Chlorination:

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:

· It is the most efficient method of making free chlorine available to raw water.
· It lowers the pH of the water slightly.
· Control is simple; testing simple; and it is not an expensive method.

The main disadvantages are:

· Chlorine gas is toxic and can combine with other chemicals to form combustible and explosive materials. · Automatic control systems are expensive.
· 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.

Figure : PERCENTAGE OF AVAILABLE CHLORINE BY WEIGHT

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

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:

· Calcium hypochlorite is not stable and must be stored in air-tight drums.
· 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.

It is important to understand the manner in which chlorine or chlorine-releasing substances behave when added to water, depending on other substances present.

· 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.
· 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.

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.
From the standpoint of disinfection, the most important form is free chlorine. Routine analysis always aims at determinin