Break point chlorination and Types of Chlorination
Breakpoint chlorination is the point where the demand for chlorine has been fully satisfied in terms of chlorine addition to the water. In other words, the chlorine has completely reacted with the bather pollutants leaving a zero chlorine residual.
Chlorine is one of the commonly used disinfectants since it is economical, reliable and can provide the residual effect needed for safe supply of water. Excess chlorine can impart a bitter taste in water.
Elemental chlorine is available both in gas and liquid form which is yellow green in colour. This is corrosive in nature so it should be transported in plastic pipes.
The above reaction is a reversible reaction controlled by the pH of the water. If the pH > 4 the reaction proceeds in the forward direction and vice versa.
Hypochlorous acid (HOCl) is also a good disinfectant.elemental chlorine is available only in low pH value, so it does not play a major role in disinfection of drinking water supply.
Hypochlorite ion (OCl- ) is also a disinfectant. The above reaction proceeds in the forward direction if the pH > 7.5 and vice versa. HOCl is having more disinfecting power than this hypochlorite (OCl- ) ion. About 80% more.
Elemental chlorine Cl2, hypochlorous acid HOCl, hypochlorite ion OCl- are combinedly called as free chlorine, which are mainly used in the disinfection process. pH is kept less than 7 for more HOCl to be available which is preferred. HOCl creates most of the disinfection.
If ammonia NH3 is present in water it affects the disinfection caused by HOCl by forming chloramines.these chloramines are also having the disinfecting capacity but they react slowly and the germicidal effects are seen only after some time. Ammonia is sometimes purposely added in the water for maintaining the residual effect.
Dichloramine is predominant in the three forms. The three forms of chloramines are collectively known as combined chlorine.
Types of chlorination
Plain chlorination: In this process only chlorination is done and no other treatment is carried out before the chlorination.chlorine can kill the bacteria, remove the organic matter and colour present in the water. Plain chlorination is done in clearer water sources like lakes,tubewells,reservoirs. They are done in emergency situations when complete treatment of water cannot be carried out.
Pre chlorination: if chlorination is done before coagulation, sedimentation , filtration process it is known as pre chlorination. The benefits of pre chlorination are ,it can reduce the load on other treatment units
Post chlorination: if chlorination is done after the filtration process it is called post chlorination. It is important for a safe supply of drinking water.
Double chlorination: both pre and post chlorination is done in this process.
Superchlorination: it is providing an excessive amount of chlorine. The concentration is kept between 5 mg/l - 15 mg/l. They are done during epidemics and disease outbreaks like amoebic dysentery caused by histolytica.
Dechlorination: After superchlorination water cannot be supplied with high concentration of chlorine so dechlorination is done.dechlorination can be done using SO2, gas, activated carbon, sodium thiosulphate.
Breakpoint chlorination
Elemental chlorine Cl2, hypochlorous acid HOCl, hypochlorite ion OCl- are combinedly called as free chlorine. They combine with NH3 ammonia to form chloramines which provide the necessary residual effect. Ions such as Fe2+, Mn2+, H2S, NO2- are reducing agents which react with the free chlorine and form Cl- which does not have any germicidal effect.
These reactions take place immediately after the addition of chlorine in the segment OA shown in figure.
In segment OB, the chlorine reacts with ammonia and forms chloramines, organic matter like phenol, fulvic acid is also oxidised in this region of the curve forming chloro organic compounds.
Segment BC shows a sudden drop, this indicates the oxidation of the organic matter present in the form of chloro organic compounds.
At point C all of the organic matter is oxidised and the chlorine added after this constitutes for the free residual chlorine. 0.1 mg/l - 0.2 mg/l is maintained as free residual chlorine. 0.2 mg/l is selected as the optimum dosage of chlorine after a contact period of 10 min.