
Chemists Work to Desalt the Ocean for Drinking Water, One Nanoliter at a Time - Claudus
http://www.utexas.edu/news/2013/06/27/chemists-work-to-desalt-the-ocean-for-drinking-water-one-nanoliter-at-a-time/
======
haldujai
Original paper: doi: 10.1002/anie.201302577

From their paper it is mentioned they have a 40 nanolitre/min flow rate (25%
desalination rate, 99% is considered safe to drink water).

They use an electric pole to generate an ion depletion zone, their problem
right now lies in the severely limited flow rate and desalination rate. From
my understanding the 25% desalination rate does not compound linearly and
decreases in efficiency as the concentration of salt ions decreases.

Additionally their flow rate is 0.4 microlitres per minute, this would equate
to needing 625 000 channels for one pass only to get 250mL / min. Scaling for
microfluidics isn't simply using a larger pipe size, microfluidic devices
largely operate with minimal forces and a Reynolds number of 1, that doesn't
hold as you get larger.

The other big problem here is that this requires pressure driven flow, to do
that they made two reservoirs of uneven height on opposite ends to drive flow.
Again this is very electrically cheap (picowatts) when dealing with such small
and perfect flow conditions.

The biggest problems they need to address are the low filtration rate and the
low flow rate, it does not seem like there is a simple answer to the first,
they do state that they are conducting a larger scale experiment that they
will publish later.

I largely suspect that the larger scale experiment will fail in efficiency as
another group (doi:10.1038/nnano.2010.34) who also uses a ion depletion zone
(albeit by a different mechanism) found after publishing that they were orders
of magnitude (10^3) too low in their predictions.

If anything this will perhaps be better than small, portable RO setups but not
replace large factories which are actually pretty efficient.

~~~
rflrob
>Additionally their flow rate is 0.4 microlitres per minute, this would equate
to needing 625 000 channels for one pass only to get 250mL / min. Scaling for
microfluidics isn't simply using a larger pipe size, microfluidic devices
largely operate with minimal forces and a Reynolds number of 1, that doesn't
hold as you get larger.

What about parallelization? Are there any obvious downsides to simply having
lots of the active units, aside from the (what seems to me largely manageable)
increase in micro fluidic chip complexity?

~~~
haldujai
Yes, parallelization is actually the currently accepted means of increasing
throughput, you'll see diagnostic devices with thousands of channels. The
reason it isn't appropriate here is because of the complexity, the design is
easy, you make a radial pattern of inlets with the splits towards the middle
so you can combine outlets of the same type. Construction is relatively easy
as well.

The problem comes from the _number_ of inlets. You can:

a) a common inlet to all of your channels, or b) independent inlet for each
channel, or for several channels

a) might seem intuitive, the problem is limitations on channel width, the
inlet would have to feed a single channel which would then split (they would
split to the inlet channel from the paper), structural limitations of PDMS and
manufacturing have maximum widths and heights in millimetres at best. Which
would not be enough to achieve the throughput required. The original 'master'
inlet would probably have to be at least in the 10-20 cm range to achieve flow
rates that could make this a household filter.

Even with another, stronger material, and perhaps manufacturing techniques I'm
not aware of you still have limitations of pressure, their channels are very
small in the paper (for a reason), pressure becomes a limiting factor to
prevent their failure.

The problem with b is sheer complexity, you're talking about 10 000 tubes and
connectors and holes punched into a chip if you even do a 1 - 60 split. The
chip with just the channels would be about the size of a tissue box, this
wouldn't be able to accomodate the channels so now you're talking about
something tens of metres x tens of metres. This is hugely cost prohibitive and
the channels from connectors would have to be really long, really long
channels need more pressure to drive the liquid requiring more energy which
reduces the efficiency significantly.

The reason parallelizaton works in pharmaceutical / DNA testing applications
is you're going from microlitres of DNA/samples to nano, or picolitres in the
channels, a single inlet can sufficiently provide that throughput (you're
talking 10 uL / hour perfusion rates)

Edit: TL;DR version: Construction constraints would make this have worse
efficiency than reverse osmosis and cost a lot more to manufacture as well.

~~~
Retric
An idea that may be useful is to place this upstream of a reverse ausmosis
filter. That way the inlets and outlets can feed into the same area. You would
still need to separate the inlet and fresh water outlet far enough from each
other and you would need to setup the pressure differential but you might be
able to do that by keeping the water moving in the high salinity tank.

The advantage being reverse osmosis is more efficient when the salinity
difference is smaller. Still I don't think the added complexity is going to be
worth slightly lower energy costs.

------
Ellipsis753
"The new method requires so little energy that it can run on a store-bought
battery."

What a silly measurement. I can buy a laptop battery or a car battery at a
store. Also it does not give any idea of how much water could be produced from
the battery. A teaspoon per battery probably isn't too great. (Although at the
moment they get only nanoliters of salty water from it of course).

~~~
Luyt
A similar silly unit is used to measure 'green' energy: "This windmill can
power 10.000 households". As if we're too stupid to understand a kWh figure.

~~~
aroch
It's nothing to do with being smart or stupid, 10K homes is a more descriptive
number (more easily visualized) than kWh for describing energy production to
the general populace.

------
bcks
The page is returning a database error for me, but the Google cache shows a
blog entry describing and linking to the low-power desalination technique
announced here: [http://www.utexas.edu/news/2013/06/27/chemists-work-to-
desal...](http://www.utexas.edu/news/2013/06/27/chemists-work-to-desalt-the-
ocean-for-drinking-water-one-nanoliter-at-a-time/)

~~~
ParahSailin
[http://libgen.org/scimag5/10.1002/anie.201302577.pdf](http://libgen.org/scimag5/10.1002/anie.201302577.pdf)

------
fernly
The paper linked from the article is behind a pay wall but the abstract[1] is
very brief: "A simple power supply is used to apply a 3.0 V potential bias
across a microelectrochemical cell comprising two microchannels spanned by a
single bipolar electrode (BPE) to drive chloride oxidation and water
electrolysis at the BPE poles. The resulting ion depletion zone and associated
electric field gradient direct ions into a branching microchannel,
consequently producing desalted water."

[1]
[http://onlinelibrary.wiley.com/doi/10.1002/anie.201302577/ab...](http://onlinelibrary.wiley.com/doi/10.1002/anie.201302577/abstract)

------
monochromatic
I'd love to see an energy-conservation analysis of this technique. How many
joules per liter does it actually require?

~~~
solox3
40 nL of desalted water per minute, for a minute, using 3.0V voltage (from
article) and 0.7 A current (standard AA, "store-bought battery" from article),
yields 3.2 GJ/L.

Even if we give it the benefit of the doubt and say we're overestimating the
current by a factor of 100, 32 MJ/L still a ridiculous amount of energy, and
~10x more than just distilling water through evaporation, which is 2.23 MJ/L.

~~~
sbierwagen
The actual paper (
[http://libgen.org/scimag5/10.1002/anie.201302577.pdf](http://libgen.org/scimag5/10.1002/anie.201302577.pdf)
) says they manage an efficiency of 25 mWhL^-1 (90 joules per litre)

Where did you get that 0.7A number?

~~~
solox3
0.7A is the current of the "store bought" (AA) battery, suggested by the
article.

To make this technology energetically favourable to boiling, it must not
exceed ~700μA in current, and it has to be far less than that to be worth its
complexity.

~~~
sbierwagen
Uh? No battery[1] delivers a constant amount of current:
[http://en.wikipedia.org/wiki/Ohm%27s_law](http://en.wikipedia.org/wiki/Ohm%27s_law)

Solving for current, Ohm's law is I = V/R. You only deliver .7 amps of current
into a 4.285 ohm load.

If you connect a 3 volt battery to a 1,000 ohm resistor, then only 0.003 amps
of current will flow, which is 9 milliwatts.

Connect a 3 volt battery to a .01 ohm load (a dead short, almost) and 300 amps
of current will flow, (900 watts!) very briefly, until the battery voltage
sags under load.

1: Or any voltage source in _general,_ barring some trickery that you can't do
with a simple electrochemical battery.

~~~
HCIdivision17
I had an electrical engineering old timer explain the misconception as the
Current Push Theory. (His industrial war stories were pretty good.)

~~~
jonmrodriguez
Googling for "Current Push Theory" only gets 5 results, none of them relevant:
[https://www.google.com/search?num=100&q=%22Current+Push+Theo...](https://www.google.com/search?num=100&q=%22Current+Push+Theory%22)

Can you please define the Current Push Theory?

~~~
HCIdivision17
The basic idea was that you can't put a 5kW drive on a 2.5kW motor: it would
shove too much power into the motor and break it. This is not quite at all how
those systems work.

~~~
jonmrodriguez
Doesn't it depend on the situation?

E.g., if you have a resistive load and an inductive power source, the power
source will keep upping its voltage until the load accepts the current that's
being shoved down its throat.

------
zw123456
It would be great if a Solar cell or small panel could power it. That could be
a life saver in emergency situations.

~~~
ars
The absolute minimum energy you need is 2.2kJ per liter. A 4x4 inch solar cell
could do that in about 2 hours.

However I suspect that you'll never actually desalinate water for that amount
of energy in a portable system.

The very best commercial systems manage to do it with about 3 times the energy
of the minimum. More common ones are about 10 times the minimum.

~~~
sbierwagen

      The absolute minimum energy you need is 2.2kJ per liter.
    

Where's that number from? For what process?

~~~
ars
Mixing saline and fresh water releases energy. To "unmix" them you have to
supply energy.

See:
[http://en.wikipedia.org/wiki/Osmotic_power](http://en.wikipedia.org/wiki/Osmotic_power)

------
usablebytes
Why do we usually utilize science to fix the wrong problem?

Shouldn't we be making more efforts on 1) Growing more trees 2) Reducing
environmental pollution 3) Controlling human population

???

~~~
ajuc
There's more than 8 000 000 000 of us, if everybody were just doing the thing
with the greatest priority it would be very ineffective division of work. Like
digging one hole with 1000 people.

As a humanity we can (and should) multitask, especially when it comes to
science and inventions, because you never know which seemingly pointless path
can lead you to a great breakthrough.

Imagine everybody since antiquity worked only on the greatest humanity
problems as they've seen it at the time. No useless stuff like music, so no
irrational numbers for example :)

~~~
BillyMaize
The population of the world is slightly under 7 billion.

~~~
CapitalistCartr
The US Census bureau says 7.097 Billion. They estimate that the world
population exceeded 7 billion on 12 March 2012.

[https://www.census.gov/popclock/](https://www.census.gov/popclock/)

[http://www.worldometers.info/world-
population/](http://www.worldometers.info/world-population/)

------
efsavage
I don't put much stock in those who make claims like "the next world war will
be over water", because we have a massive supply of water (the ocean) and a
massive supply of energy (the sun) to power whatever we need to do to make it
drinkable.

A floating desalination plant powered by an x000-acre floating solar platform,
pumping clean water back to shore, and dispersing slightly saltier water
across a wide area, doesn't seem like science fiction to me, and doesn't seem
very destructive. The process doesn't even have to be super efficient, we may
even end up using old-school techniques like distillation or electrolysis to
keep the mechanisms simple.

(granted this helps coastal regions more than inland ones, but moving
population-heavy coastal areas to this system will preserve more of the
natural water supplies for the inland needs).

~~~
sbierwagen

      The process doesn't even have to be super efficient
    

Economics are important. We didn't go back to the moon because it was too
expensive, and we're not going to see massive-scale desalination because it's
also too expensive: [http://physics.ucsd.edu/do-the-math/2012/10/the-energy-
water...](http://physics.ucsd.edu/do-the-math/2012/10/the-energy-water-nexus/)

    
    
      California uses 46 billion gallons of water per day. 
      Supplying 25% of this via desalination would require 36 GW of 
      thermal-equivalent power. California runs on 30 GW of 
      electricity, and a total energy budget of 262 GW (thermal; 
      from oil, gas, coal, hydro, nuclear, etc.—according to the 
      EIA). That’s a substantial amount for 25% of our water needs.
    

A "ten dollar glass of water" isn't even the big issue. Agriculture requires
water, and expensive water means expensive food. Expensive food means people
die, mostly from bullet wounds inflicted by people who are hungry. (I hardly
need to provide citations for the number of wars resulting from food
shortages)

~~~
adventured
These types of x-y-z is impossible due to cost assumptions have always been
made throughout modern history, and they often end up being wrong. You're
judging everything based on today's technology. When in fact technology tends
to leap forward massively at times due to necessity, and it's rarely foreseen.
The same leaps will happen again and again and again.

------
hernan604
does the ocean still clean these days ?

very nice job, expect to see it available soon

~~~
cmsmith
This is a great question. Distillation has the side effect of removing all of
the microbes from water and sanitizing the output, but if this method only
removes the salt will the output water be clean enough to drink?

~~~
ars
Do you think they clean your local city water? They don't. They filter it a
bit, then add chlorine to deal with biologicals, and that's it.

~~~
tsotha
But municipal water typically comes from rain or snowfall. The distillation
has already been done.

~~~
ars
You wish.

No, it usually comes from a river. And guess where the waste water from a
sewage treatment plant goes?

Right back into the same river, only downstream. And that's where the next
city down the line picks it up.

That water gets recycled multiple times on the way to the ocean, by each city
on the river.

That's not a bad thing! But the water can hardly be called distilled.

~~~
tsotha
Where I live the river water comes from snow packs. And nobody's allowed to
discharge treated sewage into rivers. Gross.

------
brownbat
> "The new method requires so little energy that it can run on a store-bought
> battery."

We currently have techniques that require no additional power (just using the
sun), but almost every desal operation uses a powered method.

Power consumption isn't the only factor here. A low power technique that has
throughput per area equivalent to solar stills is just worse than all
available alternatives.

If you made a device that turns the briny sludge byproduct into fuel or some
other consumable, that would be amazing.

------
sbierwagen
It's already being commercialized:
[http://www.okeanostech.com/](http://www.okeanostech.com/)

------
kunil
"consumes less energy and is dramatically simpler than conventional
techniques."

I remember a video that shows some kind of filter (it looked like a plastic
sheet). They were simply pushing water against it and you get clean water from
salty water.

I don't think it gets easier than that. However if I remember correctly the
filter gets unusable after a while

~~~
ars
You need energy to "push" the water through the membrane. A LOT of energy.

It's called reverse osmosis if you want to look it up.

------
jyf1987
its not only make ocean water drinkable, but also make ocean be livable,
especially those tropic oceans, they would build solar powered water tower and
soilless vegetable/food factories, war might break out between some countries
for the ocean

------
pipboy3
water chip? I need to buy lot of them before World War III starts. I got a
feeling I'm gona need one in the vault.

