Civil Engineering Reference
In-Depth Information
129
Width
0.45 ft
6 in (150 mm)
8
36
1
The outer paddle is positioned at a radius of 5
1
⁄
2
feet (1.68 m) to the outside edge and
the inner paddle at a radius of 3
3
⁄
4
feet (1.14 m) to the outside edge.
COAGULATION CONTROL AND MONITORING
Techniques Available
Since achieving proper coagulation has been a universal problem for water treatment
operators for many years, a wide variety of techniques have been developed for con-
trolling the coagulation process. Most of these involve in-line water quality analysis
and laboratory tests, the results of which then must be manually transferred to the full-
scale plant operation by the plant operator.
Whether the coagulant dose that provides the optimum result in the laboratory tests
will also provide optimum results on a plant scale depends on whether the same
efficiency of mixing is achieved in both cases, which is unlikely. Also, the fact that
the laboratory testing is a batch procedure results in an inherent time lag in responding
to changes in raw-water conditions. This lag may be only an hour if the operator is
on duty and alert to raw-water conditions, but it may be several hours if the operator
is off duty or involved in another task, such as equipment maintenance. This method
may give satisfactory—even if not optimum—results when applied to a raw water of
relatively uniform quality that contains only a moderate amount of organic turbidity.
For low-turbidity waters and waters containing large amounts of organic material, the
optimum coagulation dosages for particulate removal may be difficult to determine.
However, for waters that have high organic material (high TOC), higher coagulant
dosage is usually required to achieve adequate TOC removal, leaving the particulate
a lesser concern. With today's advanced in-line and laboratory particle and TOC mea-
suring equipment, the process monitoring and control can be easily achieved compared
to a decade ago.
Available coagulation control techniques fall into three general categories: conven-
tional and modified jar tests (where supernatant quality, such as turbidity, floc for-
mation time, and floc density, are analyzed); techniques based on particle charge; and
techniques based upon the turbidity of filtrate using a pilot-scale filter column.
Jar Tests
Selection of the best coagulant is most often based on jar tests. Detailed procedures
for conducting jar tests are described in AWWA Manual M37.
59
Also, a standard jar
test procedure is described in AWWA Manual M12, titled
Simplified Procedures for
Water Examination
. A standard testing plan is required to provide a meaningful com-
parison of coagulants or coagulation aids. The procedure basically consists of adding
varying coagulant dosages to several water samples contained in beakers, mixing them
simultaneously at the same speed with a gang mixer, allowing them to settle, and
measure the settled water quality, such as turbidity and color.
The jar tests defined by Manual M12 are intended to identify the chemical treatment
characteristics that would most benefit the full-scale plant. Evaluating the many chem-
