Pre-filtering Water

(Updated April 2017)

Years ago, I led a hike to a permanent stream in the Grand Canyon. Given the guaranteed water at the end of the hike, I didn't concern myself with packing any extra. As it turned out, one person in the group was Oh. So. Slow. We ran out of water and had to refill from a small bedrock pothole. The water was the color and consistency of watery pea soup, and I "filtered" it through my T-shirt. That's called "pre-filtering" these days; it removes pond scum and little critters and such, but the result isn't safe to drink.

Now suppose you find yourself in a similar situation. Suppose, for example, you run out of water and the only alternative is the muddy water in the ruts of a road (see the photo below). If you're desperate enough, you'll drink it. But if you run that water though your filter, it'll clog quickly. Is there something else you can do? I did a little experiment on pre-filtering, and will share the results here.





My goal was to measure opacity due to suspended matter in water. To do that, I taped a checkerboard gauge to the back of a square canning jar. The picture to the left shows the jar filled with tap water. As you can see, the checkerboard is still visible through the slightly wavy glass of the jar, and through 2 1/2 inches (6.4 cm) of water. 

My raw water came directly from the Rio Grande, and I added some Rio Grande mud to the sample to increase opacity (I wanted the water to be as murky as that shown in the road ruts, above). The photo to the left shows a jar full of my "raw" water. You can't see the checkerboard gauge. To put it differently, the water is so muddy that 2 1/2 inches (6.4 cm) of it is opaque. This particular "dirty" water has more sediment and less organic matter than some water you might collect in the wild. 


My next step was to pre-filter this same water three different ways, and compare the differences in opacity. My plan was to include photos where you, my blog visitor, could see for yourself how clearly the checkerboard gauge shows up. Unfortunately, the gauge doesn't show up after any of the three pre-filtering approaches! None of them removed enough suspended sediment. Instead, the pre-filtered samples all remained opaque.


I began with a SteriPEN pre-filter, which cost me $13.88 at REI. You can get the same pre-filter with a funnel to fit most water bottles, but I opted for the version that screws onto a Nalgene water bottle. This pre-filter has a 40 micron screen. Of the three approaches described here, this one was the fastest. The flow slowed as gunk accumulated—but after I did a quick back-flush, the flow picked up again. 

When all of the water was filtered, there was surprisingly little gunk left in the pre-filter—meaning that most of the suspended material had passed through.

Next I tried a common solution among ultralight hikers, a paper coffee filter. The basket-style paper filters are actually pleated disks of filter paper, and you can create a classic chemistry cone filter by flattening the pleated filter and folding it into quarters. In my test I used a lightweight plastic funnel to make the filtration easier, but for a hike that's not needed. As particulates accumulated on the filter, the flow of water  beame very slow. And, as with the SteriPEN pre-filter, water filtered through the paper coffee filter was so opaque, I couldn't see the checkerboard gauge. But at least the paper filter caught more of the suspended sediment than the SteriPEN pre-filter (see photo to left). Not surprising, since the SteriPEN pre-filter has a 40 micron mesh and paper coffee filters screen down to about 20 microns.

Finally I tried filtering through my bandanna, which I also folded to create a chemistry cone filter. Again, I used a funnel to make the filtration easier. Looks like the bandanna captured even more of the Rio Grande gunk than the coffee filter! Here again, the filtration rate slowed as particulates accumulated on the cloth, and once I filled my test jar I couldn't see the checkerboard gauge through the water. So none of the three approaches (SteriPEN filter, coffee filter, bandanna) was able to remove enough suspended sediment to make the water anywhere near clear.

Given the science, these results make sense. Clay consists of particles under 2 microns across; silt consists of particles 2 to 63 microns across, and sand consists of particles 63 or more microns across. The SteriPEN pre-filter's screen will remove silt and organic matter measuring 40-plus microns across, but not fine silt or clay. A coffee filter's roughly 20 micron pores will remove smaller silt than the SteriPEN pre-filter, but not the finest silt or clay. The bandanna (a fairly new one, as opposed to a threadbare one) performed at least as well as the coffee filter—exactly why, I don't know, but it must be partly due to the fineness of the weave.

My tentative conclusions? You can use your bandanna to remove the largest particulates , and then it's time to run your water through your fine filter to remove bacteria, protozoa, and any particulates over 1 micron. Paper coffee filters weigh almost nothing and cost about a penny each, but they're single use and, as far as I can tell, don't perform any better than a bandanna. On future hikes I won't take either the SteriPEN pre-filter or a paper coffee filter; if I have to pre-filter I'll use a bandanna, knowing that none of the approaches I describe eliminates all filter-clogging sediment.


Update, April 2017: I now use a Katadyn BeFree squeeze filter, and it does a good job of eliminating opacity from water—as you can see from my revised page on water in the wild. Plus, it's easy to clean. So if I do have to deal with especially muddy water on a hike, my current strategy is to filter out the grossest stuff with a bandanna and proceed with the BeFree.

P.S. My discussion assumes that water like that shown in the road ruts is a source of last resort. If instead you'll be doing a hike with multiple muddy water sources, you do have options for removing most of the sediment.

  • Some ultralight hikers use biodiesel filters. They can be ordered as small bags with straps, very handy for rigging a gravity feed system.
  • If your water source is a muddy stream, find a sand bar that barely rises above the stream level and dig a big hole in it. Bail out the muddy water and go away for a while, as more water seeps in. Between sand filtration and the water lying still, the water in your hole should be a lot less muddy than the water in the stream. (Please don't damage the environment by digging such temporary wells anywhere but in a sand bar that is wiped clean the next time the stream rises.)
  • Chlor-Floc includes chlorine to purify water and a flocculant to remove the solids. If you follow the instructions, you'll wind up with water that's clear and drinkable. You can get Chlor-Floc at and other online sources. Not something I'd use ordinarily, because I'm not into imbibing chemicals, but way better than drinking unadulterated water from road ruts.
Special thanks to the Rio Grande in Albuquerque, for providing the water used in the test.
Special thanks to the Rio Grande in Albuquerque, for providing the water used in the test.