From Department of the Navy
- Manual of Naval Preventive Medicine:
Chapter 6: Water Supply Afloat:
Chlorine Dosage Calculator
Disinfection of Potable Water Hoses and Appurtenances
Disinfection of Water for Drinking and Cooling
Required Halogen Residuals
Chlorine readily combines with chemicals dissolved in water, microorganisms, small animals, plant material, tastes, odors, and colors. These components "use up" chlorine and comprise the chlorine demand of the treatment system. It is important to add sufficient chlorine to the water to meet the chlorine demand and provide residual disinfection. The chlorinated water should retain a slight chlorine taste and odour.
The chlorine that does not combine with other components in the water is free (residual) chlorine, and the breakpoint is the point at which free chlorine is available for continuous disinfection. An ideal system supplies free chlorine at a concentration of 0.3-0.5 mg/l. Simple test kits, most commonly the DPD calorimetric test kit (so called because diethyl phenylene diamine produces the color reaction), are available for testing breakpoint and chlorine residual in private systems. The kit must test free chlorine, not total chlorine.
The contact (retention) time in chlorination is that period between introduction
of the disinfectant and when the water is used. A long interaction between chlorine and the microorganisms results in an effective disinfection process. Contact time varies with chlorine concentration, the type of pathogens present, pH, and temperature of the water. Contact time must increase under conditions of low water temperature or high pH (alkalinity). Complete mixing of chlorine and water is necessary.
If a system does not allow adequate contact time with normal dosages of chlorine, superchlorination followed by dechlorination (chlorine removal) may be necessary.
To treat 100 Gallons of water (standard chlorination):
5.25% Sodium Hypochlorite - Household Bleach
Contact Time -vs- 5.25% Chlorine Concentration:
2 Hours - 100 ppm (about 3 Cups or 26 ounces Bleach for 100 Gallons Water)
8 Hours - 50 ppm (about 1.5 cup or 13 oz. per 100 gal.)
24 Hours 25 ppm (about 3/4 cup or 7 oz. for 100 gal.)
65% Granular Calcium Hypochlorite Powder - HTH, Perchloron, or Pittchlor
Contact Time -vs- 65% Chlorine Concentration:
2 Hrs - 100 ppm (2 ounces per 100 gal.)
8 Hrs - 50 ppm (1 oz per 100 gal.)
24 Hrs - 25 ppm (˝ oz. per 100 gal.)
Minimum Halogen Residuals:
Standard: 0.2 ppm chlorine after 30 minutes in tanks
Disinfection: 100 ppm chlorine initially, then 50 ppm after 4 hours in tanks
Superchlorination provides a chlorine residual of 10 times the recommended minimum breakpoint chlorine concentration. Retention time for superchlorination is approximately 5 minutes. Activated carbon filtration removes the high chlorine residual.
Shock chlorination, at about 4 times the recommended minimum breakpoint chlorine concentration, is recommended whenever a tank is new, repaired, or found to be contaminated. This treatment introduces high levels of chlorine to the water. Unlike superchlorination, shock chlorination is a "one time only" occurrence, and chlorine is depleted as water flows through the system; activated carbon treatment is not required. If bacteriological problems persist following shock chlorination, a continuous chlorination system should be used.
Potable water tanks, pumps, hoses, & fittings are disinfected by filling with a solution containing not less than 100 ppm FAC. The solution must be in contact with the entire internal surface of the system for not less than two minutes. Flush the system 30 to 60 seconds with potable water prior to use.