(a) Interpolate between 5 ppm and 10 ppm on the horizontal scale to
find 7 ppm.
(b) Read vertically to the curve.
(c) Read horizontally to the left, to a SCALE READING of 1.5 gal/hr.
This is the reading to be set on the calcium hypochlorite feed indicator (see figure 5-8).
b. Chlorine Demand Test. If the chlorine demand of the raw water is higher
than anticipated, the settings given in the TM wiII very Iikely result in a finished water,
which does not have a sufficient chlorine residual. This wiII require batch chlorination of
the finished water before it may be consumed or issued. The WPU operator should,
therefore, determine the chlorine demand of the raw water before beginning the erdlator
operation. Chlorine demand is determined as follows:
(1) Reagent. Prepare a stock solution, or reagent, by mixing a known
quantity of calcium hypochlorite (70 percent available chlorine) with a known quantity of
the best water available. A mixture of 7.14 grams of Ca(C10)2 in 1 Iiter (1,000 ml) of
water produces a 5,000 ppm chlorine solution (1 oz (wt) of Ca(CI0)2 in 4 Iiters of water
wiII also produce this solution).
(2) Test dosage. Add a test dosage of known strength to a 1-liter sample of
the raw water (5 ppm, or 1 ml of the reagent to 1,000 ml water, is normally used.)
(3. Chlorine residual test. Wait 30 minutes and run a chlorine residual test
using the color comparator (see para 4-10). Subtract the chlorine residual from the test
dosage to determine the chlorine demand.
(4) Validitv. If a measurable chlorine residual is not obtained after the 30-
minute test period, the test is invalid and must be repeated by increasing the reagent by
5 ppm each time untiI a chlorine residual is obtained. If, for example, the tests were
repeated two times, the results would be recorded as follows:
Test dosage, 2 x 5 ppm =
10.0 ppm C12
5.0 ppm CI2
5.0 ppm CI2
(5) Application. To determine the calcium hypochlorite dosage required,
use the same basic formula (dosage = demand + residual). If a residual of 1 ppm is
required, and the demand is 5 ppm (as in (4) above), the dosage to be applied is 6 ppm
chlorine. Since Ca(CIO)2 is only 70 percent available chlorine, the dosage of Ca(CI0)2
required is 6 = 8.6 (9) 0.7 ppm. Entering the Ca(CI0)2 calibration chart at a
concentration of 9 ppm, the required scale reading is 2.0. This is the setting for the
calcium hypochIorite soIution feeder.