Table 7-2.Types of Odors Commonly Found in Water
Table 7-3.Dilution Series for Determining the Threshold
Sulfuretted (rotten egg)
approximate odor range by sniffing the undiluted sample,
thereby eliminating the preliminary test.
10. Based on the results obtained in the preliminary
test, prepare a set of dilutions and use the amount of the
sample diluted with odor-free water in the range
corresponding to the lowest dilution in which the odor
was detected. For example, if odor was detected in the
63-ml dilution, but not in the 16-ml dilution, use series
II in table 7-3.
11. Repeat steps 5 through 9. The threshold number is
read from table 7-4.
JAR TEST (COAGULATION)
The jar test is a reliable method for determining the
proper chemical dosages and conditions for coagulation
of water to remove color and turbidity.
The type of chemicals that should be used for
coagulating raw water can be determined by using the
results from jar tests, plant tests, or by using the data
shown in table 7-5. Theoretically table 7-5 is correct;
however, these values can be misleading when applied
to some types of raw water. The chemical content of
water may have a considerable influence on the
optimum pH range for the various coagulants. For
example, coagulation with ferrous sulfate is usually best
accomplished at relatively high pH values in the
alkaline zone. With soft, colored waters, ferric
coagulants may sometimes be used with considerable
success at pH values of 4.0 or less. Because of this wide
variation in the optimum pH range of coagulants
(caused by individual characteristics of the raw water),
the coagulant dosage and the optimum zone for floc
formation should be determined by jar tests, rather than
just relying on table 7-5.
Amount of sample
diluted to 250 ml . . . . . . . . .
The jar test is the most common method of
determining proper coagulant dosages. When there is a
question as to which chemical should be used as a
coagulant, it is often necessary to run more than one
series of jar tests. Different coagulant chemicals and pH
ranges should be used to determine which one produces
the most satisfactory results at the lowest cost. The step-
by-step procedures for a jar test are as follows:
1. Prepare a standard solution of each coagulant
selected for trial by adding 10 grams of coagulant to 1 liter
of distilled water.
2. Correct the pH of a sample of raw water to
within the optimum range for the coagulant being tested
(only if the pH is to be adjusted to the same extent in
actual plant operation). Divide the raw water into six 1-
Table 7-4.Threshold Odor Numbers
Amount of sample
diluted to 250 ml