United States Environmental Protection Agency Office of Research and Development Washington DC 20460 EPA/625/R-96/009 September 1996 Technology Transfer v>EPA Capsule Report Reverse Osmosis Process ------- Technology Transfer EPA/625/R-96/009 Capsule Report Reverse Osmosis Process September 1996 Center for Environmental Research Information National Risk Management Research Laboratory Office of Research and Development U.S. Environmental Protection Agency Cincinnati OH 45268 Printed on Recycled Paper ------- Contents Process Description 1 Applications 2 Equipment 2 Operation and Maintenance 4 Failure Analysis 6 References 9 Introduction A failure analysis has been com- pleted for the reverse osmosis (RO) process. The focus was on process failures that result in releases of liq- uids and vapors to the environment. The report includes the following: A description of RO and cov- erage of the principles behind the process. Applications of RO for treat- ment of effluent waters from the metal finishing industry. Descriptions of equipment and operating and maintenance procedures. Failure analysis that includes types of failures and causes. Key questions that can be used for software development. A bibliography on RO applica- tions in the metal finishing in- dustry. ------- Reverse Osmosis Process Process Description In the reverse osmosis (RO) pro- cess, water passes through a mem- brane, leaving behind a solution with a smaller volume and a higher con- centration of solutes. The solutes can be contaminants or useful chemicals or reagents, such as copper, nickel, and chromium compounds, which can be recycled for further use in metals plating or other metal finishing pro- cesses. The recovered water (perme- ate) can be recycled or treated downstream, depending on the qual- ity of the water and the needs of the plant. As shown in Figure 1, the wa- ter that passes through the membrane is defined as permeate and the con- centrated solution left behind is de- fined as retentate (or concentrate). The RO process does not require thermal energy, only an electrically driven feed pump. RO processes have simple flow sheets and a high energy efficiency. However, RO membranes can be fouled or damaged. This can result in holes in the membrane and passage of the concentrated solution to clean water, and thus a release to the environment. In addition, some membrane materials are susceptible to attack by oxidizing agents, such as free chlorine. The flux of component A through an RO membrane is given by Equa- tion (1): 0) where NA = Flux of component A through the membrane, mass/time- length2. PA = Permeability of A, mass-length/ time-force. DF= Driving force of A across the membrane, either pressure dif- ference or concentration differ- ence, force/length2 or mass/ length3. L = Membrane thickness, length. At equilibrium, the pressure differ- ence between the two sides of the RO membrane equals the osmotic pressure difference. At low solute con- centrations, the osmotic pressure ( p) of a solution is given by Equation (2): n = CSRT (2) where p = Osmotic pressure, force/ length2. Cs = Concentration of solutes in so- lution, moles/length3. Pressurized wastewater (dragout) o o ° Q *«nl/^ J) O ^\ VI 1 A ffj 1-1 0 . .* ° O .0 * t * O o o o _> u n 0 ,'° *.'°.* * * * * r ^_ M Concentrate Membrane DD-621 Water (permeate) Figure 1. Reverse osmosis process. ------- R = Ideal Gas Constant, (force- length )/(mass-temperature). T = Absolute temperature, °K or °R. As a mixture is concentrated by passing water through the membrane, osmotic pressure of the solution in- creases, thereby reducing the driving force for further water passage. An accurate characterization of the pres- sure to drive the RO process must be based on an osmotic pressure com- puted from the average of the feed and retentate stream compositions. The water recovery of an RO process may be expressed by Equation (3): REC = (Qp/QF)x 100 (3) where REC = Water recovery, %. Qp = Permeate flow rate, length3/ time. QF = Feed flow rate, Iength3/time. Water recovery is determined by temperature, operating pressure, and membrane surface area. Rejection of contaminants determines permeate purity, while water recovery primarily determines the volume reduction of the feed or the amount of permeate produced. Generally, for concentra- tion of waters from the metal finishing industry, greater water recoveries are desirable to obtain overall greater vol- ume reduction. Applications Nickel plating rinsewaters can be treated with RO with over 90% of the rinsewater recovered, with suitable quality for reuse. Plant payback for a 5 cubic meter per hour recovery RO plant has been estimated at 1.3 years in the case of 2,000 mg/1 nickel in the feed (Shoeman et al., 1992; Cross and Evans, 1991). There are at least 150 RO systems operating on vari- ous types of nickel baths; most use cellulose acetate membranes (Cartwright, 1984). At least 12 RO systems are operat- ing on various copper sulfate rinses. These systems use both hollow-fiber polyamide and cellulose triacetate membranes and spiral-wound, thin- film composite types, and offer a membrane life of 1 to 3 years. One effective RO system, which is being used on a zinc sulfate rinse, employs spiral-wound thin-film com- posite membranes at a feed rate of 45 gal/hr and at a water recovery of 88%. The membrane concentrate is further reduced in volume in an evapo- rator and returned to the process (Kinman, 1985; Cartwright, 1984). Approximately five RO systems are operating on various types of brass cyanide rinses. Both polyamide and cellulose triacetate hollow-fiber mem- brane elements are used. An RO system is being used after contact plating on printed circuit boards. The rinse is fed to a polya- mide hollow-fiber membrane element at the rate of 210 gal/hr. The system is operating at a water recovery of about 90%; part of the concentrate is recycled to the plating bath and the remainder is routed to the waste treat- ment system. All of the membrane permeate is reused as a rinsewater (Cartwright, 1984). Cadmium and chromium rinse- waters are also treated with RO. Mem- brane fouling has been experienced for the cadmium rinsewaters, but little fouling has been experienced for chro- mium rinsewater applications. Prelimi- nary results show that payback for 5 cubic meters per hour RO cadmium/ water and RO chromium/water recov- ery plants are three and seven years, respectively (Shoeman et al., 1992). In other industries, RO is used for production of potable water from sea- water and brines, for water recovery from landfill leachates, and for con- centration of industrial wastewaters and brines. RO is sometimes used as a pre-concentrator for evaporators to lower energy requirements and in- crease process efficiency. RO has also found many applications in the food and dairy industries; it is used in the food industry to concentrate apple juice and in the dairy industry to con- centrate cheese whey. Equipment The module is the housing that con- tains the membrane. With regard to failure analyses, module configuration is important because some types of modules are more reliable than oth- ers. Membrane modules are commer- cially available in four configurations: Plate-and-Frame Spiral-Wound Hollow-Fiber Tubular Plate-And-Frame Modules As shown in Figure 2, plate-and- frame modules use flat sheet mem- branes that are layered between spacers and supports. The supports also form a flow channel for the per- meate water. The feed water flows across the flat sheets and from one layer to the next. Recent innovations have increased the packing densities for new design of plate-and-frame modules. Maintenance on plate-and- frame modules is possible due to the nature of their assembly. They offer high recoveries with their long feed channels and are used to treat feed streams that often cause fouling prob- lems. Only recently advanced designs of plate-and-frame modules capable of operating up to 25% dissolved sol- ids and operating pressures up to 4500 psia have been placed in op- eration in Germany (Stanford and Miller, 1994). This development opens new opportunities for the use of re- verse osmosis for concentration of metal finishing wastewaters. Spiral-Wound Modules Spiral-wound modules use a sand- wich of flat sheet membranes and supports, wrapped spirally around a collection tube (see Figure 3). The feed flows in against one end of the rolled spiral and along one side of the membrane sandwich. The support lay- ers are designed to minimize pres- sure drop and allow a high packing density. Additionally, the spiral-wound modules can be designed by equip- ment suppliers to promote turbulence and therefore increase the mass trans- fer across the membrane or to pro- vide an uninterrupted flow path to decrease membrane fouling. Spiral- wound modules offer greater packing densities, but maintenance is difficult. Hollow-Fiber Modules As shown in Figure 4, hollow-fiber modules consist of small diameter membrane fibers bundled within cy- lindrical pressure vessels. The fibers are pressurized from the outside. The permeate flows to the interior bore or lumen of the fiber and down the length of the fiber to the product header. Fibers can also be pressurized from the inside, but greater mechanical strength of the fibers is necessary to prevent fiber rupture. By feeding on ------- Concentrated solution Permeate (clean water) Wastewater (dragout) DD-837 Figure 2. Plate-and-frame reverse osmosis module. Feed Product water Spacer Membrane Spacer Porous feed spacer Membrane Porous permeate spacer Membrane Permeate /I-9 Figure 3. Spiral-wound module. ------- Retentate outlet Fiber bundle plug Hollow fiber Carbon steel shell Liquid feed 1/1-10 Figure 4. Hollow-fiber module. the shell side of the fibers, a lower pressure drop is encountered down the bore of the fiber since the perme- ate flow rate is less than the feed flow rate. Hollow-fiber modules offer the greatest packing densities of the configurations described. Tubular Modules Tubular modules have membranes supported within the inner part of tubes. The operator can easily ser- vice feed and permeate channels to remove fouling layers. Tubular mod- ules are somewhat resistant to foul- ing when operated with a turbulent feed flow. This is accomplished with Permeate larger flow channels than those used with hollow-fiber and spiral-wound modules. The drawbacks of tubular modules are their high energy require- ments for pumping large volumes of water, high capital costs, and low membrane surface area per unit vol- ume of module (see Figure 5 ). Operation And Maintenance To maintain membrane perfor- mance and extend membrane life, pretreatment chemicals may be nec- essary, depending on the character- istics of the wastewater. In addition, chemicals may be required to achieve clean water specifications. Filtering wastewater may be necessary to re- move suspended solids before waste- water is fed to the RO modules. Membrane performance can be en- hanced by control of pH, removal of certain dissolved species and colloi- dal materials such as clays and oils, and dissolved or suspended organ- ics. In any RO system, depending on the capacity and size of modules, a number of parallel modules may be needed. Membrane fouling can result from the formation of a fouling layer on the membrane surface, or from internal changes of the membrane material. Both forms of fouling can cause mem- brane permeability to decline. Scaling is a form of fouling that occurs when dissolved species are concentrated in excess of their solubility limit. Chemical agents can be added to slow the formation of precipitates. Acidification is used to prevent the formation of carbonates of low solu- bility, such as magnesium carbonate. An ion exchanger is sometimes used to trade cations of low solubility salts for cations that are more soluble, for example, sodium sulfate may be traded for calcium sulfate. Prevention of biological growth is necessary to prevent damage to the membrane. Biological growth can be inhibited with chlorination, but some RO membranes are chlorine sensi- tive, so water must be dechlorinated before entering the RO module. Other disinfectants are ozone, formaldehyde, ultraviolet light, copper sulfate, and sodium bisulfate. A schematic of an RO system with four modules in par- allel, chemical pretreatment, and an up-front filtration step is shown in Fig- ure 6. Staging RO Systems RO can be used as a one or two- stage process, depending on require- ments for purity of the water removed (permeate). In the two stage process, the permeate from the first stage is "polished" by the second, producing a higher purity water than is possible with one stage alone. As indicated in Table 1, solute concentration in the permeate may be reduced from about 500 ppm for one stage to 6 ppm in two stages. The flow diagram for the two-stage RO process is shown in Figure 7. ------- Shell Feed Retentate DD-595 Baffle Header cover Permeate water Tube Figure 5. Tubular module. Wastewater (dragout) Chemicals storage tank(s) Pump 5-10 micr< filter(s) 1^ >n s) Pump hi ^ P* ^ ^^ \ 1 1 Jl r t Conce r for rec '( b- ( ( ( ntratf ;ycle DD-838 Figure 6. Reverse osmosis system. RO module RO module - RO module RO module Pump Clean water (permeate) ------- Table 1. Reverse Osmosis: One- and Two-Stage Processes, Water Recovery, and Purity Configuration Water Recovery,% Water purity, ppm ROonestage RO-two stage 77 77 500 6 1st stage RO Concentrated solution Prefiltered and treated metal finishing industry wastewaters (dragout) 2nd stage RO 1 st stage permeate DD-592 Clean water Figure 7. Two-stage reverse osmosis process. Failure Analysis A failure analysis is presented be- low for the RO process when used to treat waters from the metal finishing industry. As shown below, the fail- ures are categorized as to probability of occurrence (high, moderate, and low). To our knowledge there are no published data that further quantify the frequency of occurrence of these failures. High Probability Relief Valves (Liquid) Liquid relief valves are included in RO (and other processes) to protect the piping from overpressure. Over- pressure frequently occurs during startups, shutdowns, and upsets. Overpressures can result from con- trol valves failing in the closed posi- tion, and from the plugging of valves, piping, and membrane modules. Plate-and-frame and tubular modules are not as susceptible to plugging as hollow-fiber and spiral-wound mod- ules. Seals Seal or o-ring failures may occur in the membrane feed pump, chemical feed pump, or the air compressor that delivers instrument air to instruments ------- and control valves. Possible causes of seal failures include overheating and mechanical stress. Visual inspec- tion can confirm spraying or leaking of wastewater at the pumps or com- pressor. Valves and Pipe Fittings These failures are more prevalent in older plants than in newer ones. Causes include mechanical stress, improper maintenance procedures, and freezing during cold weather. Vi- sual observations can confirm leaks of wastewater or chemicals from valve stems and fittings. Miscellaneous Spills During Daily Operations Spills of chemicals or wastewater frequently occur when tanks are re- plenished or when the system is shut down for maintenance. For RO sys- tems, chemical spills can include ac- ids, bases, phosphates, and chlorine. Relief Valves (Vapor) Storage and run down tanks are equipped with vapor relief valves to maintain a constant pressure. These valves release contaminated vapors to the atmosphere as tank levels (and tank pressures) increase. These re- leases are small, but they can occur frequently. Moderate Probability Tank Overflows Tank overflows can result in signifi- cant releases of wastewaters or chemicals to the environment. They occur mostly during startups, shut- downs, and plant upsets. Membrane Failures Holes may develop in the mem- brane material, allowing wastewater to escape to contaminate the clean water permeate. The potting material that attaches the membrane material to the module housing may also fail and result in contamination of the clean water permeate. If the upstream filters fail, solids can escape and dam- age the membrane. And the mem- brane can be defective when it is delivered from the supplier. In addi- tion, corrosive chemicals, such as chlorine, can attack some types of membranes, though some membrane materials are more durable than oth- ers. For example, ceramics are more durable than polymer membranes. An indication of membrane failure is a sudden reduction in pressure drop across the membrane. Low Probability Tank Ruptures A tank can rupture, possibly be- cause of mechanical failure or freeze damage. Though this type of failure is rare, a rupture can result in the release of a large quantity of waste- water or chemicals to the environ- ment. Piping Ruptures Piping is typically strong and not likely to rupture. Possible causes of rupture include mechanical stress, freezing, and improper maintenance procedures. Large leaks are possible with this type of failure. A summary of the types and causes of failures and the associated ques- tions for later software development are presented in Table 2. ------- Table 2. Failure Analyses for Reverse Osmosis System Failure Cause(s) Questions for Software Development Relief valves (liquid) Seals Valves and pipe fittings Miscellaneous spills during daily operations Relief valves (vapor) High Probability - Overpressures during start- ups, upsets, and shutdowns - Key control valves failing in closed position. - Plugging of valves, piping, and membrane modules due to buildup of solids. Hollow-fiber and spiral membrane modules are most susceptible to fouling. - Overheating - Mechanical stress - Abrasive wear - Mechanical stress - Improper maintenance procedures - Freezing - Spills during filling of tanks (due to faulty gages and equipment and mistakes by operators). Spills can include pretreatment chemicals (such as acids, bases, and phosphates). - Faulty maintenance procedures - Increases in tank levels - Changes in ambient temperature What is the expected quantity of leaks through the liquid relief valves (gallons)? What is the disposition of these leaks (i.e., Do they go to a capture system, process sewer, or are they lost directly to the environment)? What is the expected quantity of leaks through seals (gallons)? What is the disposition of these leaks? What is the expected quantity of leaks through valves and pipe fittings (gallons)? What is the disposition of these leaks? What is the expected quantity of leaks from spills (gallons)? (Base on plant experience and operating records). What is the disposition of these spills? What is the expected quantity of leaks through vapor relief valves (standard cubic feet/hour)? What is the disposition of these leaks? Moderate Probability Tank overflows Membrane module failures Tank ruptures Piping ruptures - Occur mostly during unstable conditions (during startups and shutdowns). Overflows can include pretreatment chemicals (such as acids, bases, and phosphates). - Membrane defective - Module potting material defective Presence of corrosive chemicals - Presence of solids What is the expected quantity of tank overflows (gallons)? (Base on plant experience and records). What is the disposition of these overflows? What is the expected quantity of leaks through membrane modules (gallons)? What is the disposition of these leaks? Low Probability Mechanical failures Freezing Mechanical failures Freezing What is the expected quantity of releases due to tank failures (gallons)? (Be sure to include the concentrated waste if it is stored onsite). What is the disposition of these releases? What is the expected quantity of losses due to pipe ruptures (gallons)? What is the disposition of these losses? DD-839 ------- References Cartwright, P. S., "An Update on Reverse Osmosis for Metal Fin- ishing," Plating and Surface Finishing, April 1984, pp 62- 66. Cross, J. R. and P. A. Evans, "Re- cycling Rinse Waters and Re- covering Metals," Metal Finishing, 15:7, July 1991. Kinman, R. N. et al., "Reverse Os- mosis Membrane Fouling," Metal Finishing, November 1985, pp 53-55. Shoeman, J. J. et al., "Evaluation of Reverse Osmosis for Elec- troplating Effluent Treatment," Water Science and Technol- ogy, 25:10(1992) pp 79 93. Stanford, P. T., and K. A. Miller, "Cleanup of Hazardous Waste Using an Advanced Reverse Osmosis System," paper pre- sented at Emerging Technolo- gies in Hazardous Waste Management VI, Atlanta, Geor- gia, September 1994. Suggested Reading 1. Ho, W. S. and K. K. Sirkar, Membrane Handbook, Van Nostrand Reinhold, New York (1992). 2. Amjad, Z., Reverse Osmosis: Membrane Technology, Water Chemistry, and Industrial Ap plications, Van Nostrand Reinhold, New York (1993). 3. Eisenberg, T. N. and E. J. Middlebrooks, Reverse Osmo- sis Treatment of Drinking Wa- ter, Butterworths Publishers, Boston, MA (1986). 4. Belfort, G., Synthetic Mem- brane Processes, Academic Press, Inc., Orlando, FL (1984). 5. Porter, M. C., Handbook of In- dustrial Membrane Technology, Noyes Publications, Park Ridge, NJ (1990). -ftllA GOVERNMENT PRINTING OFFICE: 19% - 7SO-OOV41052 ------- SS-9 'ON lll/NH3d Vd3 QlVd S33d 4 3OVlSOd 3ivu >nna 600/96-y/929/Vd3 ooe$ JOJ ssaujsng 8929^ HO 'I jo} iu S9JBJS ------- |