MIL-HDBK-1005/9A
The top 3 inches (7.5 cm) of gravel should consist of
1/8 to 1/4 inch (0.3 to 0.6 cm) gravel. The gravel should extend
at least 6 inches (15 cm) above the top of the underdrains.
Drying beds have underdrains that are spaced from 10 to 26 feet (3
to 8 m) apart. Underdrain piping is often vitrified clay laid
with open joints, has a minimum diameter of 10 cm, and has a
minimum slope of about 1 percent. Collected filtrate is returned
to the treatment plant. Bed walls should be watertight and extend
15 to 18 inches (38.1 to 45.7 cm) above and at least 6 inches (15
cm) below the surface. Outer walls should be curved to prevent
soil from washing onto the beds. Pairs of concrete truck tracks
at 20-foot (6.2-m) centers should be provided for beds. The
influent pipe should terminate at least 12 inches (30 cm) above
the surface with concrete splash plates provided at discharge
points.
A recent trend in handling sludge in drying beds is the
increased use of mechanical lifting equipment for sludge handling.
Chemical conditioning greatly aids the dewatering process. In
many reported instances, flocculent chemicals have overcome
problems in drying beds.
c) Covers. Sand beds cab be enclosed by glass or other
material where justifiable to protect the drying sludge from rain,
to control odors and insects, and to reduce the drying periods
during cold weather. Good ventilation is important to control
humidity and optimize the evaporation rate. As expected,
evaporation occurs rapidly in warm, dry weather. Adaption of
sludge removal and handling equipment to enclosed beds is more
difficult than to open drying beds.
3.10.8.2 Drying Lagoons. Drying lagoons are technically and
operationally simple for sludge dewatering. The cost factor
depends on land availability. Since there are a fair number of
applicable regulations (such as mandatory monitoring of ground
water) to be taken into consideration, there is a higher risk
involved in choosing this method.
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