Chlorine Disinfectants

The first use of chlorine as a disinfectant in America was in New Jersey in the year 1908 (Leal, 1909). At that time George A. Johnson and John L. Leal chlorinated the water supply of Jersey City, NJ.

The principal compounds of chlorine that are used in water and wastewater treatment are the molecular chlorine (Cl2), calcium hypochlorite [Ca(OCL)2], and sodium hypochlorite [NaOCl]. Sodium hypochlorite is ordinary bleach. Chlorine is a pale-green gas, which turns into a yellow-green liquid when pressurized. Both the aqueous and liquid chlorine react with water to form hydrated chlorine. Below 9.4°C, liquid chlorine forms the compound Cl2 ■ 8H2O.

Chlorine gas is supplied from liquid chlorine that is shipped in pressurized steel cylinders ranging in size from 45 kg and 68 kg to one tonne containers. It is also shipped in multiunit tank cars that can contain fifteen 1-tonne containers and tank cars having capacities of 15, 27, and 50 tonnes.

In handling chlorine gas, the following points are important to consider:

• Chlorine gas is very poisonous and corrosive. Therefore, adequate ventilation should be provided. In the construction of the ventilation system, the capturing hood vents should be placed at floor level, because the gas is heavier than air.

• The storage area for chlorine should be walled off from the rest of the plant. There should be appropriate signs posted in front of the door and back of the building. Gas masks should be provided at all doors and exits should be provided with clearly visible signs.

• Chlorine solutions are very corrosive and should therefore be transported in plastic pipes.

• The use of calcium hypochlorite or sodium hypochlorite as opposed to chlorine gas should be carefully considered when using chlorination in plants located near residential areas. Accidental release of the gas could endanger the community. Normally, small plants that usually lack well-trained personnel, should not use gaseous chlorine for disinfection.

Calcium hypochlorite is available in powder or granular forms and compressed tablets or pellets. Depending upon the source of the chemical, a wide variety of container sizes and shapes are available. Because it can oxidize other materials, calcium hypochlorite should be stored in a cool dry place and in corrosion-resistant containers. High-test calcium hypochlorite, HTH, contains about 70% chlorine. (Available chlorine will be defined later.)

Sodium hypochlorite is available in solution form in strengths of 1.5 to 15% with 3% the usual maximum strength. The solution decomposes readily at high concentrations. Because it is also affected by heat and light, it must be stored in a cool dry place and in corrosion-resistant containers. The solution should be transported in plastic pipes. Sodium hypochlorite can contain 5 to 15% available chlorine.

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