Finer sands result in shallower zones for the retention of suspended matter. The most desirable media size depends on the suspended solids characteristics as well as the effluent quality requirements and the specific filter design. In general, rapid sand filters use sand with an effective size of 0.35-0.60 mm (0.014-0.024 in.) and a maximum uniformity coefficient of 1.7. Coarse media, often 0.6-1.0 mm (0.024-0.04 in.), are used for closely controlled coagulation and sedimentation.
CAPPING OF SAND FILTERS
Rapid sand filters can be converted for mixed media operation to increase capacity by 100%. The cost of this conversion is much lower than that of installing additional rapid sand filters.
Capping involves the replacement of a portion of the sand with anthracite. In this conversion, a 2-6 in. layer of 0.4-0.6 mm (0.016-0.024 in.) sand is removed from the surface of a bed and replaced with 4-8 in. of 0.9 mm (0.035 in.) anthracite. If an increase in capacity is desired, a larger amount of sand is replaced. Pilot tests should be run to ensure that a reduction in the depth of the finer sand does not reduce the quality of the effluent.