Section II. WATER TREATMENT PROCESSES
The purpose of water treatment is to remove or destroy enough of the impurities
in raw water to make it potable and palatable. The amount and type of treatment
required, in a garrison or field environment, depends upon the quality of the raw water,
the amount of water required, and the standards that must be met. A series of
purification processes is generally employed. Complete water purification includes the
following processes: coagulation, sedimentation, filtration, reverse osmosis (when
required), and disinfection. In practice, the processes of coagulation and sedimentation
are usually combined, so we wiII discuss these processes together. Reverse osmosis
is required in the field when water is brackish or has salt (the process may also be used
for fresh water purification). Disinfection is covered separately in Lesson 2.
Water pretreatment is the partial clarification of water by coagulation and/or
sedimentation done prior to filtration. Pretreatment prevents rapid clogging of filters and
helps to ensure the production of filtered water with uniformly low turbidity.
a. Plain Sedimentation. Plain sedimentation is the natural settling of solids that
are heavier than water. This sedimentation is caused by the force of gravity. Solids
heavier than water are held in suspension in moving water, but they gradually settle to
the bottom as the water velocity is reduced. The time required to clarify water by plain
sedimentation depends upon the following factors:
(1) Size and specific gravity of the suspended particles. Large and heavy
particles settle in a few minutes once the water has become stiII, whereas very small
particles, such as clay and silt, may remain in suspension for several days.
(2) Turbulence of the water. Particles suspended in a rapidly moving
stream will remain suspended much longer than the ones in a lake or a pond that is
(3) Temperature of the water. The viscosity of water is affected by its
temperature. The lower the water's temperature, the greater is the viscosity.
(Remember viscosity is the property of a fluid that resists internal flow.) Consider 30-
weight motor oil. Motor oil of this weight readily flows (that is, lower viscosity) at higher
temperatures but difficult to pour (that is, has higher viscosity) at lower temperatures.
The viscosity of water at temperatures above the freezing point is so slight and not
noticeable to the touch. The effect on sedimentation is also slight. At temperatures
below 45F, however, the increased viscosity of water is sufficient to retard the rate of
sedimentation of suspended solids. Therefore, sedimentation is more efficient at higher