MakeShift Challenge: Potable Water: Potable Water: “Most Plausible” Winning Entry

by William Lidwell
August 08, 2005

As I mentioned in the analysis, I am partial to a distillation solution for this challenge and this is an excellent example of one such solution. If anything, Thornton’s design is a bit over-engineered, which increases its factor of safety, but puts it at risk with regards to time. For example, the effort spent trying to adsorb the arsenic from the condensate is unnecessary because it would have been distilled out, so I can only assume he was playing it safe and going after other possible chemical contaminants. In any event, there is a lot of good stuff in here. Nice job. Congratulations Adam!

Let’s approach this problem in stages. First, we need to get the big particulates out of the water. Then we need to kill the living organisms in it, and then we want to filter the chemical toxins out of it.

I’m assuming the barrels are wood or plastic or sheet steel — something you can tap with hand tools (so, not beer kegs). Step one: fit one with a carrying pole. Put holes in it 6″ or so below the top, and run a bamboo shaft through those holes. Now two people can fill it up and carry it between them, and can carry probably 20 gallons (160 lbs) of water between them. A three-inch bamboo pole should be able to support 200 lbs with no problem. While you’re at it, fashion a coarse screen — just strips of wood, coconut palm leaves, whatever, with a mesh size of about an inch — for the top of the barrel. The only purpose of this is to keep you from getting things bigger than an inch, like dead rats, into the barrel in the first place. On the other hand, coarse cloth, like burlap — which should be in ample supply in a village#-would be even better, as it would also eliminate, say, dead medium-sized bugs.

Now you have a barrel that can easily be filled and brought back to the village, and that won’t let anything really huge into it. Tap this about 6″ above the bottom: make a stopper either from bamboo or coconut shell, and use a little rubber from one of those flat tires as a gasket to keep the seal intact. Put this in before you go get the water. Fetch the water. Put this barrel up on a table or something and let it sit for an hour or two. Then pull the stopper out and let most of the water drain into the other barrel. Pull the other barrel away or replace the stopper before the water level gets down to it. That way, you leave the heaviest contaminants, and those that float, in the original barrel.

Now you’ve got a second barrel mostly full of slightly cleaner water.

For this barrel: tap the top of it, and run perhaps two feet of bamboo tube packed with the steel wool vertically up out of it, again using rubber from the inner tube to make a decent sealing gasket. Fashion a flexible connector out of a few inches more of cycle inner tube (a section without holes) and run another tube, mostly horizontally but slightly downward, from the top of the first one. The longer you can make this, the better; we don’t have a water jacket or a long copper worm coil to aid in the condensation of the steam, so we need a good long run so that most of what comes out the far end is water, rather than steam.

This thing you have just built is a still. The steel wool will make it into a continuous-reflux still, which, while slowing distillation, will enable you to get considerably purer water out the top than a simple pot still would give you in a single run.

To operate the still, you need a heat source. Fortunately, you have an endless supply of coconuts, which are an endless supply of coconut husks, which are in turn an endless supply of tinder, once you’ve dried them out a bit. There is an endless supply of coconuts as fuel, and there’s almost certainly bamboo and other woods at hand as well. The hand drill or (probably more easily) the bicycle pedal mechanism will be one way to get enough friction to light the tinder (basically, you’d make a friction bow). Or, at least for a while, you can draw a spark off the battery using bicycle spokes with tire-rubber insulating grips as electrodes, putting the arc into the tinder. And let’s face it, it’s a village: these folks surely have some way to start a fire. (Of course, you’d think they’d have figured out to boil their drinking water on their own too…)

So now, you heat your barrel over the fire you’ve made, taking care to keep it far enough off the fire that it doesn’t actually burn the barrel. Your water will start boiling; steam will come up the column, re-condense on the steel wool, and drip back into the barrel. Eventually, however, steam will start making it out the top and you’ll get a flow out of the tube.

You probably want to throw away the first bit of liquid that comes out of there. If you’ve got benzene in there, for instance, it boils at about 80 degrees Celsius. This is, at least in ethanol distillation, called throwing away the heads. Once you get a good stream going, this is mostly potable water, and if the villagers drank it, it probably wouldn’t kill them quickly. Most things biological will have been killed by the boiling, and most chemical contaminants will have sufficiently different boiling points that, by throwing away the head and not running the still until it’s completely dry, they will generally be left behind as well. If you had to stop here, it’d be a decent stopgap solution. (In fact, if you just boiled the water for five or ten minutes, and let it cool, you’d probably have something that you could drink without getting any amoebic or bacterial infections, and unless the chemical load was really astonishingly bad, it’d be unlikely to kill anyone quickly, although it wouldn’t win any awards.)

The villagers should catch the outflow in whatever mostly clean drinking vessels they’ve got, including whatever of these water bottles we’ve got left over after the next bit. When steam production starts to taper off, dump whatever is left in the barrel (which will have a high concentration of chemicals with higher boiling points than 100 degrees Celsius), and refill and redistill from the first barrel (which should have been refilled in the meantime).

We haven’t done anything yet about the arsenic we suspect is in there (this is not quite true — the steel wool will help with it, but we’re going to do better, using the same chemical reaction, with the next step), or indeed nasty toxins with a boiling point not too far from 100 degrees Celsius (the benzene’s greatly ameliorated already, though). However, there are still a couple of tricks up our sleeves.

If the problem is arsenic, then a fairly effective treatment method is to adsorb it with ferric oxide. I bet that bike frame is pretty rusty, and I’m sure it’s steel, not aluminum. File rust off of it. And if it isn’t rusty yet, file some steel from it, and boil that in a pan, let the filings sit in the sun a while, and generally do whatever you can do to oxidize them quickly.

Ferric oxide is going to take most of the arsenic out, but it won’t taste very nice and will discolor the water. However, we can ameliorate that — and remove a great many residual nasties — by doing a final filtration with activated charcoal. Where are we going to get activated charcoal? Why, both bamboo and coconut shells are excellent sources. Make a mound of coconut husks. Cover it with coconut leaves. Start a fire under it and let it burn; occasionally pour water on it to create steam, but not enough to put it out. After awhile, the charcoal at the bottom of the mound will be activated charcoal, because it will have burned in an oxygen-depleted environment. Crush it up and you’re ready to put it in your filter. Next you’re going to make a pair of filters out of two more bamboo tubes. Tear four pieces of cloth off your shirt. Cut little transverse strips from the inner tubes to make rubber bands to hold the cloth to the tubes. Pack one tube with rust and one tube with activated charcoal. These are now filters. Use more of the inner tubes to join your filters in series: still outflow to ferric oxide filter to charcoal filter.

Now you’ve got a four-stage purification process: settlement, distillation, and two filters, one optimized for arsenic removal, and one general purpose. The material in the filters will have to be replaced every so often (every couple of weeks should suffice), but since rust and charcoal are easy to come by, this isn’t a big deal. If a pair of people can be dispatched to dump the sludge out of the first barrel and go refill it every time another still run starts, you can keep up pretty-much-continuous distillation.

Now, I’m sure it’s tempting to use that car battery on some salty water, using graphite electrodes made from charred bamboo, to electrolyze HCl into chlorine gas, which you could then dissolve in water and make a crude sort of bleach…but don’t. The boiling you do in the distillation step will do much more good, and you aren’t messing with anything hideously toxic in the process. Note that, once the village gets a decent source of water without having to resort to this system, they can ferment just about anything in their barrels, and distill that to get ethanol. If they put it through the charcoal filter, then they have a source of cheap, clean ethanol (at high proof, too; the reflux still should let them achieve about 75% ethanol once they get good at estimating where to cut the run). They could fuel internal combustion engines with it, but on the other hand, they could just drink it. Bonus! This solution leaves you with $10 in change and 6 liters of water (and a car battery you probably didn’t need). Use the clean water while you get the production process underway (not that it will go very far) and use the bottles as containers for the water output you get from the filters. As for the change, leave it in your pocket, or give it to the villagers, whatever makes you happy. Or, better, wait until the ethanol still is up and running and then play quarters. If you’re not using sparks from the car battery to start your fire, you might want to donate it to those poor schmucks in the woods in Make: 01.

> More analysis of winning MakeShift entries from Make: 02.

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