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Environment & Health

Question
1 out of 72
 

All of the following statements about purification of water are true except



A Presence of clostridial spores indicate recent contamination of water

B Coliforms must not be detectable in any 100 ml sample of drinking water

C Sodium thiosulphate is used to neutralize chlorine

D Coliforms may be detected by multiple tube method & indole production

Ans. A Presence of clostridial spores indicate recent contamination of water

Ref: Park; 22nd edi, pg – 669 TO 670

Clostridial infection or Cl.perfringens to be more precise regularly occurs in feces though in much smaller number as compared to E.coli. the spores are capable of surviving in water for a longer time than organisms of coliform group and usually resist chlorination at doses normally used in waterworks practice. Hence their presence in water denotes that fecal contamination has occurred and their presence in absence of coliforms denotes that contamination occurred at some remote time.

Bacteriological indicators for water quality

  1. Coliform organism: faecal group; E coli & non-faecal group; Klebsiella aerogens.
  2. Faecal streptococci: resistant to drying.
  3. Cl. perfringens: remote faecal contamination.

Multiple tube method also known as presumptive coliform test is used for bacteriological surveillance of water samples.

PRESUMPTIVE COLIFORM TEST (Pg-674)

The test is done to assess whether the water has been contaminated or not. Presence of coliform organisms (E coli) suggests a fecal contamination of the water.

Multiple tube method: 100 ml of water is taken and different quantities of this water are inoculated in tubes of MacConkey’s Lactose Bile Broth (0.1, 1.0, 10, 50 ml) and incubated for 48 hrs. From the number of tubes showing presence of acid and gas an estimate of most probable number of coliforms in 100 ml is obtained. This is done by consulting the Macardy’s chart.

Confirmatory test: to confirm the presence of coliform organisms (only done for chlorinated water) the tubes positive in the earlier test are subculture in 2 tubes each of brilliant green bile broth; one of which is incubated at 37 degrees and the other at 44 degree Celsius. E coli is the only organism that can produce gas from lactose at 44 deg C.

E.coli can be tested by indole test and is the only organism producing gas from lactose at 44 degree C.

Level of residual chlorine must be 0.5mg/L in the water for successful chlorination.

Environment & Health Flashcard List

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