I was talking with a pool attendant once a few years ago. She said that basically though no on ewill admit to doing it, the vast majority of swimmers will urinate in the pool, either voluntarily or involuntarily. This said when you consider the size of the bladder to the volume of water in the pool already, then the concentration of urine to water is immensely small (an oxymoron?). The chlorine is added as an antiseptic to counter the urine, but as mentioned previosuly, there's very little bacteria in urine anyway. There are certain pools in the UK where they add dyes to deter people from weeing in the pool, but not in all pools. You think this all sounds horrible swimming in urine water? then consider what we chuck into the sea, next time you go for a swim in there - personally i won't swim anywhere in the UK coastal areas except devon/cornwall, or the south west of Wales, all of which receive relatively clean atlantic waters