Environmental Engineering Reference
In-Depth Information
Factors Affecting Successful Chlorination
The factors important to successful chlorination are
• Concentration of free chlorine
• Contact time
• Temperature
• pH
• Tu r b id it y
The effectiveness of chlorination is directly related to the contact time with
and concentration of free available chlorine. At lower chlorine concentra-
tions, contact times must be increased. Maintaining a lower pH will also
increase the effectiveness of disinfection. The higher the temperature, the
faster the disinfection rate. Chlorine (or any other disinfectant for that mat-
ter) is effective only if it comes into contact with the organisms to be killed.
Good contact between chlorine and microorganisms is prevented whenever
high turbidity levels exist. For this and aesthetic reasons, turbidity should be
reduced where necessary through the coagulation and sedimentation meth-
ods previously discussed.
Chlorination Byproducts
A serious disadvantage of chlorination is the potential formation of byprod-
ucts. Chlorine, for example, can mix with the organic compounds in water
(such as decaying vegetation) to form trihalomethanes (THMs). One THM,
chloroform, is a suspected carcinogen. Other common trihalomethanes are
similar to chloroform and may cause cancer.
At the present time, on estimate, about 90% of US. water utilities use chlo-
rine to disinfect water. Although chlorine has virtually eliminated the risks
of waterborne disease such as typhoid fever, cholera, and dysentery, recent
studies have shown risks associated with byproducts of chlorine—a reason
why water utilities already have been looking at alternative methods for dis-
infecting water.
Several approaches for reducing harmful chlorination byproducts have
been used. For example, one approach is to remove more of the organics
before any chlorination takes place. This can be accomplished (to a degree)
by not chlorinating the incoming raw water before coagulation and filtration,
thus reducing the formation of THMs. Aeration or adsorption on activated
carbon will remove organic materials at higher concentrations or those not
removed by other techniques. Another approach is to reevaluate the amount
of chlorine used—the same degree of disinfection might be possible with
lower chlorine dosages. Changing the point in treatment where chlorine is
added is another approach commonly employed; rather than adding chlo-
rine as chemical feed during coagulation, sedimentation, or filtration, it can
