be physically capable of winter operations at the site under consideration at the design hydraulic residence time (HRT). Nitrogen removal is likely to be negligible at those temperatures. Similarly, if Equation 6.20 predicts a seasonal ice thickness greater than about 75%
of the design depth of a FWS wetland, the use of a wetland during the winter months may be questionable. It may be possible to increase the operating depth in these cases as long as the desired treatment results can still be achieved at <37.4°F (<3°C) and beneath an ice cover that will further impede oxygen transfer for nitrogen removal. Constructed wetlands can operate successfully during the winter in most of the northern temperate zone. The thermal models presented in this section should be used to verify the temperature assumptions made when the wetland is sized with the biological models for BOD or nitrogen removal. Several iterations of the calculation procedure may be necessary for the assumed and calculated temperatures to converge.
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