Thursday, January 14, 2016

The $3.00, 11 gram Water Filter You Probably Shouldn't Use

I was so impressed with djbarryiii's YouTube video showing how to create an ultra-light, ultra-cheap, ultra-durable water filter that I had to try it myself. The project is really simple: purchase a 1 micron bio-diesel filter off of eBay, cut out a circle and fold it using the technique you probably learned in high school chem lab to produce a sort of funnel. And you're done. Here's the filter next to a LifeStraw for comparison purposes:

Now before you chuck your water purification device of choice, here's a few reasons why the above is a bad idea:

1.The bio-diesel filter is a nominal, not absolute, filter. That means that that most objects 1 micron or larger will be filtered out. Did you catch that 'most'? A nominal filter will allow some particles larger than the size it's rated at to pass. An absolute filter, like the LifeStraw, has no space larger than it's claimed size, so it filters as advertised.

2. While 1 micron will filter out the 5 micron size Giardia and Cryptosporidium, it won't filter out other nasty stuff, like Cholera, E. Coli, and Salmonella (source:, not to mention viruses which are even smaller. The LifeStraw does far better with an absolute filter size of .2 microns.

That's not to say that this little hack doesn't have some serious advantages going for it. The obvious ones are that its relatively cheap and quite compact.

With limited experience I can tell you that the larger the disk size, the easier it is to use. When I cut out the first disk I used a tuna fish can as a stencil. The result when folded is a relatively small reservoir, that required slowly pouring water into the filter, being careful not to overflow it. The larger size, as pictured above, was created by using a Cool Whip container as a stencil (what can I say, I used what I had). The result when folded is a nice size reservoir that still requires care when filling, but allows for a descent flow rate.

Oh, and speaking of lessons learned: I originally purchased a 0.5 micron bio-diesel filter bag thinking it would be an even more effective option. Alas, the filter is so fine it has almost no flow rate. It was way too slow to be at all practical.

While the disk above is compact and lays flat, it's not quite small enough to be stashed in my wallet (alas, it would have made a sweet EDC water filter). It's definitely compact enough to store in a glove compartment, dropped into a day pack or stuffed into a under-seat bike bag. Heck, it might even find a place in my trail running kit.

Chemical purification tablets may be smaller than this filter, but they suffer from a number of annoyances. Katadyn, for example, claims to take up to 4 hours to fully work. Further more, the tablets are really optimized for quarts of water. If you want a relatively quick drink now, a filter would allow you to process just a cup full of water, versus purifying a quart and waiting for hours before the water is ready.

Another key advantage to this sort of filter is how flexible it is. I don't mean that literally, though it is indeed physically flexible. I mean that you can use the filter to rig up a water purification system using whatever materials you have handy.

Given all of the above, it doesn't look like I'll be chucking my Portable Aqua tablets anytime soon. But as a backup solution, say on a trail run when space and weight are a premium, I could definitely see putting this filter to work.

Here's the video that inspired it all:

1 comment:

  1. Cool. As you say, as a compact, lightweight backup it warrants some consideration.