Breakpoint chlorination or Dip point (Free residual chlorine):
The addition of a sufficient amount of chlorine to oxidize Organic matter, reducing substances, and free ammonia leaving behind free chlorine killing pathogenic bacteria is called breakpoint chlorination. The addition of a sufficient amount of chlorine to satisfy chlorine demand is called breakpoint chlorination. When a graph is drawn between the added chlorine to residual chlorine a dip or break is formed in the graph called breakpoint chlorination. The addition of chlorine at the dip or break is called Breakpoint chlorination. After this dip or break point chlorination, free residual chlorine is present. Usually, all tastes and odours disappear at the breakpoint.
What is breakpoint chlorination? Breakpoint chlorination is the point where chlorine levels exceed the oxidant demand, and the water begins to build a residual of free available chlorine (FAC). Theoretically, exceeding the “breakpoint” prevents increased levels of disinfectant byproducts (like chloramines). For the purposes of wastewater treatment, breakpoint chlorination is a means of removing ammonia from a solution, which changes to an oxidized volatile form. Increased chlorine residual is produced by adding chlorine to water that contains ammonia or organic matter that contains nitrogen.
These hypochlorite ions are called free chlorine. This free chlorine is responsible for the chlorination of the microbes in water. All the free chlorine is not consumed in the chlorination. This remaining chlorine is called residual chlorine.
Advantages: Breakpoint chlorination gives an idea of the amount of chlorine required to add for chlorination It
i) oxidize completely organic matter, NH3 and reducing agents,
ii) removes colours in the water,
iii) destroys completely all the disease-producing bacteria,
iv) removes odour from water,
v) prevents any growth of weeds in the water.
Disadvantages: If excess chlorine is added it leaves residual or free chlorine which imparts a bad taste and odour. Dechlorination must be done in order to remove free chlorine.
Dechlorination: Dechlorination is done by passing SO2 &sodium sulphite.
d) By using chloramines
When chloramines are added to water they produce HOCl which acts as germicide Chloramines can be prepared by passing chlorine gas into the ammonia chamber. Now a day municipalities are using this process.
Cl2+NH3 →NH2Cl+HCl
NH2Cl+H2O →HOCl + NH3
e) Disinfection by ozone (OZONIZATION).
By sending raw water through ozonizer, where the nacent oxygen liberated from ozone act as a germicide and kills the microorganisms. Ozone is unstable so easily decomposes to
O3→O+O2
Disadvantages: Equipment is Expensive
Advantages: It removes chlorine odour (smell) taste etc.if ozone is in excess, it is not harmful.