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.
Water rushes in, water rushes out. Either way, osmosis is bad news for red blood cells. Reversing the natural osmotic flow of a solution like seawater is the key to desalination by reverse osmosis. Source: Emekadecatalyst, CC BY-SA 4.0.
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.
SWRO cartridges have membranes wound into spirals and housed in pressure vessels. Seawater under high pressure enters the membrane spiral; water molecules migrate across the membrane to a center permeate tube, leaving a reject brine that’s about twice as saline as the feedstock. Source: DuPont Water Solutions.
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.
An RO rack in a modern SWRO desalination plant. Each of the white tubes is a pressure vessel containing seven or eight RO membrane cartridges. The vessels are plumbed in parallel to increase flow through the system. Credit: Elvis Santana, via Adobe Stock.
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.
Isobaric energy recovery devices can recover half of the electricity used by the typical SWRO plant by using the pressure of the reject brine to pressurize the feed water. Source: Flowserve.
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.
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