This is, by far the simplest approach in concept - just carry everything you'll need. The obvious advantages are that you know the water's safe and palatable, and you don't have to go find it while you're out. The disadvantages are that you it's bloody heavy, and scales with the length of the trip. Even for relatively short trips you'll find a desire to predict how much water you'll need to reduce weight. You'll almost always want to carry some amount though. Few people drink exclusively from streams. It is not uncommon to need to carry water for a full day or even multiple days even when using other treatments. Consider areas such as in deep deserts.

Pros

• Simple, guaranteed good water
• Can be done in areas without ground water

• Heavy and therefore impractical for longer trips
• Not a great situation if you run out of water

Filters are hugely varied. Some require pumping, some are gravity fed. Some can filter raw sewage, and some don't remove much of anything. Pore size is related to what type of stuff a filter gets out of the water. The standard rule of thumb is that you want a pore size of 0.2 microns or smaller to remove bacteria, giardia and crypto. Some filters include an activated carbon or iodine stage (affectively blending filters into chemical treatments). Activated carbon grabs ions. Thus it is good at removing pollutants affecting taste as well as industrial chemicals. Note that some pollutants taste good (and are good for you), Brita filters are activated carbon. An iodine stage will kill viruses.

I started hiking with a filter, and found that it made drinking water a treat. We were always looking around to see which stream would have the best tasting water to try. As soon as the water had filtered I'd sip some up. I stopped carrying a filter after taking a nine day trip in the Alegany forest however. On that trip our gravity fed filter sprung a small leak, and became so clogged that we were spending a significant part of the day waiting for water to filter.

Brewer has used a ceramic pump filter, these can often be force backflushed by connecting up some hoses backwards and running already filtered water backwards through the pump. With the high-quality ones you can generally always make the pump work (modulo complete failure of the pump body). They can take futzing though, and are pricey, and may take a lot of back-flushing. Also a number of models (which often get good reviews) are barely functional. Note that a pump filter is far faster than a gravity filter, but you'll have to work for that speed.

Pros:

• Delisous
• Can drink water imediately after filtering

Cons

• Slow
• Can unexpectedly fail
• Expensive
• Is the only thing that will help with polutants

Again there is a wide variety here with some chemicals working much better than others. The big advantage of chemically treating is that it's very fast to fill up, and there isn't much to break. You can also adjust the dosage based on the quality of the water. The big disadvantages are water taste, and having to wait to drink it (in which time you're probably carrying it). Chemicals won't remove any bad taste from the water itself and they tend to leave a bad taste themselves. They also don't help with polution.

Despite all those disadvantages I usually treat my water with Polar Pure. Polar Pure is a glass bottle with iodine crystals in the bottom. To use you keep it filled with water, and use this water containing dissolved iodine to treat your main water supply. Dissolving the iodine before adding it to the water means you have to use much less to achieve the same treatment, which makes it way more palatable than iodine pills. It also means that to get more solution you just fill the bottle back up and wait. They're effective as long as there are still visible iodine crystals, which is many months of continuous use. It also means you have a concentrated iodine solution for disinfecting wounds if nessiary. On the down side the iodine does impart some taste, and requires a large intake of vitamin C. (Iodine cancels vitamin C, so if you continue to intake large amounts of iodine without any vitamin C your body will eventually reject it. On the plus side adding vitamin C to water after it's finished treating neutralizes most of the taste.) It also requires you to wait ~30 minutes before drinking new water.

Don't forget that you can always run water through a bandanna before treating it. If you're pulling water from a swamp, a seep, or after pulling a rock out of a nearly dry stream bed, this trick will make you much happier.

There exist UV wands which run off batteries. They aledgedly treat water, however there is no way to know if the UV bulb has gone out, and they break and run out of batteries in a very unpredictable way. I ran into many people on the AT who ended up without any water treatment solution because they'd only been carrying a UV wand. They also don't work as well in murky water, which is arguably what needs the most treating. On the plus side they don't affect flavor at all and you can look like you come from the future.

Boiling works. It kills things. The disadvantage here is mostly time and inconvenience. Hiking in areas with sketchy water or in situations where I'm melting snow I'll often end up boiling my water to treat it because the stove is already set up. Boiling is also among the few treatment methods that you don't have to worry about freezing (e.g. freezing water in a filter can destroy it). Keep in mind though if you're boiling water you need to carry extra fuel and plan in the time to sit and wait for it to boil. This approach makes a lot more sense for static camping when you have a large fuel supply (such as wood). The great thing is, no flavor affect, and it works every time (modulo viruses).

This is the riskest of the options, but also my favorite. I only drink water straight in situations where I'm fairly confident that it's safe. For example a 50 degree spring (50 degrees is about the temperature water is when it comes out of a true spring, due to geothermal heating) where I can get the water straight from the spring instead of from a still pool beneath it. Or if the water is fast moving, cold, near the top of a mountain, and I happen to know there isn't anything bad upstream. Bad things upstream include anything that poops (especially cows, beavers, and humans), roads, or anything else unsavory. Backpacking lite actually just ran an article about drinking water straight. I haven't gotten sick from drinking water yet, though it may always just be luck.

Something to keep in mind: a number of studies have pointed to self contamination as the primary cause of most "waterborn" diseases while backpacking. Statistically before worrying about treating your water, you should worry about washing your hands after every time you use a cat-hole.