Cryptosporidium is a major cause of diarrhoea outbreaks linked to swimming pools, but little is known about the frequency of contamination. The primary aim was to investigate the occurrence and concentration, through sampling and modelling, of Cryptosporidium oocysts in leisure pools. Secondary aims were to compare detections with operational parameters, provide the evidence-base for guidance, and improve sampling capacity and interpretation for public health investigations. Up to 1000 L pool water was sampled during swim sessions once weekly for 10 weeks from 8 August 2017 at six volunteer pools. Oocysts were detected by microscopy in 12/59 (20%) pool water samples, at least once in each pool; 8/12 (66%) detections were in August when bather loads were highest. At three pools, 1 L filter backwash was sampled weekly and oocysts were detected in 2/29 (7%) samples, following detections in pool water. The probabilities of a bather contaminating the pool ranged from 1 in 1000 to over 1 in 10,000. Monte Carlo analysis showed that when high bather numbers caused contamination on over 70% of days, multiple events per day were more likely than single events. In these generally well-managed leisure pools, Cryptosporidium risk related to high bather loads. We conclude that public awareness campaigns for bather hygiene, and reminding pool operators of current guidance for managing faecal accidents, should be ahead of peak swim season.

The transfer of water from a swimming pool to the treatment location is key in determining the effectiveness of water treatment by filtration in removing turbidity and managing the risk from particulate material, including microbial pathogens, such as Cryptosporidium spp. A key recommendation for pool operators when dealing with an accidental faecal release (the likely main source of high Cryptosporidium oocyst concentrations in pools) is that the pool water should be filtered for at least six turnover cycles prior to use. This paper briefly outlines the theoretical basis of what has become known as the Gage–Bidwell Law of Dilution, which provides a basis for this recommendation, and extends the idea to account for the impact of filter efficiency. The Gage–Bidwell Law reveals that for each pool turnover 63% of the water resident in the pool at the start of the turnover period will have been recirculated. Building on this, we demonstrate that both filter efficiency and water-turnover time are important in determining filtration effectiveness and can be combined through a single parameter we term ‘particle-turnover’. We consider the implications of the Gage–Bidwell Law (as referred to in the original 1926 paper) for the dynamics of the ‘dirt’ content of pool water, whether in terms of a specific particle size range (e.g., Cryptosporidium oocysts) or turbidity.

Circulation of pool water via a treatment plant is critical for maintaining pool water in a condition that is a safe and attractive for bathers. It is also one of the pool operations that is most costly in terms of energy used and associated carbon footprint. In this study, we compare and contrast circulation rate guidelines developed for the UK and for other northern and southern European countries and, where possible, provide an explanation of the underlying rationale. We focus on the management of turbidity-forming material and Cryptosporidium oocysts. We reveal that the parameters used to assess safe operational bathing load are relatively consistent across Europe, based on physical space requirements for bathing and other constraints such as lifeguarding. Circulation rate, on the other hand, is based on either rule-based turnover times, or a more flexible approach based on the volume of water to be treated per bather. The latter offers scope for innovation and energy savings, with the proviso that safe water is maintained for bathers. The guidance in several countries suggests reducing circulation rate when the pool is not being used by bathers. We conclude that a more progressive risk-based approach to pool management, building on these findings, offers significant opportunities for pool operators to run pools better and should be a priority for future research.