Figure 1-8. Loss-of-head measurement.
(c) Advantages and disadvantages. The principal advantage of the
rapid sand fiIter is its economy of space in that it can process approximately twenty
times the amount of water that can be filtered by a slow sand fiIter with the same
surface area. Disadvantages are that pretreatment is required, that bacterial removal is
less than in the slow fiIter (90 to 98 percent as compared to 99 percent for the slow
fiIter), and that a skilled operator is required.
(3) Pressure fiIters. A pressure fiIter is essentially the same as a rapid sand
fiIter except that it is completely enclosed in a steel tank and the entire unit is operated
under pressure (see figure 1-9). The tank may be either vertical (as shown in the
illustration) or horizontal. Its characteristics and flow rate are the same as the rapid
sand fiIter except that .it operates under pressure instead of gravity. The pressure type
fiIter is usually employed in small installations or swimming pools where the cost of a
rapid sand fiIter plant would be prohibitive. It has the advantage of low initial cost and
permits the use of one pump (rather than two pumps) to raise the water from a low
elevation, through the filter, to a distribution system. Its main disadvantages are that the
operator is unable to observe the fiIter operations and that it is not equipped with wash
troughs.
MD0160
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