Explain the Procedure for Testing Quality of Water Using Presumptive Test?

Now carry out the exercise following the steps indicated herewith.

1. Make 3 series of lactose broth fermentation tubes (one each for tap water, distilled water, sewage water) each consisting of 15 test tubes, 5 tubes of double (2x) strength and 10 tubes of single (1x) strength.

2. Label 5 tubes of double strength in each series as 10, 5 tubes of single strength as 1 and other 5 tubes of single strength as 0.1, to indicate the sample volume to be inoculated. Also label the tubes for water source. One uninoculated tube each of the double and single strength is kept as a control.

3. Mix the sewage sample thoroughly and inoculate each tube in a series labeled for sewage water with 10 ml (5 tubes of 2x), 1 ml (5 tubes of 1x) and 0.1 ml (5 tubes of 1x) of the sample aseptically.

4. Repeat step 3 for tap water as well as for distilled water.

5. Incubate the tubes at 37°C for 24 hours.

6. Observe all the lactose fermentation tubes for acid and gas production. If the test is negative, incubate the tubes further for next 24 hours. Production of acid and gas after 24 hours incubation indicates a positive presumptive test for coliforms. If it develops after 48 hours of incubation, presumptive test is doubtful. No acid and gas production after 48 hours shows negative presumptive test, i.e., absence of coliforms. A positive or doubtful presumptive test suggests non-potability of water and should be tested further by confirmed test.