Swimming Pool Microbiological Testing Frequency

The microbiological criteria Heterotrophic plate count, Thermotolerant coliforms and Pseudomonas aeruginosa. Microbiological tests should be performed by laboratories accredited by the National Association of Testing Authorities (NATA).

Last updated: 18 February 2010

Microbiological criteria

The microbiological criteria are as follows:

Microbiological samples should always be collected before chemical samples to avoid accidental contamination of the swimming pool water by the sampler's body. Microbiological tests should be performed by laboratories accredited by the National Association of Testing Authorities (NATA).

Heterotrophic plate count (HPC) is also known by a number of other names, including standard plate count, total plate count, total viable count or aerobic quality count. It does not differentiate between the types of bacteria present nor does it indicate the total number of bacteria present in the water - only those capable of forming visible colonies under specified conditions on certain non-selective microbiological media. Varying the incubation temperature will favour the growth of different groups of bacteria. As it gives more meaningful information about pathogenic (disease-causing) bacteria, 35°C (or 37°C) is the preferred incubation temperature.

HPC does not necessarily indicate microbiological safety as the bacteria isolated may not have been faecally-derived but it does give a measure of the overall general quality of the pool water, and whether the filtration and disinfection systems are operating satisfactorily. Results reported by the laboratory are traditionally expressed as colony forming units per millilitre (CFU/mL) which equates to the number of bacteria in each millilitre of the original sample of water tested. A HPC count of less than1 CFU/mL indicates that the disinfection system is effective. If the count is between 10 and 100 CFU/mL, a routine investigation should be conducted as soon as possible to ensure that all the management operations are functioning properly. However, counts above100 CFU/mL are indicative of a faulty disinfection system and an urgent investigation should be conducted immediately; the pool water should be re-sampled and sent to the laboratory to be tested again as soon as possible.

Thermotolerant coliforms (TC). This group of bacteria (which includes E. coli) are normal inhabitants of the intestinal tracts of warm-blooded animals and are always present in faeces in large numbers (approximately 10⁹/g). E. coli is almost exclusively of faecal origin and does not grow in water. They are termed thermotolerant as they are capable of growth in microbiological media at the elevated temperature of 44.5°C whereas most environmental micro-organisms would be killed at the same temperature. They are also known as faecal coliforms because their detection indicates recent contamination by faecal matter. Results for TC are normally reported as CFU/100 mL of water tested. Note that the standard for TC is more stringent than HPC because an indication of faecal contamination would mean the likely presence of pathogenic micro-organisms in the pool. It is possible to have high HPC and a low TC indicating no recent faecal contamination but low overall disinfection efficiency. A high HPC and high TC indicate a disinfection system that is severely deficient, and bathers would be likely to have been at risk of contracting disease at the time of sampling. Thermotolerant coliforms are the most reliable indicator of public health risk.

Pseudomonas aeruginosa is an opportunistic pathogen commonly found in water, soil and vegetation. It also can be found in human and animal faeces. It rarely causes infection in healthy people but can colonise damaged systems, such as burn wounds and damaged eyes. Immunocompromised individuals are particularly at risk. P. aeruginosa can grow at the selective temperature of 41-42°C, where most environmental micro-organisms would not survive, allowing it to proliferate to high numbers and cause diseases like ear and eye infections and folliculitis. Although relatively resistant to a range of disinfectants, chlorination of swimming pools should be sufficient to kill the bacterium. However, in environments peculiar to spas such as water turbulence, elevated temperature and high bather loads, considerably greater care is needed to ensure their safe operation and eradication of the micro-organism. The bacterium produces biofilms and colonises drains and filter media. Laboratory results for P. aeruginosa are normally reported as CFU/100 mL of water tested. It is a more sensitive indicator than TC and may be detected in their absence. Presence of this micro-organism in the pool means that the disinfectant has not been sufficiently maintained continuously above the minimum levels. The pool may need superchlorination, or at least a shock dose of chlorine added upstream to the filter to eliminate any micro-organism that may be harbouring in the water and the filter media.

Ideally, persons collecting samples should be competent in the use of aseptic techniques. The following points should be noted concerning sampling:

Sampling technique: Aseptic techniques must be used at all times during sampling. Sterile containers of 250 to 500 mL capacity containing a few drops of sodium thiosulphate (added to neutralise the chlorine) must be used. Samples should be collected during periods of maximum bather load. Sampling frequency should be consistent with the Table below.

A sampling location close to the pool outlet should be chosen so that it is representative of the water which has already circulated through the pool. The lid from the sterile jar should be carefully removed by holding it near its base at an angle of 45°. The bottle should be filled in one slow sweeping movement, at a pool depth of at least 450 mm, with the mouth of the bottle always pointing ahead of the hand. Avoid contamination of the sample by floating debris. The container should not be rinsed to avoid washing out sodium thiosulphate. The top 15 mm of water should be tipped out of the bottle (to allow sufficient headspace for homogeneous mixing), the lid replaced, and the sample placed in a cooler with sufficient freezer bricks to keep the sample cool (2-10°C) during transport to the laboratory.

Timeliness: Once samples have been collected, it is equally important that they are delivered to the laboratory in a manner that does not alter their condition from that existing at the time of collection. For microbiological samples, it is important that there has been no increase or decline of microbial numbers during transport. Ideally, samples should arrive at the laboratory within six hours, but definitely no longer than 24 hours, after sample collection.

Interpretation: While the criteria have been explained previously, they should not be used as the basis of immediate risk assessment as it will be at least three days before results are received and inferences made. Results, however, need to be interpreted against the results of chemical testing performed immediately after the microbiological sampling.

Chemical criteria: Each chemical criterion has been rounded up to give a slight safety margin (see separate fact sheet on chemical sampling). Thus, continuous maintenance of a minimum of 1 mg/L of free available chlorine in an outdoor pool (at a pH of 7.5) which is required in the chemical criteria, is slightly more than sufficient than that needed to ensure compliance with the microbiological criteria.

Database: All microbiological sample results should be logged in a database or spreadsheet so that a history of disinfection can be developed. Once patterns have emerged, it may be possible to reduce sample frequency, or other locations can be sampled to enable comparisons.

Sampling criteria

All three microbiological criteria need to be considered at the one time, from the one sample. The recommended minimum sampling frequencies for the analysis of these indicator micro-organisms are given in the Table below. It must be recognised that the microbiological criteria are a useful guide to the performance of disinfection in a pool and that it is only an indication of past risk measured as a snapshot at one point in time. Once the snapshots have been placed in a database the whole picture of pool performance begins to emerge so that the pool operator can gain confidence in pool operations and become more competent in pool management.

Minimum sampling frequency for microbiological analysis

Micro-organism growth risk factors

The following risk factors have been associated with microbiological failure of pool water quality. Where three or more risk factors exist in a swimming pool, the pool should be changed to a more stringent category. For example, a Category 3 diving pool should be changed to a Category 2 pool where three or more of the following risk factors are observed:
• pH greater than 7.6 in a chlorinated pool
• Consistently poor disinfection (previous chemical or bacteriological failures)
• High turbidity (nutrients)
• Poor pool circulation or filtration
• High bather loads
• Presence of algae
• Regular use by birds e.g. ducks
• Easy access of foreign material e.g. litter
• Biofilms detected
• Poor quality make-up water (high in chloramines)
• Infrequent testing of disinfectant concentration
• No automatic disinfection and pH control.

When indicator micro-organisms exceed the criterion value, a thorough assessment of the pool's physical and chemical environment should be undertaken. Pool surfaces should be checked for biofilms, backwashing of the filters may be needed and disinfectant concentration, pH, cyanuric acid, total dissolved solids (TDS) and turbidity should be checked. The incoming water supply should also be checked regularly.

On most occasions microbial failure is due to inadequate continuous disinfection. Failures may also occur as a result of high turbidity and biofilms, which can shield micro-organisms from disinfectants. Biofilms within pipes and filtration units may harbour micro-organisms especially in pools not regularly super-chlorinated.

Following rectification of any pool problems, the pool water should be immediately re-sampled to confirm that the swimming pool or spa pool is safe for use.

Further Information

Guidelines for Disinfecting Public Swimming Pools and Spa Pools specify the required disinfection concentrations and other chemical parameters for public swimming pools and spa pools operations.

Public Swimming Pool and Spa Pool Code of Practice provides detailed explanations and information on disinfection, pool chemistry, risk assessment and other issues relevant to swimming pool operation.

Both the Guidelines and Code of Practice may be found at the NSW Health swimming pool website