Drinking water purification
Around the world, household drinking water purification systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking.
Such systems typically include a number of steps:
- a sediment filter to trap particles including rust and calcium carbonate
- optionally a second sediment filter with smaller pores
- an activated carbon filter to trap organic chemicals and chlorine, which will attack and degrade TFC reverse osmosis membranes
- a reverse osmosis (RO) filter which is a thin film composite membrane (TFM or TFC)
- optionally a second carbon filter to capture those chemicals not removed by the RO membrane
- optionally an ultra-violet lamp for disinfecting any microbes that may escape filtering by the reverse osmosis membrane
In some systems, the carbon pre-filter is omitted and cellulose triacetate membrane (CTA) is used. The CTA membrane is prone to rotting unless protected by chlorinated water, while the TFC membrane is prone to breaking down under the influence of chlorine. In CTA systems, a carbon post-filter is needed to remove chlorine from the final product water.
Portable reverse osmosis (RO) water processors are sold for personal water purification in various locations. To work effectively, the water feeding to these units should best be under some pressure (40 psi or greater is the norm). Portable RO water processors can be used by people who live in rural areas without clean water, far away from the city's water pipes. Rural people filter river or ocean water themselves, as the device is easy to use (Saline water may need special membranes). Some travelers on long boating trips, fishing, island camping, or in countries where the local water supply is polluted or substandard, use RO water processors coupled with one or more UV sterilizers. RO systems are also now extensively used by marine aquarium enthusiasts. In the production of bottled mineral water, the water passes through an RO water processor to remove pollutants and microorganisms. In European countries, though, such processing of Natural Mineral Water (as defined by a European Directive) is not allowed under European law. (In practice, a fraction of the living bacteria can and do pass through RO membranes through minor imperfections, or bypass the membrane entirely through tiny leaks in surrounding seals. Thus, complete RO systems may include additional water treatment stages that use ultraviolet light or ozone to prevent microbiological contamination.)
Membrane pore sizes can vary from .1 to 5,000 nanometers (nm) depending on filter type. "Particle filtration" removes particles of 1,000 nm 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. Reverse osmosis is in the final category of membrane filtration, "Hyperfiltration", and removes particles larger than .1 nm.
In the United States military, R.O.W.P.U.'s (Reverse Osmosis Water Purification Unit, pronounced "roh-poo") are used on the battlefield and in training. They come ranging from 1500 GPD (gallons per day) to 150,000 GPD and bigger depending on the need. The most common of these are the 600 GPH (gallons per hour) and the 3,000 GPH. Both are able to purify salt water and water contaminated with N.B.C. (Nuclear/Biological/Chemical) agents from the water. During a normal 24 hour period, one unit can produce anywhere from 12,000 to 60,000 gallons of water, with a required 4 hour maintenance window to check systems, pumps, R.O. elements and the engine generator. A single ROWPU can sustain a force of a battalion size element or roughly 1,000 to 6,000 soldiers.
Water and wastewater purification
Rain water collected from storm drains is purified with reverse osmosis water processors and used for landscape irrigation and industrial cooling in Los Angeles and other cities, as a solution to the problem of water shortages.
In industry, reverse osmosis removes minerals from boiler water at power plants. The water is boiled and condensed repeatedly. It must be as pure as possible so that it does not leave deposits on the machinery or cause corrosion. It is also used to clean effluent and brackish groundwater.
The process of reverse osmosis can be used for the production of deionized water.
Reverse osmosis is similar to the technique used in dialysis, which is used by people with kidney failure. The kidneys filter the blood, removing waste products (e.g. urea) and water, which is then excreted as urine. A dialysis machine mimics the function of the kidneys. The blood passes from the body via a catheter to the dialysis machine, across a filter.
In addition to desalination, reverse osmosis is a more economical operation for concentrating food liquids (such as fruit juices) than conventional heat-treatment processes. Research has been done on concentration of orange juice and tomato juice. Its advantages include a low operating cost and the ability to avoid heat treatment processes, which makes it suitable for heat-sensitive substances like the protein and enzymes found in most food products.
Reverse osmosis is extensively used in the dairy industry for the production of whey protein powders and for the concentration of milk to reduce shipping costs. In whey applications, the whey (liquid remaining after cheese manufacture) is pre-concentrated with RO from 6% total solids to 10-20% total solids before UF (ultrafiltration) processing. The UF retentate can then be used to make various whey powders including WPI (whey protein isolate) used in bodybuilding formulations. Additionally, the UF permeate, which contains lactose, is concentrated by RO from 5% total solids to 18–22% total solids to reduce crystallization and drying costs of the lactose powder.
Although use of the process was once frowned upon in the wine industry, it is now widely understood and used. An estimated 60 reverse osmosis machines were in use in Bordeaux, France in 2002. Known users include many of the elite classed growths (Kramer) such as Château Léoville-Las Cases in Bordeaux.
Because of its lower mineral content, Reverse Osmosis water is often used in car washes during the final vehicle rinse to prevent water spotting on the vehicle. Reverse osmosis water displaces the mineral-heavy reclamation water (municipal water). Reverse Osmosis water also enables the car wash operators to reduce the demands on the vehicle drying equipment such as air blowers.
Maple Syrup Production
Starting in the 1970s, some maple syrup producers started using reverse osmosis to remove water from sap before being further boiled down to syrup. The use of reverse osmosis allows approximately 75–80% of the water to be removed from the sap, reducing energy consumption and exposure of the syrup to high temperatures. Microbial contamination and degradation of the membranes has to be monitored.
For small scale production of hydrogen, reverse osmosis is sometimes used to prevent formation of minerals on the surface of electrodes and to remove organics from drinking water.
Pre-treatment is important when working with RO and nanofiltration (NF) membranes due to the nature of their spiral wound design. The material is engineered in such a fashion to allow only one way flow through the system. As such the spiral wound design doesn't allow for backpulsing with water or air agitation to scour its surface and remove solids. Since accumulated material cannot be removed from the membrane surface systems they are highly susceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any RO or NF system. Pretreatment in SWRO system has four major components:
- Screening of solids: Solids within the water must be removed and the water treated to prevent fouling of the membranes by fine particle or biological growth, and reduce the risk of damage to high-pressure pump components.
- Cartridge filtration - Generally string-wound polypropylene filters that remove between 1 - 5 micrometre sized particles.
- Dosing of oxidizing biocides such as chlorine to kill bacteria followed by bisulfite dosing to deactivate the chlorine which can destroy a thin-film composite membrane. There are also biofouling inhibitors which do not kill bacteria but simply prevent them from growing slime on the membrane surface.
- Prefiltration pH adjustment: If the pH, hardness and the alkalinity in the feedwater result in a scaling tendency when they are concentrated in the reject stream, acid is dosed to maintain carbonates in their soluble carbonic acid form.
- CO3-2 + H3O+ = HCO3- + H2O
- HCO3- + H3O+ = H2CO3 + H2O
- Carbonic acid cannot combine with calcium to form calcium carbonate scale. Calcium Carbonate Scaling tendency is estimated using the Langelier Saturation Index. Adding too much sulfuric acid to control carbonate scales may result in calcium sulfate, barium sulfate or strontium sulfate scale formation on the RO membrane.
- Prefiltration Antiscalants: Scale inhibitors (also known as antiscalants) prevent formation of all scales compared to acid which can only prevent formation of calcium carbonate and calcium phosphate scales. In addition to inhibiting carbonate and phosphate scales, antiscalants inhibit sulfate and fluoride scales, disperse colloids and metal oxides and specialty products exist to inhibit silica formation.