It is recommended that the drained wastewater sludges be removed each year. Inert chemical sludges from water treatment and industrial operations can remain in place for several years. In these cases, a trench 7 to 10 ft (2 to 3 m) deep can be constructed, so the dried solids residue remains on the bottom. In addition to new construction, the sludge freezing concept can allow the use of existing conventional sand beds, which are not now used in the winter months.
A community near Pittsburgh, Pennsylvania, is considering freezing as the dew-atering method for their estimated annual wastewater sludge production of 0.4 million gallons (1500 m3, 7% solids). Maximum frost penetration (from Table 9.8) is 38 in. (97 cm).
1. Use Equation 9.4 to determine potential design depth of frozen sludge:
2. Then, determine the bed area required for freezing:
1500 m3 2
This area could be provided by 16 freezing beds, each 7 m by 20 m. Allow 30 cm for freeboard. Constructed depth = 0.70 + 0.30 = 1.0 m.
3. Determine the time required to thaw the 0.70-m sludge layer, if average temperatures are 10°C in March, 17°C in April, and 21°C in May. Use Equation 9.3 with a sludge depth of 70 cm:
AT ■ t = (31)(10) + (30)(17) + (17)(21) = 1177°C-d
Therefore, the sludge layer should be completely thawed by May 18 under the assumed conditions.
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