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
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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.
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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.
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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)
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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.
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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
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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).
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