R.O.

Reverse Osmosis (R.O.) is a modern technique for water purification derived from the natural biological cell operation, which drives the transport of nutrients in plants and animals. The process produces pure water on a continuous basis, in a large (but reliable) quantity.

With reverse Osmosis, only one process is needed for the physical and chemical purification of water. This is a continuous pressure driven process for which no chemical additions are required. Dissolved materials are separated from the water by means of a semi-permeable membrane. Modern membranes can remove up to 99% of dissolved matter without any addition of chemicals and without frequent shut downs for regeneration.

Water produced by means of R.O. contains virtually zero dissolved solids. This prevents precipitation, discoloration, staining or other harmful effects, which could otherwise disturb an efficient water production line.



The Reverse Osmosis procedure removes:

Bacteria, viruses and pyrogens

90 to 98% of the total amount of mineral salts

95 to 98% of the heavy metals

Applications : The number of applications for reverse osmosis, for the production of high quality
water, becomes larger and larger every day.

Typical examples include:

Printing Industry : The use of water-based printing inks requires precise control of water chemistry.Reverse Osmosis is an ideal means of producing water of a consistent quality, which can then be adapted for this application.

Hotel and Catering Industry : It is evident that the use of high quality water is very important for hotels and catering companies. Industrial dishwashers will have a much longer lifespan thanks to an R.O. appliance. The dishes themselves will be visibly cleaner. The same can be said about washing machines.

Carwash : R.O. is an indispensable appliance for every carwash! The system has been manufactured with the intention to produce permeate water with a Total Dissolved Solids (TDS) level below the threshold for spot formation. Cars will shine like never before, and this with a space-saving appliance.

Horticulture : The irrigation of plants and flowers with contaminated water or water, which contains too many salts, can cause damage to plants. The use of reverse osmosis water allows better growth an a higher quality finished product.

What Is Reverse Osmosis

Reverse osmosis, also known as hyperfiltration, is the finest filtration known. This process will allow the removal of particles as small as ions from a solution. Reverse osmosis is used to purify water and remove salts and other impurities in order to improve the colour, taste or properties of the fluid. It can be used topurify fluids such as ethanol and glycol, which will pass through the reverse osmosis membrane, while rejecting other ions and contaminants from passing. The most common use for reverse osmosis is in purifying water. It is used to produce water that meets the most demanding specifications that are currentlyin place.

Reverse osmosis uses a membrane that is semi-permeable, allowing the fluid that is being purified to pass through it, while rejecting the contaminants that remain. Most reverse osmosis technology uses a process known as crossflow to allow the membrane to continually clean itself. As some of the fluid passes through the membrane the rest continues downstream, sweeping the rejected species away from the membrane. The process of reverse osmosis requires a driving force to push the fluid through the membrane, and the most common force is pressure from a pump. The higher the pressure, the larger the driving force. As the concentration of the fluid being rejected increases, the driving force required to continue concentrating the fluid increases.

Reverse osmosis is capable of rejecting bacteria, salts, sugars, proteins, particles, dyes, and other constituents that have a molecular weight of greater than 150-250 daltons. The separation of ions with reverse osmosis is aided by charged particles. This means that dissolved ions that carry a charge, such as salts, are more likely to be rejected by the membrane than those that are not charged, such as organics. The larger the charge and the larger the particle, the more likely it will be rejected.

Fundamentals of Reverse Osmosis

Osmosis is a natural phenomenon in which a liquid - water, in this case - passes through a semi- permeable membrane from a relatively dilute solution towards a more concentrated solution. This flow produces a measurable pressure, called osmotic pressure. If pressure is applied to the more concentrated solution, and if that pressure exceeds the osmotic pressure, water flows through the membrane from the more concentrated solution towards the dilute solution. This process, called reverse osmosis, or RO, removes up to 98% of dissolved minerals, and virtually 100% of the colloidal and suspended matter. RO produces high quality water at low cost compared to other purification processes.

The membrane must be physically strong to stand up to high osmotic pressure - in the case of sea water, 1000 psi (6,900 kPa) Most membranes are made of cellulose acetate or polyamide composite cast into a thin film, either as a sheet or fine hollow fibers. The membrane is constructed into a cartridge called a reverse osmosis module or element.

After filtration to remove large suspended particles, incoming water is pressurised with a pump to 150 to 1000 pri (1034 - 6900 kPa) depending on the RO system model. This exceeds the water's
osmotic pressure. A portion of the water (permeate) diffuses through the membrane leaving dissolved salts and other contaminants behind with the remaining water where they are sent to the drain as waste (concentrate).

Pretreatment is important because it influences permeate quality and quantity. It also affects the module's life because many water-borne contaminants can deposit on the membrane and foul it. Generally, the need for pretreatment increases as systems become larger and operate at higher pressures, and as permeate quality requirements become more demanding.

Permeate production increases with increasing pressure and temperature. RO systems are rated at the standard temperature of 77F (25C). When the feed water is lower than this standard, system size or the feed pressure must be increased to compensate for lower production. Membrane fouling can also reduce production by as much as 8% at 200 psi (1380 kPa) and as much as 20% at 400 psi (2760 kPa) in three years.

There are several theories about how water and salt pass through semipermeable membranes. One suggests that a membrane is porous, containing many capillaries through which water flows. Another suggests a solution / diffusion mechanism in which water continuously dissolves into the membrane on the pressurised side and diffuses out the other.

In either case, dissolved inorganic and organic matter cannot pass through the membrane to any great extent. With polyamide thin film membranes, up to 98% of the dissolved minerals can be rejected. Pure water is forced through the membrane and dissolved impurities remain on the feed side of the membrane. In systems using cellulose acetate membrane somewhere between 35 and 50% of the feed water can be drawn off (recovered) as permeate. In larger systems using polyamide membranes, recovery can reach 80%.Countertop units typically have an unpressurized storage tank; Undersink units typically have a pressurised accumulator storage tank where the water pressure tends to increase as the tank fills. This pressurised system provides sufficient pressure to move the water from the undersink storage tank to the faucet. Unfortunately, this also creates a back pressure against the membrane, which can decrease its efficiency. Some units overcome this by using unpressurised tanks with a pump to get the treated water where it is needed.

Low pressure units typically provide between 2 and 15 gallons per day of water, with an efficiency of 2-4 gallons of reject water per gallon of treated water. Water purity can be as high as 95 percent. These systems can be highly affordable, with coutertop units starting at about US $150, and undersink units starting at about US $500. These units produce water for a cost as low as ten cents per gallon once maintenance and water costs are factored in. Maintenance usually requires replacing any pre- or postfilters (typically one to four times per year); and the reverse osmosis cartridge once every two to three years, depending on usage. Look for the WQA Gold Seal (S-300) to find products that have been successfully tested to industry performance standards; and to Certified Water Specialists (CWS I-V), Certified Sales Representatives (CSR), and Certified Installers (CI) for advice on you water needs, and equipment installation.

High Pressure (Commercial / Industrial) Systems

High pressure systems typically operate at pressures between 100 and 1000 psig, depending on the membranes chosen and the water being treated. These systems are usually used in industrial or commercial applications where large volumes of treated water are required at a high level of purity.

Most commercial and industrial systems use multiple membranes arranged in parallel to provide the required quantity of water. The processed water from the firs stage of treatment can then be passed through additional membrane modules to achieve greater levels of treatment for the finished water. The reject water can also be directed into successive membrane modules for greater efficiency (see diagram below), though flushing will still be required when concentrations reach a level where fouling is likely to occur.

High pressure industrial units typically provide from 10 gallons to thousands of gallons per day of water with an efficiency of 1-9 gallons of reject water per gallon of treated water. Water purity can be as high as 95%. These systems tend to be larger and more complicated than low pressure systems, and this is reflected in their costs, which range from US $1000 through tens of thousands of dollars for a large, multi-module unit capable of providing desalinated drinking water for a resort facility or water bottling plant.

What Reverse Osmosis Treats

Reverse osmosis can treat for a wide variety of health and aesthetic contaminants. Effectively designed, RO equipment can treat for a wide variety of aesthetic contaminants that cause unpleasant taste, colour and odour problems like a salty or soda taste caused by chlorides or sulfates.

RO can also be effective for treating health contaminants like arsenic, asbestos, atrazine (herbicides / pesticides), fluoride, lead, mercury, nitrate, and radium. When using appropriate carbon prefiltering (commonly included with most RO systems), additional treatment can also be provided for such ÏvolatileÓ contaminants as benzene, trichloroethylene, trihalomethanes, and radon. Some RO equipment is also capable of treating for biological contaminants like Cryptosporidium. The Water Quality Association (WQA) cautions, however, that while RO membranes typically remove virtually all known microorganisms and most other health contaminants, design considerations may prevent a unit from offering foolproof protection when incorporated into a consumer drinking water system.

When looking for a product to treat for a given health contaminant, care should be used to find products that have been tested successfully for such purposes at a quality testing laboratory.

Conclusion

Reverse osmosis is a relatively new, but very effective, application of an established scientific process. Whether it is used to meet the needs of a typical family of four, or the needs of an industrial operation requiring thousands of gallons per day, it can be a cost effective to provide the required quantity of highly treated water. With continual advances in system and membrane design that boost efficiency and reliability, RO can be expected to play a major role in water treatment for years to come.