# GEPA Capsule Report United States Environmental Protection Agency
Technology Transfer Office of Research and Development Washington DC 20460 EPA/625/R-961009 September 1996



# Reverse Osmosis ProcessTechnology Transfer



# Capsule Report EPA/625/Fi-961009



# 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 completed for the reverse osmosis (RO) process. The focus was on processfailures that result in releases of liquids and vapors to the environment. The reoort includes the followina: A description of RO anlcoverage of the principles behind the process.Applications of RO for treatment 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 applications in the metal finishing industry.

# Reverse Osmosis

Process Process Description
In the reverse osmosis (RO) process, water passes through a membrane, leaving behind a solution witha smaller volume and a higher concentration 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 metalsplating or other metal finishing processes. The recovered water (penneate) can be recycled or treateddownstream, depending on the quality of the water and the needs of theplant. As shown in Figure 1, the water that passes through the membrane is defined as permeate and the concentrated solution left behind is defined as retentaie (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.

Pressurized wastewater (dragout) The flux of component A throughan RO membrane is given by Equation (1):



### NA
where
N A=

PA=

DF=

L = Flux of component A through the membrane, mass/timelength2. Permeability of A, mass-length/ time-force. Driving force of A across themembrane, either pressure difference or concentration difference, force/length 2 or mass/ length3. Membrane thickness, length.

At equilibrium, the pressure difference between the two sides of the RO membrane equals the osmoticpressure difference. At low solute concentrations, the osmotic pressure ( p) of a solution is given by Equation (2): 

## TC = csRT (2)
where
p = Osmotic pressure, force/ length2. C, = Concentration of solutes in solution, moles/length3.

0 l 0 0 l 0 

## 0 0 

## 0

_ * Concentrate

rn. .A n CJ

7 w w 

# l o 0 

# 0 . 0 0

l : Oî 0í 

# 0 m -. r

DD-621

Figure 1. Reverse osmosis process. Water (permeate) 

## (1)

1R = Ideal Gas Constant, (forcelength)/(mass-temperature). T = Absolute temperature, “K or “R. As a mixture is concentrated by passing water through the membrane,osmotic pressure of the solution increases, thereby reducing the driving force for further water passage. Anaccurate characterization of the pressure to drive the RO process must bebased on an osmotic pressure computed from the average of the feed and retentate stream compositions. The water recovery of an RO process may be expressed by Equation (3): REC = (Q#Q,) x 100 (3) where REC = Water recovery, %.
QP = Permeate flow rate, length3/ time. 

### QF = Feed flow rate, IengthVtime. 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 permeateproduced. Generally, for concentration of waters from the metal finishing industry, greater water recoveries aredesirable to obtain overall greater volume 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/l nickel in the feed (Shoeman et al., 1992; Cross and Evans, 1991). There are at least150 RO systems operating on various types of nickel baths; most use cellulose acetate membranes (Cartwright, 1984).At least 12 RO systems are operating on various copper sulfate rinses. These systems use both hollow-fiber polyamide and cellulose triacetate membranes and spiral-wound, thinfilm 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 composite membranes at a feed rate of 45 gal/hr and at a water recovery of 88%. The membrane concentrate isfurther reduced in volume in an evaporator and returned to the process (Kinman, 1985; Cartwright, 1984). Approximately five RO systems are operating on various types of brass cyanide rinses. Both polyamide andcellulose triacetate hollow-fiber membrane elements are used. An RO system is being used after contact plating on printed circuitboards. The rinse is fed to a polyamide 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 theremainder is routed to the waste treatment system. All of the membrane permeate is reused as a rinsewater (Cartwright, 1984). Cadmium and chromium rinsewaters are also treated with RO. Membrane fouling has been experienced for the cadmium rinsewaters, but littlefouling has been experienced for chromium rinsewater applications. Preliminary results show that payback for 5 cubic meters per hour RO cadmium/water and RO chromium/water recovery plants are three and seven years, respectively (Shoeman et al., 1992). In other industries, RO is used forproduction of potable water from seawater and brines, for water recoveryfrom landfill leachates, and for concentration of industrial wastewaters and brines. RO is sometimes used as a pre-concentrator for evaporators tolower energy requirements and increase process efficiency. RO has also found many applications in the food and dairy industries; it is used in the food industry to concentrate applejuice and in the dairy industry to concentrate cheese whey. 

# Equipment
The module is the housing that contains the membrane. With regard to failure analyses, module configuration is important because some types ofmodules are more reliable than others. Membrane modules are commercially available in four configurations:

. Plate-and-Frame

. Spiral-Wound

. Hollow-Fiber

. Tubular 

### Plate-And-Frame ModulesAs shown in Figure 2, plate-andframe modules use flat sheet membranes that are layered between spacers and supports. The supportsalso form a flow channel for the permeate 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-framemodules. Maintenance on plate-andframe modules is possible due to the nature of their assembly. They offer high recoveries with their long feed channels and are used to treat feedstreams that often cause fouling problems. Only recently advanced designs of plate-and-frame modules capableof operating up to 25% dissolved solids and operating pressures up to4500 psia have been placed in operation in Germany (Stanford and Miller, 1994). This development opensnew opportunities for the use of reverse osmosis for concentration of metal finishing wastewaters. 

### Spiral-Wound ModulesSpiral-wound modules use a sandwich 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 themembrane sandwich. The support layers are designed to minimize pressure drop and allow a high packing density. Additionally, the spiral-woundmodules can be designed by equipment suppliers to promote turbulenceand therefore increase the mass transfer across the membrane or to provide an uninterrupted flow path to decrease membrane fouling. Spiralwound modules offer greater packing densities, but maintenance is difficult. 

### Hollow-Fiber Modules As shown in Figure 4, hollow-fiber modules consist of small diametermembrane fibers bundled within cylindrical 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

2

### Concentrated solution 4-

Permeate 1-t-^- (uaal I water)

Wastewater 4 (dragout)

DD-837
Figure 2. Plate-and-frame reverse osmosis module.

Feed

\ Feed

- Membrane

Permeate

MM-9

Figure 3. Spiral-wound module.

3Retentate outlet

Fiber bundle plug

Hollow fiber

i

Carbon steel shell

Liquid feed

MM-10

Figure 4. Hollow-fiber module.

the shell side of the fibers, a lower pressure drop is encountered downthe bore of the fiber since the permeate 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 oftubes. The operator can easily service feed and permeate channels toremove fouling layers. Tubular modules are somewhat resistant to fouling when operated with a turbulent feed flow. This is accomplished with F Permeate

larger flow channels than those used with hollow-fiber and spiral-wound modules. The drawbacks of tubularmodules are their high energy requirements for pumping large volumes of water, high capital costs, and lowmembrane surface area per unit volume of module (see Figure 5 ). 

# Operation And Maintenance
To maintain membrane performance and extend membrane life,pretreatment chemicals may be necessary, depending on the characteristics of the wastewater. In addition, chemicals may be required to achieve clean water specifications. Filteringwastewater may be necessary to remove suspended solids before wastewater is fed to the RO modules.Membrane performance can be enhanced by control of pH, removal ofcertain dissolved species and colloidal materials such as clays and oils, and dissolved or suspended organits. 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 membrane 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 solubility, 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 someRO membranes are chlorine sensitive, 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 anRO system with four modules in parallel, chemical pretreatment, and anup-front filtration step is shown in Figure 6. 

### Staging RO Systems

RO can be used as a one or twostage process, depending on requirements 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.

4Shell

Membrane Baffle Header cover

Feed t

Retentate

+

DD-595

Figure 5. Tubular module. 1 ‘Tube Permeate water

Wastewater (dragout)

Pump

DD-838

Figure 6. Reverse osmosis system. t Concentrated solution for recycle (retentate)

Pump

Clean water (permeate)Table 1. Reverse Osmosis: One- and Two-Stage Processes, Water Recovery, and Purity

Configuration Water Recovery,% Water purity, ppm

RO-one stage 77 500 RO-two stage 77 6

Prefiltered and treated metal finishing industry wastewaters (dragout)

DD-592 1st stage RO Concentrated

l------ sohJt’on

1 st stage permeate

2nd stage RO

Figure 7. Two-stage reverse osmosis process.



# Failure Analysis
A failure analysis is presented below for the RO process when used to treat waters from the metal finishingindustry. As shown below, the failures 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. Overpressure frequently occurs during startups, shutdowns, and upsets.Overpressures can result from control valves failing in the closed position, and from the plugging of valves, Clean water

piping, and membrane modules. Plate-and-frame and tubular modules are not as susceptible to plugging ashollow-fiber and spiral-wound modules. 

### 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

6and control valves. Possible causes

of seal failures include overheating and mechanical stress. Visual inspection can confirm spraying or leakingof wastewater at the pumps or compressor. 

### 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. Visual observations can confirm leaks of wastewater or chemicals from valve stems and fittings. 

### Miscellaneous Spills During Daily Operations Spills of chemicals or wastewaterfrequently occur when tanks are replenished or when the system is shutdown for maintenance. For RO systems, chemical spills can include acids, 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 (andtank pressures) increase. These releases are small, but they can occur frequently. 

### Moderate Probability

Tank OverflowsTank overflows can result in significant releases of wastewaters or chemicals to the environment. Theyoccur mostly during startups, shutdowns, and plant upsets. 

### Membrane FailuresHoles may develop in the membrane 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 upstreamfilters fail, solids can escape and damage the membrane. And the membrane can be defective when it isdelivered from the supplier. In addition, corrosive chemicals, such as chlorine, can attack some types of membranes, though some membranematerials are more durable than others. 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 RupturesA tank can rupture, possibly because 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 wastewater or chemicals to the environment. 

### 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 causesof failures and the associated questions for later software development are presented in Table 2.Table 2. Failure Analyses for Reverse Osmosis System

Failure Cause(s) High Probability Questions for Software Development

Relief valves (liquid)

Seals

Valves and pipe fittings

Miscellaneous spills during daily operations

Relief valves (vapor)

Tank overflows

Membrane module failures

Tank ruptures

Piping ruptures - Overpressures during startups, 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. 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)?

- Overheating

- Mechanical stress

- Abrasive wear What is the expected quantity of leaks through seals (gallons)? What is the disposition of these leaks?

- Mechanical stress What is the expected quantity of leaks through

- Improper maintenance procedures valves and pipe fittings (gallons)? What is the

- Freezing disposition of these leaks?

- Spills during filling of tanks (due to What is the expected quantity of leaks from spills faulty gages and equipment and (gallons)? (Base on plant experience and mistakes by operators). Spills can operating records). What is the disposition of these include pretreatment chemicals spills? (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 vapor relief valves (standard cubic feemour)? What is the disposition of these leaks?

Moderate Probability

- 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 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?

- Mechanical failures

- Freezing What is the expected quantity of losses due to pipe ruptures (gallons)? What is the disposition of these losses?

DD-839

# References Shoeman, J. J. et al., “Evaluation of Reverse Osmosis for Elec

### Suggested Reading Cartwright, P. S., “An Update onReverse Osmosis for Metal Finishing,” Plating and Surface Finishing, April 1984, pp 62- 66. troplating Effluent Treatment,”Water Science and Technology, 25:lO (1992) pp 79 93. Stanford, P. T., and K. A. Miller, “Cleanup of Hazardous Waste Using an Advanced ReverseOsmosis System,” paper presented at Emerging Technologies in Hazardous Waste Management VI, Atlanta, Georgia, September 1994. 1.

2*

3. Ho, W. S. and K. K. Sirkar, Membrane Handbook, Van Nostrand Reinhold, New York (1992).

Cross, J. R. and P. A. Evans, “Recycling Rinse Waters and Recovering Metals,” Metal Finishing, 15:7, July 1991. Kinman, R. N. et al., “Reverse Osmosis Membrane Fouling,” Metal Finishing, November 1985, pp 53-55.

4.

5. Amjad, Z., Reverse Osmosis: Membrane Technology, Water Chemistw, and industrial Ap plications, Van Nostrand Reinhold, New York (1993). Eisenberg, T. N. and E. J. Middlebrooks, Reverse Osmosis Treatment of Drinking Water, Butterworths Publishers, Boston, MA (1986). Belfort, G., Synthetic Membrane Processes, Academic Press, Inc., Orlando, FL (1984). Porter, M. C., Handbook of Industrial Membrane Technology, Noyes Publications, Park Ridge, NJ (1990).

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