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 <50cfu/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 as the samples may have been taken just after someone 'released' something into the water and the system has not had time to deal with it yet. 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.

Gross Microbiological Contamination

What if the results are way outside the acceptable ranges though? At what point does the pool operator need to close the pool down due to gross microbiological contamination? The official guidance is that the pool should be closed down if any of the routine monthly microbiological test results indicate either of the following scenarios:

  • Greater than 10 E. coli per 100ml in combination with an unsatisfactory colony count (>10 per ml) and/or an unsatisfactory Aeruginosa count (>10 per 100ml)
  • greater than 50 Aeruginosa per 100ml in combination with a high aerobic colony count (>100 per ml)

You may find microbiological test reports easier to interpret by referring to the flow chart below.


As swimming pools and hydrotherapy pools are not recognised as a source for legionnaire’s disease, routine testing for legionella for these types of pools is not normally required. Spa pools however are a recognised source and therefore need to get the water sampled and tested for the presence of legionella bacteria on a quarterly basis (more frequently if there are doubts or concerns regarding the effectiveness of the pool management system). Samples should be collected from the pool and balance tank and legionella should be absent in a one litre sample. If any legionella are present, the pool should be resampled, drained, cleaned and disinfected. The pool management system should be reviewed, along with the risk assessment. The pool should be retested the day after refilling and again 2 – 4 weeks after. If the numbers of legionella detected are over 1,000 cfu’s in a one litre sample, the pool should be closed and public access restricted. The pool should be superchlorinatedwith 50mg/l of chlorine for 16 hours (with circulation on, but air blowers off). The same steps as described above should then be carried out. The pool should not be opened again until the absence of legionella has been confirmed.

Microbiological monitoring of domestic hot and cold water supplied from the mains is not usually required, unless the risk assessment or monitoring indicates there is a problem. The risk assessment should specifically consider systems supplied from sources other than the mains, such as private water supplies, and sampling and analysis may be appropriate.

Legionella monitoring should be carried out where there is doubt about the efficacy of the control regime or it is known that recommended temperatures, disinfectant concentrations or other precautions are not being consistently achieved throughout the system. The risk assessment should also consider where it might also be appropriate to monitor in some high risk situations, such as certain healthcare premises. The circumstances when monitoring for legionella would be appropriate include:

  • water systems treated with biocides where water is stored or distribution temperatures are reduced. Initial testing should be carried out monthly to provide early warning of loss of control. The frequency of testing should be reviewed and continued until such a time as there is confidence in the effectiveness of the regime;
  • water systems where the control levels of the treatment regime, eg temperature or disinfectant concentrations, are not being consistently achieved. In addition to a thorough review of the system and treatment regimes, frequent testing, eg weekly, should be carried out to provide early warning of loss of control. Once the system is brought back under control as demonstrated by monitoring, the frequency of testing should be reviewed;
  • high-risk areas or where there is a population with increased susceptibility, eg in healthcare premises including care homes;
  • water systems suspected or identified in a case or outbreak of legionellosis where it is probable the Incident Control Team will require samples to be taken for analysis. 

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