Some of the chlorine-consuming agents in the
water are nonpathogenic (nondisease-causing
organisms), but this bears no relationship to the
fact that they contribute to the total chlorine de-
mand of the water. Navy policy requires that for
field water supplies, the chlorine demand must be
satisfied and chlorine residual must be present.
Residual chlorine is the amount of unreacted
chlorine remaining at a specified time after the
chlorine compound is added. Chlorine in aqueous
solution is highly unstable. It may change
quantitatively and qualitatively under numerous
conditions, including the presence of other
elements or compounds. The total residual
chlorine in the water can be chemically divided
into several types.
. Total available residual chlorine. This is
the sum of the free available chlorine and the
combined available chlorine.
. Free available chlorine. This refers to
hypochlorous acid and hypochlorite ion present
in water. These are the most effective disinfec-
tion forms of chlorine. The free available chlorine
is a rapid-acting type, important because it can
be relied upon to destroy bacteria relatively
quickly, and thus is active during the period
immediately following chlorination. The relative
amount of each present in the water is dependent
upon the pH value of the water. It is important
to remember that when the pH is raised, the
quantity of free available chlorine required to kill
the same number of microorganisms increases.
With decreasing temperature, the same situation
of increasing dosage to maintain the same kill is
encountered. If the contact time is varied, then
the dosage applied must also be changed. For
example, to shorten the contact time the dosage
would have to be increased.
. Combined available chlorine. This results
from the presence of ammonia or organic nitrogen
that will react to form simple chloramines. Thus
the term combined available chlorine arises from
the fact that the chlorine has combined with
another substance. Chloramines are a slower
acting and less active form of disinfectant.
Therefore, a much higher concentration than that
of free available chlorine is needed to produce the
same germ-destroying effect. The specific
chloramines present are also a function of pH.
Chlorine demand in most water is likely to be
largely satisfied 10 minutes after chlorine is
added. After the first 10 minutes of chlorination,
disinfection continues but at a diminishing rate.
A standard period of 30 minutes contact time is
used to assure that highly resistant or high disease-
producing organisms have been destroyed, pro-
viding a high enough dosage has been applied.
Given a sufficiently large chlorine content, and
if certain other conditions are met, even such
special water purification problems as the presence
of amoebic cysts or schistosomes will be solved
with the 30-minute contact period.
The efficiency of the chemical disinfection
process is dependent upon numerous factors.
They include the type and concentration of
microorganisms, the pH and temperature of the
water, the presence of interfering substances, and
whether or not the organisms are protected from
the disinfection solution by being embedded in
tissue cells, or clumps of tissue cells, or other
material. Therefore, various concentrations of
disinfectants are required. Minimum concentra-
tions of disinfectants are prescribed below. Higher
concentrations may frequently be prescribed by
the medical officer on the basis of his knowledge
of endemic disease or local environmental con-
SEABEE-operated mobile and portable water
treatment units use coagulation and filtration as
a part of the treatment process. They are capable
of a high degree of removal of particulate
material. When those units are used, sufficient
chlorine will be added to the water, preferably
before coagulation, so the residual in the finished
water after 30 minutes of contact will be at least
as much as that indicated by the following table.
30-Minute Free Chlorine
Residuals in mg/1
If adequate provisions are not made for accurate
and frequent measurement of pH, 5.00 mg/1 must
be used. The following guidelines were used in
developing the above table:
. The water to be treated would be natural
surface or ground water of average composition
and not grossly or deliberately contaminated.