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 dissolved individual ions from a solution. Reverse osmosis is used to purify water and remove ions and dissolved organic molecules. It can be used to purify 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 currently in 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 osmosisreverse 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, in a concentrated brine reject water. The process of 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.
What is a R.O. system?A typical system consists of:
- One or more Pre-filters to remove silt, iron, chlorine or other particulate matter.
- A membrane to remove raw water contaminates.
- A pressure sensitive hydraulic on/off pressure valve to send pre-filtered water to the membrane to make treated water for storage.
- A small storage tank for treated water.
- May include a post-filter to polish stored water and to maximize its taste just prior to consumption.
What factors affect an R.O. system?Permeate flux and salt rejection are the key performance parameters of the reverse osmosis process. They are mainly influenced by variable parameters such as; pressure, temperature, recovery, and feed water salt concentration.
PressureIncreased feed water pressure will increase permeate flux and decrease the permeate TDS. With excessive pressure the membrane may become deformed or compacted and a decrease in product flow will result.
TemperatureIncreased temperature will increase permeate flux, which increases salt passage. It is also important to note that every unit is rated for a product flow temperature of 77°F (25°C). With a temperature decrease, the product flow will decrease. On average the membranes lose about 2% production for every degree below 77°F.
RecoveryThe recovery is the ratio of the permeate flow to the feed flow. When recovery is increased, the permeate flux will decrease and the salt passage will increase.
Feed Water ConcentrationIncreased TDS or salt concentrations will decrease permeate flux and increase salt passage. This can also lead to surface coating or fouling by the salt.
Does an R.O. system disinfect water?While the membrane used in an RO system will physically screen out and remove many microorganisms in water, including pathogenic bacteria, the system must not be relied upon to make a biologically questionable water supply safe and potable for consumption. Membrane imperfections or faulty water seals in the system could allow untreated water to pass through and over time bacteria could grow on or through the membrane. Be sure the raw water source for your RO system is safe and potable for consumption by having a sample tested by a certified water testing facility.
How does an R.O. system know when to produce water?A standard RO System has a hydraulic on/off pressure valve which is controlled by source water pressure to send pre-filtered water to the membrane to make water for storage. When water pressure in the storage tank builds up to 90% of the incoming line pressure, the standard hydraulic on/off pressure valve shuts off the flow of water to the system, stopping treated water production. When you open the RO faucet, system pressure drops as treated water exits the tank. The standard hydraulic on/off pressure valve turns on when the tank pressure is less than the source water pressure and treated water is produced through the RO system. The cycle repeats itself.
Glossary of terms
- Material to be removed from solution.
- Material on which absorption will occur.
- Air Gap:
- A clear vertical space between the end of the RO drainline and the flood level rim of the house sanitary drain system connection point.
- The portion of water that does not pass through the membrane and goes to the drain.
- Feed Water:
- An incoming water source to the RO system.
- The rate of transfer of fluid, particles, or energy across a given surface.
- A thin layer that forms a barrier, selective to the passage of chemicals.
- Portion of the raw source water, which passes through the membrane. RO system treated water for consumption.
- Reverse Osmosis:
- The movement of raw source water through a semi-permeable membrane when pressure is applied to yield treated water.
- Surface Area:
- The surface of a material available for absorption to take place.
- Thin FilmComposite (TFC):
- Thin membrane bonded to an anisotropic support layer. Synthetic fabric supports this composite. Thin film offers highest degree of selectivity, flux, and chemical stability.
- Total Dissolved Solids (TDS):
- Total amount of dissolved solids/salts in raw/treated. (TDS) water.
- Ultraviolet (UV) Light:
- Radiation (Light) having a wavelength between 100-3900 angstroms.