I learned a hard lesson my first day on the Appalachian Trail. The plan was to summit Katahadin and then hike back down to Daicey Pond Campgound. It was about a 13-mile proposition in all, though, it turns out that an ascent of Katahadin is no walk in the park, especially if you severely under estimate how much water you should take! I’m embarrassed to say that I only took one measly liter. Total noob mistake.
I made it to the top okay but was in a state of near delirium hiking down, dragging along, praying for a water source at every turn. I did finally find a trickle right in the middle of the trail. It wasn’t flowing enough to fill up a water bottle, and I didn’t have my filter with me (left it at the bottom of the mountain!), but I stuck my face in there and started slurping it up nonetheless. Giardia was a concern, but fear of dehydration won out. Thankfully I didn’t end up being plagued by the parasite, and I did make it to Daicey Pond - just in time to collapse into my tent right before a rain storm.
In my early days on the AT my water purification device of choice was a First Need Deluxe filter. It was awesome - filtering water twice as good and twice as fast as my friends with MSR filters. The only downside was it broke (twice), and so I switched to Polar Pure and have never looked back. Polar Pure is a lightweight, simple system of purification using iodine. The crystals in the bottom of the bottle seem to last forever, and as long as you keep it filled with water you’re good to go. No need to ration tablets or replace filters. I usually dump two capfuls of solution into a one liter Nalgene, wait for awhile (around twenty minutes, give or take depending on the temperature of the water), and drink! If there is a downside to this method, it would be that there is only enough solution at a time for four (or so) liters. You would then need to refill the Polar Pure bottle with water and wait an hour for the solution to reach adequate strength. It’s also not so good if you don’t like the taste of iodine, but I find that Polar Pure treated water is much more palatable than water treated with iodine tablets.
As a testament to this stuff, I used it multiple times on the Tour Divide to treat stagnant cow pond water and lived to tell about it!
If you do want to go the filter route, there are a number of good options these days. In particular, the MSR Sweetwater has a cult following. In addition to your standard giardia/cryptosporidium filter it comes with a chlorine based purifier solution to take care of the viruses. If you’re out trekking with a group or a family, it’s probably good to have one of these around. Polar Pure, Aquamira (Chlorine Dioxide), and the like are better suited for the individual. I recently purchased a MSR MiniWorks EX to use while out for multi-day hikes with the kids. The big pro with this filter is that it’s completely field-serviceable. Using tried and true ceramic filter technology, it’s pretty much fail proof.