United States
            Environmental Protection
Office of Research and
Washington DC 20460
September 1996
            Technology Transfer
v>EPA     Capsule Report
            Reverse Osmosis Process

Technology Transfer	EPA/625/R-96/009
Capsule  Report

Reverse  Osmosis
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

                                      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
                                        •    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-

Reverse  Osmosis
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):
                                                                        NA =  Flux of component A through
                                                                              the  membrane,  mass/time-
                                                                        PA =  Permeability of A, mass-length/
                                                                        DF=  Driving force  of A across the
                                                                              membrane, either pressure dif-
                                                                              ference or concentration differ-
                                                                              ence, force/length2 or mass/
                                                                        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)

                                                                        p =  Osmotic  pressure,  force/
                                                                        Cs =  Concentration  of solutes in so-
                                                                              lution, moles/length3.
o • • o • °
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^\ VI 1 A ffj

1-1 0
. .* ° • O
.0 *
• t * O
o o o _>
• n
,'° *.'°.*

* • •
• • * * *

^_ M

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)
REC  = Water recovery, %.
Qp    = Permeate flow rate, length3/
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.
  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.
  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  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


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



  Porous feed spacer


Porous permeate spacer

Figure 3.     Spiral-wound  module.

     Retentate outlet
 Fiber bundle plug
       Hollow fiber
   Carbon  steel shell
     Liquid feed
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
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
  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
   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.

                                                                              Header cover
Permeate water
Figure 5.    Tubular module.



hi ^
P* ^




r for rec




Figure 6.     Reverse osmosis system.
                                                                                   RO module
                                                     RO module
                                                     RO module
                                                     RO module
                                                                                              Clean water

Table 1.     Reverse Osmosis: One- and Two-Stage Processes, Water Recovery, and Purity
Water  Recovery,%
Water purity, ppm
RO-two  stage
                                             1st stage RO
                Prefiltered and
               treated metal
               finishing  industry
                                            2nd stage RO
                                                               1 st stage
 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
     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-


                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-

 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

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-

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
                 Questions for Software
Relief valves (liquid)
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
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
 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
  Mechanical failures
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


  Cartwright, P. S., "An Update on
      Reverse Osmosis for Metal Fin-
      ishing," Plating  and Surface
      Finishing, April 1984, pp 62-
  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
  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).

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