Hydrapack?

[OakLeaf]

I would also be afraid of parts getting brittle if they were frozen.

If I leave water in the drink tube for more than a few days it will get grody, but that's why I have the long brush. Bite valves clean up nicely with a brush (such as the kind that's sold as a toothpick, or a child-size toothbrush with the bristles cut down) plus a soak with a denture tablet.

The inside of the bladder is very easily cleaned - even brands with a narrow filler hole. Get a dishrag wet - somewhere between damp and sopping, depending on how much residual water is in the bladder. Shake a generous amount of baking soda into the bladder. You don't want it to dissolve completely, so that it acts as a mild abrasive. Close the filler cap/zipper and shake vigorously so that the dishrag goes all around the bladder. Change your hold a couple of times to make sure no corners get missed. Then fish out the dishrag and rinse. Shiny clean!

[Becky]

I've been freezing empty flat bladders for at least 5 years now, and no problems yet. That said, I completely understand wanting your freezer space for more palatable items

[malkin]

I have one made by Deuter with a big opening. I like the pack sensibilities too, but I am learning how to drink from bottles on the road too.

[jessmarimba]

I have one from North Face with a wide opening and detachable hose. Never tried a hydrapack.

I also run hot water through after I'm done, and then put a couple paper towels inside to sop up anything that's left. I've never frozen or refrigerated a bladder.

Then again, stuff here (besides month-old bread sealed in plastic) seems to dry out long before it grows anything.

[malkin]

Even the bread dries out here, unless we leave it over the steaming kettle.