Installations should consider the following recommendations for good care of drying beds:
1. A coarse, 2.0-mm-effective-size sand should be used on the surface.
2. The sludge should be drawn slowly so that a hole is not pulled in the digester sludge blanket.
3. The sludge bed should be thoroughly cleaned, removing all small bits of dried sludge before the bed is re-flooded.
4. The bed (preferably 8 in in depth) should be rotated if possible.
A report from Birmingham, England, showed a 9 in depth to be optimum after depths from 6 to 18 in were tried. These researchers also found that only 11% of the volume of sludge left the beds through the underdrains while the rest evaporated (Bowers 1957).
For tropical and semitropical areas, sludge drying beds should be substantially smaller than those in moderate or cooler climates. Because sand beds give only four loadings per year in Winnipeg, Canada and these loadings must be during the summer season, they are impractical. Drained lagoons in the same location generate no obnoxious odors and produced efficient drainage and drying in an 18-month period when poured to a depth of about 3 ft (Bubbis 1953). Without underdrains, 24 months are required. Drainage rates increase with coarser types of drainage media although at a cost of less solids in the filtrate.
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