Chemical or Microbiological Contamination Outbreaks in Swimming Pools

Chemical or Microbiological Contamination Outbreaks

A chemical or microbiological outbreak has occurred if more than two people have symptoms from the same source at roughly the same time. Written procedures are needed for dealing with an outbreak and included as part of the EAP (Emergency Action Plan) section of the PSOP (Pool Safety Operating Procedures). Inadequacies in the management approach taken with regard to swimming pools and spas has been identified as a major causal factor in a number of outbreaks in the UK. Typical problems that have led to outbreaks are listed below:

  • Inadequate water disinfection
  • Sewage contamination
  • Inadequate filtration
  • Incorrect backwash procedures
  • Inadequate disinfection of pool surround and changing room floors
  • Sharing of towels
  • Hot and cold water system design faults
  • Inadequate cleaning of showerheads
  • Release of faecal matter and/or vomit into pool and/or surrounding area
  • Lack of pre-swim showering
  • Inadequate cleaning of pool inflatables

An outbreak might be detected by pool staff, or may be picked up by local or national surveillance and reporting systems, such as those managed by Environmental Health Departments or the Health Protection Agency (HPA).

An Outbreak Control Team (OCT) is typically established by the HPA in the event of an outbreak, which is headed up by a Consultant in Communicable Disease Control (CCDC). The OCT will seek to establish the source of the outbreak and identify direct and root causes. This might well involve site visits by members of the OCT and the collection of samples of water for testing, inspection of the pool plant system, close scrutiny of documentation, and the interviewing of key staff.

Faecal Contamination

This is a difficult area, both for pool operators and for those attempting definitive guidelines. And there is potentially a lot at stake, as diarrhoea may contain the chlorine-resistant pathogen Cryptosporidium – a significant cause of gastroenteritis, particularly in pools.

If faecal contamination has only been reported, and there is some doubt about the accuracy of the report, its presence should be confirmed by pool staff. If it cannot be confirmed, pool operators should assess the risk and may decide that the risk of harmful contamination is low and allow bathing to resume. This assumes that pH and disinfection are within normal limits.

Solid faeces: the stools should immediately be removed from the pool using a scoop or fine mesh net and flushed down the toilet (not put in any pool drains). If there is any doubt that all the faeces have been captured and disposed of, and there is possible widespread distribution of the faeces in the pool, then the pool should be closed and the advice below for runny faeces followed.

Depending on the extent of the contamination, how public it has been, and how quickly it can be dealt with, operators should consider clearing the pool of bathers for, say, 30 minutes while the cleaning operation described above is taking place. Bathing should not resume until all the faeces have been removed.

All equipment that has been used in this process should be disinfected using a 1% solution of hypochlorite (1:10 dilution of commercially available sodium hypochlorite).

If the pool is operating properly with appropriate disinfectant residuals and pH values, no further action is necessary.

Runny faeces: watery, runny or soft stool (diarrhoea) is more likely to carry enteric pathogens, and be spread through the pool water. It will be impossible to remove like solid stool.

Operators are unlikely to know if Cryptosporidium is involved, so the safest option is to assume it is, close the pool and follow the procedures below.

There are in principle three procedures that will in time remove Crypto –coagulation/filtration, UV and super-chlorination. The procedures to be followed primarily depend on the efficiency of the pool’s filtration.

Pools with medium-rate filtration (up to 25 metres per hour)

This should include most public pools. Here, the main emphasis is on filtration, which if effective should remove some 99% of the Cryptosporidium oocysts in each pass of pool water through the filter.

Coagulation is critical in this: it should be continuous, and the residence time (that between the injection of coagulant and treated water reaching the filter) must be long enough for flocculation to happen – at least 10 seconds at a flow velocity no more than 1.5m/sec.

Secondary disinfection (UV or ozone) and superchlorination are also relevant – see below.

How long it takes for all the pool water to pass through the filter will depend on two factors. First is the pool hydraulics – crucially, how well mixed the pool water is. Dead spots will delay the passage of all the pool water through the filters. The second factor is the turnover period – the length of time it takes for a volume of water equivalent to the pool water volume to go from pool to plant room and round to the pool again. It might take as long as 24 hours for all the pool water to pass through the filters – based on the 3 to 4-hour turnover period common to many pools.

  1. Close the pool – and any other pools whose water treatment is linked to the fouled pool. If people transfer to another pool, perhaps from a teaching pool to a main or leisure pool, they should shower first using soap and water.
  2. Hold the disinfectant residual at the top of its set range for the particular pool (eg 2.0mg/l free chlorine if the range is 1.0 to 2.0mg/l) and the pH value at the bottom of its range (eg pH 7.2-7.4). This will maintain the normal level of microbiological protection.
  3. Ensure that the coagulant dose is correct – for continually dosed PAC, 0.1ml/m3 of the total flow rate.
  4. Filter for six turnover cycles (which may mean closing the pool for a day). This assumes good hydraulics and well maintained filters with a bed depth of 800mm and 16/30 sand. This applies also to pools with secondary disinfection.
  5. Monitor disinfection residuals throughout this period.
  6. Vacuum and sweep the pool. Cleaning equipment, including automatic cleaners, should be disinfected after use. This will at least move faecal contamination off surfaces and into the main pool water circulation, for eventual removal.
  7. Make sure the pool treatment plant is operating as it should (filters, circulation, and disinfection).
  8. After six turnovers, backwash the filters.
  9. Allow the filter media to settle by running water to drain for a few minutes before reconnecting the filter to the pool.
  10. Circulate the water for 8 hours. This will remove any remaining oocyst contamination of the pool and allow the filters to ripen. It is optional, depending on the pool operator’s confidence in backwashing procedures.
  11. Check disinfection levels and pH. If they are satisfactory re-open the pool.
  12. Any moveable floors and booms should be moved around from time to time during the whole process.

Pools with high-rate filtration (over 25 and up to 50 metres per hour)

High-rate filters do not filter Cryptosporidium oocysts, or anything else, as well as medium-rate filters. But because many pools have them, it is important to know how to deal with faecal contamination.

The main emphasis is on superchlorination. High-rate filters without coagulation remove as little as 10% of Cryptosporidium oocysts in each pass. Even with coagulation, and perhaps 50% removal, it could take two days to be safe.

  1. Close the pool – and any other pools whose water treatment is linked to the fouled pool. If people transfer to another pool, they should shower first using soap and water.
  2. If coagulation is not the norm, a supply of polyelectrolyte coagulant should be available so it can be hand-dosed in these circumstances, following manufacturers’ instructions.
  3. Superchlorinate to 20mg/l adjusting the pH to 7.2-7.4 and leave for 13 hours (or 50mg/l for 5 hours). Procedures and supplies must be in place for this.
  4. Vacuum and sweep the pool.
  5. Make sure the pool treatment plant is operating as it should.
  6. Backwash the filters.
  7. Allow the filter media to settle by running to drain for a few minutes (rinse cycle) before reconnecting the filter to the pool.
  8. Reduce the free chlorine residual to normal by dilution with fresh water or using an approved chemical. This may mean using the chemical gradually; procedures and supplies must be in place for this.
  9. When the disinfectant residual and pH are at normal levels for the pool, re-open.
  10. Superchlorination should remove any current contamination but will not guarantee future water quality. So it is important to review procedures for the control and removal of contamination by Cryptosporidia.

For further information on cryptosporidia outbreaks, refer to the Public Health Wales Publication: ‘Guidance for the investigation of Cryptosporidium linked to swimming pools’.

Blood Contamination

Pool disinfectants should kill any pathogenic microorganisms in blood or vomit, provided disinfectant residuals and pH values are within recommended ranges. But there are some precautions to take.

Small amounts of blood, from a nose bleed say, will be quickly dispersed and any pathogens present killed by the disinfectant in the water. If significant amounts of blood are spilled into the pool, it should be temporarily cleared of people, to allow the pollution to disperse and any infective particles to be neutralised by the residual disinfectant. Operators should confirm that disinfectant residuals and pH values are within the recommended ranges; bathing can then resume.

Any blood spillage on the poolside should not be washed into the pool or poolside drains and channels. Instead, like blood spillage anywhere in the building, it should be dealt with using strong disinfectant – of a concentration equivalent to 10,000mg/l of available chlorine. A 10:1 dilution of the sodium hypochlorite in use may be convenient. Using disposable gloves, the blood should be covered with paper towels, gently flooded with the disinfectant and left for at least two minutes before it is cleared away. On the poolside, the affected area can then be washed with pool water (and the washings disposed of – not in the pool). Elsewhere, the disinfected area should be washed with water and detergent and, if possible, left to dry. The bagged paper towels and gloves are classed as offensive/hygiene waste, which in only small quantities may be disposed of with the general waste.

Vomit Contamination

It is not unusual for swimmers to vomit slightly. It often results from swallowing too much water, or over-exertion, and so is very unlikely to present a threat through infection. But if the contents of the stomach are vomited into a pool, the bather may be suffering from a gastrointestinal infection. And if that is cryptosporidiosis, infective, chlorine-resistant Cryptosporidium oocysts will be present. This is a rather theoretical, unevaluated risk.

PWTAG recommends that vomit in the pool should be treated as if it were blood (ditto vomit on the poolside).