Wells are the common sources of water supply in villages so they should be disinfected regularly to keep the water fit for drinking and domestic purposes.
Previously potassium permanganate was used for disinfecting wells but now instead of potassium permanganate, bleaching powder is used for this purpose because it is cheap and quite effective disinfectant.
For disinfecting the well following steps are involved:
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(i) Measurement of well for its depth.
(ii) Amount of water in the well.
(iii) Amount of bleaching powder required.
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(iv) Mixing of bleaching powder.
(v) Addition of bleaching powder solution in the well.
(vi) Ortho-toludine test.
(i) Measurement of Well:
First of all, the diameter of the well is measured and then its depth is measured in meters by a rope tied with some weight. The rope is lowered in the well till it touches the bottom of the well. Then the rope is taken out and length of the wet portion of the rope is measured which gives height of the water column.
(ii) Amount of Water Present in the Well:
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In the next step amount of water present in the well is calculated from the following formula:
V = n r2h x 1000 liters where V = volume of water n = 22/7 r = radius (meters) h = height (meters)
The figure 1000 indicates that the volume is multiplied with 1000 to convert cubic meters into liters.
(iii) Amount of Bleaching Powder Required:
10gm of good quality bleaching powder is sufficient to disinfect 1500 liters of water. This amount of bleaching powder can vary according to the impurities present in water.
(iv) Mixing of Bleaching Powder:
Take the required quantity of bleaching powder in a bucket. To this add small amount of water so as to make a paste. Then add more of water with continuous stirring till the bucket is 3/4 full. Allow to stand the contents of the bucket for 5 to 10 minu & siso as to settle the lime.
Transfer the supernatant clear liquid (child chlorine solution) into another bucket and discard the lime sediment (the lime should not be poured into the well as it increases the hardness of well water).
(v) Addition of Bleaching Powder Solution in the Well:
Lower the bucket containing chlorine solution into the well and agitate
the water by lowering and drawing up the bucket several times and at the same time going round the well so as to mix the chlorine solution thoroughly with the well water.
Allow the chlorine solution to remain in contact with well water for at least one hour (contact period) and during this period no water should be withdrawn from the well. This will allow the chlorine water to kill the pathogenic micro-organisms.
(vi) Ortho-Toludine Test:
After one hour (contact period) ortho-toludine test is performed to know whether water has been properly chlorinated or not i.e. free residual chlorine is tested. If it is less than 0:5 mg per liter (0.5 ppm) then additional quantities of bleaching powder will have to be added.
For performing this test analytical grade ortho-toludine dissolved in 10 percent solution of hydrochloric acid is selected. A sample of chlorinated water is taken in a test tube and 2-3 drops of orthotoludine are added to it.
The appearance of yellow colour will indicate that sufficient chlorination has been done but appearance of red colour will indicate that excess chlorination has been done.
In villages no routine ortho-toludine test is performed, usually the wells are disinfected with bleaching powder solution late in the evening and water is drawn only in the next morning. This provides sufficient contact period. During cholera epidemics all wells must be disinfected with bleaching powder solution daily.
