In very cold climates with prolonged winters, the thickness of the sludge layer is not critical; however, in more temperate regions, particularly those that experience alternating freeze-thaw periods, the layer thickness can be very important. Calculations by Equation 9.2 tend to converge on a 3-in. (8-cm) layer as a practical value for almost all locations where freezing conditions occur. At 23°F (-5°C), a 3-in. (8-cm) layer should freeze in about 3 days; at 30°F (-1°C) it would take about 2 weeks. A greater depth should be feasible in colder climates. Duluth, Minnesota, for example, successfully freezes sludges from a water treatment plant in 9-in. (23-cm) layers (Schleppenbach, 1983). It is suggested that a 3-in. (8-cm) depth may be used for feasibility assessment and preliminary designs. A larger increment may then be justified by a detailed evaluation during final design.
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