Seawater => Drinking Water

[Pblais]

What do you think about getting a RO membrane for $150, then getting a pressure washer off ebay for $50, and hooking it straight up?

The on the pump is too imprecise and it can't sustain a flow that low even at high pressure even if the pressure could be regulated enough. The salt water would corrode the pressure washer really fast since they are not built for salt water. There are plans for DIY watermakers but you have to realize you are filtering to the molecular level. Aqueous salts are very very small. They are far smaller than any living organism. This has nothing to to with the RO water conditioners that you can use for household water. Those don't filter salt water.

[seandepagnier]

To solve the regulation problem, there needs to be a feedback loop. The pressure washer is on/off, so I would need it to turn on at say <850 psi, and off at 800 psi.. or maybe other numbers work better/ extend membrane life. With a high pressure tank between it and the membrane, the pressure is effectively lowpassed, so it wouldn't change instantaneously.

With the right valves, I could pump water as normal to make fresh water, then switch over to a different closed system that pumps water in a loop through the pressure washer and a tank with water (and other non-toxic solution to prevent rust and corrosion) and therefore wash the pressure washer out each time to hopefully extend life.

Maybe I should just stick to rain and solar still, heh.

[beau]

i have found on my boat that most of our fresh water is actually used as a shower at the end of the day ( I do have a woman to look after)
i was very interested in the concept of recycling shower water which would be a lot simpler than RO.
What would you suggest, filters, basic RO ???

[seandepagnier]

I thought of recycling water too, but from the head. The machine would use a centrifuge to separate liquid from solid, the solid would be completely dried and burned to heat the boat. Then the liquid parts are filtered for drinking water. It would work from the sink drain too.

[Pblais]

I thought of recycling water too, but from the head

You could be the first to do it too.

Aqueous salts are about the limit to RO and it's not like falling off a log. After that you need magic. Not all ideas are possible.

[lorenzo b]

I recently read Sailing The Farm, he goes on about solar stills and shows you how to build them. Has anyone tried them? A very amusing book, by the way, for those of us who are still trying to remember the 70s.

[Curmudgeon]

Rather than a RO watermaker, which uses amps, can break down, or can operate badly if the membrane not properly maintained, I'd be interested in a good system for catching rainwater. You would need a system that channels rain water to the tanks when a valve is opened. When it starts raining, you would wait a few minutes to let the ranwater wash away the salt from the channels, then open the valve.

[sandinmytea]

Raincatchers and solar stills all sound like the best idea for us tiny-boat, cheapskate sailors.

I still think the best way to get the pressure needed to run a DIY R.O. system that was only big enough to produce those few gallons would be run off an air-compressor forcing an extremely small aperture water-pressure device.

In other words, rather than running a cheap pressure washer pump not designed for salt water anyway, you would use air pressure to force a VERY SMALL amount of water through the smallest available membrane. As its been pointed out, aqueous salts are terribly difficult to remove from salt water, so you have to have absurd amounts of pressure. I assume the only way to get that pressure easily would be to keep a compressed air tank full with a small 12v compressor. When you want water, open the valve that would go to a water compression chamber. The water could be forced through a very small aperture that would end in your appropriate membranes (also small, and theoretically replaceable). This would discount the need for an expensive saltwater pump, but it assumes that you can physically create the necessary air-pressure to force water (and water is tough to move at high-pressure eh?).

Anyways, that's my goofy idea. I still think a solar still combined with a raincatcher is the most practical method for gaining just a few gallons a day.

[osirissail]

Originally Posted by Curmudgeon

Rather than a RO watermaker, which uses amps, can break down, or can operate badly if the membrane not properly maintained, I'd be interested in a good system for catching rainwater. You would need a system that channels rain water to the tanks when a valve is opened. When it starts raining, you would wait a few minutes to let the ranwater wash away the salt from the channels, then open the valve.

- - Rainwater catching systems are the second most popular systems seen in the Caribbean. During the 6 month "rainy season" an awful lot of water falls from the sky. Just about every boat you see has at least a bimini and most have huge awnings that cover the main deck area. These are used to get some shade and relief from the sun cooking the boat's cabin top. You will see variations/alterations made in these awning to allow them to catch rainwater and funnel it through small "thruhull" fitting in the awning and then into hoses that can be inserted into you water tank fills.
- - Some people try to channel water falling on the decks of their boats into the deck fills for the water tanks. This is a bad idea. FRG boats are covered in white gel-coat which is comprised of polyester resin mixed with calcium carbonate and titanium dioxide to get the color. The sun's UV bakes the decks of all boats and "chaulking" occurs in the gelcoat. This dust is then washed with the rainwater down into your tanks. And finally into your body by drinking or cooking with the water.
- - The awnings and specially sewn canvas "rain-catchers" can capture enormous amounts of water safely and then hoses can send it to your water tanks. The design and construction of the awnings and rain-catchers can be trickly and usually is custom done for each boat by yourself or by a good canvas/sail shop. In any case having the awnings to keep the blazing sun off you and your boat is standard amongst cruising boats.

[beau]

I have never heard of compressed air being used to build up the pressure of sea water to the 800psi necessary for RO.
Cheap 12v 4wd/truck air compressors are available which reach up to 2,000psi.
Any comments on the suitability of such a system?

[Curmudgeon]

My mainsail is an excellent rainwater catching system if I loosen the outhaul. Unfortunately it all goes on my head instead of into the tanks.

[sandinmytea]

Originally Posted by beau

I have never heard of compressed air being used to build up the pressure of sea water to the 800psi necessary for RO.
Cheap 12v 4wd/truck air compressors are available which reach up to 2,000psi.
Any comments on the suitability of such a system?

That was my thought. To pump water raw takes a lot of energy. I've always assumed forcing water with air would not be a bad idea. There are a few other applications a handy yacht-bum might be able to think of for the compressed air when its not making water as well. Like making balloon animals!

[seandepagnier]

I thought it would be good to use a scuba compressor, then you can use it to fill your scuba tanks too. The annoying part about the air compressor is you would only be able to do short runs, and then refill the tank. I'm pretty sure efficiency is lower because you have to release the air in the tank when it is still under pressure to fill with more water which is a waste of pressure.

[04 Marine]

Which do you think is smaller, salt in the water or oxygen atoms. If you try to pressurize water with air behind a membrane I suspect the air will pass right through the water and the membrane. An RO system if fairly straight forward. If you can afford a pump that can pressurize a tank to 2,000 PSI along with the relief valves etc., then you can afford to buy the components for an RO system.

Two weeks ago I purchased a Hydracell F-20 pump capable of 1,000 PSI on Ebay for $78.00 and $17.00 shipping. I can run that off of a 1/3 to 1/2HP electric motor, Marathon makes one that draws about 24amps DC. The pressure vessel is a fairly easy build and even reasonable to just buy one. Any way you go, you will still need pressure vessels, membranes etc.. Most everything else is avaiable from one of the supply houses.

Truth be told it's generally easier and cheaper to follow in the footsteps of others then to blaze new technology.

[sandinmytea]

Originally Posted by 04 Marine

Which do you think is smaller, salt in the water or oxygen atoms. If you try to pressurize water with air behind a membrane I suspect the air will pass right through the water and the membrane....

...Truth be told it's generally easier and cheaper to follow in the footsteps of others then to blaze new technology.

I wasn't suggesting you force air into an RO system. You just use air to produce that 2000psi you're talking about. First compress and store the air, and then use the air to force water in a compression cylinder.

How's the water-maker build coming?