How to do disinfection with Bleaching Powder

Bleaching powder

Bleaching powder is also referred to as T.C.L. powder. Beaching powder is nothing but a compound of Lime and Chlorine.

In fresh Bleaching powder, normally chlorine content is 33 % or more. Bleaching powder is unstable in nature and as the time passes the chlorine content reduces. To avoid loss of chlorine, it is necessary to store Bleaching powder in dry condition and away from direct sun light. If the Bleaching powder is in a bag, after use, the bag should be properly closed.

The percentage of chlorine, in the Bleaching power can be found out in a laboratory test. Such chlorine Tests are carried out in District Public Health Laboratories.

Things to note

• Contact period of minimum half an hour is required for the chlorine to act on the pathogens and kill them. Hence, it is necessary to do the chlorination, minimum one hour before the supply hours.
• The quantity of Bleaching power required for disinfection depends on the quantity of water in the GSR/ESR. It can be found out with the help of a formula.

Quantity of bleaching powder required

1. Formula for Circular Tanks:

Quantity of water in the tank (in liters) = 785 x (Tank Diameter in meter)2 x (Height of water column) – All measurement in Meters.

2. Formula for Square/Rectangular Tanks

Quantity of water in the tank (in lit) = length of tank (in m) x width of tank (in m) x height of water in the tank (in m) x 1000

Quantity of bleaching power required:

If the Bleaching powder contains 30% or more chlorine, then 5 gm Bleaching powder is required for proper disinfection of 1000 lits of water.

Let us understand this by an example.

If a square tank of size 4 m x 4 m (inner dimension) has 2.5 m depth of water in it then

– Volume of water in the tank = 4 x 4 x 2.5 x 1000 = 40,000 lit

Hence quantity of Bleaching Powder required for disinfection of this water = 40,000 x 5/1000 = 200 gms.

However, if the chlorine content of the bleaching powder is less than 30%, the quantity of bleaching powder for disinfection of the same quantity of water (40,000 lits) will have to be increased in the ratio of the theoretical content of chlorine and the actual content of chlorine.

Chlorination:

Quantity of Bleaching Powder required for disinfection as per above formula should be taken in a plastic bucket and adding small quantity of water, a thick paste should be  prepared. Then the bucket should be filled up to 50% to 75% by water and the paste should be thoroughly stirred with a wooden stick. This
solution should be allowed to stand for some time and then the supernatant should be poured in another bucket.

Then with the help of a rope, this bucket should be lowered in the GSR/ESR up to a depth of 0.3 to 0.5 m and let the chlorine solution get properly mixed in the water in the tank by stirring.

Do not mix/use the remnants of the first bucket in the tank water. It should be kept separately and disposed of properly.

Method for determination of presence of Chlorine in water

Disinfected water has residual chlorine. Presence of residual chlorine is an indicator of proper disinfection. This residual chlorine also helps disinfection of likely pollution of water in the distribution network.

How much residual chlorine is there in the disinfected water can be determined by a very simple test. This test is called “Ortho Toluidine Test” or simply “O.T. Test”. O.T Test should indicate residual chlorine in following proportion:

• 1 hr after disinfection of water in the tank – minimum 1 ppm.
• In the water from tail end of the distribution system and water sample from a public stand post – 0.2 to 0.5 ppm.

How to carryout O.T. Test

• A kit which is called “Chloroscope” is used for this O.T. Test.
• Take water sample in a Test Tube. The tube should be filled up to 3/4 height of the tube.
• Put 1-2 drops of O.T. solution in the test tube and shake it well.
• If there is residual chlorine in the water, the colour of water changes from colourless to yellow.
• Compare colour of the water in the test tube with colour disk in the chloroscope. The matched colour disk indicates the residual chlorine in the sample.
• No change in colour of water in the test tube indicates that the ‘Test is Negative’ meaning there is no residual chlorine in the sample tested.

Periodic Testing of Water Samples:

The work of periodic testing of water sample and keeping record of the same should be vested with the person responsible for distribution of water.
The person concerned should carry out the testing as given below –

• Water Sample from the GSR/ESR – Once a day, 1 hr after adding bleaching powder
• From Distribution System
  • For a village having population up to 1000 – Daily one sample from the distribution system / stand post.
  • During Epidemics – daily 5-6 samples

Use of Bleaching Powder in excess of the required quantity

There is a possibility of the source getting polluted at the start of monsoon and during heavy rains. This may result into epidemics. Hence it is very important to take utmost care in disinfection during such periods. In such situations use of Bleaching Powder, in excess of required quantity, is recommended. The extra dose of bleaching powder should be adjusted in such a way that the residual chlorine is more than 1 ppm. Adequate O.T. Test should ensure this.

Source : O&M Management and Monitoring of Swajal