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GEPA Capsule ReportUnited StatesEnvironmental ProtectionAgency

Technology Transfer

Office of Research andDevelopmentWashington DC 20460

EPA/625/R-961009September 1996

Reverse Osmosis Process

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Technology Transfer

Capsule ReportEPA/625/Fi-961009

Reverse OsmosisProcess

September 1996

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ContentsProcess Description . . . . . . . . . . . . . . . . . . . 1

Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Equipment .................................. 2

Operation and Maintenance ..... .4Failure 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 reoort includes the followina:A description of RO anlcov-

erage of the principles behindthe process.

Applications of RO for treat-ment of effluent waters fromthe metal finishing industry.

Descriptions of equipment and

operating and maintenanceprocedures.

Failure analysis that includestypes of failures and causes.

Key questions that can be used

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Reverse OsmosisProcess

Process DescriptionIn the reverse osmosis (RO) pro-

cess, water passes through a mem-brane, leaving behind a solution witha smaller volume and a higher con-centration of solutes. The solutes canbe contaminants or useful chemicalsor reagents, such as copper, nickel,and chromium compounds, which canbe recycled for further use in metalsplating or other metal finishing pro-cesses. The recovered water (penne-ate) can be recycled or treated

downstream, depending on the qual-ity of the water and the needs of theplant. As shown in Figure 1, the wa-ter that passes through the membraneis defined as permeate and the con-centrated solution left behind is de-fined as retentaie(or concentrate).

The RO process does not requirethermal energy, only an electricallydriven feed pump. RO processes havesimple flow sheets and a high energy

efficiency. However, RO membranescan be fouled or damaged. This canresult in holes in the membrane andpassage of the concentrated solutionto clean water, and thus a release tothe environment. In addition, somemembrane materials are susceptibleto attack by oxidizing agents, such asfree chlorine.

Pressurizedwastewater(dragout)

The flux of component A throughan RO membrane is given by Equa-tion (1):

NAwhere

NA=

PA =DF=

L =

Flux of component A throughthe membrane, mass/time-length2.Permeability of A, mass-length/time-force.

Driving force of A across themembrane, either pressure dif-ference or concentration differ-ence, force/length2 or mass/length3.Membrane thickness, length.

At equilibrium, the pressure differ-ence between the two sides of theRO membrane equals the osmoticpressure difference. At low solute con-centrations, 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 so-

lution, moles/length3.

0 l 0 0 l 0 0 0 0 _ * Concentrater n . .A n CJw wlo 0. 0 0

(1)

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Concentratedsolution4-

Permeate1-t-^-(uaalIwater)

Wastewater 4(dragout)

DD-837

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

Feed

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Retentate outlet

Fiber bundle plugFiber bundle plug

Hollow fiberHollow fiber iCarbon steel shellCarbon steel shell

Liquid feedLiquid feed

MM-10 F Permeate

chemicals may be required to achieveclean water specifications. Filteringwastewater 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 ofcertain dissolved species and colloi-dal materials such as clays and oils,and dissolved or suspended organ-its. In any RO system, depending onthe capacity and size of modules, anumber of parallel modules may be

needed.Membrane fouling can result fromthe formation of a fouling layer on themembrane surface, or from internalchanges of the membrane material.Both forms of fouling can cause mem-brane permeability to decline. Scalingis a form of fouling that occurs whendissolved species are concentratedin excess of their solubility limit.Chemical agents can be added to

slow the formation of precipitates.Acidification is used to prevent theformation of carbonates of low solu-bility, such as magnesium carbonate.An ion exchanger is sometimes usedto trade cations of low solubility saltsfor cations that are more soluble, forexample, sodium sulfate may betraded for calcium sulfate.

Prevention of biological growth isnecessary to prevent damage to the

membrane. Biological growth can beinhibited with chlorination, but someRO membranes are chlorine sensi-tive, so water must be dechlorinatedbefore entering the RO module. Otherdisinfectants are ozone formaldehyde

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Shell

Membrane Baffle Header cover

Feedt

Retentate+

DD-595

Figure 5. Tubular module.

1 TubePermeate water

Wastewater(dragout)

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Table 1. Reverse Osmosis: One- and Two-Stage Processes, Water Recovery, and Purity

Configuration Water Recovery,% Water purity, ppm

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

Prefiltered andtreated metalfinishing industrywastewaters

(dragout)

DD 592

1st stage ROConcentrated

l------ sohJton

1 st stagepermeate

2nd stage RO

Cleanwater

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and control valves. Possible causesof seal failures include overheating

and mechanical stress. Visual inspec-tion can confirm spraying or leakingof wastewater at the pumps or com-pressor.

Valves and Pipe Fittings

These failures are more prevalentin older plants than in newer ones.Causes include mechanical stress,improper maintenance procedures,and freezing during cold weather. Vi-

sual observations can confirm leaksof wastewater or chemicals from valvestems and fittings.

Miscellaneous Spills DuringDaily Operations

Spills of chemicals or wastewaterfrequently occur when tanks are re-plenished or when the system is shutdown for maintenance. For RO sys-tems, chemical spills can include ac-

ids, bases, phosphates, and chlorine.

Relief Valves (Vapor)

Storage and run down tanks areequipped with vapor relief valves tomaintain a constant pressure. These

valves release contaminated vaporsto the atmosphere as tank levels (and

tank pressures) increase. These re-leases are small, but they can occurfrequently.

Moderate Probability

Tank Overflows

Tank overflows can result in signifi-cant releases of wastewaters orchemicals to the environment. Theyoccur mostly during startups, shut-

downs, and plant upsets.Membrane Failures

Holes may develop in the mem-brane material, allowing wastewaterto escape to contaminate the cleanwater permeate. The potting materialthat attaches the membrane materialto the module housing may also failand result in contamination of theclean water permeate. If the upstream

filters fail, solids can escape and dam-age the membrane. And the mem-brane can be defective when it isdelivered from the supplier. In addi-tion, corrosive chemicals, such aschlorine, can attack some types of

membranes, though some membranematerials are more durable than oth-

ers. For example, ceramics are moredurable than polymer membranes. Anindication of membrane failure is asudden reduction in pressure dropacross the membrane.

Low Probability

Tank Ruptures

A tank can rupture, possibly be-cause of mechanical failure or freeze

damage. Though this type of failureis rare, a rupture can result in therelease of a large quantity of waste-water or chemicals to the environ-ment.

Piping Ruptures

Piping is typically strong and notlikely to rupture. Possible causes ofrupture include mechanical stress,freezing, and improper maintenance

procedures. Large leaks are possiblewith this type of failure.

A summary of the types and causesof failures and the associated ques-tions for later software developmentare presented in Table 2.

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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 spillsduring daily operations

Relief valves (vapor)

Tank overflows

Membrane modulefailures

- Overpressures during start-ups, upsets, and shutdowns- Key control valves failing inclosed position.- Plugging of valves, piping, andmembrane modules due to buildupof solids. Hollow-fiber and spiralmembrane modules are mostsusceptible to fouling.

What is the expected quantity of leaks through theliquid relief valves (gallons)? What is the disposition ofthese leaks (i.e., Do they go to a capture system,process sewer, or are they lost directly to the environment)?

- Overheating- Mechanical stress- Abrasive wearWhat 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 andmistakes 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 vaporrelief valves (standard cubic feemour)? What is the

disposition of these leaks?

Moderate Probability

- Occur mostly during unstableconditions (during startups andshutdowns). Overflows caninclude pretreatment chemicals(such as acids, bases, and phosphates).

- Membrane defectiveModule potting material defective

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 membranemodules (gallons)? What is the disposition of these leaks?

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References Shoeman, J. J. et al., Evaluationof Reverse Osmosis for Elec- Suggested Reading

Cartwright, P. S., An Update on

Reverse Osmosis for Metal Fin-ishing, Plating and SurfaceFinishing, April 1984, pp 62-66.

troplating Effluent Treatment,

Water Science and Technol-ogy, 25:lO (1992) pp 79 93.

Stanford, P. T., and K. A. Miller,Cleanup of Hazardous WasteUsing an Advanced ReverseOsmosis System, paper pre-sented at Emerging Technolo-gies in Hazardous WasteManagement VI, Atlanta, Geor-gia, September 1994.

1.

2*

3.

Ho, W. S. and K. K. Sirkar,

Membrane Handbook, VanNostrand Reinhold, New York(1992).

Cross, J. R. and P. A. Evans, Re-cycling Rinse Waters and Re-covering Metals, MetalFinishing, 15:7, July 1991.

Kinman, R. N. et al., Reverse Os-mosis Membrane Fouling,

Metal Finishing, November1985, pp 53-55.

4.

5.

Amjad, Z., Reverse Osmosis:Membrane Technology, WaterChemistw, and industrial Applications, Van NostrandReinhold, New York (1993).

Eisenberg, T. N. and E. J.Middlebrooks, Reverse Osmo-

sis Treatment of Drinking Wa-ter, Butterworths Publishers,Boston, MA (1986).

Belfort, G., Synthetic Mem-brane Processes, AcademicPress, Inc., Orlando, FL (1984).

Porter, M. C., Handbook of In-dustrial Membrane Technology,Noyes Publications, ParkRidge, NJ (1990).

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