The process of removing suspended matter from water by passing the water
through a layer of diatomaceous silica is known as diatomite filtration. The effluent
obtained by diatomite filtration of pretreated water has a very low turbidity and is free of
amoebic cysts and cercariae of schistosomes. Over 90 percent of all bacteria are
removed by diatomite filtration without chlorination. Even better results may be
obtained with careful operation and decreased rate of flow.
a. General. Water must be disinfected after undergoing treatment, because no
purification process or combination of processes will reliably remove all disease-
producing organisms from water. There is also danger of contamination during
rehandling and transportation before consumption. Disinfection, the process of
destroying disease agents, may be accomplished by adding chemicals such as chlorine
gas, chlorine solutions, iodine, or by boiling. Freezing does not disinfect water or make
the ice safe. Addition of coffee, tea, or beverage powders does not make water potable.
Chlorination is the most commonly used method of disinfection.
(1) Chlorine added to water becomes bound in chemical reactions with
chlorine-consuming substances (chlorine "demand") in the water. In the process,
pathogens are killed. The chlorination method of disinfection involves putting in enough
chlorine to satisfy this chlorine demand plus a small amount in excess. The excess
assures a proper kill of pathogens in the water and acts on contaminants that may enter
the water later. The ratio of chlorine to water is expressed as parts per million (ppm)--
number of units of chlorine in one million parts of water by weight--or as milligrams per
liter (mg/l)--(since a liter of water weighs one million milligrams).
(a) Chlorine dosage is the amount of chlorine added to a given quantity
of water. This amount should be sufficient to satisfy the chlorine demand as well as to
provide a residual after a specified time.
(b) Chlorine residual, or residual chlorine, is the chlorine that remains
in the water after the demand of all chlorine-consuming agents has been satisfied. A
chlorine residual of five parts per million at the point of water consumption is the
standard requirement in waters treated under field conditions, especially overseas.
Based upon a specific situation, the staff surgeon may dictate a higher residual. Only a
part of the total chlorine residual in water is in a form that will act effectively as a