Ember’s 2006 Evapotron Report

For 2006 I built four evapotrons, on four different designs, to test ideas for camp-scale graywater evaporation devices for Burning Man. For most of the week they were in the H2OK corral at the Earth Guardians pavilion. The corral was clearly a success in attracting onlookers, increasing interest in evapotrons, and connecting us with the numerous burners who have been independently developing and using their own designs.

On the playa, “graywater” or “grey water” are terms for water with contaminants that make it unsatisfactory as drinking water, but that wouldn’t normally require a septic waste system for disposal. Graywater includes ice-melt in coolers, shower water with soaps and cosmetics, kitchen waste including dishwater and scraps from food preparation (including raw meat), and toothpaste and saliva. Also a good bit of playa dust.

“Evapotron” is a highfaluting term, coined for Burning Man, for graywater evaporating devices suitable for use and operation by members of a camp. The term “evap-o-wheel” has also been used for an evapotron using rotating evaporative surfaces.

Three of my designs relied on wind power, and for them the corral was great for public attention but not for evaporative performance, because the wind and sun were often blocked by nearby structures. Non-wind-powered devices were no doubt affected as well. To get reliable estimates of performance we need to evaluate all the evapotrons in a more typical camp environment with fuller wind and sun exposure.

Each of the devices stood over a “pond” of on-edge 2×4s surrounding a plywood floor, about 24×48 inches, covered with black plastic sheet. I built the ponds and fans (salvage bike wheels with aluminum-sheet blades attached to the spokes) at home. The devices were brought to the playa as almost-flat components, then assembled.

“The Clothesline”

This design focuses on ease of construction. It is nothing more than an evaporation pond, and fabric that hangs above the pond and dips into the water. One end of the clothesline is easily detached from its support, to permit dunking the fabric fully into the water, then lifting it again to dry.

The clothesline would be a good solution for a small camp. Building it requires little construction skill or time, and in use it needs attention for a few seconds a few times a day, to dunk the fabric. The pond walls need to be taller than 3-1/2 inches; my device’s pond was too shallow to fully dunk the fabric.

“The Nonokini”

The Nonokini is a single bike-wheel fan with fabric wrapped around its nine blades. As wind rotates the fan, the fabric on each blade in turn is wetted in the pond, then lifted to evaporate or to disperse droplets downwind. The intent was to exploit the useful properties of bike wheels, without needing much mechanical tinkering.

Although the low wind made judging performance difficult, I think the Nonokini design would not perform well. The evaporative surface is relatively small, and the side of the fabric that touches the blade can’t evaporate at all. The water released as droplets may land on the ground or may annoy people downwind.


The Archie evapotron uses a bike-wheel fan to drive a bidirectional pair of Archimedes’ screws made of vinyl tubing wrapped around an inclined length of plastic pipe. Water lifted to the top of the screw is dispersed onto several sheets of fabric, to evaporate.

Archie was the most intriguing device to watch, but had several problems. The fan is close to ground level and therefore in very low wind; it is also tilted and shielded from the wind in one direction by the evaporative fabric. These problems could be overcome by making the screw longer; the fan could be elevated to 6 or 8 feet, where it would be in full wind, with a screw 12 to 15 feet long.

Another problem was that after a few days, the fabric became coated with playa dust and graywater contaminants. It lost virtually all its absorbency, and water released at the top ran straight downward without spreading within the fabric by capillary action. Evaporation diminished to near zero. I think the reason that only Archie was affected this way is that the other devices all had fabric moving in the water; contaminants could dislodge, to settle at the bottom of the pond.

I speculate that the evapotrons in several camps that trickle water over hardware cloth, rather than fabric, are successful because the metal mesh is so coarse that contaminants can’t form a continuous coating, as they do over close-mesh fabric. Perhaps a tall Archie using hardware cloth would perform well.

“Old Number One (rev. 2.2)”

Old Number One will in time get a more descriptive name. It’s a redesign of the evapotron I built in 2005. A fan carrying a clothesline pulley is mounted above a fabric-covered drum that is partially immersed in a pond. A drive belt from the fan causes the drum to rotate, dipping into the pond and carrying wet fabric up into the air and sunlight for evaporation.

To save travel space, I assembled the drum onsite. The drum ends were bicycle wheels, held apart by an 18-inch-long plastic pipe with ends notched where spokes touched. #12-24 allthread through the wheels’ hollow axles clamped the assembly together. String laced zigzag from rim to rim supported the fabric and also strengthened the assembly. In attaching the fabric, the ends were left free, which produced flaps that scavenged water from the bottom of the pond.

I made the drive belt from lengths of stockings or pantyhose legs. A very tidy knot for connecting them exploits the “hose” of the hosiery: tie an overhand knot in the end of one length; slide it into the end of another length; tie cord around the end of that length, to capture the overhand knot within. The drive belt is, naturally, very stretchy. Wet it, then stretch it substantially as you decide on the length you need.

Because a bicycle wheel rim looks like a pulley, I tried to use it as one for the drive belt. The belt repeatedly fell off the rim, requiring me to tinker with wheel alignment and belt tension. In hindsight, I realized the drive belt could have traveled on any part of the fabric drum; it didn’t need to ride on the rim.

This evapotron design was the most effective of the four. In sufficient wind, my 2005 device evaporated (or dispersed as droplets) as much as two gallons per hour. “Rev. 2.2” raises the fan from ground level to about 40 inches, where it catches more wind. The drum fabric is old sheets; I think it’s an open question whether using a more-absorbent or faster-drying fabric would make a significant difference.

Additional notes

Evapotron designs must consider disassembly and disposal as well as operation. Unevaporated water must be disposed of; this means getting it out of the pond, straining it (if that wasn’t done already), and scattering it or packing it out.

Once they were dry, I removed the fabric strips and black plastic and packed them out in my trash; apart from that, the devices were relatively clean and could be disassembled and packed out. I burned the wood of most ponds rather than taking them home to store.

Treating (disinfecting) graywater is important for camp safety, but reliable guidelines have not yet been established. Clorox is readily available but may not be the best approach. Its effectiveness is significantly reduced in water carrying suspended solids, so a concentration appropriate for purifying drinking water may be insufficient. Whenever more graywater is added, more disinfectant must also be applied.

Lining an Evapotron Tray

To hold their water, many evapotrons use a shallow tray with a floor of plywood and a rim of 2×4’s standing on edge. To make it waterproof, it’s lined with a sheet of black plastic. I wrote this to remind myself how to do the lining.

To begin with: the one important function of the plastic is to hold water. It doesn’t need to be tidy and unwrinkled, but it does need to be free of holes. Use new plastic sheet fresh off the roll, or check re-used plastic very carefully. What thickness? Thicker than a typical plastic bag, so it’s not fragile. It doesn’t need to be strong enough to support the water’s weight. The wood does that.

You will also need scissors and a stapler, the kind that can push staples into wood.

Determine the length and width you need, and cut the plastic sheet a couple of inches longer and wider. The length is the length of the plywood floor (outside measure), plus twice the height of the rim (twice 3-1/2 in. for a 2×4 rim.) Thus a floor 48 in. long requires a sheet length of 48+7 = 55 in., so cut 57 in. Similar math determines the width.

Now you will staple the plastic to the top of the rim only, but don’t start by stapling! Almost any mistake you make in positioning the plastic means a hole in the wall or floor, and you get to start over with a new sheet. Instead,

Place the plastic in the tray with a weight holding it down at each corner. Adjust the position so the plastic covers the rim tops and is fairly flat across the floor. Don’t worry yet about the plastic bunching up in the corners. Staple the sheet to the rim tops starting 3 in. from the corners, with whatever staple spacing you like.

To get tidy-looking, waterproof corners, grasp a corner of the plastic and pull upward and slightly inward. The plastic will form a fold in a straight line from your fingers down to the corner of the floor and rim. Now pull the plastic outward over the outside of the rim corner. With a little tucking, the formerly bunched-up plastic will form a neat squared shape on top of the rim corner. Staple it twice, once on each side of the diagonal. Repeat on the other three corners.

Trim the excess plastic along the rim’s outer edge, and you’re done.