b. Use of Plain Sedimentation. Plain sedimentation is not ordinarily used by
the miIitary as a separate step in water treatment because the long period required for
complete settling would call for an impractical number of settling tanks. However, in an
emergency, such as the necessity of taking water that is heavily laden with silt from a
swiftly flowing stream, special sedimentation tanks may be set up as a first step. This
initial removal of turbidity reduces the load on subsequent treatment processes. Under
normal conditions, plain sedimentation can be expected to remove about 55 percent of
the nonfiIterable residue and about 25 percent of the bacteria. Plain sedimentation is
always followed by subsequent treatment of some form. The treatment may include
chemically assisted sedimentation and/or filtration, but it always includes disinfection.
In field water treatment, plain sedimentation and disinfection may be the only treatment
given to water. In such cases, the raw water is allowed to stand in water cans, Lyster
bags, or improvised tanks until the turbidity is reduced by sedimentation. The clear
water is then drawn from the top and disinfected before use.
c. Chemically Assisted Sedimentation. Chemically assisted sedimentation
incorporates three separate treatment processes: coagulation, flocculation, and
sedimentation. Chemically assisted sedimentation can normally be expected to remove
about 80 percent of the nonfiItrable residue, 50 percent of the bacteria, 75 percent of
the color, and 25 percent of the taste. Compared to plain sedimentation, this
clarification takes place in a relatively short time. Optimum results are usually obtained
in about 30 to 40 minutes.
(1) Coagulation. Coagulation is the process of adding chemicals to water,
rapidly mixing the solution, and causing the particles to cluster and settle out. The
clustering of these particles causes them to increase in weight and settle to the bottom.
Various chemicals, but most notably, ferric chloride and aluminum sulfate, are used and
mentioned in paragraph 1-11.
(2) Flocculation. After coagulation, particles are assisted to cluster faster by
the addition of chemicals that create an insoluble, jelly-Iike substance called floc. The
floc/sticky or gelatinous solution entraps fine turbidity particles, which contain bacteria
and other impurities, causes them to stick together, and settle as heavy clumps. Floc
particles settle more rapidly and absorbs color. This process is called flocculation.
(3) Sedimentation. Sedimentation is the settling of the floc by gravity after
the movement of the water has been retarded. It is essentially the same as plain
sedimentation, but much faster.
1-11. CHEMICALS USED FOR COAGULATION
Several different chemicals may be used in coagulation, depending upon the
characteristics of the water being treated. In some waters, combinations of two or more
chemicals produce better results than any one chemical alone. In order to determine
which chemical or combination and how much of it should be used, it is usually
necessary to perform coagulation tests in the laboratory (see para 5-10). The pH of the