Page 272 - Water and wastewater engineering

COAGULATION AND FLOCCULATION 6-49

Comments:
1. Because G is tapered, n must be calculated for each of the following chambers as well as
the first chamber.
2. A motor power of (1.5)(357 W) 535.5 or 540 W or larger is recommended.
3. Add 0.60 m to depth of tank for freeboard.

6-8 OPERATION AND MAINTENANCE
The most important operation and maintenance (O&M) task in coagulation is the selection of
the appropriate chemicals and the adjustment of the dose to ever-changing raw water quality and
plant flow. Frequent jar testing is the standard technique for adjusting the chemical dose. Because
of the time delay in conducting the test, other techniques such as zeta potential measurements and
the use of a streaming current detector (SCD) have been used to augment the jar test. None of
these relieve the operator of the necessity of a significant amount of attention.
Monitoring of the chemical feed system to detect clogging of the lines and maintenance of
the mixers ranks second in the need for close O&M oversight. Appropriate mixing energy is an
important part of optimization of the chemical dose. Excess dosing with coagulant to compensate
for inefficient mixing not only is uneconomical in terms of chemical usage, it is expensive in
terms of sludge production.

Hints from the Field. Suggestions from operators include the following:
• All of the design calculations in this chapter were based on the design flow, that is, the maxi-
mum daily flow rate at the end of the design life. Because the minimum flow rate at start-up
will probably be considerably less than the design flow, operational problems may be severe.
It is highly recommended that the design be evaluated at minimum flow conditions.

• Most drive failures are caused when the unit is started at the top rotational speed. O&M
manuals should note that mixers should be started at low speed to avoid very high torque
force and a high power requirement.

Visit the text website at www.mhprofessional.com/wwe for supplementary materials
and a gallery of additional photos.

6-9 CHAPTER REVIEW

When you have completed studying this chapter, you should be able to do the following without
the aid of your textbooks or notes:
1. Explain what NOM is and why it may be one of the goals of coagulation/flocculation to
remove it from drinking water.
2. Differentiate between the terms coagulation and flocculation.
3. Explain how colloidal particles become negatively charged.