Reverse Osmosis, Forward Osmosis & Molecular Screening

While it is not as widely used as Multistage Flash Distillation, Seawater Reverse Osmosis (SWRO) has become the most popular new method of desalination. Osmosis somewhat mimics the way cells separate salt from water, by exploiting a semi-permeable membrane. This allows water to pass through a molecular filter which leaves a concentration of lower salinity on one side of the screen and higher salinity on the other side of the screen. But lowering the concentration of salt in water means that entropy (i.e. disorder) is decreased in the total system, so the Laws of Thermodynamics demand that energy is required to lower this total entropy. This means that the water cannot filter itself, but rather a certain amount of energy, either in the form of a pressure differential or electrostatics is required to force the water through the screen.

There are different methods of osmosis, some use pressure to drive the system, some use a difference of electric potential, some force the salt-heavy brine through the screen while others force the lower-salt water through. All of these variations strive for greater efficiency, and the effect of osmotic research in the field of desalination has been effective ... the most common form of this molecular screening, called Seawater Reverse Osmosis (SWRO) has dramatically lowered the cost of desalinated water over the last fifteen years.

Molecular screens usually require careful maintenance to prevent fouling from pollutants or tiny sea creatures that make it past prefilters.

Typically, this type of desalination is well-suited for large-scale municipal projects since recent advances in the molecular screens have allowed these systems to operate as a higher efficiency than conventional distillation systems.

Solar Desalination

In its simplest form, the sun is used to heat a volume of water, which causes evaporation. Water from the volume is absorbed into the air where it condenses and then drips into a collection system. The process is so simple that it can be accomplished with just a sheet of transparent plastic and a large hole in the ground. But the limitation of solar desalination is that there is a theoretical maximum amount of water which can be evaporated by the sun in a given area. Traditional solar desalination structures were typically built similar to greenhouses. But the amount of water produced by these large structures was small enough that the cost of construction was prohibitive. There are other methods of solar desalination, for instance where photovoltaic panels are used to directly drive small SWRO devices. This process works, but extremely large solar panel arrays are required to run functionally-large SWRO systems. Other solar desalination systems use focusing mirrors to create a devices to create devices similar to MSF. Because of these limitations, solar desalination accounts for less than 1% of the world's desalinated water.

Multistage Flash Distillation

If you have ever cooked a pot of spaghetti and seen the water that collects on the inside of the lid then you've seen flash distillation in action. Water is heated, it is converted to steam and then it condenses out of the air onto a collection surface. Multistage Flash Distillation (MSF) uses multiple stages to convert a small portion of feed water to steam at one time. The subsequent stages then continue to convert liquid water to steam. The steam is condensed and collected to make distilled water.

One of the differences between distilled water and screened water is that distilled water is pure water, and the only contaminants present in the distilled water are those that condense out of the air with the water. But osmosis-produced water retains some of the minerals in the original feed water. In some cases, it is desired to keep a small cross-section of the original minerals in the water, but distilled water can be mixed with a small portion of the original source water to create a mineral profile identical to the original water, but in a lower concentration.

While MSF is gradually falling out of favor due to its relatively high energy costs, it is still the most widely-used form of desalination, producing about 85% of all desalinated water in the world.

Electromagnetic Deionization

This is an exotic and still experimental method where direct current, alternating current, magnetism or a combination of these is applied to salt water. Using a variety of electromagnetic effects, the ionic components of the salt are separated and removed from the solution.

Chemical Desalination

Chemicals are added to water which have an affinity to the dissolved salts already in the water. The chemicals cause the salt to precipitate from the water. This method is ideal for emergency desalination in life-rafts or for uses where a large quantity of desalinated water is not desired. But now, small hand-operated reverse osmosis systems are more common than this method.

SEA Panel

SEA is an acronym for Solar Evaporation Array. SEA Panels operate similarly to conventional solar desalination plants, but rather than requiring concrete foundations, glass, steel and building techniques, these panels are made of plastic and can be set up in minutes. The main advantage of SEA Panels over other methods is that it can be configured to collect salt crystals instead of emitting toxic salt brine. Also, SEA Panels have a much lower cost per unit area than conventional solar desalination systems since they are made from thermoformed plastic. SEA Panels are powered by 100% sun energy which gives them a good long-term cost advantage over conventional systems. Further, SEA Panels can be used away from electrical infrastructures.