Figure 1-7. Typical sand filter.
(2) Adsorption. Suspended particles adhere to the grains of the fiIter
medium and cover the surfaces with a sticky coating.
(3) Sedimentation. Void spaces between individual grains of fiIter medium
act as tiny sedimentation chambers. Small-suspended particles settle out caused by
(4) Biological action. Depending on the rate of flow through the fiIter, Iimited
bacterial action may occur within the filtration mat. This action destroys bacteria and
other microorganisms by oxidation.
b. Types of FiIters. FiIters used in municipal or fixed military installation water
treatment facilities include slow sand fiIters, rapid sand fiIters, pressure fiIters, and
(1) Slow sand fiIters. Slow sand filters are no longer being constructed
because more-efficient methods have been developed. However, there are stiII many
slow filtration plants in existence. Slow sand fiIters are usually used when coagulation
is not included in the treatment process. Their capacity is about 2 to 10 million gallons
per day (gpd) per acre. This rate of fiItration requires about 500 square feet of fiIter
surface to maintain a water production rate of 3,000 gallons per hour (gph), the capacity
of the Army's largest standard mobile, field water purification unit.
(a) Construction. The slow sand filter is constructed as shown in
figure 1-7, except that it does not have wash troughs. It consists of about 12 to 18
inches of graded gravel (the largest sizes on the bottom) covered by about three feet of
sand. The cover of the structure should be at least six feet above the surface of the
sand to provide for an adequate depth of water over the sand and to allow sufficient
head room for cleaning.