TOTAL WATER
- Industrial Reverse Osmosis Technology:
Softeners
Dealkalizers
Rinse
Boiler Feed
Reverse Osmosis
Degasifier
Strong Base Demineralizer
Depth Filtration
Carbon Filtration
Demineralizers and Deionizers
Ultraviolet (UV) Systems
Industrial Reverse Osmosis
Reverse Osmosis (RO) technology can be a complicated subject, particularly without an understanding of the specific terminology that describes various aspects of RO system operation and the relationships between these operating variables.
In the reverse osmosis process, the water from a liquid with a high concentration of dissolved solids is forced to flow through the membrane to the low concentration side where this water can be collected. The process is achieved by applying enough pressure to overcome the natural osmotic pressure forces on a membrane. The semi-permeable membranes used in the process are engineered to only allow the passage of the water molecule. The result is high quality water.
How reverse osmosis works
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The heart of the RO system is the semi-permeable membrane which acts as a molecular filter to remove up to 99% of all dissolved solids. The semi-permeable membrane allow water molecules to pass through while blocking other salt molecules. So as pressure is applied to the concentrated solution, water is forced through the membrane from the concentrated side to the dilute side. The dissolved and particular materials are left behind.
Water molecules penetrate the thin layer of the membrane and diffuse through it molecule by molecule. Dissolved salt ions do not diffuse through this layer because the solubility of the salt ions is much less than that of the water. Thus, the water moves through more readily and separation from the other molecules present occurs. The driving force is furnished by both the pressure and the concentration of differentials across the membrane. For water, the pressure effect is the most important, and for dissolved minerals the concentration difference is most important. Therefore, increases in pressure increase the product water flow without a corresponding decrease in the quality of the product water. This process removes up to 99% of most dissolved mineral salts, virtually all of the particulate matter, and many dissolved organic compounds.
The semi-permeable membrane must be made of a highly durable material since it must withstand pressure higher than the pressure differential between the concentrated side and the diluted side of the membrane which can be very high – as in the case of seawater, where it is up to 350 psi (25kg/cm2)
- Some RO systems utilize line pressure only and therefore must be maintained at 50-60 psi to keep a driving force across the membrane to produce high quality, low mineral content water. The recovery rate on these systems is typically 20-30%.
- Larger RO systems utilized a pressure pump to maintain the driving force. These systems normally use pumps that maintain 125-250 psi. The recovery rate for these systems is typically 25%-75%.
The correct choice in pretreatment is very important as it influences the quality and quantity of the product water and above all, the life-span of the membrane. Improperly pretreated systems can experience scaling and/or fouling which will greatly reduce the capacity and life of the membrane.
Typical uses for reverse osmosis
Ultra-fine filtration of water for drinking water applications, food & beverage servce, ice production/drinking water, humidification, boiler pretreatment, vehicle wash and water jet cutting machines.
Untreated water can cause poor tasting food and beverages, improperly carbonated beverages, increased costs, spotted vehicles, increased maintenance costs and increased utility bills.
Typical applications for reverse osmosis
- Food and Beverage: Superior taste and increased cost savings
- Ice Procduction/Drinking Water: Reduces scaling, improves taste and clarity
- Restaurants: Improves steamer operation
- Humidification: Reduces scaling and dusting
- Grocery Stores: Prevents scaling in vegetables misters
- Horticulture: Reduces leaf spotting and mineral build up in potting soils
- Water Jet Cuttng: Improves operating efficiency and extends orifice life.
- Pretreatment: For use in deionization systems
- Boilers: Improves energy efficiency
- Vehicle Wash: Provides spot-free rinses
Listed below are definitions of some of these key terms which also provides a brief overview of the factors that affect the performance of the RO membranes, including pressure, temperature, feedwater salt concentration, permeate recovery, and system pH.
Definitions
Recovery - the percentage of membrane system feedwater that emerges from the system as product water or "permeate". Membrane system design is based on expected feedwater quality and recovery is fixed through initial adjustment of valves on the concentrate stream. Recovery is often fixed at the highest level that maximized permeate flow while preventing precipitation of super-saturated salts within the membrane system.
Rejection - the percentage of solids concentration removed from system feedwater by the membrane.
Passage - the opposite of "rejection," passage is the percentage of dissolved constituents (contaminants) in the feedwater allowed to pass through the membrane.
Permeate - the purified product water produced by a membrane system.
Flow - feed flow is the rate of feedwater introduced to the membrane element, usually measured in gallons per minute (gpm). Concentrate flow is the rate of flow of non-permeated feedwater that exits the membrane element. This concentrate contains most of the dissolved constituents originally carried into the element from the feed source. It is usually measured in gallons per minute (gpm).
Flux - the rate of permeate transported per unit of membrane area, usually measured in gallons per square foot per day (gfd).
Dilute Solution - purified water solution, RO system product water.
Concentrated Solution - brackish water solution such as RO system feedwater.
Effect of Pressure
Feedwater pressure affects both the water flux and salt rejection of RO membranes. Osmosis is the flow of water across a membrane from the dilute side toward the concentrated solution side. RO technology involves application of pressure to the feedwater stream to overcome the natural osmotic pressure. Pressure in excess of the osmotic pressure is applied to the concentrated solution and the flow of water is reversed. A portion of the feedwater (concentrated solution) is forced through the membrane to emerge as purified product water of the dilute solution side.
Water flux across the membrane increases in direct relationship to increases in feedwater pressure. Increased feedwater pressure also results in increased salt rejection but the rejection is less direct than for water flux.
Because RO membranes are imperfect barriers to dissolved salts in feedwater, there is always some salt passage through the membrane. As feedwater pressure is increased, this salt passage is increasingly overcome as water is pushed through the membrane at a faster rate than salt can be transported.
However, there is an upper limit to the amount of salt that can be excluded via increasing feedwater pressure. As the plateau in the salt rejection curve exceeds a certain pressure level, salt rejection no longer increases and some salt flow remains coupled with flowing through the membrane.
Effect of Temperature
Membrane productivity is very sensitive to changes in feedwater temperature. As water temperature increases, water flux increases almost linearly, due primarily to the higher diffusion rate of water through the membrane.
Increased feedwater temperature also results in lower salt rejection or higher salt passage. This is due to a higher diffusion rate for salt through the membrane.
The ability of a membrane to tolerate elevated temperatures increases operating latitude and is also important during cleaning operations because it permits use of stronger, faster cleaning process.
Effect of Salt Concentration
Osmotic pressure is a function of the type and concentration of salts or organics contained in feedwater. As salt concentration increases, so does osmotic pressure. The amount of feedwater driving pressure necessary to reverse the natural direction of osmotic flow is, therefore, largely determined by the level of salts in the feedwater.
If feed pressure remains constant, higher salt concentration results in lower membrane water flux. The increasing osmotic pressure offsets the feedwater driving pressure. There is an increase in salt passage through the membrane (decrease in rejection) as the water flux declines.
Effect of Recovery
Reverse osmosis occurs when the natural osmotic flow between a dilute solution and a concentrated solution is reversed through application of feedwater pressure. If percentage recovery is increased (and feedwater pressure remains constant), the salts in the residual feed become more concentrated and the natural osmotic pressure will increase until it is as high as the applied feed pressure. This can negate the driving effect of feed pressure, slowing or halting the reverse osmosis process and causing permeate flux and salt rejection to decrease and even stop.
The maximum percent recovery possible in any RO system usually depends not on a limiting osmotic pressure, but on the concentration of salts present in the feedwater and their tendency to precipitate on the membrane surface as mineral scale. The most common sparingly soluble salts are calcium carbonate (limestone), calcium sulfate (gypsum), and silica. Chemical treatment of feedwater can be used to inhibit mineral scaling.
Effect of pH
The pH tolerance of various types of RO membranes can vary widely. Thin-film composite membranes are typically stable over a broader pH range than cellulose acetate (CA) membranes and, therefore, offer greater operation latitude. Membrane salt rejection performance depends on pH. Water flux may also be affected.