The same basic facility can be used for water treatment sludges and wastewater sludges. The area can be designed as either a series of underdrained beds, similar in detail to conventional sand drying beds, or deep, lined, and underdrained trenches. The Duluth, Minnesota, water treatment plant uses the trench concept (Schleppenbach, 1983). The sludges are pumped to the trenches on a routine basis throughout the year. Any supernatant is drawn off just prior to the onset of winter. After an initial ice layer has formed, sludge is pumped up from beneath the ice, spread in repeated layers on the ice surface, and allowed to freeze. The sand bed approach requires sludge storage elsewhere and application to the bed after the freezing season has begun.
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