(b) Maintenance. To clean the slow sand fiIter, the first step is to take
it out of service and allow the water to drain from it. The fiIter is then cleaned by
manually removing the top 3 or 4 inches of sand. The fiIter may be cleaned several
times before any sand is replaced; however, the depth of sand must never be less than
24 to 30 inches.
(c) Advantages and disadvantages. The principal advantage of the
slow sand fiIter is that, when properly operated, it removes amebic cysts and
approximately 99 percent of the bacteria. This removal is partly due to biochemical
action within the fiIter medium. Its action closely approximates natural purification. The
principal disadvantages of the slow sand fiIter are the large surface area required and
the time lost when the filter is taken out of service for cleaning.
(2) Rapid sand fiIters. The rapid sand fiIter is the most commonly used fiIter
today in municipal and garrison water treatment facilities. Rapid sand fiIters are
designed to fiIter about 2 gallons per minute (gpm) per square foot (ft2), which is roughly
equivalent to 25 square feet of fiIter surface for a 3,000 gph production rate. This is
about twenty times the capacity of the slow sand fiIter. Because of the increased rate of
flow, water must be pretreated before filtering.
(a) Construction. The construction of a rapid sand fiIter is as shown in
figure 1-7. The under drain system supports 18 to 24 inches of graded gravel. The
gravel is about 24 to 30 inches of fiIter medium. The sand used in a rapid sand fiIter is
somewhat coarser than that used in the slow sand fiIter (0.35-0.55 mm in the rapid fiIter,
as compared to 0.25-0.35 mm in the slow fiIter). The coarser sand permits a faster rate
of flow. The rapid sand fiIter has a provision for reversing the flow of water in the under
drain system for washing the fiIter, a feature not found in the slow filter.
(b). Maintenance. Cleaning of the rapid sand filter is necessary when
the loss of head is 7 to 9 feet or when the fiIter has been operated continuously for 100
hours. By "loss of head," we mean a differential in the pressure of the influent and the
effluent equivalent to the weight of 7 to 9 feet of water (3 to 4 pounds per square inch).
Loss of head is measured by a gauge that can be read directly in feet of water (see
figure 1-8). Washing is accomplished by taking the fiIter out of service, opening the
waste valve, and running clean water through the under drainage system in a reverse
direction at seven or eight times the rate of filtration. The clean water is forced upward
through the gravel and sand. The water causes expansion that loosens and agitates
the sand grains and washes accumulated floc and foreign matter off the grain surfaces.
The wash water rises into the wash troughs (see figure 1-7) and is discharged through
the sewer, carrying foreign matter with it. The filter is returned to service when the wash
water is free of foreign matter.