DISINFECTION by CHLORINATION

Disinfection is a process designed to kill harmful (pathogenic or disease causing) organisms. Sterilization is the complete destruction of all organisms in a water stream, which is not only unnecessary but it is also costly, and quite impractical. Fortunately, the pathogenic bacterial organisms are somewhat less resistant to chlorine and other disinfectants than most saprophytes, so that disinfection can be effected without the need for sterilization.

There are three important operational factors involved in disinfection with chlorine. They are:

The first is TIME/CONCENTRATION. These are two mutually related parameters. With long contact times, a low concentration of disinfectant suffices, whereas short contact times require higher concentrations to accomplish equivalent kills. The destruction of microorganisms, is directly related to these two mutually related parameters:
If time is represented by "t" and concentration is denoted by "C", then organism destruction is proportional to t x C or stated:
"Kill" <proportional> t x C

This means that if the time of contact with chlorine is decreased, then the concentration of the chlorine residual must be increased to ensure that the "kill" remains the same. It stands to reason that if the time of contact with chlorine is increased then the concentration of the chlorine residual may be decreased for a given "kill".

The pH of the water has been shown to be a disinfectant factor also. A lower pH, which favors the formation of HOCL over OCLG, is more effective for disinfection.

The final factor is that of temperature. The warmer the process water stream to be disinfected is, the faster the disinfecting action will be.

In Summary: A warmer, slightly acidic process stream, with a long detention time in the chlorination basins requires less chlorine to disinfect than a cold, basic process stream with a short detention time. In order to disinfect the basic (or alkaline), cold, short detention time stream properly, you must add larger amounts of chlorine.

Dosage/Demand/Residual: The chlorine applied (or added) at the mixing point is called the dosage. It is usually expressed in terms of mg/l or lbs/day of chlorine that is added. There is also a certain demand or usage of chlorine by impurities in the process stream that consume chlorine after it is added to the process stream. If there is any chlorine remaining in the process stream, this excess chlorine is called the residual, which does the disinfecting. The residual is the result of the demand amount being subtracted from the dosage amount. If your demand is greater than your dosage, you obviously need to increase your dosage till you exceed the demand, and produce a residual, so that disinfection may take place.

OPERATION
Fusible plugs on the cylinders or ton containers melt at 158 to 165 degrees F. Can withdraw up to 40 lb/day gas from 150 lb cylinder before freezing. Heat applied to a container of chlorine expands the gas inside, so containers are kept cool and in the shade. Chlorine gas added to water vapor (moisture) forms hydrochloric acid and hydrochlorous acids which cause corrosion and chlorination system components to fail. Check for leaks with ammonium hydroxide, which form white gas in contact with chlorine. Chlorine is 2.5 times heavier than air. It can expand from one part liquid chlorine to 450 parts gas chlorine. Prechlorination is the dosage of chlorine prior to any other treatment process; may increase THMs in raw water containing precursor compounds; used to control algae and microbial growths, mudball formation, reduce taste and odors, increase chlorine contact time for disinfection, and for improved coagulation. Breakpoint chlorination is the term applied to the point when the chlorine demand of the water has been met, and further additions of chlorine proportionally increases the chlorine residual. Free chlorine is not bound to other atoms; combined chlorine is used to describe chlorine bound to ammonia to form monochloramine, dichloramine, and trichloramine depending on the pH of the water and the ratio of chlorine to ammonia. Chloramines are a longer lasting residual, but not as effective as a free chlorine residual. Generally: spore-forming bacteria are more resistant than non-spore forming bacteria; viruses and cysts can be very resistant to certain types of disinfectants; the resistance varies greatly among the species and the disinfectant types, which is why disinfection alone can not provide reductions necessary for a safe product. Chloramines (especially dichloramine) cause taste and odor problems. THMs (trihalomethanes) are formed when three (tri-), halogen family (ie chlorine, fluorine, bromine) atoms are attached to an atom of methane. THMs are carcinogenic.

INTERACTION:
When the process water to be disinfected is very low in turbidity (less than 1.0 NTU) most of the bacteria has been removed, and is therefore easier to disinfect. Slight residual necessary to protect the potable water distribution system and prevent/reduce re-growth of microorganisms.

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