Disinfectant

Chlorine (CL) gas is a greenish yellow in colour and about 2.5 times heavier than air. Liquid chlorine is amber coloured and about 1.44 times heavier than water. CL gas is moderately soluble in water with a maximum solubility of about 1 % at 10°c. When chlorine is added to water, it first reacts with any iron, manganese or hydrogen sulphide that may be present in water. Then Cl will react with organic substances (including bacteria). Some of these reactions, which consume chlorine, are able to kill microorganisms in the water. The chlorine will react with organic compounds in water and form trihalomethanes (THM). It will also react with reducing agents such as hydrogen sulphide, ferrous ions, manganese ions, and nitrite ions. Chlorine may be added to water in the form of chlorine gas, hypochlorite, or chlorine dioxide. All types of chlorine will kill bacteria and some viruses, but only chlorine dioxide will effectively kill Cryptosporidium, Giardia, protozoans, and some viruses. Chlorine gas is compressed into a liquid and stored in metal cylinders. The gas is difficult to handle since it is toxic, heavy, corrosive, and an irritant. At high concentrations, chlorine gas can even be fatal.

The reactions depend on the type of chlorine is added to the water ( Water Treatment by Chlorine ) as well as on the pH of the water itself. Each of the above reactions uses up the chlorine in the water and produces either chloride ions or hydrochloric acid, which have no disinfecting properties. The total amount of chlorine which is used up in reactions with compounds and destroying pathogens in the water is known as the chlorine demand. A sufficient quantity of chlorine must be added to the water so that, after the chlorine demand is met, there is still some chlorine left as free residual chlorine to take care of any further contamination.

 Reactions of Chlorine with Water

When chlorine gas enters the water, the following reaction occurs:

The chlorine reacts with water and breaks down into hypochlorous acid and hydrochloric acid.

Chlorine + Water Hypochlorous Acid + Hydrochloric Acid

Cl₂ + H₂O  –> HOCl + HCl

Hypochlorous acid may further break down, depending on pH:

Hypochlorous Acid <-> Hydrogen Ion + Hypochlorite Ion

HOCI                       <-> H⁺ +           OCl^–

The double-sided arrows in the equations mean that the reaction is reversible. Hypochlorous (HOCL) acid may break down into a hydrogen ion and a hypochlorite (OCL) ion, or a hydrogen ion and a hypochlorite ion may join together to form hypochlorous acid.

Advantages and Disadvantages of Chlorine

Advantages of Chlorine as disinfectant are:

• It can be easily stored for long time without any risks of it getting deteriorated.
• It is highly soluble in water.
• It is quite inexpensive compared with other chemicals, and easily available.
• The chlorine dose to be added can be easily measured and controlled, and hence chances of overdosing or under-dosing are minimized.
• It is a very powerful disinfectant, and may remain in water as free residual for good enough time.
• The operation of the plant does not require much skilled supervision.
• No sludge is formed in its application, as may be formed when bleaching powder is used for disinfection.

Disadvantages of chlorine as disinfectant are:

• The storage of chlorine and its use must need careful handling.
• Chlorine forms a very explosive mixture when mixed with carbon monoxide gas. Provision should be made for forcing air into storeroom from top, so as to drive out the heavier chlorine gas through the ventilators provided at the floor level.
• Larger chlorine residual may cause bad taste.