
A super-thin slice of wood can be used to turn saltwater drinkable - based2
https://www.newscientist.com/article/2212346-a-super-thin-slice-of-wood-can-be-used-to-turn-saltwater-drinkable/
======
abdullahkhalids
20 kg/m^2/h seems enough for a house near the sea to be self-sufficient. You
could pump salty ground water and filter this through such a setup. A person
uses about 350-450 liters of water every day. And 20 kg/m^2/h * 1 m^2 * 24 h =
480 liters. So one square meter is sufficient for a single person.

~~~
jaclaz
You are IMHO a tad bit optimistic, the process needs power, so - usually in
remote areas - it is solar powered, and that - unless you add the complication
of energy storage/batteries, limits the output to a much smaller number of
hours per day.

See (as an example):

[https://en.wikipedia.org/wiki/Membrane_distillation#Solar-
po...](https://en.wikipedia.org/wiki/Membrane_distillation#Solar-
powered_membrane_distillation)

On the other hand, 350-400 lt per person is on the very high side of water
consumption in a very "first world" scenario (such as the US) a more real
estimate is 150-200 lt per person per day this is UK:

[https://www.ccwater.org.uk/households/using-water-
wisely/ave...](https://www.ccwater.org.uk/households/using-water-
wisely/averagewateruse/)

Germany and Spain:

[https://water-for-africa.org/en/water-consumption.html](https://water-for-
africa.org/en/water-consumption.html)

[https://www.researchgate.net/figure/Water-consumption-
liters...](https://www.researchgate.net/figure/Water-consumption-liters-per-
person-per-day-reprinted-with-permission-from-3-Copyright_fig1_267434851)

With a minimum of attention/care 100 lt per day per person can be enough.

And a large part of this is of course not drinking water, so it needs less
desalinisation.

In a "built today" home, the water flushed from the basin/shower/bath (which
is relatively clean) is re-used to supply the toilet and - in some cases - the
washing machine.

Newish toilets have typically double flush at 6 and 9 liters each, so if you
re-use only those it is likely to be in the 30 liters per day per person
saving.

~~~
abdullahkhalids
Thanks for the detailed answer. I am from a developing country and my water
usage is indeed closer to 150l than 400 l. Personally, this peaked my
attention because I moved to a new city just three days ago; the city is on
the coast and struggles with water supply. I was wondering how expensive such
a system is today.

I did think of energy usage but didn't find easy number to quote. The wiki
link suggests 6.5 m^2 of thermal concentrate and 75 W solar panel is enough
for the energy needs of 150l/day in 2011. This is not that difficult for a
remote or urban house, and will cost less than a few hundred dollars
[https://kenbrooksolar.com/price-list/solar-water-heaters-
pri...](https://kenbrooksolar.com/price-list/solar-water-heaters-price)

I don't know the capital costs of membrane distillation.

~~~
jaclaz
Well, if we use a "normal" (non solar) plant, using this as a base comparison:

[https://www.lenntech.com/applications/emergency-seawater-
des...](https://www.lenntech.com/applications/emergency-seawater-desalination-
units.htm)

it's 15 KW for 2000 lt/hour, assuming the efficiency scales linearly (I
strongly doubt it does), a 40 lt/hour that would be 300 W (which I don't
believe), it is more likely to be in the (not very accurate measure) "around 1
kW" range, which in laymans terms should mean some 6 or 7 sqm of solar panels,
possibly a little bit more.

About the cost, you can buy today commercial systems (only the desalination
part) in the US$ 5,000 range:

[https://www.echotecwatermakers.com/beach_house_desalination_...](https://www.echotecwatermakers.com/beach_house_desalination_systems.htm)

The smallest model has 5.4 A @230V which seems just in line with what we
calculated above, around 1 kW.

If we assume that the cost of the needed solar plant (the additional part
needed to supply the desalination plant) would be around 500 US$ or less, the
price to beat is 5,500 US$, but considering that added solar power surface
produces power that might be used, I would say that a more reasonable cost
would be below US$ 4,000, which is still a lot.

~~~
abdullahkhalids
Thanks for the detailed answer again. Clearly, there exists a business
opportunity to innovate on these systems, make them cheaper and sell them in
my new city.

Both the systems you link to are reverse osmosis systems which require
electric power to run the pumps etc. Solar radiation -> electric power is only
20% efficient. My understanding is that membrane distillation tech uses hot
water (60 degrees in OP link), which can run on solar thermal systems + small
amount of PV. Solar radiation -> thermal energy of water can be 80%+
efficient, so the price of the whole system might come down significantly from
the $5000.

~~~
jaclaz
Yep, but it was a comparison aimed to the real world and costs, not abstract
efficiency.

Given that you need for both 5-10 square meters of surface exposed to the sun
and assuming (and it is not necessarily given) that solutions "A" and "B" have
the same "current" (maintenance, spares, consumables) costs, I don't care if
solution "A" is more efficient than solution "B", I only care if solution "A"
costs less than "B".

If it does, then it should cost much less, as solution "B" might have a
tangible advantage (once you have purified enough water the solar energy can
be used for other uses).

Now, be nice, check the actual Solar Spring Gmbh site /the spin-off of the
Fraunhofer Institute for Solar Energy Systems mentioned in the wikipedia
article ), and see how - strangely enough - they build/sell both "A" and "B"
solutions:

[https://solarspring.de/en/products-and-
services/#pg-356-2](https://solarspring.de/en/products-and-services/#pg-356-2)

[https://solarspring.de/en/solar-
purification/](https://solarspring.de/en/solar-purification/)

In any case high efficiency - usually - means more sophistication, added
components, and what not, so rarely exists a solution that is more efficient
AND costs less.

------
Someone
As always with filters, there’s the question of longevity. FTA: _”The water
vapour then travels through the pores in the membrane toward its colder side
and leaves the salt behind”_

⇒ chances are that salt stops this from functioning after some time. How much
time, and how easy is it to remove the salt, clearing the filter?

