Introduction to Microbiological Testing


All non-domestic pools should be getting the pool water tested at a UKAS-accredited laboratory for microbiological contamination. In most pools this should be done on a monthly frequency, but certain pools, such as hydrotherapy pools, it should be done on a weekly basis. Whenever a microbiological sample is taken it is important that a pool water chemical test of free and combined chlorine and pH is taken at the same time as a reference. The water clarity and the bather load should also be noted.

 The four standard tests and the acceptable levels for each are:

  • Aerobic Colony Count: < 10cfu/ml
  • Total Coliforms: <10cfu/100ml
  • E. Coli <1cfu/100ml
  • Pseudomonas Aeruginosa <10cfu/100ml

If you get the lab results back and any of them are outside of these ranges, you need to get the water tested again.

If the repeat tests are still not within the acceptable ranges, the pool operator should take this as an indication that the pool water treatment and/or management system is not functioning as it should.

The system and arrangements for managing the pool water quality will need to be looked at with a view to pinpointing exactly what is wrong and then putting it right.

Colony Count

This is a count of all types of bacteria that have been able to form a colony on the laboratory media under the test conditions.

Total Coliforms

Total coliforms are a particular type of bacteria. They exist in animal (and human) intestines as well as being present in the environment (on vegetation and in soil). The presence of total coliforms in the water may indicate that faecal or environmental contamination.

E. Coli

A type of coliform bacteria. This particular type of bacteria cannot multiply in water, but they are normal inhabitants of the intestinal tract. Therefore, the presence of this bacteria in the water is an indication of faecal contamination.

Pseudomonas Aeruginosa

These bacteria can cause folliculitis (a type of skin infection) and are widely distributed in the environment. The presence of these bacteria indicates colonisation of part of the system (probably the filters). They can be present in a sample, even in the absence of coliforms, hence their inclusion in the standard microbiological tests.


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