Civil Engineering Reference
In-Depth Information
The sludge dewatering process occurs by two mechanisms:
Gravity drainage through the sludge cake and sand-filter
•
Air drying from the surface of the sludge cake by evaporation
•
Usually both processes must be functioning for the sludge to reach a condition in
which it may be removed from the drying bed for transport to a point of ultimate
disposal. The design of drying beds should consider sludge characteristics affecting
gravity drainage and air drying rates and the extent to which sludge may penetrate
into and through the sand bed during the initial drainage phase. Excessive penetration
requires frequent sand replacement and produces unacceptable direct filtrate discharge.
9
Organic polymer conditioning increases compressibility and reduces penetration.
Gravity drainage rates for water treatment sludges vary considerably with the nature
of the sludge, the extent of conditioning, and the applied depth. Generally, softening
sludges drain rapidly, iron-based coagulant sludges show intermediate drainage prop-
erties, and unconditioned alum sludges show relatively poor drainage characteristics.
The specific resistance of the sludge correlates well with the gravity drainage rates;
thus, physical or chemical conditioning significantly improves drainage characteristics
of poorly draining sludges.
9
Sand Drying Bed Operating Results.
Effective organic polymer conditioning substan-
tially decreases the time required for the gravity drainage phase of dewatering. For
example, King et al. found that for well-conditioned alum sludge the time can be
decreased by 50 to 70 percent.
48
For sludges with a low specific resistance, drainage
can be satisfactory at applied depths of 2 to 3 feet (0.6 to 0.9 m). For poorly draining
sludges, applied depths of 1 foot (0.3 m) or less are required, unless conditioning
agents are used.
Air drying is normally necessary for a drained sludge on a sand drying bed to reach
a state in which it can be removed. Although sludge drying rates vary through the
depth of sludge, with the top layers drying most rapidly, Novak and Langford found
that sufficient moisture is normally lost in air drying to render the entire cake handle-
able.
49
However, they also note that some sludges, especially those that have not been
conditioned, may form a dry surface crust that prevents further evaporation. Novak
and Langford believe that the completeness of drying throughout the sludge cake is
dependent on the completeness of drainage, particularly for well-conditioned sludges.
Neubauer reports that with a 5 mph (8.1 km / h) wind, temperatures of 69 to 81
F
(21 to 27
C), and humidities of 72 to 93 percent, solids concentrations of 20 percent
were achieved from an alum sludge in 70 to 100 hours, with 97 percent capture of
solids and a solids loading of 0.8 lb / sq ft (3.9 kg / m
2
).
45
Use of effective sand sizes
of 0.38, 0.50, and 0.66 mm made little difference in total drying time.
The sizing of drying beds should be based on the effective number of uses per year
that may be made of each bed and the depth of sludge that can be applied to the bed.
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