For a world covered in oceans, getting a drink of water on Planet Earth can be surprisingly tricky. Fresh water is hard to come by even on our water world, so much so that most sources are better measured in parts per million than percentages; add together every freshwater lake, river, and stream in the world, and you’d be looking at a mere 0.0066% of all the water on Earth.
Of course, what that really says is that our endowment of saltwater is truly staggering. We have over 1.3 billion cubic kilometers of the stuff, most of it easily accessible to the billion or so people who live within 10 kilometers of a coastline. Untreated, though, saltwater isn’t of much direct use to humans, since we, our domestic animals, and pretty much all our crops thirst only for water a hundred times less saline than seawater.
While nature solved the problem of desalination a long time ago, the natural water cycle turns seawater into freshwater at too slow a pace or in the wrong locations for our needs. While there are simple methods for getting the salt out of seawater, such as distillation, processing seawater on a scale that can provide even a medium-sized city with a steady source of potable water is definitely a job for Big Chemistry.
Biology Backwards
Understanding an industrial chemistry process often starts with a look at the feedstock, so what exactly is seawater? It seems pretty obvious, but seawater is actually a fairly complex solution that varies widely in composition. Seawater averages about 3.5% salinity, which means there are 35 grams of dissolved salts in every liter. The primary salt is sodium chloride, with potassium, magnesium, and calcium salts each making a tiny contribution to the overall salinity. But for purposes of acting as a feedstock for desalination, seawater can be considered a simple sodium chloride solution where sodium anions and chloride cations are almost completely dissociated. The goal of desalination is to remove those ions, leaving nothing but water behind.
While thermal desalination methods, such as distillation, are possible, they tend not to scale well to industrial levels. Thermal methods have their place, though, especially for shipboard potable water production and in cases where fuel is abundant or solar energy can be employed to heat the seawater directly. However, in most cases, industrial desalination is typically accomplished through reverse osmosis RO, which is the focus of this discussion.
In biological systems, osmosis is the process by which cells maintain equilibrium in terms of concentration of solutes relative to the environment. The classic example is red blood cells, which if placed in distilled water will quickly burst. That’s because water from the environment, which has a low concentration of solutes, rushes across the semi-permeable cell membrane in an attempt to dilute the solutes inside the cell. All that water rushing into the cell swells it until the membrane can’t take the pressure, resulting in hemolysis. Conversely, a blood cell dropped into a concentrated salt solution will shrink and wrinkle, or crenellate, as the water inside rushes out to dilute the outside environment.
Reverse osmosis is the opposite process. Rather than water naturally following a concentration gradient to equilibrium, reverse osmosis applies energy in the form of pressure to force the water molecules in a saline solution through a semipermeable membrane, leaving behind as many of the salts as possible. What exactly happens at the membrane to sort out the salt from the water is really the story, and as it turns out, we’re still not completely clear how reverse osmosis works, even though we’ve been using it to process seawater since the 1950s.
Battling Models
Up until the early 2020s, the predominant model for how reverse osmosis (RO) worked was called the “solution-diffusion” model. The SD model treated RO membranes as effectively solid barriers through which water molecules could only pass by first diffusing into the membrane from the side with the higher solute concentration. Once inside the membrane, water molecules would continue through to the other side, the permeate side, driven by a concentration gradient within the membrane. This model had several problems, but the math worked well enough to allow the construction of large-scale seawater RO plants.
The new model is called the “solution-friction” model, and it better describes what’s going on inside the membrane. Rather than seeing the membrane as a solid barrier, the SF model considers the concentrate and permeate surfaces of the membrane to communicate through a series of interconnected pores. Water is driven across the membrane not by concentration but by a pressure gradient, which drives clusters of water molecules through the pores. The friction of these clusters against the walls of the pores results in a linear pressure drop across the membrane, an effect that can be measured in the lab and for which the older SD model has no explanation.
As for the solutes in a saline solution, the SF model accounts for their exclusion from the permeate by a combination of steric hindrance (the solutes just can’t fit through the pores), the Donnan effect (which says that ions with the opposite charge of the membrane will get stuck inside it), and dielectric exclusion (the membrane presents an energy barrier that makes it hard for ions to enter it). The net result of these effects is that ions tend to get left on one side of the membrane, while water molecules can squeeze through more easily to the permeate side.
Turning these models into a practical industrial process takes a great deal of engineering. A seawater reverse osmosis or SWRO, plant obviously needs to be located close to the shore, but also needs to be close to supporting infrastructure such as a municipal water system to accept the finished product. SWRO plants also use a lot of energy, so ready access to the electrical grid is a must, as is access to shipping for the chemicals needed for pre- and post-treatment.
Pores and Pressure
Seawater processing starts with water intake. Some SWRO plants use open intakes located some distance out from the shoreline, well below the lowest possible tides and far from any potential source of contamination or damage, such a ship anchorages. Open intakes generally have grates over them to exclude large marine life and debris from entering the system. Other SWRO plants use beach well intakes, with shafts dug into the beach that extend below the water table. Seawater filters through the sand and fills the well; from there, the water is pumped into the plant. Beach wells have the advantage of using the beach sand as a natural filter for particulates and smaller sea critters, but do tend to have a lower capacity than open intakes.
Aside from the salts, seawater has plenty of other unwanted bits, all of which need to come out prior to reverse osmosis. Trash racks remove any shells, sea life, or litter that manage to get through the intakes, and sand bed filters are often used to remove smaller particulates. Ultrafiltration can be used to further clarify the seawater, and chemicals such as mild acids or bases are often used to dissolve inorganic scale and biofilms. Surfactants are often added to the feedstock, too, to break up heavy organic materials.
By the time pretreatment is complete, the seawater is remarkably free from suspended particulates and silt. Pretreatment aims to reduce the turbidity of the feedstock to less than 0.5 NTUs, or nephelometric turbidity units. For context, the US Environmental Protection Agency standard for drinking water is 0.3 NTUs for 95% of the samples taken in a month. So the pretreated seawater is almost as clear as drinking water before it goes to reverse osmosis.
The heart of reverse osmosis is the membrane, and a lot of engineering goes into it. Modern RO membranes are triple-layer thin-film composites that start with a non-woven polyester support, a felt-like material that provides the mechanical strength to withstand the extreme pressures of reverse osmosis. Next comes a porous support layer, a 50 μm-thick layer of polysulfone cast directly onto the backing layer. This layer adds to the physical strength of the backing and provides a strong yet porous foundation for the active layer, a cross-linked polyamide layer about 100 to 200 nm thick. This layer is formed by interfacial polymerization, where a thin layer of liquid monomer and initiators is poured onto the polysulfone to polymerize in place.
Modern membranes can flow about 35 liters per square meter every hour, which means an SWRO plant needs to cram a lot of surface area into a little space. This is accomplished by rolling the membrane up into a spiral and inserting it into a fiberglass pressure vessel, which holds seven or eight cartridges. Seawater pumped into the vessel soaks into the backing layer to the active layer, where only the water molecules pass through and into a collection pipe at the center of the roll. The desalinated water, or permeate, exits the cartridge through the center pipe while rejected brine exits at the other end of the pressure vessel.
The pressure needed for SWRO is enormous. The natural osmotic pressure of seawater is about 27 bar (27,000 kPa), which is the pressure needed to halt the natural flow of water across a semipermeable membrane. SWRO systems must pressurize the water to at least that much plus a net driving pressure (NPD) to overcome mechanical resistance to flow through the membrane, which amounts to an additional 30 to 40 bar.
Energy Recovery
To achieve these tremendous pressures, SWRO plants use multistage centrifugal pumps driven by large, powerful electric motors, often 300 horsepower or more for large systems. The electricity needed to run those motors accounts for 60 to 80 percent of the energy costs of the typical SWRO plant, so a lot of effort is put into recovering that energy, most of which is still locked up in the high-pressure rejected brine as hydraulic energy. This energy used to be extracted by Pelton-style turbines connected to the shaft of the main pressure pump; the high-pressure brine would spin the pump shaft and reduce the mechanical load on the pump, which would reduce the electrical load. Later, the brine’s energy would be recovered by a separate turbo pump, which would boost the pressure of the feed water before it entered the main pump.
While both of these methods were capable of recovering a large percentage of the input energy, they were mechanically complex. Modern SWRO plants have mostly moved to isobaric energy recovery devices, which are mechanically simpler and require much less maintenance. Isobaric ERDs have a single moving part, a cylindrical ceramic rotor. The rotor has a series of axial holes, a little like the cylinder of an old six-shooter revolver. The rotor is inside a cylindrical housing with endcaps on each end, each with an inlet and an outlet fitting. High-pressure reject brine enters the ERD on one side while low-pressure seawater enters on the other side. The slugs of water fill the same bore in the rotor and equalize at the same pressure without much mixing thanks to the different densities of the fluids. The rotor rotates thanks to the momentum carried by the incoming water streams and inlet fittings that are slightly angled relative to the axis of the bore. When the rotor lines up with the outlet fittings in each end cap, the feed water and the brine both exit the rotor, with the feed water at a higher pressure thanks to the energy of the reject brine.
For something with only one moving part, isobaric ERDs are remarkably effective. They can extract about 98% of the energy in the reject brine, pressuring the feed water about 60% of the total needed. An SWRO plant with ERDs typically uses 5 to 6 kWh to produce a cubic meter of desalinated water; ERDs can slash that to just 2 to 3 kWh.
Finishing Up
Once the rejected brine’s energy has been recovered, it needs to be disposed of properly. This is generally done by pumping it back out into the ocean through a pipe buried in the seafloor. The outlet is located a considerable distance from the inlet and away from any ecologically sensitive areas. The brine outlet is also generally fitted with a venturi induction head, which entrains seawater from around the outlet to partially dilute the brine.
As for the permeate that comes off the RO racks, while it is almost completely desalinated and very clean, it’s still not suitable for distribution into the drinking water system. Water this clean is highly corrosive to plumbing fixtures and has an unpleasantly flat taste. To correct this, RO water is post-processed by passing it over beds of limestone chips. The RO water tends to be slightly acidic thanks to dissolved CO2, so it partially dissolves the calcium carbonate in the limestone. This raises the pH closer to neutral and adds calcium ions to the water, which increases its hardness a bit. The water also gets a final disinfection with chlorine before being released to the distribution network.
I imagine renewables changes the power equation.
To some degree. But, like most industrial processes, it works best and most reliably when it’s always running, not when it starts and stops with an intermittent power source.
There is a new kid on the block for water desalination, Ion Concentration Separation. This also uses a membrane but requires an electric field rather than a high-pressure pump. It is less energy efficient than RO, but is better suited to powering from renewables. It doesn’t require any sort of pressurisation beyond gravity to make the water drop through the device, so intermittent operation doesn’t have the same pressure-cycling effect that you get when you power RO plant from intermittent sources.
People are ignorant and foolish, one does not simply pull the salt from the ocean with no consequence. You imbeciles cry about bs global warming yet want to accelerate the exact problem glacial melt water would cause.
Have you actually done or even seen any of the math on this? For example, how does a desal plant compare to the salinity costs associated with pulling fresh water from a stream or river and preventing that water from getting to the ocean? I haven’t. But I suspect the amount, even if used to replace all of humanities freshwater needs (rather than the few percent proposed) would result in immeasurable changes to seawater salinity – particularly a hundred meters or more away from the outlet. And if that’s right, then the environmental benefit from desal water, and reduction to the need for massive and growing freshwater infrastructure, could be significantly more than any harm.
With high enough pressure of compression (achievable in regions of oceanic hyperdepth) anything can become Diesel if you want it – for example bodies of OceanGate Titan crew were literally turned into CO2 and ash because air combined with fat in their lungs acted like fuel injected into truck engine.
In the future I think most energy could be generated by Diesel plants located under the ocean, the only problem will be shipping fuel deep down enough to reach without cracking.
???
Water desalination problem was solved in early 60s by using salt water to directly cool nuclear reactor. It’s turned into steam and then filtered to normal drinking water.
Someone should inform California then, because they are constantly having issue with access to potable water…..
Well that’s a different problem, The problems with water in California is on purpose. Rerouting water so it runs into the ocean instead of LA, a lack of maintenance in the last few decades resulting in water systems running dry, creating all sorts of problems with reservoirs. It’s a huge mess. That’s why, well over a year ago, before the fires, all insurance companies in the state upped the rates on homes and lowered the payouts as all the problems are compounding resulting in the state being quite a fire hazard.
There is so much fresh water in California. Almost the entire state has access to smaller rivers, creeks, ponds, lakes, all with fresh water.
I’m not American but I started reading on the history of California’s water issues and it’s quite interesting how this was caused knowing exactly what would happen. And then a perfect storm happened, that they were warned about.
If you have a car and you refuse to do oil changes, don’t be surprised when your engine fails after a while. Same goes for everything, water supplies, power grids, etc. And California is known for not doing maintenance. So many fires have started due to grid failures and it’s only getting worse. The days before the fires I saw a bunch of video’s out of California with fires starting over power grid problems. It’s a huge mess.
65 year-old, native Californian here. Bob’s right. This happens not only with water that should be flowing to LA but the SF bay area too. I’ve seen what he’s describing happen. Voters gave the state $7B to spend for water retention projects probably a decade or so ago. Nearly all of that went into systems to ease the flow of water into the ocean. This causes the droughts which Sacramento blames on “climate change” (can’t be their fault, you see).
N Cal here.
Nobody will let LA establish water rights because of what they did to the Owens Valley.
LA has nobody to blame but themselves.
You can’t act friendly until you get a foot in the door, then just outvote the locals, take all the water and expect anybody to ever let you get a foot in the door again.
Tool is right about LA!
https://youtu.be/rHcmnowjfrQ?si=UWQ5CvVefAVFuWRp
Those living in known repeated burn areas (e.g. coastal LA) should have insurance bills that reflect that risk. Not subsidized by the rest of the state, as it is now.
The latest LA fires are not a ‘perfect storm’ or ‘climate change related’.
They were historically predictable and fairly routine.
Happened lots of times before, just less populated then.
The firefighting effort was one arm behind back…(e.g. leaving fire trucks with last years engines parked because they weren’t clean enough.)
But it wouldn’t have mattered, those winds have always been a bitch and fuel buildup is just inevitable, given rainy season.
Fun fact: The LA river runs more than enough water into the ocean each year to supply LA area. There is just no place to put it during rainy season.
Good news!
There are no dry season water rights available in N Cal anymore.
I mean they, can be claimed, but you’d get water 1 year in 50.
Last in line.
LA needs to learn to live without lawns or dam up a dry valley or two and pump.
They choose to live there, deal.
All the good dam sights in the state are already done.
Most of the mediocre ones are also done.
You do want to leave some fish spawn rivers anyhow.
Like everything, it’s a balance.
Not as the primary coolant loop right? That would make the water radioactive.
For a loose definition of “solved”. Scaling and fouling, material compatibility, etc.
The BN-350 reactor in Shevchenko (now Aktau, Kazakhstan), which began operation in 1973, is a notable exception, providing both electricity and desalinated water for decades. However, even this project faced its own set of technical and operational challenges.
https://inis.iaea.org/records/j3tty-ej229
HAD columns never disappoint!
“we’re still not completely clear how reverse osmosis works, even though we’ve been using it to process seawater since the 1950s.”
In case you would like to make RO a subject of big chemistry please do!
Interesting. Thank you.
I like how the new model is basically just filtration
First for hermit lifestyle on deserted island with solar powered desalinator, small permaculture vegetable and animal farm.
Society has nothing more to offer me
Where are you going to get new tools when they break?
I feel like that is part of the dream behind Reprap and any other ‘fully printed’ design.
I think there is a dream for us to have a little box like a replicator that can spit out all the parts we need for something including itself. Then we can fix anything and even more ideally recycle the broken parts to base materials to use later.
If you really want to see tremendous pressures, search the web for ethylene hypercompressors.
Interestingly, this same isobaric recovery device principle is used in one type of automotive supercharger known as the pressure wave supercharger that I found out about recently. Super interesting if you want to look into it. I thought I had seen it all. Only documentation I have seen of it is the 88 Mazda 626 Capella. Definitely worth reading about.
Trade name Comprex.
Used in Valmet Tractors (411CX engine in the 80’s).
Ferrari experiment with their F1 26C (https://www.forza-mag.com/issues/217/articles/under-pressure)
Peugeot, Opel, and Mercedes-Benz: (https://www.osti.gov/etdeweb/biblio/6812900)
Rinspeed Topaz Comprex: show car in 82.
Greenpeace SmILE concept car.
NASA and Antrova AG developing Comprex 2.0. (https://ntrs.nasa.gov/citations/20050176387)
“about 27 bar (27,000 kPa),”
Check your conversion. 27 bar is 2700 kPa.
So, not actually “enormous” pressure after all. Easily achieved by a rather small pump, like you will find in nearly every kitchen under-sink RO system.
Easy to get to useful units.
1 bar ~= 1 atm ~= 15 psi.
If you own a grease gun, you own a 10-20 kpsi hand pump.
For some definition of “useful”. Though imperial units for pressure are particularly annoying and user-abusive to do calculations with.
But I suppose if a few percent of the human populace prefers it we should consider allowing it to exist. We need have diversity and inclusion, after all.
Indeed. The CP3 injection pump on my diesel truck achieves ~345 bar at idle and north of 1700 bar at max rail pressure.
What is the increase in salt content of the “waste” water. Because my brain instantly thinks free high purity feedstock for additional chemical processes.
Not high purity. And probably unusable as feedstock for any serious process. There are mountains of salt lying out there (Monte Kali https://en.wikipedia.org/wiki/Monte_Kali ) and people don’t know what to do with it. According to wiki, there are 900 tonnes being added every hour.
*Water this clean is highly corrosive to plumbing fixtures *
Not true. Not at all. Stop perpetuating this myth.
RO, distilled, DI, ultrapure water erode metallic pipes even less quickly than “ordinary” tap water.
The myth comes from the insistence by purveyors of ultrapure water to use non-metallic plumbing because the tiny amount of metal ions the water does get degrades the water quality, not the pipe quality.
Where is water still routinely chlorinated? Best I know this is not done often because customers don’t like the taste, and the chlorine is kept in storage to be used in “emergencies” only.
A few years ago I read some research paper of another way to desalinate water. The idea is to use a DC current to get the ions out, but do this inside a pipe, and then split off the center of the stream. (Most of) the ions don’t even reach the electrodes. They are washed away in the stream. I think they were able to get around 75% of the salt out of the water this way and the research was ongoing.
You’ll have a hard time finding a large metropolitan area in any developed country that does not routinely use chlorine, or chlorine dioxide, to keep their water distribution system safe.
My municipal supply provides water to almost 1M people. I routinely check chlorine levels as part of my filter maintenance. I find the municipal supply varies from barely detectable at 0.1 ppm to 1 ppm. I have not found any correlation to season or temperature or rainfall. Yes, 1 ppm is easily detectable by taste. Hence my filter.
‘our’ water treatment plan processes about a million tons of water per day, implying they use a ton of chlorine per day. Bonkers.
My city uses oxygenation, but it’s a small “spa city” in Poland. Most of other cities use chlorination too.
An advantage of chlorination over ozone or other oxygen treatment is that the chlorine remains in the water much longer, providing residual treatment. Ozone quickly decomposes.
My small town uses chlorination, though the level is high enough that I can barely tolerate it. Thank the Flying Spaghetti Monster for fridge-filtered water!
There’s so many people on earth that we need gorillions of dollars worth of high tech just for them to have something to drink, yet people will still call you Hitler if you suggest “maybe 8 billion is a bit much”. Wild stuff.
Maybe you would prefer to be called Malthus instead?
It’s actually all physics, virtually no chemistry invovled.
… which is just applied physics :-)
obxkcd: https://xkcd.com/435/
“sodium anions and chloride cations”
This is backwards in the article. Cations refer to positively charges ions (like Na+) and anions refer to negatively charged ions (like Cl-).
“0.0066% of all the water on Earth” is fresh? All the source I can find say it’s around 3%. Several orders of magnitudes larger. Still relatively small, but not that small.