Dealing with a faecal incident in a swimming pool - why 6 turnovers?
When a pool is subject to an accidental faecal release one of the key recommended actions, following closure of the pool, is that the pool water should be filtered for 6 turnover cycles (PWTAG Technical Note - Faecal Contamination).
But why 6 turnover cycles? A justification for this recommendation was outlined in a trade magazine article by Brian Croll in 2004 based on simple modelling of water circulation and filtration efficiency.
But the origins go back to a report published 78 years previously in 1926 (Gage et al 1926) on the findings of a committee comprising members of the American Public Health Association and the Conference of State Sanitary Engineers (chaired by Stephen DeM Gage), which proposed a set of standards for the design, construction, equipment and operation of swimming pools. Many of the recommendations would be recognised in today’s codes of practice, but the really interesting bit is hidden away in Section XVI “Proportioning the water interchange for recirculation and flowing through pools” and is concerned with the purification of water by dilution or filtration as water is recirculated through a pool.
The report points out that this purification process proceeds according to the Gage and Bidwell “law of purification by consecutive dilution”, referred to now as the Gage-Bidwell Law of Dilution. This law is presented in the form of an abstract of a paper in preparation at the time, which states that (in a well-mixed pool) in each pool turnover, 63% of the water resident in the pool at the start of the turnover period will have been recirculated through the pool. This law, proposed by Gage and Bidwell in 1926 in just 625 words, has for almost a century underpinned the recommendations e.g. the WHO 2006 guidelines for the clean-up of pool water following an accidental faecal release.
Look out for our paper on this in 2020.
How dirty can bathers really be?
As well as the cleaning up of pool water following a contamination event, the other aspect of the performance of a pool filtration system that is of interest to designers, operators and those responsible for producing industry guidelines is the maximum concentration of contaminants (in particular turbidity arising from the particles washed off bathers in a pool) that is likely to occur in a pool with different bathing loads.
But how much dirt is actually washed off a typical bather when they swim in a pool? A useful source of data is a paper presented by Stefan Stauder and Matthias Rödelsperger at the International Swimming Pool and Spa Conference in Portugal in 2011. They used particle counting and turbidity measurements on a very busy outdoor pool over the summer months to monitor changes in dirt content in the pool water. They then used the the data on number of bathers in the pool to estimate how much dirt each bather was dumping in the pool.
And the answer for this particular pool was … between 0.25 and 0.5 NTU (bather/m³).
So how much would the turbidity (NTU) of a pool rise if 40 dirty bathers entered the pool (let’s say a 200 m pool)?
If the input per bather is 0.5 NTU/(bather/m³) then the answer is 0.5 x (40 / 200) = 0.1 NTU.
This assumes all the added turbidity remains in the pool - we would hope there is effective filtration and a significant proportion of this dirt would be removed by the filters as the water is circulated!
To help put this figure into context, the upper limit for turbidity for pools (as recommended by WHO and PWTAG) is 0.5 NTU. So without effective filtration it wouldn’t take long for the turbidity of the water to be exceeded.
We’re currently using these data to test our own model for predicting changes in turbidity in any given pool with different bathing loads and filtration systems. The next step will be to use our Mobile Lab to get data for a range of pools and spas in order to find out how dirty bathers really are!