ECONOMIC AND DESIGN FACTORS IN THE APPLICATION OF REVERSE
OSMOSIS TO METAL FINISHING SOLUTE RECOVERY
By Peter
S.
Cartwright, P.E.,
President
C 3 International, Inc.
2019 W. County Rd. C, Roseville, M N 55113
Abstract
Reverse osmosis is a membrane separation process which has as
its greatest application to date the desalting or purifiThis ability to remove both ionic and
cation of water.
organic solute from relatively dilute streams make it a
viable candidate
in
metal finishing
waste
treatment
applications.
In certain situations, reverse osmosis can be
applied
directly to the
rinse water from
a particular
bath
concentrating the salts enough to return them to the plating
bath and the purified water back to the flowing rinse.
Many
factors affect the use of this technology in such "zero
discharge" applications, and these are examined in detail.
Economic considerations are also examined.
New discharge
regulations ,
coupled
with
more
rigid
enforcement of the older regulations, have required that many
existing precipitation/clarification systems be revised or
replaced.
Reverse osmosis can be used to "dewater" or
concentrate the mixed stream prior to the clarifier, thereby
reducing the hydraulic load. Also, this technology can be
used to "polish" the clarified effluent to ensure that the
final stream will safely meet all requirements. Design and
economic considerations are detailed.
Backsround
Alt.hough reverse osmosis has been commercially utilized since
only about 1969, the basic concepts have been understood for
years, and the origins of the process are based on osmosis, a
fundamental action of nature. When a semipermeable membrane
such as a living cell wall separates two solutions with
differing concentrations of dissolved solids, pure water will
flow from the solution containing the lower concentration of
solute through the membrane into the solution containing the
higher concentration of solute.
This movement of water
through the cell wall (semipermeable membrane) is a result of
the fact that the solution containing less solute is at a
higher energy state than the more concentrated solution.
In
order to attain an equilibrium of energy, the movement of
water results.
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P 04154
By applying pressure to the more concentrated solution, the
normal osmotic flow is reversed, and pure water is forced
through the semipermeable membrane into the l e s s concentrated
solution,
In the process of reverse osmosis, applied
pressure is usually provided by a pump and amounts to adding
energy to the more concentrated (lower-energy) side to cause
the movement of water.
Osmotic pressure is the difference between the potential
energy of any solution and that of pure water.
It is a
function of the specific solute and its concentration in
water. In practical terms, it. is the minimum pumping energy
required to produce the first drop of pure water from a
solution of a given solute at a specific concentration.
An extreme example of osmotic pressure can be found in the
desalination of sea water. Typical sea water has an osmotic
In order to get the first drop of
pressure of 4 0 0 psi,
potable water from sea water, the applied pressure must
exceed 400 p s i . On a practical basis, the pumping pressure
should be in the range of 600 p s i .
R e v e r s e osmosis ( R O ) specifically involves the separation of
dissolved ionic materials from water. The exact mechanism of
this separation, or "repulsion", is not fully understood, and
disagreement exists between the leading theorists on this
issue; but it is well-documented that the higher the ionic
charge (valence) of an ion, the greater its tendency to be
repelled from the surface of the memebrane. This means that
monovalent salts such as sodium (Na+) and chloride (Cl-) will
tend to p a s s through the membrane into the pure water side
(permeate) at a higher rate than multivalent salts such as
The
calcium (Ca++), nickel (Ni++) and sulfate (SO4--).
typical pore size of an RO membrane is on the order of 5
angstrom units (5 x 10-4 micrometers).
*
Figure 1 illustrates the mechanism of reverse osmosis.
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Figure 1 , Reverse Osmosis Mechanism
Although reverse osmosis can be discussed in terms of a
"filtration" process, it does not involve the aspects of
conventional (dead-end) filtration where the entire liquid is
HO utilizes a different
pumped through the filter media.
process, known as "crossflow filtration" where the solution
to be filtered flows tangentially (parallel) across the
surface of the filter media, and, under pressure, a portion
of this stream is forced through the filter media forming the
permeate stream.
The portion feed exiting without passing
through the media is known as "concentrate" or "rententate."
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Figure 2 illustrates these two filtration processes.
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=------
-
--
1
0
Particle-freepermeate
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Figure 2 , Filtration Processes
Only certain plastic polymers
have the properties
effectively perform as reverse osmosis membranes; these
be characterized as follows:
to
can
POLYMER
Polyamide
Cellulose Cellulose
Acetate
Triacetate
Thin Film
Composite
pH stability
4-11
2-8
Oxidat ion
resistance
Poor
Good
Fair-Good
Biological
resistance
Good
Poor
Fair-Good
35
35
30
50
90
90
>90
Temperature
limit (C)
Typi cal reject ion
of ionic species ( 8 ) > 9 0
4-7.5
2-12
Fair-Good
Good
The membrane
can
be "Packaqed"
element
in
several
configurations
("devices"j,
each
offering
par ti cular
advantages depending on the application.
Tubular- Manufactured from ceramic, carbon, or a number of
porous plastics, these tubes have inside diameters ranging
from 1/8 inch up to approximately 1 inch.
The membrane is
typically coated on the inside of the tube, and the feed
solution flows through the interior from one end to the
other, with the "permeate" passing through the wall to be
collected on the outside of the tube.
~
Hollow Fiber- Simi.lar to the tubular element in design,
hollow fibers are generally much smaller in diamet.er and
require rigid support .such as is obtained from the "potting"
of a bundle inside a cylinder. Feed flow is either down the
interior of the fiber or around the outer diameter.
Spiral Wound- This device is constructed from an envelope of
sheet membrane wound around
a permeate tube that
is
perforated to allow collection of the permeate or filtrate.
Plate and Frame- This device incorporated sheet membrane that
is stretched over a frame to separate the layers and
facilitate collection of the permeate.
5
Figure 3 illustrates these devices.
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PLATE AND FRAME
SPIRAL WOUND
ROLLTO
POROUS
SHEET
MEMBRANE
CONCENTRATE OUT
PERMEATE OUT
PERMEATE SIDE BACKING MATERIAL
WITH MEMBRANE ON EACH
SIDE AND GLUED AROUND
EDGES TO CENTER TUBE
TUBULAR
POROUS TUBE
MEMBRANE
CONCENlRATE
HOLLOW FIBER
c
PERMEATE OUT
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F i y u r e 3 , MeiiIbrdi,e L l r i i i e r i t C o n f i g u r a t i o r i s
For a given membrane polymer, the total volume occupied by
the elements is dependent upon the area of membrane contained
in each, or "packing density".
The ability of the membrane
element to resist fouling from suspended or precipitated
solids is an extremely important property, as this is the
greatest single cause of element failure.
These characteristics are summarized for each element in
following table:
Element
Configuration
Tubular
Hollow fiber
Spiral wound
Plate and frame
Packing
Density*
Low
High
High
Fouling
Resistance
Good
Poor
Fair
Good
Low
* Membrane area per unit volume of space required
6
the
Because of the propensity of suspended or precipitated
materials to settle out on the membrane surface and plug the
membrane pores (fouling), turbulent flow conditions must be
maintained (Reynolds numbers in excess of 2100). For most
waste treatment applications, this requires recycling a
percentage of the concentrate back to the feed side of the
pump. The addition of this concentrate stream into the feed
solution
obviously
increases
the
dissolved
solids
concentration further increasing osmotic pressure.
Svstems Desiun Considerations
In order to treat an effluent stream, it must be
analyzed for the following properties:
Total solids
Suspended
Dissolved
Dissolved
thoroughly
content
(TSS)
organic (TOC)
ionic (TDS)
Specific chemical constituents
Oxidizing chemicals
Organic solvents
Operating temperature
Usually, the goal is to "dewater" the feed stream as much as
possible; that is, to remove water to facilitate either reuse
or removal of the concentrated solute.
Of importance also
is the possible reuse of the purified water.
These two
considerations are significant in .determining both
the
process and membrane device to be used.
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Figure 4 illustrates a general schematic for the reverse
osmosis process. Note that the term "recovery" is defined as
the percentage of feed volume that is pumped through the
membrane and comes out as permeate (purified water).
...
c
FEED
>TREAY
'F
\
2-
.. .
\
cF
.
PERMEATE S T R E A M
QP
CP
>
\
CCNCENTAATE STREAM
LJF
-
CF
-
Uc
-
CC
-
up
Cp
F e e d flow r e t e
h o l u t e coocentrstion
PerneaZe
flor
5 o l u t a concsntrotlon
Concentrata
in f a a d
rete
flo-
In p e r m e a r e
veta
S o l u t e concentration
In concentcmte
Figure 4
namorene
~ r o c c s s i n qS c h e m e t i c
In virtually all applications, it is desired to otain as high
osmotic
a recovery as possible. Two factors limit recovery
pressure and permeate quality.
-
defined
earlier,
osmotic
pressure
increases
as
concentration increases.
The force available to
pump
permeate through a membrane can be related to osmotic
pressure a s follows:
As
Net driving force (psi) = pump pressure - osmotic pressure
From a practical standpoint,
usually 1000 psi.
the maximum
pump pressure
is
No membrane is perfect in that it rejects 100% of the solute
on the feed side, and this solute leakage through the
membrane is known as "passage."
Expressed as "percent
passage," the total quantity of solute which passes through
the membrane is a function of the concentration of solute on
the feed side.
Under high recovery conditions,
the
concentration of solute on the feed side is increased and
therefore the actual quantity of solute passing through the
membrane also increases. Because most effluent applications
demand that, in addition to a minimum concentrate volume, the
permeate quality be high enough to allow reuse or meet
discharge regulations, the "Catch-22" predicament of permeate
quality decreasing as recovery is increased can impose design
limitations.
Zero Discharge Applications
Figure 5 illustates the layout for a reverse osmosis system
installed on the 1st countercurrent rinse in a standard
electroplating line.
Under the right conditions,
the
concentrate flow will equal the evaporation rate from the
plating tank, and since this usually accounts for the only
loss of bath volume, the plating salts which are concentrated
from the first rinse by the reverse osmosis system are almost
The permeate from the
all returned to the plating bath.
reverse osmosis unit is usually pure enough to be directed to
the last rinse for total recycle.
Make-up water, in the
same volume as that lost though evaporation from the plating
tank, is also added to the last rinse. This is usually tap
water purified by deionization or reverse osmosis, and
typically amounts to less than 108 of the feed flow to the
zero discharge reverse osmosis unit.
1
PL4TlNG
TANU
V
.,
REVLRSE
?
PERME4TE
In studying this application, it soon becomes apparent that
those plating baths which operate at higher temperatures are
the best candidates for zero discharge reverse osmosis
treatment. Watts nickel baths are excellent in this regard,
and with the high value of these plating salts,
the
investment in the recovery equipment can usually be paid off
within 18 months, The chemistry of the bath can play a major
role in the performance of the membrane, and to date, those
baths with
high oxidizing
chemistry present
chemical
compatibility problems for the available membrane polymers,
Because of its high packing density and relatively high
resistance to fouling, the spiral wound element configuration
is used in virtually all existing systems.
Plating baths operating at temperatures below 50 degrees C
(120 degrees F ) offer a multitude of problems because of
their low evaporation rates.
If reverse osmosis is used
directly, it may be impossible to attain the recovery
required because of the high osmotic pressure effect.
This
effect can be significant when considering that the increase
in salts concentration between recoveries of 95% and 98% is a
factor of 2.5. to 1. To minimize this problem, techniques
plating bath,
partial
such as air
agitation of the
the concentrate
stream, directing
the
evaporation of
concentrate stream to an air scrubber and bleeding part of
the bath directly into the reverse osmosis unit have been
used with success.
Figures 6 Through 10 illustrate the relationship of recovery
to osmotic pressure f o r rinse streams from various baths.
These rings contain approximately 3000 mg/l of plating salts.
Athough the osmotic pressure increases with
increasing
recovery for all bath chemistries, there is considerable
variation in the curves.
IO
10
30
LO
50
REC)VER'I,
60
I
70
80
90
1
LOO
0
1
0
10
10
30
&O
50
MCOVERI.
F I GURt
60
10
80
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t t F t C I OF PLCOVLIlY ON O S W T I C PRLSSURt :
Z I N C CYAVIDL R I S E WATER
90
LOO
EFFT.CT OF RECOVERY 01 O S W T I C PRfSSVRE :
BRACS CIAVIOL RIRSC VATLR
"U
Id
20
XJ
10
WCOVERI.
r i v d
I)ECORATI\'E C H R O r
HARU CHI(OXE
50
60
70
80
90
100
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Fiqure 11 summarizes test data on zero discharge applications
for 6 common bath rinses.
RO Test Data For Six Common Plating Bath Rinses
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#.tiw
Bath
Toxk
1.mpmhrre. 'CPF
contamlnmt'
Pucull
n)rctkn"
Walts Ni
60f140
Ni"
g9t
Copper cyanlde
60/140
CU'
CN-
85
40
Zinc cyanide
27/80
a*
m
baul
Brass cyanlde
27/80
CN-
80
a*
cu'
CN-
98
97
97
Oecoratwe Cr
43/110
Cr"
90
Hard Cr
55/130
Cr"
82
'Rlnse flow adjusted to glve a tot61 salt concentration of approximately3000 mg/L.
"Rejectlon of toxfc contamlnant in permeate calculated as follows: [l-(Conc. In PermeaWConc. in Feed)] 100.
The following have been installed in North America to date:
Bath Treated
Number of Existing Systems
150
Nickel
Acid Copper
Acid Zinc
Copper Cyanide
Hexavalent Chrome
12
1
1
1
Feed TDS: 2-3000 mg/l
Typical feed rate: 120-600 gph
"End of Pipe" Mixed Effluent Treatment
Because of its ability to concentrate all dissolved solids
out of water solutions, reverse osmosis has found application
in "dewatering" effluent streams, both before and after
conventional waste treatment systems.
Figure 12 illustrates the application of reverse osmosis to
dewater mixed rinses prior to chemical precipitation/
of
clarification.
This approach offers the advantage
reducing hydraulic loading to the precipitation/clarification
process, thereby conserving space when a treatment system
requires expansion.
It also offers the advantages of
producing pure water that c a n be recycled to the process for
rinsing, boiler feed, or other purposes.
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. .nt.ratc
s 1anl-tldr
c l a r i f icr
Figure 12
I.uxif I I I
t o
Figure 13 illustrates the applications of reverse osmosis to
clarified effluent.
This is particularly useful when a
change in discharge regulations, process chemistry or an
upset in the precipitation/clarification system results in
unacceptable concentrations of hazardous wastes remaining in
the clarified effluent. A l s o , there are situations where the
high concentration of sodium and calcium salts from the
clarifier preclude either reuse of the treated water or
discharge to the POTW (Publically Owned Treatment Works). By
treating the clarified
effluent with reverse
osmosis,
purified water can be obtained representing 9 0 - 9 5 % of the
feed flow, with the concentrate portion accounting for only
5-10%.
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5-LO?
Figure 13
"E"*
of
@io." .fFl"ont
tI-..tlld"t
with zero discharge applications, osmotic pressure and
permeate quality are the limiting factors for
maximum
recovery. Usually, the goal is to dewater the stream as much
as
possible in order to
have a minimum quantity
of
concentrate for further treatment or disposal. The intended
use of the permeate will typically dictate its quality
requirements, and
this,
along with
osmotic
pressure
considerations, will set the maximum recovery for each
application.
As
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Figure 14 illustrates the approximate capital costs of the
reverse osmosis systems as a function of feed flow.
These
uninstalled costs apply to high recovery systems for b o t h
zero discharge and total effluent dewatering applications.
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1.
-ljure 14
.
1187 I
fccd
Rate l g p h l
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Operating costs are a function of the following factors:
electrical consumption
membrane element life
cleaning frequency
pre-filter maintenance
other system maintenance
$
The most significant operating cost factors are electrical
usage and membrane replacement; at an electricity cost of
3.06 per kwh and a two year life for the membrane elements,
(Typicaloperating costs are $1.50 to $2.00 p e r 1000 g a l l o m
of feed.
Conclusion
The purpose of this paper has been to illustrate the
versatility of reverse osmosis for effluent treatment in
metal finishing applications.
As
the advantages of this
unique separation technology are more thoroughly understood,
it. will find greater application in the metal finishing
industry,