Simple, solar-powered water desalination (2020) | Hacker News Reader

Simple, solar-powered water desalination (2020)

news.mit.edu

591 pointsdrran227 comments

An issue with desalination systems of this type (that have water vapor in a carrier gas, so called HDH or Humidification-Dehumidification desalination systems) is the retardation of mass transfer to the cold surface by the carrier gas. Multieffect distillation (MED) systems usually operate without a carrier gas for this reason, but this means they operate below atmospheric pressure, so they have to be sturdy to resist pressure loads.

A few years ago, some people at MIT solved this problem with a bubble heat exchanger: the humidified carrier gas is bubbled up through trays of water (at progressively lower temperatures). The surfaces of the bubbles provide a very large surface area for heat/mass transfer.

https://news.mit.edu/2013/produced-water-cleanup-0205 https://dspace.mit.edu/handle/1721.1/86334 http://web.mit.edu/lienhard/www/papers/reviews/HDH-Desalinat...

Some of those involved went on to form a company to commercialize the technology, which has been used to recycle/purify brine from natural gas wells and other industrial tasks. I think they ended up being bought out after several years.

The opposite of bubbles, mist, might also work. Extremely high surface area.

For those curious what the 385% refers to

> the team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.

Honestly I still don't know what that means, or how efficiency can be over 100%.

Usually, numbers over 100% mean that you are putting in less energy than is needed for the process. In this case, that would mean putting in a little over a quarter of the needed energy. That does not imply free energy or anything. The rest of the energy has to come from the environment.

A heat pump is another common example with efficiency numbers in the same ballpark. With a heat pump, the heat is being moved from outside to inside (or vice versa for air conditioning and refrigerators). In that case, it requires, for example, 1kW of electricity input to move 3.85kW of heat.

Efficiency is still the wrong name. What you are describing is usually referred to a coefficient of performance, abbreviated to COP.

See https://en.wikipedia.org/wiki/Coefficient_of_performance

The Deja vu sensation of this conversation happening almost identically 2 days ago (everything from wondering how >100% efficiency works, someone explaining it's from the environment, and then someone else explaining efficiency is inappropriate and COP exists) is kinda wild.

Nothing to worry about. Its people alpha-testing the Copilot internet commenting plugin.

Can't wait for all our text input boxes on the web to be GPT-3-enabled, and then all comments on HN and Reddit and Twitter will be just people accepting the defaults, and it will end up just being GPT-3 talking to itself, and we can all go back to doing something productive.

Pretty sure FB has several instances of itself with all the users played by GPT-3 trained on the users’ previous activities so they can monte carlo various changes, like pre-A|B testing or estimating impact of various new ad types or congressional testimonies.

Reminds me of a post here where it was some neural network chat/excuse generator. Just two people back and forthing about why they couldn’t meet up.

I lost it when one of the excuses was, 3 weeks later “oh, I can’t meet for lunch on July 29th, I have to go to my mother’s funeral”

Amy way you can link to that conversation? I tried to find it on Google.

I think the convo is different each time. You’d have to search here for the chat itself. It was 2-3 months Ago

Relevant xkcd: https://xkcd.com/810/

I wonder if concentrating solar cells should have similar nomenclature.

The 100% level refers to a system where all energy goes to heating up water in order to evaporate it, and then letting it cool down to condense it. All the energy that was spent to heat up the water, is lost to the environment in the "cooling down" step.

If some of the heat is instead recovered during condensation to heat up the next batch of water, then you have >100% efficiency.

Does it have to be batches?

Can't it be like the reverse of a rocket engine where they use regenerative cooling from the fuel to cool the rocket nozzle, but just in reverse.

Or like they way my grandpa was doing moonshine -- the alcohol vapors pass through a serpentine in a water tank, condense, at the end you obtain alcohol, the water in the tank gets warmer -- instead, heat water coming from the water cooling tank that is preheated by the vapors of water that is condensing in the serpentine pipe.

Unless you ignore energy sources from the environment you cannot exceed 100% efficiency. And that would be incorrect, applying that same standard to photovoltaic panels would result in infinite efficiency. That doesn't make any sense. Edit: Also, when you take heat/energy from a previous step in the process, you also need to account for the energy put into that previous step. In the end that will again be <100%.

As someone else already pointed out, this would be called Coefficient of Performance. Efficiency is clearly defined and cannot exceed 100% without breaking laws of physics. Call it "3.85 times more efficient than before" or something along those lines and it won't sound like a free energy claim.

> applying that same standard to photovoltaic panels would result in infinite efficiency

No it wouldn't. The equivalent for panels would be like... you want to run some number of watts through a diode, and you're using solar panels to collect this power. The diode happens to give off waste light. By aiming this light at your panels, you can recapture most of it back into electricity, and reuse it 2.85 more times.

the full sentence is:

the [..] device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.

it seems quite clear how the 3.85 ratio is obtained

Others already explained where the >100% efficiency comes from, but I want to point out that a good 1/4th of the article is repeatedly explaining how this works, over a couple of paragraphs.

Reading it in the comments was still more efficient than reading the article.

220% more efficient.

My understanding: As the water condenses onto the next surface layer in the stack, the solar heat is recycled. This is because the transition from gas to liquid releases heat.

Really clever stuff!

Edit: mixed up evaporate/condense

For that to work, each successive layer will need to remain cool enough for water to condense on it.

It’s due to changes in pressure. https://upload.wikimedia.org/wikipedia/commons/thumb/3/33/Ph...

At 5ATM water condenses at a higher temperature than it boils at 4ATM.

I think it means 3.85x more water is evaporated compared to just leaving the water outside in the sun.

The paper[0] goes into more detail about how the efficiency is calculated (scroll down a bit to see the actual formula).

[0] https://pubs.rsc.org/en/content/articlehtml/2020/ee/c9ee0412...

It quantifies the heat re-used between stages. In the supplement, they note 600% is the maximum. The derivation is at the bottom of p834 from the journal pdf.

It's basically vapor produced at the measured average temperature divided by energy input.

If 600% is the maximum, then what does this mean?

> Theoretically, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700 or 800 percent, Zhang says.

I think it tracks how well the materials they're using move heat between stages or lose it to the atmosphere. Their modelling (supplement figure 2, mentioned on page 4) depends on their specific construction. I wonder what it would do with gold as the plate material and aerogel everywhere else...

Exposing a deep body of water to sunlight is probably the reference.

edit: deep as in practically having unlimited heat capacity and the heat conductivity of water.

I think maybe it means "3.85 times as efficient as sunlight in evaporative filtering" ?

> Honestly I still don't know what that means, or how efficiency can be over 100%.

They explain it in their article[1]:

"the solar-to-vapor conversion eﬃciency, defined as the ratio of total vaporization enthalpy to total solar energy input, for most previous studies has been limited to below 100% as the vaporization enthalpy is lost to the ambient environment."

[1] https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee0...

My guess is that they get 3.85 times as much water evaporated as the energy used to just normally evaporate water. They did that by also using the energy from condensation to be put in the process again.

Its disingenuous. We know what the theoretical best efficiency of desalination is, by thermodynamics. You can calculate it by assuming a completely ideal reverse osmosis setup. Compared with that, the efficiency would be less than 100%. This article takes 'efficiency' as compared to just evaporating the water and not reclaiming any energy on condensation.

I thought it would be something impressive like somehow using the obtained salt for further powering of the device but nope, another clickbait trip into bullshitland

I'm not even sure what 'the energy of water evaporation' means? Gravitational potential of the mass of water evaporated (and condensed at some height)?

I think they mean latent heat/enthalpy of evaporation.

I think of it as taking the combustible fuel energy required for a car's motor to drive it somewhere, divided by the energy required to turn the ignition key and press the pedal.

It's an example of a good old scientifically flavored clickbait.

The energy efficiency of anything cannot exceed 100% (until I missed something groundbreaking in physics).

How can it be "click bait" if neither the title nor the headline contains that number? You have to read at least the first three paragraphs of the article to stumble about that number. The focus of the article is on the inexpensive design, not on its efficiency

The HN title has been changed. It used to refer to the 385 percent claim. Probably GP thought this was from the title.

It very much can exceed 100% and it’s not a measure invented for click bait purposes.

Look up heat pumps. You can make things hotter by moving heat than you could by directly heating it by burning fuel.

As another comment pointed out, it’s usually called the Coefficient of Performance in this case.

Which is to say this is click bait, because saying it has a COP of 3.85 isn’t anywhere near as sexy, whilst being technically more accurate.

The ‘more than 100%’ is only in comparison to a less efficient process. A heat pump itself is operating below 100% efficiency - it’s just kind of “cheating” by using an external source of external energy as one of its inputs in addition to the electricity running it.

I guess water evaporates naturally, adding sunlight speed up the process.

And my perpetual motion machine uses naturally running hamsters.

FTA: "Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers."

The capabilities are freaking interesting, but let's say someone builds a big enough settlement on a coastline or island in which every building has one or more of these devices on its rooftop. Would this release back enough salt so that the surrounding seawater becomes hostile to its previous lifeforms? Also, since then the water used in the process would contain more salt, how much would that render the device less efficient?

This chunk from the article is, unfortunately, an outright lie.

By definition, removing salt from seawater leaves you with water and salt. The article handwaves the salt away by supposing that the desalinator will float on top of the ocean.

In actual production configurations other than towing it behind your sailboat, you will end up with a brine pool that needs to be disposed of, or a concentration in the wicking material that prevents low-concentration seawater from entering, or a pumping system.

This is not just a common negative externality, but a casual lie about it.

I don't think they are talking about the impact on the environment. I think they are talking about the procedures for operating the equipment.

The way I read it, they mean whoever operates this device does not have to periodically go empty some bin/tank of salt or brine.

An analogy would be to a frost-free refrigerator. When you say the freezer compartment of your fridge is frost-free, you don't mean that it never generates frost. You mean it does generate frost, but it also automatically removes it. It's "free" of frost in the sense that you are free of doing a chore that you have to do with a freezer that lacks this feature.

If you operate it directly on the open ocean, towing it behind your sailboat, you don't have to do anything special.

If you operate it in a commercial, protected context where you pump water through it, you will generate brine.

If you operate it en-masse in the ocean, the things that are insignificant at the scale of 1 and 10 square meters may become significant at the scale of 1 and 10 square kilometers.

All of these things are true for other desalinization systems, too. Claiming "you don't have to worry about brine" because it floats on the water surface is misleading at best. You can put any system you want on the water and make it float -- we have concrete and steel hulls, no problem -- but that doesn't solve the concentration problem.

It sounds like they're targeting small deployments in the developing world, at the scale of 1 to 10 square meters, not large commercial systems.

From the article:

> In production, they think a system built to serve the needs of a family might be built for around $100... The hope is that it could ultimately play a role in alleviating water scarcity in parts of the developing world where reliable electricity is scarce but seawater and sunlight are abundant.

The problem with brine from desalination plants is that it's released at high concentration in one spot. This would not suffer from that problem.

Water in open ocean is not still - there are currents. I suppose if you had a large enough operation you could create a gradient of sufficient size to be self-sustaining without further mechanical agitation (such as your example of towing behind a ship). It would be an interesting exercise to see a large collection of these in operation. Heck even if you had to pump ocean water on land and back out to dilute the brine this process would still use significantly less energy overall than current desalinization plants.

Speaking of current plants, a recently constructed one in Carlsbad CA leverages the cooling outflow of a power plant to handle the dilution. The power plant was already there and so was the outflow - these things can be planned to leverage overlapping uses to further increase efficiency.

> supposing that the desalinator will float on top of the ocean.

I think that as its solar, you only have 50% duty cycle (more or less) so the idea is that you "just" slow down the water supply to remove the salt over night.

I wonder if the fresh water used in such a way wouldn’t eventually be released back into the ocean thus somewhat mitigating the effect.

There are two counterpoints to the issue of excess salt being an environmental hazard.

A) brine can be introduced back into the ocean combined with waste water. In the Cape Town drought of 2017 some small desalination plants were brought into service very quickly, and the brine was expelled in the same pipe as the outflow from a (treated waste) sewerage plant.

B) the ocean is big - very big - and at least by our coast seldom "calm". So outflow of anything would disperse very quickly. Hot-water outflow from a nuclear power station dissipates very quickly for example - typically within tens of metres of the outflow.

C) the natural salinity of the ocean varies a fair bit at the very local level - think river mouths - storm water - evaporation etc. Outside of specifc closed bodies of water (Dead Sea etc) we'd need desalination on a massive scale to even measure the impact.

Why release the salt back into sea water? This is salt production alongside desalination.

We already have a massive oversupply of salt from existing desalination. The cost of table salt is basically all transportation at this point.

> The cost of table salt is basically all transportation at this point.

Using the salt on site would save the transport, though?

Can be buried also.

In large enough systems some kind of “salt removal” mechanism might be added - I think it wasn't added to a PoC just to make it as simple and cheap as possible.

Salt release is already a huge environmental concern with exiting desalinization systems. Energy consumption and installation costs aren't the sole (and probably not the main) problems. Desalinization is inherently a pollution.

Why not release the salt back to the sea? Presumably it would not increase the salt content of the ocean very much considering the vastness of the ocean and the sheer quantity of fresh water that is being added to it by melting glaciers, etc? What am I missing?

The salt you release isn't instantly diluted in the whole ocean and we're speaking, for a plant, of millions cubic meters of salt enriched water per day. The release area is just dying.

Can you provide a source for that? I dove near a desalination plant exit pipe in Saudi, the impact on the marine live did not seem to big in the area. But it was a small plant for a few thousand people and we did not do a proper assessment.

I don't doubt there is an effect, but am not convinced that the "release area just dying" is correct.

https://www.keiken-engineering.com/en/brine-disposal-how-eff...

Brine disposal is indeed a complex issue. Should be noted the above URL is from a desalinization industry proponent so I would say it's probably a little more optimistically biased but it was one of the better summaries I found so it still has value from that perspective. If you do more digging around each of the solutions they discuss you can find more pro's/con's for each and you will quickly find out that it is a pretty significant issue.

The link does not really show any evidence that marine ecology at brine outlets is disturbed. It does however suggest that pouring the brine on land or roads (for de-icing) are alternative solutions, which I doubt ;).

I was more thinking of proper studies, such as reviewed in [0], which says in the abstract: "Ecological monitoring studies have found variable effects ranging from no significant impacts to benthic communities, through to widespread alterations to community structure in seagrass, coral reef and soft-sediment ecosystems when discharges are released to poorly flushed environments. In most other cases environmental effects appear to be limited to within 10s of meters of outfalls."

So, impact yes, but "release area is just dying", probably not.

[0] https://www.sciencedirect.com/science/article/abs/pii/S00431...

This makes sense to me. I wonder how feasible it would be to release the saltier water over a broader area to reduce the salinity in any one area? Also, there are at least some ecological benefits to increasing the amount of fresh water. An extreme example is the lake that appeared in the middle of a desert in the United Arab Emirates. I have no idea how we weigh one against another.

Now you’re paying for salt transportation again.

Not if you have many small desalination plants distributed over a larger area. This would work well for less-densely populated areas.

That is acceptable

Why not just collect the salt, and sell it as... salt? Even if its not ok for cooking, it should be fine for roads? I am sure there are other industrial uses for salt.

Salt from roads is seriously contaminating our environment and significantly impacting farming in more and more areas.

Ever hear of the Romans salting the earth of people the conquered? It was so they couldn't grow crops - yet we routinely do this to ourselves - pretty daft.

We also farm in a way that is almost optimized for erosion of topsoil. We’ve already lost a third of it and it only renews on geological timescales. Daft is too polite.

Actual salt is rarely used on the roads in the USA anymore. Majority use magnesium chloride now. Don’t know about the rest of the world.

Lots of reasons, pros and cons. I work in automotive and hate it.

A small part of it can be avoided by combining this with a salt evaporation pond (https://en.wikipedia.org/wiki/Salt_evaporation_pond)

Problem will be that we’ll get way too much salt (a few grams of salt per liter of water), so it won’t help much.

> 1.5 gallons of fresh drinking water per hour for every square meter

7 litres per m^2 per hour.

A comparison with reverse osmosis:

2.46kWh/m^3 (energy consumed per unit of water produced) is claimed for reverse osmisis [1]. This equates to 8.86MJ/m^3.

Output for this still is 7L/(m^2.h)

Assume a solar flux of 1kW/m^2.

Energy consumed per unit of water is 1kW/m^2 / [7L/(m^2.h)] = 3.6MJ/0.007m^3 = 514MJ/m^3.

Assuming the above is correct (check anyone?), the still uses 58 times more energy than reverse osmosis. The solar energy may be "free" but with a 20% PV cell efficiency a reverse osmosis system would produce about 11 times more water per unit area of solar collector?

[1] http://www.ijesd.org/papers/243-B20001.pdf

> In production, they think a system built to serve the needs of a family might be built for around $100.

I think this is much more important to a great many people than the theoretical efficiency of reverse osmosis. Percentages greater than 100% always do well in media reports about these topics, but they're not necessarily the point of the exercise. If the goal was to produce a system that's as efficient as possible, the researchers wouldn't have used household-style supplies but more expensive, advanced materials.

Reverse osmosis is great for a central area such as a large city in a place with reliable distribution, but in many places around the world, an independent, affordable system that can turn seawater into drinking water for a family or two has much more value.

Nothing about reverse osmosis can't be scaled down in size or cost.

The actual membranes are $9 for enough for 100 gallons per day (retail prices, [1]).

High pressure pumps and hoses scale linearly. Solar panels scale linearly. Filters scale linearly.

In fact, using this solar fountain [2] as the basis for the design, and switching the pump impeller for a high pressure version, and the nozzle for an RO membrane and hose, you immediately have a drinking water machine for a few people for $20. The fountain already has a pre-filter built in.

[1]: https://www.aliexpress.com/item/32669709750.html [2]: https://www.aliexpress.com/item/1005002883892948.html

I have first-hand experience that a) you need positive displacement pumps of a certain quality and power for RO and the cheap Ali ones self-destruct in minutes/hours b) you still need housing, plumbing, receptacles, etc c) fouling is a non-trivial issue.

Expect to pay around $300 minimum for a small RO solar plant, and a couple bucks a month for upkeep. And logistics. Granted that's USA prices (upstate NY, not SV) but that prices out a lot of developing regions.

Even these numbers are too small if you're going to RO seawater. I suspect what you have in upstate NY isn't access to seawater but access to some kind of less than amazing mostly-not-saltwater.

The osmotic pressure you need to overcome to perform desal is directly proportional to the concentration. So 500-1000ppm water that you don't love is a lot less challenging than seawater which is about 3.5% or 35000ppm.

You can do "I want extra pure drinking water from 'normal' water RO" for a few hundred, sure because the pressures are in the range of 20-60 psi. Reasonably efficient seawater starts somewhere around 400-600psi and I've heard of plenty of systems that work at more like 800-1000psi. Different pumps, membranes, membrane housings, piping, all of it. It's a couple thousand dollars plus a fair amount of energy unless you buy even more expensive energy recycling pumps that use the pressure in the brine output to help on the input.

Exactly, there's a pressure/flow/salinity tradeoff triangle. Higher salinity needs higher pressure and/or waste bypass. This was running around 100-150 psi, was a few years back, don't quite remember.

But a few hundred bucks will still get you from either "potable but not great" to fairly pure, or brackish to potable. If you have effectively infinite seawater and power, you can still pump at lower pressure and get pure permeate, it's just less efficient per unit pump and filter lifetime. If you had a super reliable pressure system (low friction ceramic pumps for example, not cheap but last forever) and cheap first-pass filters, you can run it for quite some time. But you're spot on in that you get to a point of engineering-give-a-mouse-a-cookie and it just makes sense to optimize the whole stack.

Fouling can be dramatically reduced by putting a small electrical current through the saltwater, freeing Chlorine ions which quickly kill biological things before they can adhere to your membrane.

That combined with reverse flow flushing will probably last many years. And it can all be controlled by a 10 cent microcontroller, one pump, one valve, and a pressure sensor.

I don't doubt that high pressure pumps are bad... but that's just a design issue - there is nothing theoretically expensive about them.

Commercial RO membranes (based on the poly(m-phenylenediamine trimesic acid), a.k.a. polyamide) chemistry have zero tolerance to free chlorine and will rapidly degrade under such conditions. The chlorine tolerance is listed as “nil” on many manufacturer’s spec sheets. Even using tap water on a polyamide membrane without some kind of prefilter (like GAC) will cause noticeable reduction in both the permeance and rejection of the membrane.

There was a big push towards developing chlorine-resistant chemistries a few years ago, but as far as I can tell, that has fallen into a “researchers don’t know what they’re doing, the plants are already designed around this problem” narrative. Of course, those plants are huge investments, and maybe it’s correct that one wouldn’t be able to take advantage of chlorine-resistant membranes until a new plant was built.

Cellulose triacetate RO membranes have chlorine tolerance, but have inferior chemical stability, productivity and selectivity to polyamide membranes, so it is sometimes used where chlorine tolerance is an issue. CTA membranes are also available in hollow fiber format, while polyamide membranes are essentially exclusively found in spiral wound configuration (some operators want fibers for higher fouling/solids feed). CTA is limited to a much smaller pH window (like 4-6 versus 2-12), and are not suitable for more aggressive cleaning methods, so I’m not sure if it overall provides a benefit versus polyamide RO with more aggressive cleaning cycles.

Heh, this is reminiscent of the infamous HN dropbox comment.

Free chlorine (technically assorted chlorine oxides like hypochlorite) attacks RO membranes, so now you have to deactivate your reactive species first. Usually UV lamps, you use sun, but now you need UV-clear tubing, not easy.

Reverse flow flushing can be done with the components you mention, that's another $20/50 plus sourcing logistics.

Yes, the theoretical expense in quality pumps is quality. There are tighter tolerances, beefier components, better polymers, and more QA. It all adds up.

It's still all fairly cheap by Western standards, but it's a tall ask for a lot of places that barely have potable water.

Ah, so what you are saying is that this is already deployed on a large scale to solve distributed water issues?

What he’s actually saying is he found some residential grade nonsense from China that only seems economically feasible because of the slave labor and subsidized transport.

That's a good point, but! I wonder if it couldn't be more efficient overall, if you look at it globally. Is an equivalent reverse osmosis installation cheaper or more expensive to produce, in terms of embodied energy that goes into manufacturing and maintaining all equipment, including control equipment?

Also, I wonder which system would be easier to slap together in a garage out of leftover junk. Not everyone can rely on access to commercial-grade, turn-key solutions to a problem. A design that could be reasonably DYIed could be better in certain contexts, even if less efficient.

The MIT system appears to output clean water from each 'stage' of the system at variable temperatures. Thats energy lost - if a counterflow heat exchanger could be devised such that the output water was all cool, the final efficiency would be much higher.

They also don't seem to keep good control of salinity within the stages - I suspect after a few hours operation the efficiency will drop as the salinity gets higher and higher within the paper towels.

Back of the envelope - say that you could get 7-8 hours of sunlight a day that could destill water. That would mean ~50 liters/day. ~20 Days to get 1m^3. Reverse Osmosis costs approx $0.50/m^3 water, so your payback on a $100 system would be ~200 * 20 Days or 4,000 Days to equal what you could get for spending $100 on buying water from a reverse osmosis system. The objective here isn't large scale economics, but self sufficiency.

There are a lot of quite liveable islands that are deserted only due to lack of fresh water. This could change that.

Or maybe we don't have to spread humans everywhere like a malignant cancer, leaving at least these Islands to nature.

Maybe don't spread your Malthusian bullshit like a malignant cancer. Some of us like being human.

Hey, me too! Never said I didn't. Just that it's nice to leave some parts of the world to itself.

>> malignant cancer

> it's nice

come on dude, have some self-awareness

The only thing I was saying is - just because humans can live everywhere doesn't mean we need to.

If you look at any map of the modern world, there is almost nowhere that we haven't transformed into farmland, cities, etc, and it's pretty good (nice) that we have left the uninhabitable islands to nature. We've consumed most of the land, which is both pretty amazing and also quite bad for biodiversity, as everyone knows.

I'm not debating the "cancer" metaphor which is definitely harsh - it has been catastrophic for other life, but great for us. Maybe that wording can be less harsh, but IMO that's not the point.

Looking at a 10+ year time frame, I feel that maintenance cost would become the dominating factor. For both.

I think the difference here is that the materials used are cheap and easy to procure. That sounds like maintenance should be straightforward. The difference between producing water locally and shipping/trucking it in is probably huge in places without water pipes. A couple of gallons a day goes a long way. Scale it up a little and now you are irrigating your vegetable garden as well.

The Reverse osmosis cost estimate was based on a 10 year committed Hyflux Desalination commercial contract with the PUB in Singapore (from about 5 years ago, so might be less expensive now, actaully) - so includes maintenance costs.

Regardless - this isn't a scale able solution, but doesn't need to be - should work fantastic for a small family with negligible environmental impact.

5.78 liters per square meter actually, per the article.

So in ideal conditions one could expect to provide about 12 to 15 people with drinking water per square meter. For me this is an impressive number.

If I calculate one unit with 300 dollars this would mean 5.5 cents per person per day for one year for fresh drinking water. And after that only a little bit for maintenance probably.

Hmmm, yes it is. It's nearly too impressive!

From the article "The team estimates that a system with a roughly 1-square-meter solar collecting area could meet the daily drinking water needs of one person.", which sounds very different.

On this Q&A page [1] Zhang is quoted "Our current strategy, for example, is to use an assembly of 100 of these devices to achieve an area of 1 m2, which will increase the total production by 100 times to create 10-20 liters of clean water per day."

I don't want to sound too negative, but something doesn't add up here. Still, enough water for one person per square meter of desalination plant is an impressive result I think. The oceans are big.

[1] https://www.techbriefs.com/component/content/article/tb/feat...

96.3 um/s! That's enough to drain a 6 foot deep swimming pool in 5.27 hours.

Really thanks for the conversion.

American gallons are smaller than U.K. gallons, which is where I suspect you got the 7l (6.8) from

Nearly 2ml per second?

(My faith in my own maths is weak ..)

Assuming 1.5 US gallons, this converts to 5.678 litres.

Now we have 3 numbers... US units are great...

https://en.wikipedia.org/wiki/United_States_customary_units

American gallons are smaller than U.K. gallons.

Thank you.

This device "could" produce 1.5 gallons per day, per square metre of solar collecting area. The researchers estimate a system suitable for a family might be built for $100, so double or triple that after profits etc.

I assume that the 1.5 gallons was the highest yield they would have had from a single day, had they had a 1 square metre solar panel, and the expected average yield would be less (accounting for cloud cover etc.)

But even with reduced yield, for very small communities of a few dozen people, this definitely could work out.

The article says "per hour", not per day. Although that would seem able to support dozens of users rather than just 1.

Thanks for spotting that.

I'm a little confused then, later in the article it says "The team estimates that a system with a roughly 1-square-meter solar collecting area could meet the daily drinking water needs of one person." So something doesn't add up.

[Edit: the paper is available at https://pubs.rsc.org/en/content/articlehtml/2020/ee/c9ee0412.... It says, "To meet the average daily water intake for one adult (≈3.2 L),49 100 TMSS devices can be placed into a 10 × 10 array, filling an 1 m2 area, which would provide approximately 10–20 L of clean water every day depending on the weather condition."

The lower bound of 10l is outdoor performance on a partly sunny day. So I think they are just being conservative by saying it would meet the requirements of one person - coastal areas are frequently cloudy, and in some locations, there might be little sun for extended periods of time.]

'drinking water' often doesn't mean the water a human literally drinks, but all the clean water necessary for living everyday life (bathing, flushing toilets, washing dishes, cleaning clothes, cooking etc.).

I am pretty sure if you had to get your water from one of these devices you would not be using it to flush toilets or other non-essential use.

And actually it does seem like 'drinking water' is actually drinking water and not for all those other uses you mentioned. As 3.7 litres is the amount required for a human male each day.

https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-h...

>I am pretty sure if you had to get your water from one of these devices you would not be using it to flush toilets or other non-essential use.

Are you suggesting that toilets, showers, washing machines use salt water?

If their toilets used clean water, then I have a much more efficient way to produce clean water : drink from the toilet and shit in a hot house.

Is there a problem with flushing a toilet with salt water?

Offhand I'd say that you are doubling the plumbing system plus you are using a much more corrosive fluid.

It would be an interesting experiment, since toilets use a great deal of water. Take a beach town, build a separate pressure water and sewer system, see if it's worth it. It's certainly more complicated than a sailboat's set up.

From Wikipedia:

> Drinking water, also known as potable water, is water that is safe to drink or use for food preparation.

40-50L per day would seem to about cover one person's essential uses of potable water.

The average US citizen uses 82 gallons of potable water per day.

Sounded astonishingly high, but a quick search from a few sources (one cited [1]) checks out. Now to go down a rabbit hole of how this 82 gallons breaks down and what I can do to reduce my consumption...

[1] https://www.usgs.gov/special-topic/water-science-school/scie...

" Using a low-cost and free-of-salt accumulation TMSS architecture, we experimentally demonstrated a record-high solar-to-vapor conversion efficiency of 385% with a production rate of 5.78 L m−2 h−1 under one-sun illumination, where more than 75% of the total production was collected through condensation. "

Abstract is available here:

https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee0...

The original journal article is available here as a pdf:

https://pubs.rsc.org/en/content/articlepdf/2020/ee/c9ee04122...

"Tests on an MIT building rooftop showed that a simple proof-of-concept desalination device could produce..."

So doesn't look like they've even hooked it up to the sea yet, so still a bit to go for a real implementation, but still sounds interesting.

I harbour a small dream that I could buy some seaside land someday on a Greek island and hook one of these desalination devices up to the sea and build myself a small oasis.

The device itself would not be "on" the sea, it would just get water from it with a pipe/pump. I don't really see any reason it wouldn't work the same way by the sea: the main problem seems to be how to remove the salt, efficiently, not how to get water into the device, which is easier. Getting enough sun might even be harder than getting sea water.

Yep, and that's what I basically meant by 'hooking it up'.

I guess you would need a pump to bring the seawater to the device, something to pump it elsewhere (for storage, or irrigation), and also some mechanism to dump the brine back in the sea when you're no longer desalinating.

And for maximum ecological efficiency that could be powered by solar panels.

I wonder how maintenance free you could build such a device, and just leave it to work away by itself for months on end.

I had the "stupid moment" of the day. The thermal effects of evaporation on a surface is that the surface actually cools down -- so I could not get the "it produces heat" part.

After drinking my morning coffee I realized that the heat transfer is from the surface to the water droplets/vapors that then carry it to the next layer of this still.

Ergo: coffee makes you smarter and I shouldn't be on HN so early in the morning.

Does anyone know of any designs for DIY passive desalinators? My mum has a house on an island that doesn't have a natural source of fresh water (except rain of course), and it just occurred to me that a desalinator would be a fun thing to build there. Normally if there's a long drought she would have to have drinking water delivered by sea.

The most simple ones ("survival" type, but it won't produce enough water unless you scale up) is spanning clear plastic over a pool of (shallow) salt water, weigh it down with a rock in the middle, and put a collector in the center underneath so the evaporated water runs down it.

But I'm sure there's commercial solutions out there.

I'm sure you'll get better answers but I really liked the idea of ventilating greenhouses with salt water.... could be a good way to get the extra liquid needs..a quick google found me this https://www.sciencedirect.com/science/article/pii/S001191641...

I imagine a trillion of these floating along the coast of the western United States, little landing pads (and solar powered charging stations) for a trillion drones that can move the freshly desalinated water from the ocean to the nearest forest fire. You need a drone cloud 500 million strong to deliver 100 million gallons of fresh water to X location immediately? No problem. Need a drone cloud to water your 20,000 acre vineyard? Coming right up. Who needs clouds?

Water is very heavy though. For watering crops I think you would do better in terms of energy efficiency with land-based infrastructure. At the very least, if you're going with drones, you ideally want something more energy-efficient than quadcopters.

A trillion? Well I guess efficiency doesn't concern you...

It's wild ass guess, but imagine a drone fleet capable of delivering that quantity of water on demand.

We will see this in our lifetimes. Rain on demand.

I don't think we will. Except in the case of fighting fires in remote areas, if you want water in some area, you use pipes.

For fighting remote fires, we already have piloted vehicles and in some cases uncrewed vehicles delivering water in bursts ... so perhaps.

As I understand desalination, a huge bottleneck besides cost is what to do with the brine. Why can't they build a gigantic pool/lake out in the desert and pump the brine there and let it dry into salt and other minerals. A 10 square mile pit/pool with a clay or other barrier to prevent it from sinking into the aquifers would allow for a massive amount of brine to be safely contained in one place wouldn't it?

brine disposable seems like a made up problem to me. We're not going to raise the salt content of the world's oceans by pumping the salt back from whence it came. I understand we don't want to put the high salt discharge near sea life, but that is not a hard problem.

Seriously, so many possible solutions.

You can sell it.

You can give it away for free.

You can turn it into something else and use it/sell it.

You can mix it with treated sewage water flowing into the ocean.

You can mix it with fresh water flowing into the ocean.

You can discharge it over a larger area.

It's not a "real" problem.

> We're not going to raise the salt content of the world's oceans by pumping the salt back from whence it came.

Sorry, maybe dumb question but why not? We're taking out water and leaving behind salt

I meant the salinity content per liter of the world's oceans would rise an insignificant amount, especially since fresh water flows back to the sea for the most part. If someone can tell me otherwise, I'm ready to be educated.

It takes a while to disperse. Meanwhile salinity in the region has gone up, say 2%. Apparently sea life is very sensitive to it.

I agree there are a number of solutions though. Adding to treated sewage is a good one.

I grew up in a town with a gray water pipe to the ocean. When the environmentalist movement took hold, it was decided the pipe should be extended further into the ocean so as not to disturb the ecosystem. They got it done, so I know even a small town can do this.

Partly we're taking out small amounts of water (compared to the size of oceans) - secondly some of the water is likely to come back via eg sewer treatment plants.

The logistics would be a challenge. You’d have to transport the brine from the coast to the desert. In some places this would be a relatively short trip (Israel, North Africa, California) but in other places you’d be shipping a lot of heavy fluid over long distances. And you don’t even have someone buying the stuff on the other end; you’re just disposing of it.

Over a decade ago I saw simple solar stills installed in India with the cascades built of concrete and a glass panel. This was at Maharana Prtap university in Udaipur.

So “proud” of my Alma mater university for developing an exotic technology that uses aerogels and such ... sure, it’s more efficient, but those folks In Udaipur, without a big PR office like MIT’s, built something that could be built by anyone using everyday material.

Would it be possible to re-green deserts with this technology? Does anyone know of any studies on using this technology to combat climate change?

Greening deserts is not a good solution for climate change. In fact it might worsen climate change due to albedo changes. A nice video about this: https://www.youtube.com/watch?v=lfo8XHGFAIQ

Challenge with that video is it takes a very industrial approach to green deserts - mono cultures for trees, massive desalination plants to provide water etc

Some of the original ideas for greening deserts are permaculture based (https://www.youtube.com/watch?v=sohI6vnWZmk) which are a way more sustainable approach but won't bring around carbon reduction fast enough.

Yishan Wong, of Reddit CEO infamy, has been working on this for a few years: https://www.terraformation.com/

I seriously considered taking a job there (I did not for personal reasons, namely not wanting to move) and would recommend looking at them to others.

I think they are poised to be a key player in the forestry and reforestation space in the next 2 decades.

But... the reforestation occurs naturally for last 2 decades (at least, more like for 3 or even 4)

Not likely. This is much less cost efficient at scale than desalination through reverse osmosis (RO). And RO itself is generally too expensive to be used for large scale irrigation.

What's the point here? Is electricity even the bottleneck of water desalination? I thought the biggest issue with these plans is the brine. That is the major pollutant, and no amount of solar panels will solve that.

Reading the article might help:

> Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers.

Your rely is not helpful. The problem with any desalination system is that it creates brine. And brine in large amounts is toxic to the marine environment.

The only difference with this example is its small scale. Once it is scaled up it will have the same problem as any existing desalinating plant.

But you would never scale this system up - it's entirely inefficient compared to reverse osmosis, on a cost basis. This is the type of system that you would use for just a few people, and as such - brine would never be an issue.

Yes. So it is not an issue for the device. However, the salt does end up back in the ocean where at scale the salt levels will be higher leading to the (edit: local) environmental issues mentioned by the GP.

The sea is a big place. I could imagine perhaps some very localised effects near the plant, but even this isn't obvious.

> imagine > perhaps > but even this isn't obvious

So basically you're admitting you don't know anything about the subject, but you're making a conclusion anyway. It's OK to admit you don't know enough about a subject, and it's OK to not comment or theorize based on no knowledge besides an imagination.

A man once said, "imagination is more important than knowledge."

That man's name? Albert Einstein.

I didn’t know that making assumptions while clearly saying you are making assumptions is forbidden on HN.

Is the water cycle not a thing any more?

Not an expert but I could imagine some local negative effects. Salt concentration may get higher in the area where the brine is returned. Depending on volume, currents, etc...

A large part of the fresh water generated may be used for watering crops or gardens, and would thus evaporate in part. The evaporated water in desert areas would probably rain back down far way, and take long to mix with the sea near the plant again. For example, the Red Sea has much higher salinity that other oceans due to high evaporation and narrow connections to the other oceans. I doubt the effect is large though.

Indeed - the dead sea has stupid high salinity because the only way water leaves is via evaporation :)

It is a very odd sensation to "swim" in the dead sea. Even floating is pretty hard because you are so buoyant. It's probably the closes to true weightlessness I will ever get.

It makes no difference how concentrated the output brine is if it's the diluted to same concentration at the outlet and the same amount of fresh water is extracted.

You completely miss the point. Extracting fresh water from salt water creates brine and in a large scale plant it is extremely difficult to dilute it sufficiently.

The only answer is to pipe the brine into areas with strong currents, or dilute it by distributing it over a very large area.

Brine is the biggest issue with some desalinators, not this one.

Power can be a constraint, too. It depends on how much power you have available or can pay for, how much fresh water you need and how pure, how much you can buffer, etc. In short, power/cost is a constraint sometimes.

This doesn’t use electricity at all, the “solar power” is heat from the sun causing the water to evaporate. It’s a still, not a filter. According to the article the salt/brine freely flows back into the body of water.

Which by definition will increase the salinity in the local environment.

The article directly addresses your point.

It doesn't. Any salt extraction creates brine which must be disposed of somehow.

The trick is to flow seawater past the still and take only, say, 1% of the water from each litre that flows past, which you can do with a still as opposed to a filter.

It's a bit like slingshotting a space probe past venus. Technically to move Venus a bit closer to the sun, but not enough to make a difference.

This reminds me of how many places we have on the planet which are beautiful but inhospitable. Fixing the inhospitable part will come its own environmental impact, and the places will end up being less "beautiful". I vote we go to live elsewhere and leave the planet as a giant touristic resort :) .

This article is 17 months old. Has anything happened in the meantime? (Aside from Covid, of course.)

What is this aerogel thing, and this "capillary wick"?

Not bad for $100, but I'm more interested in durability, if parts needs to be replaced and maintained, and if yes what is required to make those parts.

The capillary wick is literally a paper towel.

It sounds like they figured out how to make the distillation version of a compound steam engine [0]. Like others, I’m confused by the efficiency number. Are they producing 385% more water for the same energy compared to the best single state result?

[0] https://en.m.wikipedia.org/wiki/Compound_steam_engine

That's already a thing.

https://en.wikipedia.org/wiki/Multi-stage_flash_distillation

Discussed at the time:

Simple, solar-powered water desalination - https://news.ycombinator.com/item?id=22269115 - Feb 2020 (192 comments)

This is essentially a solar still that uses the natural water cycle to produce fresh water.

Learning about desalinization should be taught in all schools for more innovations in this area.

It would be a cosmic joke to run out of water on a water planet, we'd look like universal dunces.

USGS site has a great overview of desalinization that is a good place to start, you can even try your own solar still in your backyard.

USGS Desalination site [1]

Build your own backyard desalinization system (solar still) [2]

> You can make your own personal desalination plant

> Remember looking at the picture at the top of this page of a floating solar still [3]? The same process that drives that device can also be applied if you find yourself in the desert in need of a drink of water.

> The low-tech approach to accomplish this is to construct a "solar still" which uses heat from the sun to run a distillation process to cause dew to form on something like plastic sheeting. The diagram to the right illustrates this. [2] Using seawater or plant material in the body of the distiller creates humid air, which, because of the enclosure created by the plastic sheet, is warmed by the sun. The humid air condenses water droplets on the underside of the plastic sheet, and because of surface tension, the water drops stick to the sheet and move downward into a trough, from which it can be consumed.

> You can try this at home! [2]

> - Dig a pit in the ground

> - Place a bowl at the bottom of the pit that will be used to catch the condensed water

> - Cover the pit loosley with a plastic sheet (you can use stones or other heavy objects to hold it in place over the pit

> - Be sure that the lowest part of the plastic sheet hovers directly over the bowl

> - Leave your water "trap" overnight and water can be collected from the bowl in the morning

We need to put tons of money in desalinization. California is already a leader in that but we need more. Israel and Saudi Arabia are also pretty good at desalinization due to more dire water situations.

Additionally we need geoengineering in terms of helping create moisture/rain in areas that feed the Colorado.

The better bet is desalinization that uses the nature water cycle, it makes for cleaner water as well. Saudi Arabia is doing a solar dome to test this [4], we need more of this.

[1] https://www.usgs.gov/special-topic/water-science-school/scie...

[2] https://www.usgs.gov/media/images/how-build-your-own-solar-s...

[3] https://www.usgs.gov/media/images/a-floating-solar-still-des...

[4] https://wired.me/science/environment/desalination-solar-dome...

Is the water pumping also passive? I'm just assuming that you'll have salt water on a lower level, and you'll need to lift it up to this installation to process it.. with a pump.

The article suggests it could float:

> In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers.

> which was tested on an MIT building rooftop. The system delivered pure water that exceeded city drinking water standards, at a rate of 5.78 liters per square meter

I would be very interested to see data on this vs. sunlight and climate conditions, in what weeks/month of the year they tested it. I think its effectiveness would be highest at MIT's location from late April to end of September and considerably less in colder/overcast/less sunny weather and winter.

How much would it cost on industrial scale? (for irrigation system for farming purposes traditional/hydroponic/aquaponics (considering that aqua systems use 1/10th of the water of soil-based gardening)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4483736/

Thinking about California drought problem and what scale of investment would it be needed to fix it.

This is the winning sentence for me:

Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of.

Or is it?

This is fine for a small scale demonstration unit, but with bigger plants you will again run into the problem of over-salinating seawater, destroying the environment (and reducing your still's efficiency).

So once you get past a certain scale, you'll again need to redirect the wick into some waste brine tank and figure out logistics for disposing it.

The factor for me is that they are at least thinking about it. It feels as though the waste is an afterthought every time I read about desalination.

That would have to be a very small sea indeed.

I'll assume the device isn't using an infinitely long wick, so you face the same outflow problems as traditional plants: https://en.wikipedia.org/wiki/Desalination#Outflow

Can salt be compressed and then just buried?

Sure, it's just that in this regard it has no advantage over other desalination technologies. You always have to deal with the excess salt, MIT is dishonest in waving away these concerns.

Yes, the waste always has to go somewhere, the choices are really about how quickly and how concentrated the waste is. (And where it goes, I suppose)

How can we couple this with carbon sequestration?

Use byproducts (sodium, chloride, calcium, etc) as feed stock for useful compounds. eg Phosgene COCl2 is a valuable industrial product.

Create artificial salt water marshes, which are pretty good at carbon capture.

Everyone's trash is someone else's gold. People are clever. They can find good use for the salt.

I like the idea that brine is disposed over time at night, which will greatly reduce the disposal problems.

The article in question: https://pubs.rsc.org/en/content/articlepdf/2020/ee/c9ee04122...

The 385% seems to refer to obtaining de salination in lesser solar collection area requirements.

When the $100 system can run for 5 years of continuous production get back to me. I would be surprised if the system, as described, would work beyond a couple of days. Other than for emergency situations this isn't useful.

California is building the largest desalination plant in the US.(2015) Is this ready yet ??

https://news.ycombinator.com/item?id=10343296

Largest SOLAR desalination plant in the US. The article doesn't seem to list the numbers but this article https://www.businessinsider.com/california-waterfx-solar-des... seems to indicate the next step up from their demo plant was a plant capable of 2M gal/day.

Hopefully they are still developing the technology, it seems like it could be a life-changer for small farming communities around the world.

By contrast the biggest 'traditional' desalination plant in the US makes 50M gal/day and the biggest plant in the world makes around 260M gal/day (US gallons).

The comments here are all upset about the efficiency number presented in the article, but I was hoping for more commentary on the 1.5 gallons per square meter thing with respect to improving lives in the developing world.

You need exactly 3 litres per day. Or rather: I do and I did it for several years. This would mean tiny portable and durable device and it would make many places totally habitable. Not just Baja California.

Another interesting use for this system, and I didn't see it mentioned, is for water purification. That may be even more useful across the world especially in remote regions.

Amazing. I wonder if there has been an update since 2020, in particular with regards to mass-production...

It's incredibly disappointing to see MIT put its name to some apparently PhD-level research that claims over-unity efficiency, and apparently (according to the picture) using a few baking trays and tinfoil.

It also does not explain how to deal with the increased salination of the water source, nor how water can continue to condense on the successive layers of the device as they are heated due to that condensation.

This is for small scale, family-sized units, where power and cost are the limiting factor, so brine will not be an issue at this scale.

As for condensation, the plate is hot, but colder that the vapor, and heated by the vapor only, so when it is too hot, condensation stops, the temperature drops quickly due to evaporation on the other side, and condensation can continue.

At equilibrium, each successive plate is colder than the next, the last one using the sea water as a heat sink.

This is actually innovative and the optimization of the design parameters (e.g. The distance between the plates) is not trivial.

I think you are overly dismissive of this solid piece of engineering.

Presumably you could use the same technology on liquid effluent to create a 'toilet to tap' cycle

There's still the issue of what you do with all that salt

put it back into the ocean where it came from?

Brine is toxic unless it can be diluted. Which ends up being a huge problem in large plants.

[Feb 2020]

Does anyone actually believe a word of this?

Yes. Why wouldn’t you?

A healthy exercise in skepticism.

An exercise in skepticism is actually asking a specific question that bothers you about the article. And not just a random "I don't believe this".

I have a healthy skepticism about your skepticism

> The system delivered pure water

That's toxic! We can't drink pure water. You have to maintain _some_ of the minerals in there, you just want to take most of the NaCL out.

Does the system avoid this over-distillation? i.e. is the water properly potable, or is it just not-salty?

I drink distilled water all the time, it's not toxic in normal quantities.

Ok, fair enough, but if that's all the water you drink than you will have to get a bunch of minerals elsewhere in your diet.

Here: https://naturalhealthfundamentals.com/is-distilled-water-saf...

it says:

Those who may need to be cautious when drinking it:

* Anyone who is already deficient in minerals. You may be someone who would benefit from the little extra minerals water can give you. Although if you are already deficient in minerals, and have health issues, make sure to get your water from a good source to avoid all the chemical additives from most tap waters.

* There also isn’t much information on how well we absorb inorganic minerals from water. There is also the possibility that the minerals present in the water are not doing us much good. But since it could be helpful it probably doesn’t hurt to have the minerals there.

* Someone who has health issues or malabsorption problems.

* Anyone who has an extremely poor diet and doesn’t get many minerals from their food should remineralize distilled water if they are drinking it.

It is my understanding that it is incredibly difficult to keep water pure. Transporting it any amount of distance from the source is likely to leech quite a few minerals from the available pipes.

Sorry, what? Pure water isn't toxic. And you can get minerals from other sources.

That's like saying tap water is toxic because it doesn't contain a balanced diet of vitamins.

Distilled water isn't toxic per se, but it's definitely got harmful effects. It'll leach minerals out of your teeth, for instance.

Just add some artificial minerals, it can come in some powder form I guess.

A few years back Coca-Cola got in trouble by doing exactly this. What they were doing was taking tap-water, and purifying it so they could sell it as "Dasani" - not a mineral water product, exactly, but a "lifestyle drink" they called it.

The thing about water when it's pure enough is that it's remarkably odd. It's unpleasant stuff: it'll weaken concrete if it leaks onto it, apparently it'll damage brass and steel, and it'll leach the minerals out of your teeth, but only until the water is no longer pure enough to do so. So what people do is, as you say, add buffer minerals back into the purified water to take the edge off.

What Coca-Cola got wrong was the dosing: they put ten times the amount of buffer minerals into the water as they should have, and made it toxic. So they'd taken a perfectly safe tap-water supply and ruined it.

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