Solar Water Distillation
If there’s one thing orchids need, it’s clean water. Generally, the cleaner, the better. Better water gives the opportunity for stronger roots to grow, affecting the rest of the plant as well. There are
also benefits for the leaves, for media longevity, and nutrition.
There are several issues behind water quality, most of which I will not discuss here. However, water quality across the US is suffering. In coastal areas, excessive groundwater pumping has caused so much fresh water
to be taken out that salt water has started to contaminate some wells. In the desert southwest and other regions, very hard water causes roots to die. Some orchids require very pure water (less than 200 ppm TDS),
which may not be locally available.
Generally, growers resort to using expensive bottled water, or home reverse osmosis (RO) water treatment systems. One requires paying money to lug water home, the other requires paying more water for a system that
wastes 3-4 gallons (or more) for every gallon produced, causing water quality problems to get worse.
What to do?
Fortunately, there is one system that pays for itself after a few months in operation. Using the power of the sun, solar water distillation can allow you to produce one or more gallons of water per day- water of much
higher quality than even the best reverse osmosis system can produce. Instead of rejecting 3-4 gallons for every gallon of product water, solar DI rejects about one gallon per gallon produced. The water produced by
solar distillation is clean, and inexpensive- but it does require good quantities of sunlight- found throughout the southern United States, where water quality can be the worst.
I built my still from plans produced by EPSEA, the El Paso Solar Energy Association. The biggest problem is that the plans are sketchy, poorly drawn, and refer to products that can be hard to get- such as FDA-grade black silicone caulking, and silicone primer. Everything else can be found with one or two trips to the hardware store, and a glass store.
The net upshot is that you build a plywood box, line the inside
with isocyanate foam insulation, then prime it with the silicone primer. Cover the interior with black, FDA-grade silicone caulking (a total of three layers). A glass pane covers the top, and
slopes towards a trough where the product water collects.
Here’s the still in its final position. The PVC tube supports a polyethylene tube (which is flexible), which then feeds into a 5-gallon collection tank in the shed. Here’s another view of the still.
It’s supported from below on stacks of bricks. The black tube
that snakes up from under it connects to the sprinkler system. It comes on every other day, and fills the still so that it overflows about 2 gallons out of a tube in the back. This puts in some more
fresh water, so that the still doesn’t clog up with precipitates quite so fast.
This is a picture of the still from above. You can see the
way the water flows down the underside of the glass. At the bottom, there is a collection “groove,” where the condensate falls. You can also see how black the still is.
It gets so hot that you can’t hold your hand on the surface of the glass in the afternoon here in Phoenix.
So far, I’ve been averaging about a gallon of water per day, with a 6 square foot still, which is a pretty small one. Most people make them around 18 square feet, which is
only 3’ x 6’, and produce around 3 gallons per day in a desert environment. We use our water for drinking-
making orange juice that doesn’t taste like saltwater, iced tea which doesn’t look like it’s made with peat
moss- and watering some orchids. Before we use it, I put it through a Britta water filter, as although the
still removes organics, I’m not convinced there aren’t some compounds leaching out of the insulation.
Now, the insulation is all sealed behind 2-3 layers of food-grade silicone caulking, so it SHOULD be just fine. Although this design has been thoroughly tested, we’re playing it safe.
The water is incredible. It’s sweet- not flat, like bottled water. Iced tea made with the water is great. Best
of all- it’s free, or darned close to it. Twice as expensive as tap water, really. Let’s do some math.
A gallon of distilled water at the grocery store costs me $1.39. The still cost me $120 in parts (plus about
two weekends in labor) to make. At that rate, after a hundred gallons, I’ll break even. At a gallon per day, that’s less than 3-1/2 months. After that, the still pays for itself.
Let’s say you’re one of the lucky few that can get bottled water at $.25 a gallon. Even if it cost you $160
to make a solar still this size, break-even would be after 640 days, or less than two years. Stills like this supposedly last about 5 years, with cleaning required every two years or so.
Compare this with a reverse osmosis system, costing several hundred dollars, and giving you “good” water (50-150 ppm TDS). Solar distilled water runs about 1 ppm TDS (or less), with an initial cost of
$100-500, depending upon size. RO systems need to have membranes replaced. Stills don’t.
Anyway. This is the way we did it, and I love it. Better than buying bottled water. We’re slowly going to
incorporate solar DI into our flasking operations- we’re very picky about the water we use, for obvious reasons. Even with FDA-grade silicone and polyethylene tubing being the only things touching the
product water, we’re going to play it slow. We’ll be sure to share our results here.
If you have any questions about solar stills, see the EPSEA site, or Agua del Sol. The former sells plans,
kits, and completed units. The second sells assembled and unassembled units.
If you grow disas, they’ll love you for it!