disinfectant. This part of the residual is termed free available chlorine. Unless
otherwise stated, all chlorine residual values given in this text are in terms of free
available chlorine. Chlorine residuals required at water points or other spots where
chlorine is added to water are discussed with treatment processes described in
subsequent paragraphs. Chlorine residual equals the chlorine dosage minus the
(c) Chlorine demand is the amount of chlorine dosage that reacts with
and is consumed by organic material, bacteria, and other material in the water.
Chlorine demand varies with the nature and quantity of chlorine-consuming agents, the
pH value and temperature of the water, and the contact period. Some disease
organisms will be destroyed by the time chlorine demand is satisfied, but a moderately
stable chlorine residual should be permitted to act for an additional length of time to
effect satisfactory destruction of resistant organisms such as the virus of hepatitis, the
cysts of Entamoeba histolytic (the causative agent for amoebic dysentery or amebiasis),
and the larvae of schistosomes. Chlorine demand equals the difference between the
chlorine dosage and the chlorine residual.
(d) Disinfecting time is the time required for chlorine in a water to effect
satisfactory disinfection. Chlorine demand in most waters is virtually satisfied 10
minutes after the chlorine is added. When a satisfactory chlorine residual remains after
the 10-minute contact period, satisfactory disinfecting action usually results after an
additional 20 minutes of contact between water and chlorine. It is for this reason that,
for field chlorination, water having a satisfactory chlorine residual after 10 minutes is
allowed to stand an additional 20 minutes before being consumed.
(2) Calcium hypochlorite Ca(ClO)2 is the chemical most commonly used for
disinfection of water in the field. When calcium hypochlorite powder is dissolved in
water, it reacts with the water to form a disinfectant (hypochlorous acid, HClO) plus
other products. Only 70 percent of the Ca(ClO)2 is available for the formation of HClO;
therefore, when calcium hypochlorite is used in bulk, chlorine dosage imparted to a
water is computed on the basis of this available part, which is referred to as available
chlorine. For example, 1,000 grams of Ca(ClO)2 supplies a chlorine dosage of 700
Section III. QUARTERMASTER WATER TREATMENT
5-11. CONTINUOUS FLOW TYPE WATER PURIFICATION EQUIPMENT
Quartermaster water purification equipment (figure 5-2) is designed for continuous
purification of natural raw water by the solids contact method of water treatment.
Suspended solids contact units, sometimes called upward flow units, combine mixing,
flocculation, and clarification in the same structure.