First time filter user.

It is effective at filtering micro organisms, but not things like heavy metals, chemicals, or other toxic substances. Also really murky or silty water is more likely to clog a filter.

Stagnant is OK, mine runoff not so much. I just pick the best source I can and trust the filter when it comes to organisms.

Basically I drink water filtered from a stagnant puddle if that is the best available source. I have resorted to that quite a few times and never had a problem, but given the choice a nice ice cold mountain stream or spring would be my preference.

I have used the squeeze and more recently the mini since their release and consider myself very lucky to have always found relatively clean and seemingly safe water sources. The squeeze will remove everything except viruses, which I have been led to believe are rare in North American water supplies so yes, you would likely be safe drinking from a stagnant pool. I have always carried 10 katadyn tablets on the chance that I would not be so lucky in my water selection options. If you can filter the water, and tablet it, there will be nothing left alive.

Chemicals, (heavy) metals, etc. are another story and require a purifier to remove. The only backpackable filter I know of that is an (EPA certified) purifier is the First Need, however that level of water processing will cost you over $100 and once used weighs over 20 oz.

The limitation with the Squeeze is that you don't know if the filter is/has been damaged by freezing. So don't let yours freeze.

When people are talking about a stagnant pool they are referring to natural water sources (such as rain puddles). As noted, these filters will not remove heavy metals and chemicals, so do not drink from downstream/runoff from agriculture or livestock, the Sawyer will not work against that.

Some people use a pre-filter to catch the larger "floaties" so that the filter itself doesn't clog too quickly. Pre-filter can be straining through a bandana, a piece of sponge, or a filter mesh material.

As always, the purists will correct accordingly, but roughly put, back country filtration breaks down into 3 main catagories.

Filtration: A membrane type of physical filter that simply doesn't let any particles larger than size X pass through. This kind blocks cysts, like Giardia and Crypto, as well as general nasty skankiness of biomass polluted water. (Sawyer)

Purification: A method to kill living organisms. Things like viruses, that are much too small to be blocked by a physical barrier, die fairly easily with chemicals like chlorine, chorine-dioxide, iodine, etc. Cysts have a hard shell that make them hard to kill with chemicals though. Should really be used in conjunction with filtration, but usually isn't. (Aqua Mira)

Adsorption: Molecular level chemicals like pesticides and fertilizers are bonded to activated charcoal, which is why aquariums use it to pull ammonia out of the water. (Charcoal cartridges)

A full blown, old-school, backwoods filtration system consists of a pre-filter sponge, a main membrane filter that's been treated with a chemical virus killer, like iodine, then a charcoal cartridge to remove chemicals, including much of the treatment chemicals used.

I didn't bring up boiling. Although it will kill both the cycts, and the viruses, it doesn't remove anything, so it's not a "filtration" method I guess.

Of course this is way overkill for most cases, and as with all UL practices, a bit of wisdom goes a long way. Choose water sources carefully, and treat accordingly.

The Sawyer is a membrane filter only. It will not kill viruses, and it will not bond chemicals from out of the water. Agricultural and livestock runoff are bad choices to try to drink from.

Can you stick the Sawyer in a mud puddle and drink from it? Not along side a highway, but out in the woods, probably pretty easily. But remember, it's just a filter. One that's designed to filter out microscopic organisms. If you start filtering out stuff like dead bugs, grass, sand, silt, decayed leaves, etc., you won't get much water out before it's too plugged up to use. Prefilter dirtier water first, then use the Sawyer for its intended purpose of filtering out finer matter, and it will serve you well.

A well rounded system would include prefiltering heavily particulated water, a .1 micron absolute membrane main filter, like a Sawyer, then treat the filtered water with chlorine dioxide drops like Aqua Mira for 15 minutes. You won't see many people on these forums doing all these steps, but also remember that most of the vocal community here gets their water from high elevation mountain springs, which are a pretty pure source. Start drinking from low elevation beaver ponds full of tanin tainted stagnation, and you might consider using both filtration and purification.

Great answer Glenn.

Would the steri pen qualify as purification?

Thank you for the insightful comments! Glenn, that answer was exactly what I was looking for.

Here in the UK it's not hard to find fresh water sources, it's more the heavy farming of the land that I would see as a problem, as sheep roam pretty much everywhere you go, but only lower fields are used for crops and sprayed with chemicals, which I would imagine puts most rivers out of use without boiling.

@ Glen
>>
A full blown, old-school, backwoods filtration system consists of a pre-filter sponge, a main membrane filter that's been treated with a chemical virus killer, like iodine, then a charcoal cartridge to remove chemicals, including much of the treatment chemicals used.
>>
My first Hiker Pro from PUR was marketed for third-world use and included all the components you name: prefilter, membrane with iodine, filter, then charcoal post filter to remove the iodine taste. I took it on some trips to India and used it for the intended purpose.

Query: If you suspect pesticide or herbicide contamination, I would think none of the commercially available cartridges contain enough charcoal to do the job. The one Platypus sells as an add on to its gravity filter kit for example is marketed for improving the taste.

Another factor to consider is blue-green algae contamination, which makes the water toxic. As a general rule, avoid any water which has a color. Tea-colored water is generally ok though, as long as upstream vegetation explains the color.