Freezing Technology

2.5.2.3.1. Break-point chlorination

When small amounts of chlorine are added to water, first, chlorine reacts with certain impurities present in the water. These impurities are substances responsible for chlorine demand. Chlorine reacts with nitrogenous compounds to form chloramines or other chloronitrogen compounds. On further addition of chlorine, a free residual chlorine appears. On still further addition, the free residual chlorine gradually increases until it reaches a particular concentration. The concentration of free chlorine is determined by the physical and chemical nature of the water. When the level of free chlorine is increased beyond this concentration, oxidation reaction occurs between free chlorine and chloronitrogen compounds.

The level of the free residual chlorine is decreased during the oxidation of chloronitrogen compounds. When the oxidation is completed, the addition of chlorine to water results in corresponding increase in chlorine concentration. The point after the first rise in concentration at which the free residual reaches its lowest level is known as the breakpoint.

The advantages of break point chlorination are the following:

• Complete oxidation of ammonia and other compounds in the water
• Correction of tastes and odours of biological origin
• Correction of tastes and odours due to phenol and other substances
• Reduction of colour due to organic matter
• Improvement of bacterial quality of water