the pressure fiIter may be found. Crushed anthracite (hard coal) is sometimes used to
replace the sand in a rapid sand fiIter, or a layer of fine anthracite may be placed as a
cover for a sand bed. Graded anthracite may also be used to replace the gravel.
Among the advantages claimed for anthracite are lower head loss, less wash water
needed, a lower rate of application of wash water, a higher rate of fiItration, and lower
filter runs. Chlorination always follows filtration.
c. Diatomite Filtration. Diatomite fiItration is not often found as the primary
means of fiItration in municipal water supply systems. They are, however, frequently
found where (1) auxiIiary means are needed to meet seasonal demands, (2) temporary
faciIities are needed whiIe permanent faciIities are being buiIt; or (3) treatment by
fiItration is considered necessary, but chemical coagulation is not warranted. In
municipal use, diatomite fiItration is not generally used in conjunction with chemical
pretreatment. Diatomite fiItration is always followed by chlorination.
1-17. FIELD WATER TREATMENT UNITS
Field water treatment differs from municipal or fixed installation water treatment
in that the equipment used must be portable so that it can be rapidly moved and set up
to support tactical operations. It must also be capable of treating water from a poorer
source than is generally available to a fixed installation or a municipality. Accordingly,
field water purification equipment usually incorporates both chemical pretreatment and
diatomite fiItration. Although field water treatment is discussed in Lesson 5, the
equipment w II be described here in general terms
a. Erdlator. The term "erdlator" is a nickname for the standard Army field water
purification unit that was coined by its developers (Engineer Research and
Development Laboratories). The erdlator may only be used to purify fresh water.
(1) Principles of operation. All of the erdlators incorporate the following
(a) A continuous flow solids contact process utilizing an upflow clarifier,
are commonly referred to as the erdlator tank. In continuous flow equipment, water
enters, is treated, and leaves at a constant rate of flow. This flow can be maintained for
24 hours a day, if necessary.
(b) Chemical pretreatment combined with disinfection, using ferric
chloride as a coagulant, pulverized Iimestone as a coagulant aid, and calcium
hypochlorites as a disinfectant.
(c) With the exception of the 10,000--gph WPU (see para (2) (f),
below), fiItration of coagulated water from the erdlator is done by means of diatomite