Rotating-drum vacuum filters are the conventional sludge dewatering equipment. The filtrate is returned to the plant influent, and the sludge cake is disposed. Sludge drying beds are common in smaller plants for disposal. Weather conditions are an important factor when open drying beds are used. Some wastewater treatment facilities use glass-covered drying beds to exclude precipitation. Drying beds have underdrain networks of tile fields laid in sand and gravel. The discharge from this network should be disinfected when it is not returned to the treatment plant. Sludge treatment and disposal are usually performed in separate treatment plants (or plant sections). Figure 7.21.14 shows a typical sludge treatment plant.
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