Environmental Engineering Reference
In-Depth Information
Types of Coagulants
Two types of coagulants are used in the coagulation process: coagulants
and coagulant aids. Generally, the types of coagulants and aids available
are defined by the plant process scheme. To determine optimum chemical
dosages for treatment, jar tests are normally used. We discuss jar tests in the
following section.
DID YoU KNoW?
Selection of the proper coagulant and coagulant aid is critical for a
water supply. A jar test for water should be run to determine which
coagulants and flocculants aid are economical and most effective.
Jar Tests
Jar tests are widely used to simulate a full-scale coagulation and flocculation
process to determine optimum chemical dosages—the cost-effective dose of
a coagulant for the time and intensity of agitation selected. Such tests have
been used for many years by the water treatment industry; the test condi-
tions are intended to reflect the normal operation of a chemical treatment
facility and allow evaluation of the type and quantity of sludge and physical
properties of the floc. The test can be used to
• Select the most effective chemical.
• Select the optimum dosage.
• Determine the value of a locculant aid and the proper dose.
The testing procedure requires a series of samples to be placed in testing jars
and mixed at 100 rpm. Varying amounts of the process chemical or speci-
fied amounts of several flocculants are added (one volume/sample container).
The mix is continued for one minute. The mixing slows to 30 rpm to provide
gentle agitation, then the floc is allowed to settle. The flocculation period and
settling process are observed carefully to determine the floc strength, settle-
ability, and clarity of the supernatant liquor (the water that remains above the
settled floc). The supernatant can then be tested to determine the efficiency of
DID YoU KNoW?
Rapid mixing is the fast and thorough mixing—flash mixing—of vari-
ous chemicals, such as coagulants and coagulant aids, with water to
produce the chemical reactions required.
