What is a membrane
A membrane is a layer of material which allows some atoms or molecules to pass through but not others. In water treatment applications such as Reverse Osmosis, water molecules are allowed to pass through the membrane, while salts and other contaminants cannot.
What is osmosis and reverse osmosis?
Osmosis and reverse osmosis are similar to one another and the two processes are described and illustrated in the diagram below.
The diagram on the left shows the process of osmosis. A semi-permeable membrane (such as an RO membrane) is placed between two chambers. One chamber has a very dilute solution (such as pure water) and the other chamber has a very concentrated solution (such as salt water). The process of osmosis occurs when pure water naturally flows from the pure water solution in one chamber through the membrane into the salt water solution. This will cause the water level to rise in the chamber with the salt water solution. The water flow will stop when the pressure caused by the increased column of water equals the driving force caused by the difference in the salinity of the dilute and concentrated waters.
Reverse osmosis makes this natural process go in reverse. Instead of the water flowing from the dilute side to the concentrated side, with reverse osmosis the water flows in the opposite direction. This requires a force to be applied to the concentrated water (typically with a pump). If pressure is applied to the salt water side, the reverse flow produces pure water from the salt solution. And the salt solution will become ever saltier.
What type of water is recovered from Reverse Osmosis (RO)?
RO employs a semi-permeable membrane that allows water molecules to pass through, while rejecting larger molecules. Water which enters the RO system is separated into two streams: pure product water (permeate) and salty water containing unwanted compounds (concentrate). The permeate water is virtually free of all organics and typically contains less than 5% of the dissolved minerals originally present. RO systems typically operate between about 100 psi to over 1000 psi. Higher pressures are required as the water becomes saltier.
What type of water is recovered from Nanofiltration (NF)?
NF operates essentially the same as RO except that the membrane allows more of the dissolved solids to pass through. NF membranes also operate at lower pressure than RO membranes. Applications where final water quality is not as critical, feed water pressure must be minimized, or where there is a desire to pass more minerals (as in some drinking water applications), NF may be the correct technology. NF systems typically operate between about 50 psi to 100 psi.
What type of water is recovered from Ultrafiltration (UF)?
UF is also a pressure driven membrane process, though when compared to other membrane processes (such as reverse osmosis), UF systems require a much lower amount of pressure for operation. Typically, an ultrafiltration system will operate between 7 and 45 psi (0.5 and 3 bar). Ultrafiltration is typically used for the removal of suspended solids, organics and bacteria from the feed water. Due to the much larger pore sizes associated with the UF membranes, dissolved solids (such as salts) are not removed. UF is often used for drinking water applications, especially where the water source is a surface water.
What type of pretreatment is typically used for Reverse Osmosis (RO)?
Various types of filtration of chemical additions are often used in RO systems. Some of the most common pretreatment steps are described below.
Primary Function: Removal of larger suspended materials from the feedwater.
The multimedia filters are designed for the removal of larger suspended particles from the process stream, and therefore protects the RO membranes from fouling. As the filters accumulate material removed from the water, the pressure drop across the units increases. At a certain point, the units will require backwashing. At this point, the filters are then taken off-line and backwashed prior to being returned to service.
Process description: Most raw water supplies contain a variety of contaminants. While some of them are dissolved, e.g., salts and organics, others float or are suspended in water. A multimedia filter will remove particles greater than about 20 micron in size. (A 20 micron particle is 0.0008 inch.) The multimedia filter traps particulate matter within the media as the water passes through. (Over time, sufficient particulate matter will accumulate to reduce the flow of water through the filter. The pressure across the filter will increase and a backwash of the filter is required to remove the trapped particulate particles.) Typically, the differential pressure across the multimedia filter should not exceed 10 psi. Multimedia filters are backwashed either based on the differential pressure or based on time. The frequency of the backwash depends on the quality of the water, though typically occurs every 1 to 14 days.
Primary Function: There are two primary reasons why RO Prefilter is employed.
- Removal of small suspended particles from the feed water.
- Protection of RO membranes from fouling.
Process description: Most raw water supplies contain a variety of contaminants. While some of them are dissolved, e.g., salts and organics, others float in water. Both a bag and/or cartridge filter, rated for 5 microns, will remove any particulate matter five (5) microns or larger in size. (A 5 micron particle is 0.0002 inch.) The filter traps particulate matter on the filter surface as the water passes through. (Over time, sufficient particulate matter will accumulate to reduce the flow of water through the filter. At that point, the filter must be replaced.) If the differential pressure across this filter exceeds 7 – 10 psi, the filter should be changed.
Antiscalant Addition System
Primary Function: Metered injection of chemical antiscalant into the process stream, thereby preventing the precipitation of sparingly soluble salts like CaSO4 and silica in the process and on the membrane surfaces. This unit operation allows sustained and enhanced performance of membranes.
Process description: During the Reverse Osmosis process, feed water is separated into two streams: pure product water (permeate) and salt water (concentrate). Some of the concentrated salts are less soluble in the water and tend to precipitate out on the membrane surfaces. The precipitation and scaling of sparingly soluble salts can and will deteriorate the performance of the membranes. To keep the sparingly soluble salts from precipitating, the feed water is constantly injected with antiscalant chemical. The injection of antiscalant chemical is accomplished by a small dosing pump. Typically, enough antiscalant chemical is added to the feed water so that the concentration of the antiscalant chemical is between 2 and 5 mg/liter of the feed water. The dosage of antiscalant in the feed water depends upon the chemistry of the feed water.
Sodium Bisulfite Addition System
Primary Function: Metered injection of sodium bisulfite into the process stream, thereby protecting the RO membranes from oxidizing agents.
Process description: Sodium bisulfite is a strong reducing agent and is, therefore, able to remove oxidizing biocides (chlorine and chloramine) from the water. Such oxidizing agents must be removed prior to the RO unit in order to protect the RO membranes.
Acid Addition System
Primary Function: Metered injection of acid into the process stream to lower the pH, thereby protecting the RO membranes from carbonate scaling.
Process description: Acidification of the RO feedwater helps prevent the precipitation of sparingly soluble salts in the RO concentrate by converting carbonate and bicarbonate to carbon dioxide. The carbonate and bicarbonate are then not available to combine with free cations to form precipitated solids. A pH controller, located in the RO control panel, is typically used and modulates the pump output rate. This controller adjusts the pump output rate in order to maintain the proper pH level in the RO inlet stream.
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