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I'm wondering how long before one needs to clean their personal water bladder/reservoir while on a thru-hike, extended backpacking trip, or survival scenario.
I know it's advised to clean after every use but sounds excessive while in the wilderness for that amount of time.
Perhaps some hard data on how long it would take for mold, algae, and/or bacteria to grow in such a scenario might be helpful (given that the water wouldn't be stagnant for that long during the day-to-day).
On a trip where you are constantly using a bladder or water bottle where it's not sitting around, I would limit the cleaning to filling it up and then dumping it once before refilling.
Mould takes time to grow, and on most trips, you are refilling at least once a day if not more.
If you're in a part of a trip where you aren't going to use a bladder for a couple of days, make sure it dries out before it gets packed away.
Usually, the problem with these comes from being in storage, not out on the trail.
If you try to grow mold or fungus (think: mushrooms), you will find that it generally takes at least 7 days under near-ideal conditions for a spore to reproduce to the point where it's visible. This is why most food will be edible in a refrigerator for at least a week (not necessarily fresh or tasty, but at least visibly mold-free). It's also why most food with at least some moisture will show mold after 14 days or so (mold spores are pretty much always in the air, so the moment you expose food to it, the countdown begins).
Ideal conditions for sporulating microbes include a stable temperature range, aqueous environment with a particular pH, consistent food source, and for some microbes, oxygen. Thus, non-ideal conditions include desiccation, food scarcity, and movement. The parts of your hydration system that get regularly cycled with water and have little exposure to air (like the bladder) will be relatively inhospitable to microbes, due to the poor food supply (assuming you get mostly clean water) and low oxygen. I've never seen biofilm on the inside of my water bladders.
On the other hand, the parts which are exposed to oxygen and get a regular "food supply" (like the mouthpiece) are much more likely to accumulate biofilm. Fortunately, people drink out of water bottles with visible biofilm every day, with little to no ill effects. But in my experience, it noticeably changes the taste of water. So if you can taste the biofilm in your system, it's a good time to clean it out (which means, not a bad idea to clean it out before you go on your trip, if you haven't for a while). If you can't taste the biofilm, then you're gonna live. Your Stone Age ancestors were exposed to much more mycotoxin than that, and yet, here you are!
If you're really paranoid, just wiping off your mouthpiece in clean water periodically will keep the biofilm to below-detectable levels. But you'd need to be pretty paranoid to do this more than once a week. The tiny internal bits in the hoses are more difficult to clean, but unless you're backwashing bits of food into your water system, it shouldn't build up much.
First of all, I don't think a one-size-fits-all number can be given. Some influencing factors I'd consider:
What water do you put in there? Molten snow? Untreated stream/source? Tap water including proper levels of anti-microbial treatment? Micro-filtrated puddle?
I find no problem quickly rinsing and refilling a water bottle over and over again even in summer with tap water in Germany. But I've had a bottle of "nice water from our* own well" visibly and smellibly going wrong within less than a day (Poland, so pretty much the same temperatures).
While I think "your stone age ancestors" arguments should be taken with care, one thing that I adhere with is that if my pre-stone-age chemosensors (smell, taste) tell me the stuff is not good, I trust them.
Keep in mind that we still have about 1.1 mio. deaths annually (world wide) due to dysentery - that's the same order of magnitude as malaria.
And the recommended most important measures for avoiding it are food hygiene, personal hygiene (both somewhat more difficult in outdoor conditions than at home) and water hygiene which we're trying to push towards its limit in this thread. So that's against us. On our side is that we're usually going on tours with in good health and that there are few people around (e.g Shigella are contracted from humans/primates; E. coli in general may be more often encountered). Also keep in mind that quite a lot of what we're calling "tropical" diseases aren't really bound to the tropics and are not of a concern in the "western" world just because our style of living mostly allows us very good hygiene (which we leave at home in good parts for outdoor tours) and medical treatment (which may not be at hand).
If you want your water bag to last as long as possible without cleaning, then maybe choose your storage device to be easily rinsed and dried out and operate it in old fashioned non-contact mode: no mouthpiece, no tiny hoses/tubes. Big clear bottle (allows visual inspection as well) and either learn to drink without sucking or bing a mug and use that.
There's also the intermediate possibility to use water from a bladder that gets slightly suspicious only for cooking. (We call it minestrone...)
You may not need to keep all your bottles and water bladders moist all the time. If e.g. you need the big bladder only for fetching water in the evening/morning, consider whether it could be carried openly so that it is dry inside most of the time in between.
If your bladder stands boiling water, you may consider giving it a fill of boiling water if needed (possibly drying it before).
Metal containers are often far more hostile to microbes than, say, rubber bladders. Plus, you can use them for boiling. I have used my alu pot to fetch stream/lake water that was anyways to be boiled in order to not contaminate my plastic bottle or bladder with unfiltered and otherwise untreated water.
Personally, I may bring some chlorine or iodine tablets so that they are at hand if needed, but I may not be using them all the time. I've also successfully been distinguishing between "clean" and "not-so-clean" bottles (the latter e.g. for cooking water).
Last but not least, remember that hands are often the problem, not the content of the water bottle (at least not until those hands are in contact with vital parts of said bottle).
I may say that so far I've only once had trouble that I consider related to drinking suspicious water (from a tap at the end of the line, summer in Italy - being used to that region having reliable water treatment, I had put my mouth under the tap before opening it. Spitting out apparently wasn't sufficient - and of course there was no clean water around to wash out my mouth. Lesson learned.) I haven't been around in tropic regions, though. And I've been on tour with friends who did have trouble that may have had to do with water. I've also been doing plain dumb things in terms of food hygiene - which fortunately didn't have bad consequences (realized in time that there was a mistake and spent some thought on the situation and also some hand disinfectant).
*not ours, but the place's where we asked for water
