We like to think the water we drink is totally pure and clean. It generally is, but after a year, the effects of the chlorine or anything else that may have been used to limit the growth of stuff would have been overwhelmed. There are some algae and bacteria that could produce slime. As to whether you can sanitize it to then use it, I'd guess yes, but don't know. You might call the manufacturer and see if they have any suggestions on how to sanitize it so you can then use it. Some of that may have gotten into the pipes. You could probably hook up a pump, and run some bleach through it and the pipes, but I'd talk to someone that may know how best to handle this..