Chlorination: Fresh or sea water can be chlorinated using
either chlorine gas or hypochlorites. Chlorinated water minimizes slime
development on working surfaces and helps control odour.
The main advantages of using chlorine gas are:
Hypochlorites are generally available in two forms - sodium
hypochlorite solution normally available at 10% concentration and
calcium hypochlorite available as a powder.
The main disadvantages of using hypochlorites are:
From the standpoint of disinfection, the most important form is free chlorine. Routine analysis always aims at determinin
Figure : CHLORINATION TREATMENT
· It is the most efficient method of making free chlorine available to raw water.The main disadvantages are:
· It lowers the pH of the water slightly.
· Control is simple; testing simple; and it is not an expensive method.
· Chlorine gas is toxic and can combine with other chemicals to form combustible and explosive materials. · Automatic control systems are expensive.Figure : PERCENTAGE OF AVAILABLE CHLORINE BY WEIGHT
· Chlorine cylinders may not be readily available at small centres.
· Chlorine expands rapidly on heating and hence the cylinders must have fusible plugs set at 70°C. It also reacts with water, releasing heat. Water should not therefore be sprayed on a leaking cylinder.
COMPOUND
|
CHEMICAL COMPOSITION
|
% CHLORINE BY WEIGHT
|
Chlorine gas |
Cl2
|
100.0
|
Monochloramine |
NH2Cl
|
138.0
|
Diochloramine |
NH4Cl2
|
165.0
|
Hypochlorous Acid |
HOCl
|
135.4
|
Calcium hypochlorite |
Ca(OCl2)
|
99.2
|
The main disadvantages of using hypochlorites are:
· Calcium hypochlorite is not stable and must be stored in air-tight drums.It is important to understand the manner in which chlorine or chlorine-releasing substances behave when added to water, depending on other substances present.
· Sodium hypochlorite is quite corrosive and cannot be stored in metal containers
· Sodium hypochlorite must be stored in light proof containers.
· It is difficult to control the rate of addition of hypochlorites in proportion to water flow.
· Hypochlorites raise the pH in water.
· They are more expensive than chlorine gas.
· When water contains reducing substances like ferrous salts or hydrogen sulphide, these will reduce part of the added chlorine to chloride ions. · When water contains ammonia, organic matter, bacteria and other substances capable of reacting with chlorine, the level of free chlorine will be reduced.When chlorine reacts chemically as in the first two cases, it loses its oxidising power and consequently its disinfecting properties. Some ammoniacal chlorides however still retain some disinfecting properties. Chlorine present in this form is termed residual combined chlorine or combined chlorine.
· If the quantity of chlorine added is sufficiently large to ensure that it is not all reduced or combined, a portion of it will remain free in the water. This is termed as residual free chlorine or free chlorine.
From the standpoint of disinfection, the most important form is free chlorine. Routine analysis always aims at determinin
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