If the stool is watery, runny or soft (something like diarrhoea), the risk of infection is greater:
the perpetrator is more likely to be carrying enteric pathogens, and if so they are likely to be spread through the pool water. It will certainly be impossible to remove the faecal material as it is with solid stool.
The infectious causes of diarrhoea include viruses, bacteria and protozoa. (Other causes include alcohol, emotion, diet and medicine side effects.)
Most bacteria and viruses that cause diarrhoea – E coli, Shigella, norovirus, for example – are killed within minutes in a satisfactorily disinfected pool water.
But if the diarrhoea contains oocysts of the chlorine-resistant protozoa Cryptosporidium, normal levels of chlorine will not be effective.
Crypto is a significant cause of relatively serious gastroenteritis, particularly in pools. Young children are both the likeliest sources of the infection, and those worst affected (along with the immunocompromised).
Investigations of outbreaks linked to pools frequently reveal inadequate design and management issues, which would have made the pool vulnerable to an outbreak following contamination with diarrhoea.
In most cases of diarrhoea in a swimming pool, the operator will not know if Crypto is involved. So the safest option is to assume that it is and immediately close the pool.
There are in principle three procedures that will in time remove Crypto – coagulation/filtration, UV and superchlorination.
The procedures to be followed primarily depend on the efficiency of the pool’s filtration. These procedures are endorsed by Public Health England and the national Cryptosporidium Reference Unit which is part of Public Health Wales.
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 ocysts 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.
This, then, is the procedure.
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, 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.