I bet I'll get shot down in flames for not doing it correctly but I just use Milton sterilising tabs dissolved in 5 lt of water a couple of times, let it stand, run the tap, then rinse. As I don't drink the water I think it's simple and safe (i'm OK so far). I make cider and have milton sterilised all the demijohns and glass bottles for years with no issues. Just empty the waste tank. I'm sure someone has a better way though?