Revisiting the Gage–Bidwell Law of Dilution in Relation to the Effectiveness of Swimming Pool Filtration and the Risk to Swimming Pool Users from Cryptosporidium

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