I first saw this water filter made from a wooden stick long time ago on somebody’s blog. I’ve since read a lot of comments and opinions about its usefulness. Turns out, this idea comes from a publication in a free-access online scientific journal, which means everyone can download and read it for free. So I decided to read the article and test the filter so you don’t have to do it 🙂
First of all, I must admit I wasn’t able to replicate the experiment with a 100% accuracy as the biggest bottle I could find (5 litres) was not big enough to exert enough pressure for the filter to work efficiently. It did work but way too slow. That’s ok though. My point wasn’t to make it work exactly like in the paper. I simply wanted to illustrate what this might look like when made from simple readily available items. And other than the fact it wasn’t fast enough, it worked pretty well. It filtered water, didn’t leak along the walls (as far as I can tell), didn’t cost much or take long to build and required little effort and attention to keep it going.
Lets get to the questions.
– What wood and why?
The authors used white pine but they point out that it doesn’t have to be this particular species. What it does have to be, however, is a coniferous tree. Why? Because conifers have much shorter vessels and thicker xylem layer than deciduous trees, which allows us to use those short sticks effectively and safely. In other words, due to extremely long vessels of broadleaf trees water would simply pass through damaged xylem without being filtered properly.
Conifers are able to filter particles down to 200 nm in diameter, which means their vessels will stop bacteria, protozoans, cysts and eggs of some nasty worms. Unfortunately, they won’t stop viruses.
How fast does it filter water? According to the paper, pressure of 5 psi (35 kPa) should allow several litters of water to pass through the filter every day. Enough for at least one person even in a hot environment. Again, to exert such pressure you will need a bigger container than the one I used here and you’ll have to keep toping it up not allowing for the pressure to drop too dramatically and to slow the filtration down.
So is this a viable survival method? First of all, I don’t think it was supposed to be. The authors say the idea was to address “the need for pathogen-free drinking water in developing countries and resource-limited settings”. Ok, the ‘resource-limited settings’ can be interpreted as a survival situation. And yes, if you stay in one place long enough and if you have the materials required to build such a filter it should serve you well. But if you just wont something that you can shove in your pack and use like a straw straight out of river, you can forget about it. I know because I’ve tried it. So I suppose it all depends on a situation.