Treating Water with Alum

Experienced GC guide Drifter Smith posted this originally on, and it also appeared in the Waiting List, V.3 #3.


There's an easy way to settle water using alum as a settling agent. I do this in Grand Canyon and on the San Juan all the time when the river is muddy. First, you need to buy some alum, which you can find (usually) at the drugstore in small quantities, or you can buy larger quantities from a restaurant or food supplier, swimming pool chemical supplier, or chemical company. A pound or so would be a very generous amount to carry on a multi-week river trip with a couple dozen people if you settled all your drinking and dish water:  odds are, you'd actually use less than half a pound. Alum, by the way, is aluminum ammonium sulfate. It is approved for use in food products, for example it's used to keep pickles crisp. It's the same stuff that's called "baking powder," the aluminum kind... It's also used in municipal water treatment plants to help remove particulates: it's a flocculent.  It causes small suspended particles to clump together, so they will settle out.

If you have a friend who works in a municipal water treatment plant, you might be able to score a 100 lb sack of the stuff cheap: enough to last you and all your river running buddies a couple lifetimes.

My preferred method is to prepare a saturated solution by filling a container about 1/3 full of alum powder or granules, then adding very hot water. (Shake it up to disolve the alum...if you add too much powder or granules, and they don't all disolve, you can add more water later after you've used some of the saturated solution). On commercial trips I carry a quart sized Nalgene bottle - the kind with a small (narrow) cap - of this pre-mixed solution and I use the cap to measure the amount to settle a five gallon (bailer sized) bucket of water. A tablespoon of this solution - about half a capful - is stirred into a 5 gallon pail of muddy water: be sure to mix it thoroughly. If the water is very muddy you'll see the flocculation begin within a minute or two. In a half hour or so, almost all of the mud and 99%+ of the alum will have settled out on the bottom of the pail. Then you can decant the clear water into another container, or carefully filter the settled water off the top.

The prepared solution is easy to measure and goes to work instantly; sprinkling and stirring the dry powder will also work, but is less accurate and more time consuming: odds are, you'll end up using more alum than you need, and you'll definitely have to wait longer for the water to clear. It's a good idea to use the minimum amount that will work; if you use too much there will be enough left in solution to degrade the taste of your drinking water.  This works best in very muddy water: if it's only a little muddy, it's not so effective.

Even if you have reservations about adding chemicals to your drinking water (who doesn't?), you might want to consider the advantages of having clear water for washing your dishes.

Most of the bacteria present in muddy water are found attached to the mud particles: if you get rid of the mud, you also get rid of most of the bacteria. When the water runs muddy, I fill a couple bailers and stir in the alum as soon as I've unloaded my boat at camp. At dinner time, the dishwater is clear, regardless of how muddy the river is."

- share water
- Drifter Smith

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