per square foot. Pressure filters (fig. 9-17) have
the filter bed enclosed in a pressure vessel. Water
is either pumped into the vessel and forced
through the filter or is drawn into the vessel and
through the filter by a pump. The diatomite filter
is classified as a pressure filter.
Besides coagulation, sedimentation, and filtra-
tion, water must undergo an additional treatment
step; disinfection. This is necessary because no
combination of the other three steps can be relied
upon to remove all disease-producing organisms
from water; also because there is danger of recon-
tamination during handling before consumption.
Residual disinfection using chlorination is the final
step in all water treatment processes (including
distillation). Under emergency or field conditions,
water may be disinfected with iodine or by boiling.
The most satisfactory means of water disinfec-
tion and provision of a residual is by means of
a chemical disinfectant. The efficiency of the
disinfection process is dependent upon numerous
Figure 9-17.Pressure filter.
factors. These include the chemical used, the con-
tact time, the type and concentration of
microorganisms, the pH and temperature of the
water, the presence of interfering substances, and
the degree of protection afforded organisms from
the disinfecting solution by materials in which they
are imbedded. Therefore, various concentrations
of disinfectant are required depending upon the
local environmental conditions and the amount
of particle removal effected.
Chlorine is the most commonly used chemical
for disinfection of water. It is used in field water
supply in the form of calcium hypochlorite, a
standard item in the supply system (commercially
known as HTH powder). When the calcium hypo-
chlorite is dissolved, the chlorine goes into solu-
tion and a calcium carbonate sludge settles out.
The chlorine is present in the solution as
hypochlorous acid or hypochlorite ion (depending
on the pH). Both forms are powerful oxidizing
substances. The chlorine available in either form
rapidly oxidizes the organic and inorganic
matter, including the bacteria in the water. In this
reaction the chlorine is converted to chloride and
is no longer available as a disinfectant. The
organic matter as well as such material as iron and
manganese consumes the chlorine. The use of
chlorine makes it possible to introduce an
accurately measured dosage to ensure the destruc-
tion of disease-producing organisms and provide
a readily measured residual to safeguard against
recontamination during further handling.
Chlorine dosage is the amount of chlorine
added to water to satisfy the chlorine demand as
well as to provide a residual after a specified time.
The amount required to disinfect water varies with
the organic content and pH value of the water,
the temperature, the time of contact, and the
chlorine residual required. The dosage is usually
stated in terms of parts per million (ppm) or
milligrams per liter (mg/l).
The chlorine demand of water is the difference
between the quantity of chlorine applied in water
treatment and the total available residual chlorine
present at the end of a specified contact period.
The chlorine demand is dependent upon the
nature and the quantity of chlorine-consuming
agents present and the pH value and temperature
of the water. (High pH and low temperatures
retard disinfection by chlorination.) For com-
parative purposes, it is imperative that all test
conditions be stated. The smallest amount of
residual chlorine considered to be significant is
0.1 ppm. The relationship of the demand to the
length of the contact period is discussed below.