Filters are a favorite method with backpackers. They have a good reason for liking filters. High quality filters do a great job of removing contaminates and debris while retaining that fresh, clean stream taste. Ever tasted water from a cold mountain stream? Oh my! It's wonderful!
The most important thing to look for when shopping for a filter is its pore size. This tells you how small of a particle it will filter out. Protozoa are about 1 microns, so just about any filter will take care of them. However, bacteria can be as small as .2 microns. Few filters can remove particles this small. To give you a point of reference, a period at the end of a sentence is usually around 500 microns. Buy the filter with the smallest pore size you can afford.
Filters do not work well against viruses. They can be as small as .018 microns. Thankfully, in developed countries it is very rare to find viruses in the water. Also, they do not remove all pollutants. Do not solely rely on a filter in third world countries.
There are a TON of different types of filters.
Hand pump filters are probably the most common. They are easy to use, but are heavy, bulky, and expensive. A good one can set you back over $300. Decent ones can be found for less than $100 though. These are a great choice for a back country camp or to keep in your car, but the average hunter will not want to lug this around with them on a day hunt.
Although they are less expensive and easier to use than hand pumps, gravity filters are even bulkier. Most of them work by filling a container with water and letting it flow through a filter into a drinking container. They are simple and work great, but are most useful for a campsite. I doubt any hunter will want to lug one around all day. You can find them for less than $100.
These are basically a straw filter attached to a bottle. They are perfect for a hunter IF you can find one that filters out bacteria. That is a big if. I have not found one that does. They generally cost less than $50.
Water purifiers are the ultimate choice if you want really safe water. They use chemicals or electricity to kill anything, including viruses, that makes it past the filter. Like hand pumps, they are heavy, bulky, and expensive. I have been told that they do not work well in cold weather, although I have not personally experienced this. These normally cost more than $100.
There are a couple other types of filters, but they are not practical for a hunter. I can see why campers and backpackers like filters, but I do not think they are the best choice for a hunter. Hunters are hard on their gear and need to be able to move quickly and quietly. Most filters are moderately delicate, heavy, and bulky. I can see carrying one if you know that you will be needing it, but they are impractical as a "just in case" item.
Next we will look at a method more suited for the hunter. Stay tuned!