5-7.
SEDIMENTATION
The process of allowing suspended solids to settle out of the water without the
addition of chemical coagulants is called plain sedimentation. The turbidity of water
from swiftly flowing, silt laden streams can be reduced considerably by holding the
water in plain sedimentation tanks or basins. The rate of settling of the particles being
removed varies with their diameter and specific gravity. The larger particles settle out
quickly in still water, but small particles such as silt may require a period of days to
settle out. For complete treatment, the retention periods would be excessive.
Therefore, plain sedimentation is not ordinarily used by the Army as a separate step in
water purification.
5-8.
COAGULATION
a. Pretreatment. Certain chemicals, when added to water, cause the small
suspended particles to coagulate into larger ones which, because of their weight, settle
out more quickly. The partial clarification of water by coagulation prior to filtration is
known as pretreatment. Proper pretreatment reduces the amount of suspended matter
in water and is especially important for very turbid sources. Pretreatment prevents rapid
clogging of filters and ensures a filtered water with a uniformly low turbidity.
b. Flocculation. The coagulant can be added to water in tanks, such as the
standard 3,000-gallon tank, or it may be added continuously to water by mechanical
feeding devices. In either case, the water must be stirred after the chemicals have been
added. Initially, a "jellylike" substance appears in the form of small particles that give
the water a milky appearance. These small particles will not settle out. It is necessary
to continue stirring the water until the small particles unite to form larger particles that
will settle rapidly. The formation of the large jellylike particles, or "floc," through
agitation is called flocculation. A heavy, rapid-settling floc is desirable for efficient
treatment.
c. Optimum pH. The optimum pH value of water is that value at which the floc
is heaviest and forms fastest. This value, which is dependent on the water
characteristics, can be determined by the jar test. In this test, varying amounts of
coagulant and alkali are added to water in six jars. The samples are stirred and the
appearance of the floc, its settle ability, and the pH value of the water are noted.
Because the water quality fluctuates considerably, the pH value should be checked
frequently.
d. Color Removal. Vegetable dyes from decomposing vegetation cause colors
in water that can be removed by careful control of pH in the acid range in the
coagulation process. Activated carbon will also absorb and remove most true color
from water.
MD0008
5-9