4 ms·

A super-thin slice of wood can be used to turn saltwater drinkable

18 points by based2 7y ago

abdullahkhalids 7y ago
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.
- deleted 7y ago
  [deleted]
- jaclaz 7y ago
  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-powered_membrane_distillation https://en.wikipedia.org/wiki/Membrane_distillation#Solar-po... 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/averagewateruse/ https://www.ccwater.org.uk/households/using-water-wisely/ave... 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-per-person-per-day-reprinted-with-permission-from-3-Copyright_fig1_267434851 https://www.researchgate.net/figure/Water-consumption-liters... 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 7y ago
    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-price https://kenbrooksolar.com/price-list/solar-water-heaters-pri... I don't know the capital costs of membrane distillation.
    - jaclaz 7y ago
      Well, if we use a "normal" (non solar) plant, using this as a base comparison: https://www.lenntech.com/applications/emergency-seawater-desalination-units.htm https://www.lenntech.com/applications/emergency-seawater-des... 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_systems.htm https://www.echotecwatermakers.com/beach_house_desalination_... 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 7y ago
        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.
Someone 7y ago
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?

18 points by based2 7y ago

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.

deleted 7y ago
[deleted]
jaclaz 7y ago
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-powered_membrane_distillation https://en.wikipedia.org/wiki/Membrane_distillation#Solar-po... 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/averagewateruse/ https://www.ccwater.org.uk/households/using-water-wisely/ave... 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-per-person-per-day-reprinted-with-permission-from-3-Copyright_fig1_267434851 https://www.researchgate.net/figure/Water-consumption-liters... 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 7y ago
  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-price https://kenbrooksolar.com/price-list/solar-water-heaters-pri... I don't know the capital costs of membrane distillation.
  - jaclaz 7y ago
    Well, if we use a "normal" (non solar) plant, using this as a base comparison: https://www.lenntech.com/applications/emergency-seawater-desalination-units.htm https://www.lenntech.com/applications/emergency-seawater-des... 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_systems.htm https://www.echotecwatermakers.com/beach_house_desalination_... 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 7y ago
      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.

Someone 7y ago

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?