Science of Water : Concepts and Applications

Water Treatment 299
CHEMICAL FEEDER CALIBRATION
Chemical feeder calibration ensures effective control of the treatment process. Obviously, chemical
feed without some type of metering and accounting of chemical used adversely affects the water
treatment process. Chemical feeder calibration also optimizes economy of operation; it ensures
the optimum use of expensive chemicals. Finally, operators must have accurate knowledge of each
individual feeder’s capabilities at specifi c settings. When a certain dose must be administered, the
operator must rely on the feeder to feed the correct amount of chemical. Proper calibration ensures
chemical dosages can be set with confi dence.
At a minimum, chemical feeders must be calibrated on an annual basis. During operation, when
the operator changes chemical strength or chemical purity or makes any adjustment to the feeder,
or when the treated water fl ow changes, the chemical feeder should be calibrated. Ideally, any time
maintenance is performed on chemical feed equipment, calibration should be performed.
What factors affect chemical feeder calibration (feed rate)? For solution feeders, calibration
is affected any time solution strength changes, any time a mechanical change is introduced in the
pump (change in stroke length or stroke frequency), or whenever fl ow rate changes. In the dry chemical
feeder, calibration is affected any time chemical purity changes, mechanical damage occurs
(e.g., belt change), or whenever fl ow rate changes.
In the calibration process, calibration charts are usually used or made up to fi t the calibration
equipment. The calibration chart is also affected by certain factors, including change in chemical,
change in fl ow rate of water being treated, and a mechanical change in the feeder.
Example 9.2
To demonstrate that performing the chemical feed procedure is not necessarily as simple as opening
a bag of chemicals and dumping the contents into the feed system, a real-world example is provided
below.
Problem:
Consider the chlorination dosage rates below:
Setting (%) Dosage (mg/L)
100 111/121 0.93
70 78/121 0.66
50 54/121 0.45
20 20/121 0.16
Solution:
This is not a good dosage setup for a chlorination system. Maintenance of a chlorine residual at the ends
of the distribution system should be within 0.5–1.0 ppm. At 0.9 ppm, dosage will probably result in this
range—depending on the chlorine demand of the raw water and detention time in the system. However,
the pump is set at its highest setting. We have room to decrease the dosage, but no ability to increase
the dosage without changing the solution strength in the solution tank. In this example, doubling the
solution strength to 1% provides the ideal solution, resulting in the following chart changes.
Setting (%) Dosage (mg/L)
100 222/121 1.86
70 154/121 1.32
50 108/121 0.90
20 40/121 0.32