Is solar water distillation not economically viable for a whole home water supply

October 19, 2010 3:24 PM

Is solar water distillation not economically viable for a whole home water supply? Some friends live outside the city limits and don't get city water. They have a well, but it produces smelly, sulfur (?) water that isn't potable (or at least that isn't desirable). I figured solar distillation would be the way to go, but most stills I've found only produce 3 liters a day. Is there anything that can do 400+ gallons a day that the average US home uses?

They've tried bombing the well with chlorine, but that only fixed it for a few days. The water destroys any fixtures it comes in contact with. Seems their only viable option may be to drill a new well, but there's no guarantee that will fix the issue since they'll probably be tapping the same pool.

posted by willnot to home & garden (19 answers total)

Is there anything that can do 400+ gallons a day that the average US home uses?

The average home probably has a lot of high-flow toilets and shower heads and other inefficient devices. With efficient water fixtures and checking for leaks, a household can reduce its daily use to about 45.2 gallons per person. If your friends have to provide their own water treatment infrastructure, they're going to want to reduce that even further.

I dunno how many people live in your friends' home, but efficiency is definitely a good idea. Given the price of any kind of distiller, it's probably going to be cheaper to buy new toilets and fixtures than to get an increasingly large distiller or other treatment system.

Anyway, you're not going to get anywhere near even 100 gallons a day with a solar distiller, especially during the winter, unless you built one to cover the whole roof and then some. There's just not enough energy coming in from the sun. If your friends want to be energy efficient, buy an electric distiller or other conventional water treatment system and get a solar water heating system.
posted by jedicus at 3:35 PM on October 19, 2010 [1 favorite]

Here's a press release that just came out today about a Portable Solar-Powered Water Desalination System from MIT. Not really solar distillation but getting there - but only 80 gal/day costs $8000.

That said, your friends have power, so wouldn't a standard RO & filter setup work. Here's a guy that actually talks about treating smelly Sulphur water.
posted by Long Way To Go at 3:36 PM on October 19, 2010 [1 favorite]

Have you looked into water filters? I don't know much about them myself but both my mother and brother use them. I Google's for. "Water filter sulphur " and the first image result looks just like the filter my brother owns.
posted by phil at 3:39 PM on October 19, 2010

Forgive my phones auto correction
posted by phil at 3:40 PM on October 19, 2010

You don't need water that is going down the toilet or gong on the garden to be potable so capacity doesn't need to include that. Efficiency measures can reduce this still further, and generally speaking efficiency should always be the first step in considering any renewable energy scheme at home.
posted by biffa at 3:41 PM on October 19, 2010

Filtration is a lot easier and less energy-intensive than distillation. First they need to think about how much water they actually want filtered — just drinking water? If so, it's only a few gallons a day, and they could probably get away with an under-the-counter tap filter (charcoal or RO) or maybe even a Brita-type pitcher. If they want to filter bathing/showering water, then it's going to require a much bigger system.

But you can have degrees of filtration for different purposes. My parents have a well and have a whole-house sediment filter (cheap) where the line runs into the house, which strips out the worst of the stuff and keeps it from eating the pipes, and then run drinking water through an under-the-counter activated charcoal filter in the kitchen for drinking and making coffee. They just decided that removing the sulfur from bathing water wasn't worth the cost. Your friends should consider their options and weigh them against how much they care. You can buy a lot of replacement faucets and toilet flapper valves for the price of a whole-house RO system or even a good high-volume filter.
posted by Kadin2048 at 3:43 PM on October 19, 2010

How large a solar collection area do you need to distill a given volume of water?

On average, you can get about 6 kWh / m2 / day from the sun (assuming 100% collection efficiency). 1 kWh is 3600 kJ, so you get about 21600 kJ /m2 / day from the sun.

The heat of vaporization of water (energy required to boil water) is 2257 kJ / kg. So you can distill about 10 kg water / m2 / day using solar energy. 1 kg of water is about 3 gallons, so even for the low use case of 45 gallons per person, you'd need about 15 square meters of solar collectors.

Obviously, this is a relatively crude calculation, but it tells you why you can't find any solar distillation systems that handle the volume of water you want - the energy just isn't there.
posted by pombe at 3:49 PM on October 19, 2010

My water isn't bad enough to corrode things. However, it's bad enough to "smell funny" and it has both Fe and Mn (as oxides) in it. So I use an ozonator. This takes out all the smell and all the metal oxides. It does not take out calcium carbonate.
posted by jet_silver at 4:07 PM on October 19, 2010

How abouut a reverse osmosis unit? You can get them in configurations that produce water in batches or you can get units to serve the entire home. I realize they 'waste' water to produce water but it might be an avenue to explore.

SandPine
posted by sandpine at 4:32 PM on October 19, 2010

Obviously, this is a relatively crude calculation, but it tells you why you can't find any solar distillation systems that handle the volume of water you want - the energy just isn't there.

That back-of-napkin calculation is assuming that you throw away your energy once you boil the water. I had a look at some designs online, and after boiling the water, they take the heat out of the boiled water and use it heat up the yet-to-be-boiled water, recapturing and reusing the same energy over and over, so they are vastly more energy efficient than this calculation suggests.
posted by -harlequin- at 5:12 PM on October 19, 2010

The other issue to consider, even if all the other technical difficulties are overcome, is storage to bridge the gaps between daylight and cloudy days. It could be that you need a 5 day tank to maintain service.


One of the problems the asker mentioned is that the water is literally eating all the fixtures. So a drinking water-only solution won't help.

My advice:

0- First, they should get the water tested to see what exactly is in there. Much easier to fight what you know than what you think. Also, the well could be contaminated with Various Bad Things.

1- The pipes in the house are probably in terrible shape. They may need to be replaced. That may well be part of the problem: there is so much crud in the pipes that the water may be coming out of the well in decent shape, but as it goes through the piping, picks up bad smells and flavors. Same with any pressure tanks, etc. Even the well itself: the water might not be so bad, but the pipe or the lining is f-ed up and making things worse.

2- Filters don't remove dissolved stuff.

3- Activated charcoal works good to start with, but quickly turns into a waste of time as the charcoal gets "used up". They would almost literally need to change the canister on a monthly basis.

4- They probably will need a big honking water softener. Besides "softening" the water, they also replace various awful ions (like sulfates) with less awful ones like sodium.

5- There could also be galvanic and/or ph issues going on.

6- And yes, reverse osmosis would be a good solution for the drinking water part of the problem. But even RO units need a lot of upkeep, and if the water is truly terrible, those cartridges will need to be changed quite often as well.

Off the wall solutions:

A- Treat the well water as best as you can so that it doesn't ruin fixtures, but then just buy drinking water. Might be cheaper than maintaining RO units.

B- Collect rainwater and use that for all the non potable needs. Or at least to sort of dilute the well water so it isn't quite so foul. And then purchase drinking water.

C- Do like municipal water does: buy some swimming pools and create settling and/or aeration tanks. Collect the water that evaporates as your distilled drinking water, and use the water that has "aired out" as non potable bathing/washing/flushing water.
posted by gjc at 5:14 PM on October 19, 2010

Seconding the suggestion to get the water tested. Find out what you are dealing with.

Look into aeration. Hydrogen sulfide remediation in drinking water from wells can be
as simple as spray aeration as it is pumped from the well into a holding tank. Automatic
chlorination will also work on hydrogen sulfide.
posted by the Real Dan at 5:44 PM on October 19, 2010

If you decide to go with filtration, maybe try Aquasana. They can get one of the faucet filters for $100, try that out first (they're good for 6 months). If it's up to grade for what they need, grab the $1000 home filtration unit for the whole house.
posted by yeloson at 7:04 PM on October 19, 2010

Sulfur water is stinky but it sure makes skin soft. And refrigerating it takes away the odor, in my experience. It tastes fine.

I was at a sulfurous spring a couple of months ago, Indian Springs in Georgia, met an old lady who was filling endless gallon jugs. She does it once a month. When she told me her age I was astonished, she looked twenty years younger. Fountain of youth.
posted by mareli at 7:12 PM on October 19, 2010

That back-of-napkin calculation is assuming that you throw away your energy once you boil the water. I had a look at some designs online, and after boiling the water, they take the heat out of the boiled water and use it heat up the yet-to-be-boiled water, recapturing and reusing the same energy over and over, so they are vastly more energy efficient than this calculation suggests.

Congratulations! You just invented perpetual motion!

You can recycle some of the heat, but you cannot recycle all of it. The Second Law of Thermodynamics won't let you.

The original calculation was a rough one, and represented a low bound because it assumed 100% efficiency, which is also forbidden. You may make some gains through reuse of some of the heat, but your system will still be less efficient than the napkin calculation. It's a good estimate to show the approximate size of the problem.

For all that solar power is hip and sexy, it's really rather diffuse. And, of course, it's drastically attenuated on cloudy days. (And not present at all at night.)
posted by Chocolate Pickle at 7:36 PM on October 19, 2010

1 kg of water is about 3 gallons

erm, 1 kg is (by definition) 1 litre, which if you're talking US gallons is 0.26.

Also need to reinforce the early point - water efficiency of 150 litres (40 gallons) per person per day should be a very reasonable, easy target.

But that's roughly 20 m2 solar evaporator per person per day. So no, it's not going to happen that way.
posted by wilful at 10:10 PM on October 19, 2010

How about rainwater? I grew up too far from the city to get on the mains, and my parents had two huge concrete tanks that provided all the water we could ever need. Tasted way better than Adelaide water as well.

Even if you don't want to drink rainwater, it should be easy to get your house plumbed so that drinking water comes from a separate source to non-drinking water.
posted by A Thousand Baited Hooks at 1:14 AM on October 20, 2010

If part of their water needs include irrigation or sprinkler systems, they could consider a greywater system. The idea is that water used for laundry and baths can be effectively recycled into water good enough to sprinkle your lawn with, even if you wouldn't want to drink it.
posted by musofire at 10:57 AM on October 20, 2010

Congratulations! You just invented perpetual motion!

The original calculation was a rough one, and represented a low bound because it assumed 100% efficiency, which is also forbidden. You may make some gains through reuse of some of the heat, but your system will still be less efficient than the napkin calculation.

You're being silly. I'm talking about real-world designs, and they are far more efficient than the napkin numbers because that assumed no reuse of energy at all. Throwing in "perpetual motion" suggests you're not thinking it through. Assuming substantial losses, the efficiency can be orders of magnitude greater than the napkin number.
If what you said was true, hybrid cars would get lower gas mileage than the regular model, not to mention half of industry wouldn't work. Recycling energy works.
(You may also be surprised at how close to 100% efficiency you can get when the form of energy you need is heat, rather than heat differential, motion, chemistry, electricity, etc.)
posted by -harlequin- at 8:49 PM on October 24, 2010