(7) The sample with the least number of FeC13 drops showing clear water
above the flocculent layer contains the optimum coagulant concentration for the water
(8) Each drop of FeCI3 solution is equivalent to 10 ppm in a 200 ml sample;
therefore, multiply the number of drops in the sample jar selected as having the best
settling rate by 10 to determine the optimum coagulant dosage in ppm. Determine the
solution feeder setting in the same manner as for Ca(CI0)2 described in paragraph 5-
10a(2) using the ferric chloride solution calibration chart.
(9) If, after (6) above, none of the jars has clear water above the flocculent
layer, repeat the test using concentrations of 4, 5, 6, and 7 drops, respectively, in step
(10) If the water quality control set is not available, the above test may be
improvised using as reagent a stock solution made of 2 grams of FeCI3 per Iiter (2,000
ppm) of distilied or demineralized water. Adding 1 ml of reagent to a 200 ml sample
thus represents a coagulation dosage of 10 ppm.
5-11. EVALUATION OF ERDLATOR OPERATION
The environmental health specialist should be able to observe a water
purification unit in operation and evaluate its operation. Many of the things he wiII look
for wiII be based upon common sense, such as cleanliness and orderliness. Other
important points wiII be based on a general knowledge of the principles of operation.
The following are some common operational checks.
a. Water Quality. The basic and essential criterion for evaluating the operation
of a WPU is the quality of the water.
(1) Chlorine residual and DH. The inspector should determine the FAC
residual and pH of the finished water in the storage tanks. If the FAC is less than the
values discussed in paragraph 2-7 (or as prescribed by competent medical authority),
additional calcium hypochlorite must be added manually as described in paragraph
2-10c (batch chlorination). A 30-minute contact time should be allowed before the water
(2) Color, taste, and odor. If the finished water has an objectionable color
(other than as described in (3), below), odor, or taste, activated carbon should be added
to the Iimestone compartment of the slurry feeder in the amount prescribed by the
(3) Appearance of coagulated water in erdlator. The appearance of the
coagulated water at the top of the erdlator tank (above the slurry blanket) is an
indication of whether chemicals are being fed at the proper rate.