Note that if you anticipate filtering from very dirty sources with high particulate matter, you may want to double-treat: bring a bandana or another strainer to first remove the particulates before passing water through your filter, this preserves the life & effectiveness of your filter.
Choose an appropriate water source
These are the important things to consider (an ironic acronym could be CRUD, since you actually want CRUD-free water, whatever helps you remember it!)
- Clear - the more clear, the better! Large rivers can often be silty (high in particulate matter), so it's often better to take water from a smaller stream or tributary if available.
- Running - running water doesn't collect contaminants, avoid stagnant water if at all possible
- Upstream - move away from trails, roads, or camps to avoid runoff
- Decontaminated - minimize contamination risk by avoiding areas close to agricultural or animal husbandry or ranching use
For more tips on locating water & also staying hydrated more generally, check out this resource.
Notes on physical filtration: All physical filters are susceptible to freezing. If it freezes, ice crystals can expand and crack the filter cartridge, compromising its effectiveness. Unfortunately there's often no sure way to tell if the filter has been damaged in this way, so it should be replaced. If you believe the temperatures will drop below freezing at night, you should sleep with your filter inside a ziplock bag in your sleeping bag, close to your body heat. That said, always attempt to store your filter away from temperature extremes.
Notes on chemical treatment: When using chemicals to clean your water, try to ensure that the rim of the water container is not exposed to dirty water. If it is exposed, then try to treat it as well: after the chemicals have been added to the water, unscrew your container slightly, shake it upside down, and wait until a some of the water drips through to clean the threads and rim of your bottle. Also note that prolonged use of chemicals to clean water may have consequences on health.
Notes on UV filtration: The most important tip for battery-operated devices: take out your batteries! There are too many stories where the device's on-button was inadvertently pressed, and so it was completely drained by the time it was needed. Not to mention, in cold environments, batteries will drain (you should, in these cases, keep your batteries in an interior pocket, close to your body heat), and in hot environments they can explode (you should also keep them in a pocket in this case, somewhere out of the sun).
Notes on boiling water: Per US Center for Disease Control, "Water should be brought to a rolling boil for 1 minute. At altitudes greater than 6,562 feet (greater than 2000 meters), you should boil water for 3 minutes".
Notes on the MSR Trailshot filters that we carry! Sometimes the filter can seem to run really slow or water can leak out from the bulb. We realize these issues can be hard to troubleshoot, so we have published the training videos we use on how to check-in the filter. We strongly recommend reviewing before taking these filters into the backcountry. Note, the same videos apply to the MSR Trailbase filter, since it uses a Trailshot as its core filtration system.