Total Coliform Count

Total coliform count indicates the microbiological quality of water by measuring a group of bacteria used as indicators of possible contamination. Because coliforms occur in human and animal intestines as well as in soil, vegetation and other environmental sources, their presence does not by itself prove faecal contamination or the presence of disease-causing organisms.

What the count measures

Testing for every waterborne pathogen is difficult, so water-quality assessment uses indicator organisms to help identify contamination and failures in treatment or protection of water supplies.

• Total coliforms comprise a broader bacterial group than faecal or thermotolerant coliforms; not all members originate in faeces.
• Results are commonly reported as the most probable number (MPN) per 100 mL, which is a statistical estimate obtained from a culture-based test.
• Membrane-filtration methods can instead report colony-forming units per 100 mL, so the testing method and reporting unit should accompany the result.

How to interpret the result

A positive result is a warning of possible contamination, not a direct count of pathogens. Its significance depends on whether the sample comes from a river, a treatment plant or a drinking-water distribution system.

• In treated drinking water, detection can indicate inadequate treatment, entry of contaminated material, or bacterial regrowth within the distribution system.
• Escherichia coli (E. coli) is a more specific indicator of faecal contamination than total coliforms because it is associated with human and animal intestines.
• A higher total coliform count does not translate directly into a corresponding disease risk, because the test does not identify particular pathogens.
• A negative result does not establish complete safety: chemical contaminants and pathogens not reliably represented by this indicator require separate assessment.

How water-quality standards use it

Acceptable results depend on the intended use of water; a bathing-water criterion must not be treated as a drinking-water standard.

• The BIS drinking-water specification, IS 10500, requires total coliform bacteria to be undetectable in any 100 mL sample of treated water entering the distribution system.
• Under CPCB Class B criteria for organised outdoor bathing, total coliform organisms must be 500 MPN per 100 mL or less; dissolved oxygen, biochemical oxygen demand and pH criteria must also be satisfied.