Hey, guys…
I was a microbiologist in a previous life, and I can tell you with all due certainty that freezing will not kill microbes. It will slow down their metabolism and reproduction, but it definitely will not kill them. They’ll resume their normal function as soon as they warm up.
The only way to actually kill bacteria and mold in your CamelBak effectively is to use one of the commercially available detergents and/or sterilizing agents.
Leo