Domestic Cistern Disinfection
April 2000
Calculating
Water Storage Volume
Before
treatment, you should calculate the actual volume of water stored since useable
tank capacity is usually less than total tank volume.
Useable tank capacity can be estimated by multiplying the depth of the
water (in feet) by the length and width (also in feet) of the tank interior and
then multiplying by 7.48 to gallons.
Round tank capacity can be estimated by multiplying the length (in feet)
by the square of the tank radius, then by 3.14, then by 7.48 to get gallons.
Shock
Chlorination
Shock
chlorination means adding chlorine to reach the 10 to 50 ppm range.
The actual chlorine concentration needed depends on water chemistry, tank
characteristics, and degree of contamination. Generally,
shock chlorination is reserved for disinfecting new tanks or tanks with unknown
disinfection history, or for disinfection of heavily contaminated tanks.
The amount of 5.25% household bleach in 1000 gallons of water required to attain 10 ppm chlorine is about 32 fluid ounces. The contact time should be 24 hours.
Care should be exercised in disposing of heavily chlorinated water. It must not be allowed to enter a septic system or state waters. As an alternate, the water may be held in the tank until the chlorine has dissipated to less than 2 ppm at which time it can be available for consumption.Maintenance Chlorination
Maintenance chlorination means adding chlorine to reach the 2-5 ppm range during initial treatment with a target of 0.2 to 0.5 ppm in the finished water. The actual chlorine concentration needed depends on handling, water chemistry, and tank characteristics. Generally, maintenance chlorination is intended to control any bacteriological contaminants introduced to the tank during the transport and filling process.
The amount of 5.25% household bleach in 1000 gallons of water required to attain 2 ppm chlorine is about 6.5 fluid ounces. The actual dose required should be adjusted based residual testing of the finished water. Test strips may available from a chemical supply or a local pool supply (see YellowPages).
REFERENCES
Household Drinking Water Protection and Treatment. EB102. Montana State University Extension. Bozeman MT 1991
AWWA Standard for Disinfection of Water-Storage Facilities. ANSI/AWWA C652-92. Denver CO. 1992
Ground Water Manual for Small Water Systems. Montana Department of Environmental Quality. Helena MT. 1998