Most of the bugs in water that might cause harm to a human in large doses will die if the water is kept in the dark. So if it's kept in the dark it will become almost sterile. Kept in the dark, in good containers and not near a source of pollution (most plastic containers will permit many chemicals to migrate through them) then drinkable water will stay drinkable more or less indefinitely. There's a good reason that many of the water bottles you see aren't transparent.
Light will allow green algae to grow in water unless it was sterile to start with which is probably difficult to achieve. Puritabs, chlorine etc. will do that but may leave a nasty taste.
I keep 2 litre PET bottles full of water in the loft just in case we lose the supply. There are fifty or so up there at the moment. Many of them have been there for years.
Later on today I'll get one down for you and drink it.
Not all tap water in the UK is treated with chlorine. A couple of years ago, when staff at work complained of a bicarbonate flavour in the water at work in Derbyshire, I asked about it at our water supplier's technical department. They said they pump it out of the ground, put it straight in the pipe and that's it. No chlorine, no anything. Just water straight out of the boreholes. They had no idea what was causing the strange taste in the water. Scary.
