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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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