
Freshwater from salt water using only solar energy - diggernet
http://news.rice.edu/2017/06/19/freshwater-from-salt-water-using-only-solar-energy-2/
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bencollier49
6 litres / metre-squared per hour.

We require 130 litres / person per day in Europe. A town of 10,000 people
would require 1,300,000 litres / day = ~54,000 litres / hour = 9000 metres-
squared.

A facility of 100 x 100 x 1 m seems feasible. Based on those calculations,
this all seems quite practical. I do wonder how frequently the filters need to
be changed, though.

~~~
Kayou
I thought 130 litres/person was quiet high and probably included things like
water to grow vegetables, but no, one shower is between 30 et 50 litres and
each time you go to the bathroom, it's 10 litres of drinkable water.

Of course, I you have saltwater and no freshwater, you would probably use
saltwater in your toilets, so that could reduce the daily usage of at least 20
litres. But then, if you consider agriculture and manufacturing, I guess that
the number of 130litres/person would blow up completely.

~~~
stult
Wouldn't salt water in toilets require two sets of pipes leading into each
home? Seems like an expensive proposition unless you're building a new
community from scratch.

~~~
Jedd
It does, but it isn't.

Similarly multiple out-going pipes to split off various levels of grey waste
(some direct to garden, others, to soakways, or to septic processing systems).

Capex for these kinds of systems is ultimately dwarfed by savings on opex -
just like we find with mandatory double-glazing & other insulation
regulations.

~~~
nerdponx
Makes sense. But out of curiosity, where are you getting that info? Personal
experience? Research in some field?

~~~
Jedd
Mostly personal experience including some stories from permaculture types
who've been approaching these problems for a while. I'm designing a habitat
that will have an additional complication of both low-pressure gravity fed and
electric-pump (city pressure) circuits.

Regulations lag behind optimum configuration, though parts of EU, especially
in the north, legislated triple-glazing (f.e.) quite a few years ago (can't
find a cost analysis of break-even points, unfortunately). I'm sure there's
plenty of assumptions wrapped up in these regulations around estimated abode
longevity, energy pricing over that period, etc. But as to multiple water
sources into abodes, I haven't seen much formal work on it. I think the idea
gets touted in London periodically - it'd be a nice solution to the dual
problems of high water table and (surprisingly) occasional potable water
shortages. But retrofitting this to an existing, massive, badly planned
metropolis would be a nightmare.

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ajarmst
Dear God, do you realize what we've done? We've invented evaporation! (Sorry,
the title struck me as funny.)

~~~
Qwertious
Not really - evaporation is used to trap the residue _in_ the water, and
dispose of the water itself.

This is the reverse - dispose of the residue, keep the water.

~~~
ajarmst
I appreciate your recognition of my plea for a refresher in middle-school
science. Most people can't perceive my pain in the midst of the sarcasm.

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dredmorbius
The general problem with membrane-based desal methods is membrane fouling and
lifetime.

I'm not sure that increasing the complexity of the membrane substrate itself
is a positive step here, or that a complex heating mechanism offers
significant wins.

The alternative of more traditional membrane reverse-osmosis processing
focusing on cheap substrates, whilst provisioning power separately
(conventional solar PV would be suitable, and could be located on-site or
remotely) seems rather more tractable.

~~~
msl09
Yeah. After the initial excitement my first thought was if using
nanotechnology could make the amortized cost of a panel prohibitively
expensive.

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rdlecler1
Solar desalination is not new. KKR invested $100b in SunDrop farms which uses
solar desal that produces energy (steam) and water for greenhouses in the
desert.

~~~
smoyer
Solar desalination is not new ... I performed the same feat in the '70s during
Boy Scout camp-outs in Virginia Beach (Fort Story) [0]. What is new is the
level of efficiency!

[0]
[https://en.wikipedia.org/wiki/Solar_still](https://en.wikipedia.org/wiki/Solar_still)

~~~
logfromblammo
IMHO, scout troops should teach the kids how to acquire fresh water before how
to make fire, but for some odd reason the kids find fire to be more
interesting.

Who wants to worry about wells and sump holes and and filters and bleach tabs
when you can burn things?

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SubiculumCode
Frequently I hear how persons stranded at sea or on a small island die of
thirst, and I wonder if they could have used the evaporation method in
conjunction with the sun or fire, and a cloth set above to capture the
evaporated water.

~~~
totallynotcool
METTC dependant, a 'solar still' could work. With the right vegetation and a
plastic bag you could even collect transpiration. Off topic, but interesting
nonetheless.

[1]
[https://en.m.wikipedia.org/wiki/Solar_still](https://en.m.wikipedia.org/wiki/Solar_still)

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malloryerik
Dumb question: would it be possible to turn the desert green? I'm imagining
something done little-by-little, through farming and maybe public trusts. If
this kind of technology were to continue to fall in cost, could we have farms
and forests in the Sahara?

~~~
kbart
Water is not the only problem. You'd also need soil as not much can grow in a
sand.

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Overtonwindow
Desalinization is becoming better understood, as well as cheaper, but the
biggest challenge is still what to do with the leftover salt. We can't pour it
back into the ocean because it will kill everything. Hopefully someday we find
a way to clean and safe way to dispose of salt.

~~~
ori_b
> _We can 't pour it back into the ocean because it will kill everything._

That's simply false. The amount of water that would need to be permanently
removed from the ocean in order to measurably increase the salinity would be
astounding. Let alone the amount that would need to be permanently removed to
kill anything.

If local salinity was a problem (which it isn't) there's an extremely easy
solution: Simply pump more water, remove less salt from it. Dump it in over a
wider area of shore. Let mixing take care of the rest.

~~~
bluGill
Most of the water we desalinate gets dumped right back in the ocean, add the
salt back. Even the water that doesn't get into the sewage system ends up back
in the ocean again via the water cycle/rainfall. (meaning we need to worry
about local salinity issues but not global)

Actually most sewage systems output water that is safe to drink so we can put
that water right back into our drinking water system and forget the whole
problem over an over again. (If you ban lawns there might even be water left
over as food is turned into water and the food probably isn't grown with city
water). People generally don't like the idea of drinking sewage though so this
will never happen.

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anotherbrownguy
Let's not forget that solar cannot be used for anything critical where
reliability is expected. It requires backup from a reliable source to be
practical.

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regularfry
Any particular reason you couldn't do a siphon-pumped multistage flash still
with solar heating? That would be just glasswork.

