Science of Water : Concepts and Applications

322 The Science of Water: Concepts and Applications
Operators should purchase hypochlorites in small quantities to ensure they are used while still
strong. Old chemicals should be discarded safely.
A chemical metering pump’s pumping rate is usually manually adjusted by varying the length
of the piston or diaphragm stroke. Once the stroke is set, the hypochlorinator feeds accurately at that
rate. However, chlorine measurements must be made occasionally at the beginning and end of the
well pump cycle to ensure correct dosage. A metering device may be used to vary the hypochlorinator
feed rate, synchronized with the water fl ow rate. Where a well pump is used, the hypochlorinator
is connected electrically with the pump’s on-off controls to assure that chlorine solution is not fed
into the pipe when the well is not pumping.
Determining Chlorine Dosage
Proper disinfection requires calculation of the amount of chlorine that must be added to the water
to produce the required dosage. The type of calculation used depends on the form of chlorine being
used.
The basic chlorination calculation used is the same one used for all chemical addition
calculations—the pounds formula. The pounds formula is
Poundsmg/L8.34MG where
pounds = pounds of available chlorine required
mg/L = desired concentration in milligrams per liter
8.24 = conversion factor
MG = millions of gallons of water to be treated
Example 9.3
Problem:
Calculate the number of pounds of gaseous chlorine needed to treat 250,000 gal of water with 1.2 mg/L
of chlorine.
Solution:
Pounds1.2 mg/L8.340.25 MG
2.5 lb
(9.12)
Note: Hypochlorites contain