memberresources REVERSE OSMOSIS (RO) After storage, water may next undergo reverse osmosis treatment. Reverse osmosis is used to remove excess dissolved solids and a variety of organic contaminants. Reverse osmosis systems generally reject 90-% of dissolved salts (ionic contaminants) and -100% of dissolved organic contaminants (except small molecules such as THMs) in source water. This is particularly useful with source water high in dissolved solids (800 ppm or greater). To produce purified water, reverse osmosis may be followed by deionisation through a mixed-bed deioniser. Reverse osmosis, or R.O., refers to a technology relying on a high pressure pump and special membrane, called semi-permeable membranes, to reverse the natural phenomenon of osmosis. The following illustrates osmosis and osmotic flow. The skin of a raisin is a natural semi-permeable membrane, e.g., placing a raisin in water causes it to swell. Water passes through the raisin skin from a place of relatively pure water to the inside with its high concentrations of protein and other large, soluble chemicals. The large molecules cannot pass through the skin (membrane), while smaller water molecules can. The force or pressure generated by the water as it passes through a semi-permeable membrane is called osmotic pressure. By applying pressure against the osmotic flow of water, pure water can be forced back through the membrane. This flow is the opposite, or reverse, of osmosis. Reverse Osmosis employs synthetic membranes, which are more selective than natural membranes. The synthetic materials used in semi-permeable membranes allow water molecules to pass through but block most ions or organic molecules. The amount of pressure needed to reverse the osmotic flow depends on the concentration (ppm) of contaminants. For example, the osmotic pressure of sea water is about 400 psi; therefore, a pressure of 400 psi must be applied to sea water to force even one drop through a semi-permeable membrane. As a rough guide, osmotic pressure equals about 1 psi per 100 ppm TDS. In an R.O. unit three flows can be identified-feed, concentrate, and permeate. Feed refers to water entering the R.O. unit for treatment. Concentrate, or reject Solution, refers to the concentrated solution of contaminants which is unable to pass through the membrane. Permeate, or recovery, refers to the higher purity water which passes through the membrane. As was noted before, the removal of contaminants is not 100%; a small fraction pass through the membrane and is referred to as leakage or passage. Types of RO Membranes There are four classes of polymers used to produce synthetic semi-permeable membranes for use in RO: polyamide; thin film composite (TFC); cellulose acetate; and cellulose triacetate.
The thin film composite membrane is constructed of polyamide-type materials. Each type of membrane material has its advantages and applications. However, for bottled water production, the most common membranes are either cellulose-acetate, polyamide or TFC. Table 1 provides a comparison of the operational characteristics of these membranes. Table 1 - Reverse Osmosis Membrane Characteristics Membrane Material Cellulose Acetate ph Stability Chlorine Resistance Biological Resistance Temperature Limits 2-8 1.5 ppm Poor 2-35 C Polyamide 4-11 0.1 ppm Good 2-35 C Thin Film Composite Cellulose Triacetate 2-11 0.1 ppm Good 2-45 C 4-8 1.5 ppm Fair-good 2-30 C Cellulose acetate costs less than polyamide-based membranes. But, as noted in Table 1, cellulose-based membranes are susceptible to degradation by bacteria and other micro-organisms. Cellulose acetate has a low chemical resistance and cannot be sued above ph 8. This also limits the ability to clean the membrane, because strong cleaning agents cannot be used. On the other hand, cellulose acetate membranes are more resistant to oxidizers such as chlorine and ozone than are the polyamide-based membranes. The new TFC membranes exhibit good chemical resistance and the highest temperature stability; however, TFC membranes are more prone to fouling and are degraded by oxidizers. Special TFC membranes have been developed to remove certain contaminants, such as nitrates, not readily removed by traditional membranes. RO membranes are usually supplied in a spiral-would element, because other configurations are more susceptible to fouling. Contaminant Removal Efficiencies by RO Table 2 provides typical percentage removal of inorganic constituents by Reverse Osmosis. Table 2 - Typical Percentage Removal of Selected Inorganics by Reverse Osmosis Contaminant Arsenic Barium Bicarbonate Boron Bromide Cadmium Calcium Chloride Chromium Copper Cyanide Fluoride Iron Percent Removal 83 82 Page 2 of 5
Lead Magnesium Manganese Mercury Molybdenum Nickel Nitrate Nitrite Permanganate (<2 ppm) Phosphate Potassium Radium Selenium Silicate Silver Sodium Sulphate Strontium Uranium Zinc 90+ 90 96 The actual performance of an RO system depends largely on the particular membrane selected. The variability of membrane effectiveness is especially pronounced when trying to remove organic contaminants. Table 3 provides the range of percent removal reported. In general, thin film composite membranes are more effective than cellulose acetate or polyamide membranes for organic removal, especially of solvents. Table 3 - Removal of Organics by Reverse Osmosis Contaminant Acrylamide Aldicarb Aldicarb Sulfone Aldicarb Sulfoxide Carbofuran 1, 2-Dichloropropane cis-1, 2-Dichloropropane 2, 4-D Ethylene Dibromide (EDB) Lindane Methaoxychlor Monochlorinated Polychlorinated byphenyls (PCBs) Trichloropropane ylenes Range of Percent (%) Removal 0-94- 94- - 86-4-88 0-20 1-65 37-84 52-73 90+ 50-0-85 86 Page 3 of 5
Trouble Shooting R.O. Units The symptoms of a poorly operating R.O. system include low production volume, poor product quality, and changes in pressure drop across the membrane. Common troubleshooting symptoms are provided in Table 4B- 6, and some of these items are discussed. Table 4 - Troubleshooting Reverse Osmosis Units Failure Low Flow/Low High Flow/Low Low Flow/High Increased Pressure Drop O-ring or seal Fouling (minor) Fouling (Severe) Low H 2 O Temp. High H 2 O Temp. (<30 C) Low System Pressure Irreversible Membrane Compaction Membrane Hydrolysis High Reject Flow Low Reject Flow Membrane Degradation Decreased Pressure Drop Feed Water Because R.O. never removes all of a contaminant, the quality of an R.O. system can be affected by changes in feed water. For example, if an R.O. unit rejects 90% of minerals, and feed water has TDS of 1,000 ppm, then the product water will have a TDS of 100 ppm. A doubling of feed water TDS to 2,000 doubles the product water TDS to 200 ppm. Fouling of Membrane Pre-treatment of feed water is essential to prevent membrane fouling. All R.O. units must be preceded by particulate filtration. For water from municipal sources, a polishing filter is usually sufficient. A 10 μm filter is recommended. For cellulose acetate membranes, an acid feed is often required to prevent formation of carbonate scale. For hard feed water, a water softener may be used, or a polyphosphate feed keeps the hardness minerals from precipitating out on the membrane. It is advisable to periodically flush the system with a detergent solution to remove built-up solids. Temperature of Feed Water RO units are temperature-sensitive. Since membranes will degrade mush more quickly at elevated temperatures, water temperature must be kept below the temperatures specified in Table 4B-3. However, lowering feed water temperature reduces dramatically the production rate of product water. As a rule of thumb, a 2 C reduction in water temperature reduces the flow of product water by 3%. Page 4 of 5
Membrane Hydrolysis Membrane hydrolysis occurs most frequently because of a lack of ph control of feed water. A second cause of membrane hydrolysis occurs most frequently because of a lack of ph control of feed water. A second cause of membrane hydrolysis is high temperature. Limits on ph and temperature are given in Table 4B-3. Degradation of Membranes Chemical oxidizers and microbial growth can lead to degradation of RO membranes. Ozone and chlorine readily degrade many membranes; see Table 4B-3. Bacteria can readily degrade cellulose acetate membranes, especially when a polyphosphate feed is used. Monitoring R.O System To maintain a consistent quality of product water, an RO system must be carefully monitored. Records of hourly measurements are recommended for: 1. ph 2. Feed Water Temperature 3. Feed Water TDS 4. Chlorine 5. TDS 6. Pump pressure 7. Flow 8. Brine Flow 9. Polyphosphate Feed (if used) For Further Information: Australian Beverages Council Ltd info@australianbeverages.org Correct as at 17th October 2012 Page 5 of 5