Reverse Osmosis Plants ..
 
What is ....Reverse Osmosis

Anyone who has been through a high school science class will likely be familier with the term osmosis. The process first described by a French Scientist in 1748, who noted that water spontaneously diffused through a big bladder membrane into alcohal. Over 200 years later, a modification of the process known as reverse osmosis allows peop throughout the world to affordably convert undesirable water that is virtually free of health or aesthatic contaminants. Reverse osmosis systems can be found providing treated water from the kitchen counter in a private residence to installations used in manned spacecraft.

Reverse Osmosis is a technology that is found virtually anywhere pure water is needed; common uses include:
 
  • Drinking Water
  • Humidification
  • Ice-Making
  • Car Wash Water Reciamation
  • Rinse Waters
  • Biomedical Applications
  • Laboratory Applications
  • Photography
  • Pharmaceutical Production
  • Kidney Dialysis
  • Water used in chemical processes
  • Cosmotics
  • Animal Feed Hatcheries
  • Restaurants
  • Greenhouses
  • Metal Plating Applications
  • Wastewater Treatment
  • Boiler Water
  • Battery Water
  • Semiconductor production
  • Hemodialysis
How Reverse Osmosis Works.

A semipermeable membrane, like the membrane of a cell wall or a bladder, is selective about what it allows to pass through, and what it prevents from passing. These membranes in general pass water very easily because of its small molecular size; but also prevent many other contaminants from passing by trapping them. Water will typically be present on both sides of the membrane, with each side having a different concentration of dissolved minerals. Since the less concentrated solution seeks to dilute the more concentrated solution, water will pass through the membrane the lower concentration side to the greater concentration side. Eventually, osmotic pressure (seen in the diagram below as the pressure created by the difference in water levels) will counter the diffusion process exactly, and an equilibrium will form.
 
The process of reverse osmosis forces water with a greater concentration of contaminants (the source water) into a containing water with an extremely low concentration of contaminants (the processed water). High water pressure of source side is used to "reverse" the natural osmoic process, with the semi-permeable membrane still permitting the passage of water while rejecing most of the other contaminants. The specific process through which this occurs is an ion exclusion, in which a concentration ions at the membrane surface form a barrier that allows other water molecules to pass through while excluding other substances.

Semipermeable membranes have come a logn way from the natural pig bladders used in the earlier osmosis experiments. Before the 1960's, these membranes were too inefficient, expensive, and unreliable for practical applications outside the laboratory. Modern advances insystematic materials have generally solved these problems., allowing membranes to become highly efficient at rejecting contaminants, and making them tough enough to withstand the greater pressures necessay for efficient operation.

Even with these advances, the "reject" water on the source side of Reverse Osmosis (RO) system must be periodically flushed in order to keep it from becoming so concentrated that it forms a scale on the membrane itself. RO systems typically require a carbon prefilter for the reduction of chlorine , which can damage an RO membrane; and a sedime prefilter is always required to ensure that fine suspended materials in the source water do not permanently clog the membrane, Hardness reduction, either through the use of water softening for residential units or chemical softening industrial use, may also be desirable in hard water areas.
 

Low Pressure (Residential) Systems

Low pressure RO systems generally refer to those systems with a water feed pressure of less than 100 psig. These the typical countertop or undersink residential systems that rely primarily on the natural water pressure to make the reverse osmosis process function; a typical system is shown schematically below.


Contertop units typically have an unpressurized storage tank; Undersink units typically have a pressurized accumulation storage tank where the water pressure tends to increase as the tank fills. This pressurized system provides sufficient pressure to move the water from the undersink storage tank to faucet. Unfortunately, this also creates a back pressure against the membrane, which can decrease its efficiency. Some units overcome this by using unpressurized 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 Raw water per gallon of treated water. Water purity can be as high as 95 percent. These systems can be highly affordable with countertop units starting at about US $350, and undersink units starting  at about US $600. 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 osomosis cartridge on 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), Certified Installers (CI) for advice on your water needs, and equipment installation.

High pressure systems typically operate at pressures between 100 and 1000 psig, depending on the membranes conditions and the water being treated. These systems are usually used in industrial or commercial apllications 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 quant water. The processed water from the first stage of treatment can then be passed through additional membrane module membrane modules for greater efficiency (see diagram below), though flushing will still be required when concentration reach a level where fouling is likely to occur
 
Commercial Multiple Membrane Module RO System
High pressure indutrial 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 percent. These systems to be larger and more complicated than low pressure systems, and this is reflected in their costs, which range from $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 aesthatic contaminants. Effectively designed, RO equipment can treat for a wide variety of aesthatic contaminants that cause unpleasant taste, color, and odor problems like a sour soda taste caused by chlorides or sulfates.

RO can also be effective for treating health contaminants like arsenic, asbestos, astrazine (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 "volatile" 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 syatem.

When looking for a product to treat for a given health contaminants, 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 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 systems and membrane design that boost efficiency and reliability, RO can be expected to play a major role in water treatment years to come.
 
 
 
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