e. Prechlorination. Prechlorination is the practice of adding chlorine to water
prior to other treatment processes. Prechlorination has several advantages:
It may improve coagulation.
It will reduce tastes and odors caused by sludge in this dimentation tank.
(3) It helps to keep filter sand cleaner and increases the length of filter runs
by reducing algae and other organisms.
f. Postchlorination. Postchlorination is the practice of adding chlorine after
filtration. When combined with prechlorination, the process is referred to as double
chlorination. When prechlorination is practiced, postchlorination (double chlorination in
that case) is still recommended as a safety precaution against recontamination.
VARIABLES AFFECTING EFFICACY OF CHLORINE
a. Under normal operating conditions the only acceptable procedure for the
routine disinfection of water is chlorination. Six major variables have been shown to
affect the efficacy of chlorine disinfection. These are:
The types and concentrations of the chlorine forms present.
(2) The equilibrium relationships between coexistent chlorine forms. (This
factor is governed largely by the pH of the water.)
(3) The type and density of organisms (virus, protozoa, heiminth, or others)
The time of contact of the organisms with the chlorine.
(5) The temperature of the water. (This factor influences the rate of
reaction of chlorine compounds and the rate of kill of organisms.)
The concentration of substances exerting chlorine demand.
b. It is generally accepted that, of the waterborne diseases, those that are
caused by bacteria are the most susceptible to chlorine disinfection. On the other
extreme, certain pathogenic organisms such as the cysts of the protozoa E. hiystolytica
are the most resistant. Therefore, two parallel recommendations for chlorine residuals
are often made; the lower one sufficient for bactericidal purposes and the higher one for
cysticidal purposes. Available information suggests that cysticidal residuals are also
veridical. Bactericidal levels are routinely used for all water supplies at fixed
installations in CONUS, since waterborne bacteria are likely to be the most prevalent
organisms. Cysticidal levels should be instituted whenever epidemiological evidence