You're right Anna, they apparently only contain "solids" - so, even worse, if a child has diarrhoea, it'll go right through... Of course, you and I would not take a child with gastro into a public pool, but some people would...
Then again, I reckon most preschoolers pee in the pool anyway.
Hmmm, I guess we're very lucky we know someone with a heated indoor pool!