TM 5-814-3/AFM 88-11, Volume III
(1) Design factors. Area requirements can be interpreted in terms of the per capita values in table 16-6.
These values are very arbitrary and depend largely on climatic conditions. Embankment heights will be 12
to 14 inches, using concrete or concrete-block walls. Underdrains are to be provided with lateral tiles 12 feet
apart, and their transported leachate must be returned to the head of the treatment plant. An 8-18 inch bed
of gravel, ranging in size from 0.1 to 1.0 inches, is placed on the underdrains. The sand placed on the gravel
will have a depth of 18 inches, with the sand being washed and dirt-free. The sand will have an effective size
between 0.3 and 0.75 millimeters, with a uniformity coefficient of not more than 4.0. Sludge distribution can
be of various design, although an impervious splash plate of some kind is always provided. Sludge cake
removal can be by hand or mechanical means. Bed widths may range from 15 to 25 feet, with lengths of 50
to 150 feet. if polymers are added for conditioning, the bed length can be reduced to 50-75 feet to prevent
poor sludge distribution on the bed. Multiple beds provide operational flexibility and will be used if
appropriate. Enclosed beds will have sides no higher than 18 inches so as not to shade the sludge. Open sides,
forced ventilation and artificial heating are possible modifications. Usually, a combination of open and closed
beds performs best in average situations. Odor and insects can be a problem unless the sludge is digested
completely. Land requirements and sludge cake removal costs are other disadvantages.
c. Vacuum filtration. Vacuum filtration reduces sludge moisture content by applying a vacuum (10 to
25 inches mercury) through a sludge layer, using various equipment configurations. Vacuum filters can be
drum type, belt type, string discharge type or coil type. The use of coagulant pretreatment is necessary for
good dewatering efficiencies. FeCl3 is the coagulant aid most commonly used. Generally, the higher the feed
solids concentration, the higher the filtration rate and filter yield. Feed solids, however, will be limited to 8
to 10 percent to prevent difficulties in handling the sludge. Figure 16-3 shows typical vacuum filter
applications.
16-10
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