Question

As per drinking water specification of IS 10500, thermotolerant coliform bacteria or E. coli shall not be detectable in any __________ ml sample.

• A. 15
• B. 50
• C. 100
• D. 250

Hint:

Remember the standard microbiological drinking water rule: E. coli or Coliforms = "Absent/Nil in 100 ml" under IS 10500 and WHO guidelines.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Potable water must be free from pathogens to prevent waterborne disease epidemics.
Coliform bacteria, particularly Escherichia coli, are used as primary biological indicators of fecal contamination in drinking water.

Step 2: Detailed Explanation:
The Bureau of Indian Standards (BIS) has established the national drinking water specification under standard code IS 10500.
To ensure microbiological safety, the standard mandates a strict zero-tolerance policy for fecal coliforms in treated drinking water:
1. Thermotolerant coliform bacteria or Escherichia coli must not be detectable in any 100 ml sample of drinking water.
This 100 ml sample size is the universally standardized volume used in membrane filtration and multiple-tube fermentation techniques to evaluate potable water safety globally (including WHO and US EPA standards).
Detecting even a single coliform cell in a 100 ml sample indicates fecal contamination, requiring immediate corrective action (such as chlorination) and declaring the water supply unsafe for consumption.

Step 3: Final Answer:
Thus, E. coli shall not be detectable in any 100 ml sample.