 |
 |
WHAT IS REVERSE OSMOSIS
The technology of Reverse Osmosis can be found anywhere pure water is needed; You can find Reverse Osmosis in the following:
|
|
|
|
|
- Drinking Water
- Humidification
- Ice-Making
- Car Wash Water Reclamation
- Rinse Waters
- Biomedical Applications
- Laboratory Applications
- Photography
- Pharmaceutical Production
- Kidney Dialysis
- Water used in chemical processes
|
- Cosmetics
- Animal Feed
- Hatcheries
- Restaurants
- Greenhouses
- Metal Plating Applications
- Wastewater Treatment
- Boiler Water
- Battery Water
- Semiconductor production
- Hemodialysis
|
How Does Reverse Osmosis Work?
A semipermeable membrane is a membrane that will pass
some atoms or molecules but not others. Saran wrap is
a membrane, but it is impermeable to almost everything
we commonly throw at it. The best common example of a
semipermeable membrane would be the lining of your
intestines, or a cell wall. Gore-tex is another common
semipermeable membrane. Gore-tex fabric contains an
extremely thin plastic film into which billions of
small pores have been cut. The pores are big enough to
let water vapor through, but small enough to prevent
liquid water from passing (see this page for more
information on Gore-tex fabric).
In the figure above, the membrane allows passage of
water molecules but not salt molecules. One way to
understand osmotic pressure would be to think of the
water molecules on both sides of the membrane. They
are in constant Brownian motion. On the salty side,
some of the pores get plugged with salt atoms, but on
the pure-water side that does not happen. Therefore,
more water passes from the pure-water side to the
salty side, as there are more pores on the pure-water
side for the water molecules to pass through. The
water on the salty side rises until one of two things
occurs:
The salt concentration becomes the same on both sides
of the membrane (which isn't going to happen in this
case since there is pure water on one side and salty
water on the other).
The water pressure rises as the height of the column
of salty water rises, until it is equal to the osmotic
pressure. At that point, osmosis will stop.
Osmosis, by the way, is why drinking salty water (like
ocean water) will kill you. When you put salty water
in your stomach, osmotic pressure begins drawing water
out of your body to try to dilute the salt in your
stomach. Eventually, you dehydrate and die.
In reverse osmosis, the idea is to use the membrane to
act like an extremely fine filter to create drinkable
water from salty (or otherwise contaminated) water.
The salty water (Reject Water)is put on one side of
the membrane and pressure is applied to stop, and then
reverse, the osmotic process.
Even with these advances, the "reject" water on the
source side of a Reverse Osmosis (RO) system must be
discarded in order to keep it from becoming so
concentrated that it forms a scale on the membrane
itself. RO systems will require a carbon prefilter to
reduce the damage that chlorine that can cause to the
RO membrane. A sediment prefilter is prerequisite to
ensure that fine suspended materials in the source
water do not permanently clog the membrane. When the
source water is hard, the use of a water softener is
used as well.
|
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 Reclamation
- Rinse Waters
- Biomedical Applications
- Laboratory Applications
- Photography
- Pharmaceutical Production
- Kidney Dialysis
- Water used in chemical processes
|
- Cosmetics
- Animal Feed
- Hatcheries
- Restaurants
- Greenhouses
- Metal Plating Applications
- Wastewater Treatment
- Boiler Water
- Battery Water
- Semiconductor production
- Hemodialysis
|
How Does Reverse Osmosis Work?
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 water in the less concentrated solution seeks to dilute the more concentrated solution, water will pass through the membrane from 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 tank containing water with an extremely low concentration of contaminants (the processed water). High water pressure on the source side is used to "reverse" the natural osmotic process, with the semi-permeable membrane still permitting the passage of water while rejecting most of the other contaminants. The specific process through which this occurs is called ion exclusion, in which a concentration of 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 long 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 in synthetic 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 necessary for efficient operation.
Even with these advances, the "reject" water on the source side of a Reverse Osmosis (RO) system must be discarded in order to keep it from becoming so concentrated that it forms a scale on the membrane itself. RO systems also typically require a carbon prefilter for the reduction of chlorine, which can damage an RO membrane. A sediment 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 for industrial use, may also be desirable in hard water areas.
Inside A RO Membrane
Water enters the RO membrane under pressure and travels from the feed end to the reject end. Some of this feed water passes through the membrane and becomes purified. This water is called the RO product water. The contaminants that were previously in the product water exit the membrane in the reject water.
Several layers of membrane material are sandwiched between spacer material to form leaves with a feed / reject channel and a product channel. These leaves are then rolled around a central product collection tube. This assembly is referred to as a spiral wound membrane element.
The spiral wound membrane element is installed in a pressure vessel. A seal between the outside of the membrane and the inside of the pressure vessel prevents the feed water from flowing between the membrane and pressure vessel. This is called a brine seal. Membranes should always be installed with the brine seal on the feed end of the vessel.
The reject water exits the vessel and feed the next vessel or is sent to drain. The product water exits the vessel and is sent to a storage tank or point of use. An o-ring seal prevents the reject water from mixing with the product water.
Low Pressure (Residential) Systems
Low pressure RO systems generally refer to those systems with a feed pressure of less than 100 psig. These are the typical countertop or under sink residential systems that rely primarily on the available line pressure to make the reverse osmosis process function; a typical system is shown schematically below.
Countertop units typically have an unpressurized storage tank; Undersink units typically have a pressurized accumulator 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 the 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 5 and 50 gallons per day of water, with an efficiency of 2-4 gallons of reject water per gallon of treated water. These systems typically remove greater than 90% of the dissolved solids found in the feed water. 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 post filters (typically one to four times per year); and the reverse osmosis cartridge once every two to three years, depending on the feed water and usage.
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.
Commercial / industrial systems use a pump to provide the pressure necessary to drive the reverse osmosis process. These systems typically use multiple membranes modules arranged in parallel to provide the required quantity of water. The reject water from one module can be directed into successive membrane modules for greater efficiency (see diagram below). An adequate reject flow must be maintained to prevent membrane fouling and scaling from occurring.
High pressure commercial / industrial units typically provide from 2 gallons to 400 gallons per minute of water with an efficiency of 0.3 - 6 gallons of reject water per gallon of product water produced. These systems typically remove greater than 95% of the dissolved solids found in the feed water. These systems tend to be much larger and more complicated than low pressure systems.
Factors That Effect the Operation of RO Systems
1. Total Dissolved Solids (TDS) of the feed water
The natural osmotic pressure of the feed water is approximately 1 psi for every 100 ppm. This means that if the feed water TDS is 500 ppm it will take more than 5 psi of pressure just to begin the reverse osmosis process.
2. Feed water temperature
Reverse osmosis is temperature dependent. The RO process occurs slower with cold water. All RO membranes are rated with a feed water temperature of 77° F. Higher operating pressures can be used to compensate for colder water temperatures. For feed water colder than 77° F, it will take approximately 1.5% more pressure or the membrane will produce approximately 1.5% less water for each degree F.
3. Feed water pressure
Reverse osmosis is pressure dependent. Raising the feed pressure produces more product water while lowering the pressure produces less product water.
4. Pretreatment
The RO feed water must be pretreated in order to prevent membrane damage and/or fouling. Proper pretreatment is essential for reliable operation of any RO system.
Pretreatment requirements vary depending on the nature of the feed water. Pretreatment equipment is sold seperatly. The most common forms of pretreatment are described below.
Media Filter - Used to remove large suspended solids (sediment) from the feed water. Backwashing the media removes the trapped particles. Backwash can be initiated by time or differential pressure.
Water Softener - Used to remove calcium and magnesium from the feed water in order to prevent hardness scaling. The potential for hardness scaling is predicted by the Langelier Saturation Index (LSI). The LSI should be zero or negative throughout the unit unless approved anti-scalents are used. Softening is the preferred method of controlling hardness scale.
Carbon Filter - Used to remove chlorine and organics from the feed water. Free chlorine will cause rapid irreversible damage to most membranes.
Chemical Injection - Typically used to feed antiscalant, coagulant, or bisulfite into the feed water or to adjust the feed water pH.
Prefilter Cartridge - Used to remove smaller suspended solids and trap any particles that may be generated by the other pretreatment. The cartridge(s) should be replaced when the pressure drop across the housing increases 5 - 10 psig over the clean cartridge pressure drop.
The effect of suspended solids is measured by the silt density index (SDI) test. An SDI of five (5) or less is specified by most membrane manufacturers and three (3) or less is recommended.
Iron & Manganese - These foulants should be completely removed (less than 0.1 ppm). Special media filters and/or chemical treatment is commonly used.
pH - The pH is often lowered to reduce the LSI.
Silica: Reported on the analysis as SiO2. Silica forms a coating on membrane surfaces when the concentration exceeds its solubility. Additionally, the solubility is highly pH and temperature dependent. Silica fouling can be prevented with chemical injection and/or reducing the recovery.
|
|
|
|
|
|
