Reverse osmosis
Reverse osmosis is a water purification process that uses a semi-permeable membrane to separate water molecules from other substances. RO applies pressure to overcome osmotic pressure that favors even distributions. RO can remove dissolved or suspended chemical species as well as biological substances, and is used in industrial processes and the production of potable water.
RO retains the solute on the pressurized side of the membrane and the purified solvent passes to the other side. The relative sizes of the various molecules determines what passes through. "Selective" membranes reject large molecules, while accepting smaller molecules.
Reverse osmosis is most commonly known for its use in drinking water purification from seawater, removing the salt and other effluent materials from the water molecules. As of 2013 the world's largest RO desalination plant was in Sorek, Israel, outputting. RO systems for private use are also available for purifying municipal tap water or pre-treated well water.
History
A process of osmosis through semi-permeable membranes was first observed in 1748 by Jean-Antoine Nollet. For the following 200 years, osmosis was only a laboratory phenomenon. In 1950, the University of California at Los Angeles first investigated osmotic desalination. Researchers at both UCLA and University of Florida desalinated seawater in the mid-1950s, but the flux was too low to be commercially viable. Sidney Loeb at UCLA and Srinivasa Sourirajan at the National Research Council of Canada, Ottawa, found techniques for making asymmetric membranes characterized by an effectively thin "skin" layer supported atop a highly porous and much thicker substrate region. John Cadotte, of Filmtec corporation, discovered that membranes with particularly high flux and low salt passage could be made by interfacial polymerization of m-phenylene diamine and trimesoyl chloride. Cadotte's patent on this process was the subject of litigation and expired. Almost all commercial RO membrane is now made by this method. By 2019, approximately 16,000 desalination plants operated around the world, producing around. Around half of this capacity was in the Middle East and North Africa region.In 1977 Cape Coral, Florida became the first US municipality to use RO at scale, with an initial operating capacity of 11.35 million liters per day. By 1985, rapid growth led the city to operate the world's largest low-pressure RO plant, producing 56.8 million liters per day.
Osmosis
In osmosis, the solvent moves from an area of low solute concentration, through a membrane, to an area of high solute concentration. The driving force for the movement of the solvent is the reduction in the Gibbs free energy of the system in which the difference in solvent concentration between the sides of a membrane is reduced. This is called osmotic pressure. It reduces as the solvent moves into the more concentrated solution. Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is similar to other membrane technology applications.RO differs from filtration in that the mechanism of fluid flow is reversed, as the solvent crosses membrane, leaving the solute behind. The predominant removal mechanism in membrane filtration is straining, or size exclusion, where the pores are 0.01 micrometers or larger, so the process can theoretically achieve perfect efficiency regardless of parameters such as the solution's pressure and concentration. RO instead involves solvent diffusion across a membrane that is either nonporous or uses nanofiltration with pores 0.001 micrometers in size. The predominant removal mechanism is from differences in solubility or diffusivity, and the process is dependent on pressure, solute concentration, and other conditions.
RO requires pressure between 2–17 bar for fresh and brackish water, and 40–82 bar for seawater. Seawater has around 27 bar natural osmotic pressure that must be overcome. As for their energy consumption, seawater RO systems typically require 2.9-5.5 kWh/m3, although state-of-the-art systems are around 2.3 kWh/m3.
Membrane pore sizes vary from 0.1 to 5,000 nm. Particle filtration removes particles of 1 μm or larger. Microfiltration removes particles of 50 nm or larger. Ultrafiltration removes particles of roughly 3 nm or larger. Nanofiltration removes particles of 1 nm or larger. RO is in the final category of membrane filtration, hyperfiltration, and removes particles larger than ~0.2 nm.
Fresh water applications
Drinking water purification
Around the world, household drinking water purification systems, including an RO step, are commonly used for improving water for drinking and cooking.Such systems typically include these steps:
- a sediment filter to trap particles, including rust and calcium carbonate
- a second sediment filter with smaller pores
- an activated carbon filter to trap organic chemicals and chlorine, which degrades certain types of thin-film composite membrane
- an RO thin-film composite membrane
- an ultraviolet lamp for sterilizing any microbes that survive RO
- a second carbon filter to capture chemicals that survive RO
The cellulose triacetate membrane rots unless protected by chlorinated water, while the thin-film composite membrane breaks down in the presence of chlorine. The thin-film composite membrane is made of synthetic material, and requires the chlorine to be removed before the water enters the membrane. To protect the TFC membrane elements from chlorine damage, carbon filters are used as pre-treatment. TFC membranes have a higher rejection rate of 95–98% and a longer life than CTA membranes.
To work effectively, the water feeding to these units should be under pressure.
Though Portable RO Water Purifiers are commercially available and extensively used in areas lacking cleaning potable water, in Europe such processing of natural mineral water is not allowed. In practice, a fraction of the living bacteria pass through RO through membrane imperfections or bypass the membrane entirely through leaks in seals.
Solar-powered RO
A solar-powered desalination unit produces potable water from saline water by using a photovoltaic system to supply the energy. Solar power works well for water purification in settings lacking grid electricity and can reduce operating costs and greenhouse emissions. For example, a solar-powered desalination unit designed passed tests in Australia's Northern Territory.Sunlight's intermittent nature makes output prediction difficult without an energy storage capability. However batteries or thermal energy storage systems can provide power when the sun does not.
Military
Larger scale reverse osmosis water purification units exist for military use. These have been adopted by the United States armed forces and the Canadian Forces. Some models are containerized, some are trailers, and some are themselves vehicles.The water is treated with a polymer to initiate coagulation. Next, it is run through a multi-media filter where it undergoes primary treatment, removing turbidity. It is then pumped through a cartridge filter which is usually spiral-wound cotton. This process strips any particles larger than 5 μm and eliminates almost all turbidity.
The clarified water is then fed through a high-pressure piston pump into a series of RO vessels. 90.00–99.98% of the raw water's total dissolved solids are removed and military standards require that the result have no more than 1000–1500 parts per million by measure of electrical conductivity. It is then disinfected with chlorine.
Water and wastewater purification
RO-purified rainwater collected from storm drains is used for landscape irrigation and industrial cooling in Los Angeles and other cities.In industry, RO removes minerals from boiler water at power plants. The water is distilled multiple times to ensure that it does not leave deposits on the machinery or cause corrosion.
RO is used to clean effluent and brackish groundwater. The effluent in larger volumes is treated in a water treatment plant first, and then the effluent runs through RO. This hybrid process reduces treatment cost significantly and lengthens membrane life.
RO can be used for the production of deionized water.
In 2002, Singapore announced that a process named NEWater would be a significant part of its water plans. RO would be used to treat wastewater before discharging the effluent into reservoirs.
Food industry
Reverse osmosis is a more economical way to concentrate liquids than conventional heat-treatment. Concentration of orange and tomato juice has advantages including a lower operating cost and the ability to avoid heat-treatment, which makes it suitable for heat-sensitive substances such as protein and enzymes.RO is used in the dairy industry to produce whey protein powders and concentrate milk. The whey is concentrated with RO from 6% solids to 10–20% solids before ultrafiltration processing. The retentate can then be used to make whey powders, including whey protein isolate. Additionally, the permeate, which contains lactose, is concentrated by RO from 5% solids to 18–total solids to reduce crystallization and drying costs.
Although RO was once avoided in the wine industry, it is now widespread. An estimated 60 RO machines were in use in Bordeaux, France, in 2002. Known users include many of elite firms, such as Château Léoville-Las Cases.