How to disinfect distilled water for a humidifer?

Over the summer I ran a dehumidifier in my basement and collected the distilled water in a drum for use in my humidifier in the winter. It's the ultrasonic style humidifier so I wanted water with zero mineral content. The dehumidifier was brand new so it was completely clean, and the drum I used is a 55 gallon plastic drum that originally contained a disinfecting caustic cleaner in it, which I very thoroughly rinsed out with city water (which has a high chlorine content). I thought everything would be perfectly sanitary and I'd have a collection of perfectly distilled water, but when I went to use it for the first time I found it smells like a musty dish rag.

Is there anything I can put into the drum of water to disinfect it and eliminate the smell which will be safe when it's vaporized through the humidifier? I've considered bleach but I've read several sources that said that's hazardous to do.