4. reverse osmosis

4
Reverse Osmosis
Richard G. Sudak
INTRODUCTION
The first large industrial application of reverse osmosis occurred in 1970
when a 100,000 gallon per day (GPD) system was placed in operation at Texas
Instruments’ electronics manufacturing facility in Dallas, Texas. In this applica-
tion, the reverse osmosis plant was used to pretreat Dallas municipal watdr which
was being converted to ultrapure processwater by ion exchange resins. It was re-
ported by the facility engineer that the use of reverse osmosis increased the yield
in the manufacturing operation to such an extent that the resultant savingspaid
for the reverse osmosis plant in about two weeks. Now, virtually every electron-
ics plant in the United States usesreverse osmosis as pretreatment in the prepara-
tion of ultrapure water.
I The worldwide total reverse osmosis operating capacity by the end of 1970
was 880,000 GPD. By the end of 1976, reverse osmosis operating capacity had
grown to 167,000,000 GPD’ and, by the middle of 1980, this capacity had more
than doubled to 390,500,OOO GPD.’ Reverse osmosis continued to expand and
it was reported3 that the operating capacity by the end of 1984 was 524,000,OOO
GPD.
Figure 4.1 shows that the rate of growth in reverse osmosis capacity was
slow between 1970 and 1973. From 1974 through 1980, there was an accel-
erated rate of growth during which time the reverse osmosis capacity increased
from 62,500,OOO GPD to 412,500,OOO GPD or an increase of 560% in the 7-
year period. Between 1981 and 1984 (4 years), the total operating capacity in-
creased from 412,500,OOO to 524,000,OOO GPD, an increase of 27%.
Reverse osmosis finds applications predominantly in the following areas:
Industrial - Used to prepare industrial process water or to Proc-
esswastes.
260

Reverse Osmosis 261
Irrigation - Used to upgrade waters for agricultural purposes.
Military - Used for military purposes.
Municipal - Used to upgrade waters to municipal drinking water
levels.
Power - Used to prepare process water in electric power sta-
tions.
Miscellaneous - Used for various purposes.
ooo-
YEAR
Figure 4.1: Reverse osmosis capacity vs. year.
Table 4.13 below shows the uses of reverse osmosis as described above.
Table 4.1: Reverse Osmosis Product Water Usage
Product Water Use Capacity.MCD*
Industrial 158
Irrigation 2
Military 13
Municipal 191
Miscellaneous 10
Power 55
Yuma DesalLing Plant 73
Total 502
* MGD- One million gallons per day
Percent of Total
31.5
0.4
2.6
38.0
2.0
11.0
14.5
100.0

262 Handbook of Industrial Membrane Technology
Reverse osmosis finds much of its application in the production of potable
water for municipal water supplies. However, the use of reverse osmosis to pre-
pare industrial process waters and to treat industrial waste is in a close second
place. Agricultural irrigation accounts for only a very small percentage of total
capacity because current sources of agricultural water are much cheaper than
water from reverse osmosis. The military use of the reverse osmosis processwas
small as of the end of 1984 but the U.S. military serviceshave discovered reverse
osmosis to be a process ideally suited to providing drinking water in the field.
The process not only is suitable for converting brackish and seawaters into pot-
able supplies but it will also remove toxic and biological agents which result
from germ, chemical and nuclear warfare. Consequently, military use of reverse
osmosis should increase in the future. Power plants have found reverse osmosis
very useful in preparing boiler feedwater. In Yuma, Arizona, a desalting plant
is being constructed by the U.S. Government to improve the quality of the
Colorado River prior to its flowing into Mexico. That plant alone accounts for
14.5% of the worldwide capacity of the reverse osmosis process.
Reverse osmosis is a process that transforms an unusable water supply
into a usable resource. It is capable of renovating a broad spectrum of feed-
waters from municipal water supplies that need polishing for industrial purposes
to seawater that is refined into a potable water supply. Table 4.23 shows the dif-
ferent types of feedwater being processed by reverse osmosis units. Seawater
is considered to have a nominal total dissolved solids (TDS) content of 35,000
mg/!L Wastewater is from industrial or municipal sources and the TDS is vari-
able. Brackish water is defined for the purposes of Table 4.2 only as a water that
may have a TDS from that of municipal water supplies up to 10,000 mg/Q.
Table 4.2: Source of Reverse Osmosis Feedwater
Capacity,MGD Percent of Total
Sea Water 67.9 13.0
Waste Water 26.5 5.0
Brackish Water 82.0
Total 524.0 100.0
As of the end of 1984, the desalination of brackish water accounted for
82% of capacity. This is due to the fact that early reverse osmosis membranes
were incapable of single stage seawater desalination and, thus, they were limited
to brackish water desalination. Within the last 10 years, significant advances
have been made in both the flux and rejection capability of membranes and re-
verse osmosis is technically able to desalt seawater in a single stage. In the re-
cent past, it has been an effective competitor to the distillation process in sea-
water desalination. In fact, reverse osmosis is now beginning to replace existing
distillation capacity in the Middle East.4
Although reverse osmosis is a relatively new technology, there is sufficient
operating capacity in a number of varied applications to warrant confidence
in the process. From a technical and economic point of view the process is ca-

Reverse Osmosis 263
pable of desalting a broad range of feedwaters from municipal water supplies to
seawater. It has economic viability in a large number of industrial applications.
BASIC PROCESS CONSIDERATIONS
An industrial reverse osmosis plant usually will consist of three separate sec-
tions which are shown in Figure 4.2. The first section is the pretreatment sec-
tion in which the feedwater is treated to meet the requirements of reverse os-
mosis element manufacturers and the dictates of good engineering practice.
Following pretreatment, the feedwater is introduced into the reverse osmosis
section where the feedwater is pressurized and routed to the reverse osmosis
elements which are in pressure vessels.The feedwater flows acrossthe membrane
surface where product water permeates through the membrane and a predeter-
mined amount remains behind as reject. The reject is discharged to waste while
the product water is routed to the posttreatment section. The third or post-
treatment section treats the product water to remove carbon dioxide and adds
chemicals as required for industrial use of the product water.
I I
PRETREATMENT
PRODUCT
* TO INDUSTRY
Figure 4.2: Industrial reverse osmosis.
Although operating reverse osmosis plants are commonplace, the subject
of salt and water transport is still controversial. Several transport models have
been proposed and these will be discussed briefly below. A more thorough
presentation is made in a paper by Soltanieh and Gill.’

The sieve mechanism is the simplest and easiest model to understand. This
model proposes that salt and water are separated due to physical size differences
by a membrane with a pore size that lies between the sizes of salt and water.
While most laymen prefer this concept, it is unfortunate that for solutions, such
as sodium chloride in water, the sizesof the salt and water molecules are almost
the same. This fact would seem to rule out the sieve mechanism model.
Another model proposed is the wetted surface mechanism or the water
clustering concept of transport. It is generally recognized that reverse osmosis
membrane material is wettable and that water tends to be absorbed on the
membrane by hydrogen bonding. In this concept, it is theorized that the water
film at the surface of the membrane obstructs the pores and prevents salt from
entering the pores. The water passesthrough the membrane by passingfrom one
absorbed site to the next until it reaches the other side of the membrane barrier
layer. The energy requirements for solvent migration are much less than the
energy requirements for salt migration and, thus, the separation of salt from
water takes place.
Another concept of water and salt transport in reverse osmosis is the pref-
erential sorption-capillary flow mechanism. In this model, the surface of a mem-
brane is microporous and heterogeneous at all levels of solute separation. It is
hypothesized that, due to the chemical nature of the membrane skin layer in
contact with the aqueous solution, a preferential sorption for the water causes
a sorbed water layer to be formed at the skin. This layer of purified water is
then forced through the capillary pores by pressure.
The solution-diffusion model of transport assumesa nonporous, homogene-
ous membrane surface layer. Each component in a pressurized solution dissolves
in the membrane and then diffuses through the membrane. The flow of water
and salt through the membrane is uncoupled, i.e., they are independent of each
other, and the water transports through the membrane at a more rapid rate than
the salt.
The product water flow through the membrane is defined as follows:
F, = A*(AP-An)
where F, = water flux through the membrane
A = water transport coefficient
AP = pressure differential acrossthe membrane
An = osmotic pressure differential acrossthe membrane
The flow of water through a reverse osmosis membrane is primarily dependent
on the applied pressure differential and the osmotic pressure differential across
the membrane. The osmotic pressure is directly dependent on the salt concen-
tration of the process stream. ks a rule of thumb, each 100 mg/Q of dissolved
solids is roughly equivalent to one psi of osmotic pressure. Since the product
stream usually has a very low salt content, the osmotic pressure of that stream is
negligible. In addition, the product stream normally leaves the reverse osmosis
pressure vesselsat near atmospheric pressure so that the applied pressure differ-
ential is the feed pressure. Consequently, the term “net applied pressure” has
come to mean the applied pressure minus the feed osmotic pressure.

Reverse Osmosis 265
The flow of salt or dissolved solids across the membrane is dependent on
the following equation:
F, = B*AC
where F, = salt flux
B = salt transport coefficient
AC = salt concentration gradient acrossthe membrane
The salt transport is primarily dependent upon the concentration of dissolved
solids on each side of the membrane.
The solution-diffusion model seems to represent the performance of a re-
verse osmosis membrane. Figure 4.3 shows the salt rejection and flux of a low
pressure polyamide membrane as a function of applied pressure. The membrane
was operated on a 5,000 mg/ll aqueous solution of sodium chloride at 25°C. As
can be seen, there was no product water flow until the applied pressure exceeded
the osmotic pressure (50 psi). After this, the flux increased linearly as would
be predicted by the above water flux equation. Rejection is poor at lower pres-
sures and increases rapidly until it reaches an asymptote at an applied pressure
of about 150 psig. This can be attributed to a near constant flow of salt with
a rapidly increasing product water flow which results in a more dilute product
or in increased rejection. These data tend to substantiate the assertion of the
solution-diffusion model that flow is uncoupled.
APPLIED PRESSURE, PSIG
Figure 4.3: Membrane flux and rejection vs. applied pressure.
It is noted that the data shown in Figure 4.3 was derived in membrane test
cells with near perfect small areas of membrane. In a practical system, there
would be a number of imperfections in the membrane and the salt flow through
these capillaries would contribute to the total salt flow. Therefore, a practical

266 Handbook of industrial Membrane Technology
salt transport model must also take into account the contribution of the mem-
brane imperfections to salt flow.
The water transport coefficient (A) is not a constant in that it varies with
temperature. The product flow as a function of temperature may be estimated’s
by using the following equation:
Q 25/ Qt = ex
where Q 25 = flow at 25°C or 77°F
Qt = flow at temperature T, “C
e = 2.71828
X = U [l/(T+273) -l/2981
T = Temperature in “C
u = 2723 (for cellulose acetate membranes).
Figure 4.4 graphically shows the ratio of water flux at 25°C to water flux at
other temperatures.
2.4
2.2
2.0
1.2
1.2
1.4
1.2
1.0
0.2
0.6
0.4
0.2
0 10 20 20 40 60
TEMPERATURE OC
Figure 4.4: Temperature correction factor.
As a rule of thumb, the product water flow with constant net applied pres-
sure will increase about 3% for each degree centigrade increase in feedwater tem-
perature. Salt flux through the membrane is also directly proportional to tem-
perature and the ratio of salt flux to water flux is essentially constant at differ-
ent temperatures. This results in little or no change in rejection as a function of

Reverse Osmosis 267
temperature. For some of the newer composite membranes, the water and salt
permeation coefficients also vary as a function of pH.
Reverse osmosis is a cross-flow membrane separation process which sepa-
rates a feed stream into a product stream and a reject stream. The recovery of a
reverse osmosis plant is defined as a percentage of feedwater that is recovered as
product water. As all of the feedwater must be pretreated and pressurized, it is
economically prudent to maximize the recovery in order to minimize power con-
sumption and the size of the pretreatment equipment. Since most of the salts
remain in the reject stream, the concentration of salts increases in that stream
with increased recovery. For instance, at 50% recovery, the salt concentration
in the reject is about double that of the feed and at 90% recovery, the salt con-
centration in the reject is nearly 10 times that of the feed. In cases of sparingly
soluble salts, such as calcium sulfate, the solubility limits may be exceeded at a
high recovery. This could result in precipitation of the salt on the membrane sur-
face resulting in decreased flux and/or increased salt passage. In addition, an in-
crease in recovery will increase the average salt concentration in the feed/reject
stream and this produces a product water with increased salt content. Conse-
quently, the recovery of a reverse osmosis plant is established after careful con-
sideration of the desired product quality, the solubility limits of the feed con-
stituents, feedwater availability and reject disposal requirements.
While the above equations are helpful in describing the reverse osmosis
process, the water and salt transport coefficients seldom are used to describe
membrane performance. Most manufacturers test reverse osmosis membrane
with standard solutions as described below.
(I) Brackish Water Membrane-The flux and rejection of the mem-
brane is determined when the membrane is tested on a feedwater
of an aqueous sodium chloride solution with a concentration of
2,000 mglQ at an applied pressure of 420 psig (net applied pres-
sure = 400 psig) with a temperature of 25°C and a feed pH of from
5.0 to 6.0 for 30 minutes prior to data collection.
(2) Seawarer Membrane-The seawater test is similar to the brackish
water test except that the feedwater is an aqueous sodium chloride
solution with a concentration of 35,000 mg/R and the test pressure
is 800 psig.
The results are reported as gallons per day per square foot of membrane
area (membrane flux) and as rejection of sodium chloride. Rejection is calcu-
lated as follows:
R = (I-CPKF) 100
where R = Rejection, percent
CP = Product water concentration
CF = Feedwater concentration
The sodium chloride rejection differs from that of other inorganic and or-
ganic dissolved solids, and membrane manufacturers will provide information
and rejection data that are available for their specific membrane. Table 4.3

shows typical results for a composite membrane when tested on a multicom-
ponent solution. The rejection of the divalent ions such as calcium and sulfate
is much better than the rejection of the monovalent ions such as sodium and
chloride. If salt passageis defined as product concentration divided by the feed
concentration, or one minus rejection, then it can be seen that the salt passage
for the divalent ions is about one-fifth of the salt passagefor the monovalent
ions.
Table 4.3: ionic Rejections
IOn- Feed, mp,/l Product, mg/l Rejection, X
Calcium 61 0.2 99.6
Sodium 150 3.0 98.0
Potassium 12 0.3 97.4
Bicarbonate 19 0.7 96.2
Sulfate 189 0.4 99.8
Chloride 162 2.9 98.2
Nitrate 97 3.5 96.4
TDS 693 11.0 98.4
The abovedescribed tests are conducted at a low recovery rate to minimize
the effects of concentration polarization which isdescribed below, For example,
membrane tests above are conducted at less than 1% recovery and tests with
spiral wound elements are conducted at recoveries from 5 to 10%.
Reverse osmosis is a cross-flow process and, as in any dynamic hydraulic
process, the fluid adjacent to the membrane moves slower than the main stream.
While the main stream flow may be turbulent, the layer next to the membrane
surface is laminar. This thin, laminar flow film is called the boundary layer.
When water permeates through the membrane, nearly all of the salt remains be-
hind in the boundary layer next to the membrane. The salt must then diffuse
acrossthe boundary layer and back into the bulk stream. This results in a bound-
ary layer with a salt concentration which is more concentrated than the bulk
stream. The effect has been termed concentration polarization, and it is de-
fined by the following equation:
p = CBICM
where /3 = Concentration polarization
CM = Concentration in the main stream
C8 = Concentration in the boundary layer
Concentration polarization increasesthe salt concentration at the membrane sur-
face, and this results in an increased osmotic pressure at that surface. The in-
creased osmotic pressure causes a drop in water flow as shown in the following
equation:

Reverse Osmosis 269
Fw = A*(P-/3n)
where Fw = the water flow
A = the water transport coefficient
P = applied pressure
P = concentration polarization
n = osmotic pressureof main stream
The increased salt concentration at the membrane surface will also increase the
tendency of sparingly soluble salts to precipitate on the membrane.
The flow of salt also increases and this can be simply shown in the following
equation:
Fs = B*@Cl-C2)
where Fs = salt flux
B = salt transport coefficient
P = concentration polarization
Cl = main stream salt concentration
c2 = product water salt concentration
In the imperfect membrane with a small number of pores, the increased salt con-
centration at the membrane surface would also result in increased salt passage
through the pores which would be directly proportional to concentration polari-
zation.
The information shown in Table 4.4 below assumes a membrane operating
on various feedwaters with a total dissolved solids (TDS) content of 2,000,
5,000 and 35,000 mg/Q. It is assumed that the membrane will deliver 20 gallons
per square foot per day at a 400 psi “net applied pressure” and have a rejection
of 99% when there is no concentration polarization, i.e., fl= 1.0.
Table 4.4: Effects of Concentration Polarization
Feed TDS, mg/l
Flux, gfd
B = 1.0
8 - 1.1
8 = 1.5
8 = 2.0
Rejection, %
S= 1.0
f3 = 1.1
B = 1.5
8 = 2.0
?,ooo 5,ooo 35,000
20.0 20.0 20.0
19.9 19.8 18.3
19.5 18.8 11.3
19.0 17.5 2.5
99.0 99.0 99.0
98.9 98.9 90.0
98.5 98.4 97.3
97.9 97.7 84.0

The penalty of a high concentration polarization is not as severe with a low
TDS feedwater as it is with a high TDS feedwater. The recommendations as to
minimum flows or maximum recoveries which are specified by the reverse os-
mosis element manufacturer should be followed at all times, especially when the
application is the desalination of high TDS waters.
Concentration polarization cannot be eliminated, but it can be minimized
by decreasing boundary layer thickness. This is done by increasing the flow rate
across the membrane surface or introducing turbulence promoters into the
feed/reject stream. In order to achieve optimum performance, most membrane
manufacturers will recommend a minimum feed rate to or from their elements
and a maximum recovery in order to minimize the effects of concentration po-
larization.
REVERSE OSMOSIS MEMBRANES
Osmotic phenomena have been observed since the middle of the eighteenth
century. The first experiments were conducted with animal membranes and it
wasn’t unit1 1867 that artificial membranes were employed. In the early 1950’s,
research workers at the University of Florida demonstrated, with thick films,
that cellulose acetate possessedunique salt and water transport properties which
made it potentially attractive as a reverse osmosis desalination membrane. Dur-
ing the 1960’s, Loeb and others at the University of California at Los Angeles
developed techniques to prepare cellulose acetate membranes with an econom-
ical water flux and salt rejection at moderate driving pressures.With this devel-
opment, reverse osmosis became a practical possibility.
The early cellulose acetate membranes were only suitable for brackish water
desalination as they were not capable of operating at the high pressuresrequired
for seawater desalination (see below). In addition, the membrane rejection was
insufficient to desalt seawater in one stage. The first membranes were fabricated
from cellulose diacetate which is subject to hydrolysis at either a low or high pH,
biological attack and compaction at temperatures in excess of 85°F. Cellulose
acetate also compacts (densifies) at pressures above 400 psig and temperatures
below 85°F. Suitable feedwater pretreatment lessensthe adverse effects of tem-
perature, pH and biological organisms to the point where the membrane can be
used in practical reverse osmosis plants.
A considerable amount of research has been done to develop a membrane of
cellulose triacetate as this material is more stable to extremes of temperature and
pH and it will better withstand chemical and biological attack. While the cellu-
lose triacetate membranes operated quite well on a short term basis, they were
prone to compact at an operating pressure of 400 psig, with the resultant lossof
flux to an impractical level, in a short period of time.
It has been demonstrated that a blend of cellulose diacetate and cellulose
triacetate provides an improved membrane in that:
(1) it is more stable than the cellulose diacetate membranes;
(2) it has a better flux and rejection than the cellulose diacetate; and
(3) it is more resistant to compaction than either the diacetate or
triacetate.

Reverse Osmosis 271
Incidentally, all of the cellulose acetate membranes will tolerate a limited
amount of residual chlorine which allows chlorine to be used for feedwater
disinfection.
The cellulose acetate membranes are asymmetric and fabricated from a
single polymer. The use of electron microscopy in the 1960’s demonstrated
that the cellulose acetate membranes consisted of a relatively thin dense layer
and a thicker porous layer of the same material. The membrane thickness is
usually about 100 micrometers with the dense layer accounting for about
0.2% of the thickness and the remainder being an open cell porous matrix (see
Figure 4.5).
/
Porous Matrlx
Figure 4.5: Single component asymmetric membrane.
During the 1960’s, the DuPont Company screened numerous polymers to
determine the suitability of materials other than cellulose acetate for use in re-
verse osmosis desalination. The results of this work indicated that aromatic
polyamides were the “choice as the best polymer type for use in the DuPont
commercial permeators”.’ The company was most successful in developing an
asymmetric aromatic polyamide reverse osmosis membrane in a hollow fine
fiber configuration which successfully competed with cellulose acetate in the
market place.
Shortly after the concept of an asymmetric membrane was established,
composite membrane research was initiated. A composite membrane is also
asymmetric but it consists of two polymer layers which are the membrane
barrier layer and the porous support layer (Figure 4.6). The porous support
is formed separately, by conventional membrane casting techniques, from one
polymer. The porous support has a thickness of between 75 and 100 micro-
meters and its porosity is due to numerous small perforations through the sup-
port. The membrane barrier layer is a dense thin film of another polymer that is
formed or deposited in a subsequent operation on the porous support. The
membrane barrier layer varies in thickness from 400 to 1,000 angstroms.

Polyamide
Membrane Serrler Layer
c”
P5
s
r4”
I 1400-.l.OOOAn~strome
i
I-76 Mlcrone
Porous Support
Figure 4.6: Two component composite membrane.
Several polymers have been used as porous supports. One of the earliest
composite membrane systems was a porous support formed from cellulose
nitrate-cellulose acetate with a membrane barrier layer of cellulose triacetate.
While this membrane successfully desalted seawater, it was fragile and expen-
sive. To a large degree, present day commercially available composite membranes
use a polysulfone porous support.
Membrane barrier layers have been formed on porous supports in the fol-
lowing manner:’
(1) solution,
(2) thin film polymerization and
(3) interfacial polycondensation.
The solution coating technique was used in the preparation of the cellulose
triacetate membrane discussed above. A solution of cellulose triacetate in chlo-
roform was deposited on the porous support and the solvent was then evapo-
rated leaving a thin film on the porous support. Thin film polymerization was
used to prepare a polyfuran membrane barrier layer on polysulfone. In this case,
the monomer furfuryl alcohol is polymerized in situ by adjustment of pH and
temperature. This membrane proved to be highly susceptible to oxidizing agents
and is of limited value. By far the most valuable technique in the formation of
membrane barrier layers is interfacial polycondensation. In this method, a poly-
mer is formed on the porous support surface at the interface of organic and
aqueous phases by reaction of specific molecules dissolved in each phase. It is
by this method that a number of polyamides and polyurea membrane barrier
layers have been formed on polysulfone. Elements containing these membranes
are available commercially.
There has been very little progress in the last five years in improving the Per-
formance of single polymer asymmetric membranes. Meanwhile, the composite
membranes have been improved and they exhibit a higher flux and better re-

Reverse Osmosis 273
jection at lower operating pressuresthan is available with the single component
asymmetric membranes. The polyamide and polyurea composite membranes
can withstand higher temperatures and larger pH variations than the cellulose
acetate based membranes and they are immune to biological attack. On the
other hand, as noted above, the cellulose acetate membranes can tolerate a lim-
ited concentration of residual chlorine while the polyamide and polyurea mem-
brane barrier layers are subject to disintegration by residual chlorine and other
oxidizing agents. Consequently, alternate methods of water disinfection or chlo-
rination followed by dechlorination are employed in reverse osmosis systems
using these membranes.
While the cellulose acetate membranes are compacted at the moderate
pressures required for brackish water desalination, the thin film composite
membranes exhibit ho compaction at these pressuresand a very low compaction
rate at pressuresof 1,000 psig.
The superior flux and rejection capabilities of the thin film composite mem-
brane has been demonstrated at the municipal wastewater reclamation facility
of the Orange County Water District in California. Both asymmetric cellulose
acetate and thin film composite membranes were tested on lime clarified second-
ary effluent. The pilot plants were operated at 85% recovery and the rejections
reported in Table 4.5 are the percent rejection of the constituents in the feed-
water and not the rejection of the average concentration of the specific constitu-
ents in the feed/reject stream. Use of the average concentration would give a
higher rejection in both cases.
Table 4.5: Rejection Comparison
Constituent
Na
NH4
SO4
Cl
NOa
COD
TDS
CAMembranetg TFC PolyamideMembrane(lO1
93.8 99.0
89.0 94.8
99.3 99.8
93.3 95.2
71.0 97.7
93.6 >94.8
94.4 97.5
The thin film composite membrane exhibited superior overall rejection per-
formance in these tests, with ammonia and nitrate rejection showing an out-
standing improvement. It has also been reported that silica rejection by the thin
film composite membranes is superior to that of cellulose acetate. While the
above data indicates a marginal improvement in the rejection of chemical oxygen
demand (COD), which is an indication of organic content, other tests conducted
by membrane manufacturers show that the polyurea and polyamide membrane
barrier layers exhibit an organic rejection that is clearly superior to that of cellu-
lose acetate. Reverse osmosis element manufacturers should be contacted for
rejection data on specific organic compounds.

MEMBRANE PACKAGING
Reverse osmosis membrane is produced in sheet form-up to 60 inches wide
and lengths up to 1,500 feet-and as a hollow fine fiber. The asymmetric cellu-
lose acetate was originally produced as a sheet and later as a hollow fine fiber.
The asymmetric aromatic polyamide was originally produced as a hollow fine
fiber and later in sheet form. The composite membranes with polyamide or poly-
urea membrane barrier layers are produced in sheet form as of the end of 1987,
but research has been and will continue to be done to produce the composite
reverse osmosis membranes as a hollow fine fiber.
The Office of Saline Water, U.S. Department of Interior, sponsored much of
the development of membrane packaging configurations for reverse osmosis
membrane. The configurations which have been developed and evaluated are as
follows:
(I) plate and frame,
(2) spiralwound,
(3) tubular and
(4) hollow fine fiber.
Plate and Frame
The plate and frame configuration is much like the conventional plate and
frame filtration concept except that, in reverse osmosis, a typically higher fluid
operating pressure is used. Figure 4.7 is a schematic of the plate and frame ap-
proach. The membrane package is installed in a pressure vesseldesigned and fab-
ricated to withstand operating pressures from 400 to 1,000 psig. A stack of par-
allel porous plates are used to support the membrane on each side of the porous
plate, Feedwater under pressure enters the top of the pressure vessel and flows
between the parallel stacks of membrane/porous plates. Product water passes
through the membrane and into the porous plate to be routed to the product
water collection system and then out of the pressure vessel. As the feed/reject
stream passes across the membrane, it becomes concentrated and eventually
leaves the pressure vessel as concentrate or reject. More advanced designs utilize
external tie bolts which hold together sufficiently thick end plates which contain
the pressurized feed/reject stream. The plate and frame packaging configuration
has an advantage in that only the membrane must be replaced when a membrane
becomes defective. It has disadvantages of complex flow patterns and high costs.
The plate and frame configuration can achieve a packaging density of up to 150
square feet of active membrane area per cubic foot of pressure vessel. Overall,
the plate and frame concept has not been economically competitive and there
are no remaining manufacturers of this type of equipment in the United States.
Spiral Wound
The spiral wound membrane packaging configuration is shown in Figure
4.8. Basically, the spiral wound element consists of two sheets of membrane
separated by a grooved, polymer reinforced fabric material. This fabric both
supports the membrane against the operating pressure and provides a flow path

Reverse Osmosis 275
for egress of the product water. The membrane envelope is sealed with an ad-
hesive on three sides to prevent contamination of the product water. The fourth
side is attached to a product water tube which has perforations within the edge
seal so that product water can be removed from the porous product water car-
rier material. The membrane envelope is rolled up around central product water
tube, with a plastic mesh spacer between the facing membrane surfaces, in a
spiral. The mesh spacer not only serves to separate membrane surfaces but it
provides a flow path for and turbulence in the feed/reject stream of each ele-
ment. The elements have an outer wrap to contain the feed/reject stream in the
mesh passageway and brine seal to insure that the feed/reject stream goes through
the element and not around it. Spiral wound elements are available in lengths
from 12 to 60 inches and diameters from 2 to 12 inches. Packaging densities of
up to 300 square feet of membrane to 1 cubic foot of pressure vessel have been
attained. Reverse osmosis plants with a capacity from 100,000 to 5,000,OOO gpd
of product would normally use elements with an 8-inch diameter by a 40-inch
or 60-inch length.
WI WdlW In
a
Fresh
’ PorousPIales
Figure 4.7: Plate and frame packaging.

Fcedwalec6rine Spacer
I
Feedwaler
to Fresh Water by Passage
through Membrane
Product Waler Flow
(atler Passage
through Membrane)
PIO~UC~Water Side
Sackinq with Membran
Membranes
Membrane
Product Tube
End view
oi Each ‘&da
Figure 4.8: Spiral wound packaging.

Reverse Osmosis 277
Spiral wound elements are installed in a pressure vesselwhich is usually fab-
ricated from fiberglass reinforced plastic. The pressure vessel inside diameter is
sized to match the outside diameter of the element brine seal. The pressure ves-
sels are designed and fabricated to accommodate from 1 to 6 elements and op-
erating pressures from 50 to 1,000 psig. Figure 4.9 shows a pressure vesselwith
6 elements installed. Feedwater enters one end of the pressure vessel and flows
through the first element in which about 10% of the feed permeates through
the membrane and flows through the product water carrier material into the
product water tube. The reject from the first element flows to and through the
second element and the reject from this element becomes the feed to the next
element, etc. The reject from the last element is routed from the pressure vessel
to the high pressure reject manifold. The first and sixth element product water
tubes are sealed to the pressure vesselend caps by O-ring devices to prevent con-
tamination by the high pressure feed or reject to the purified product water
stream. The element product water tubes in the pressure vesselare connected to
each other with interconnectors which again are O-ring devices whose seals pre-
vent product water contamination. The product water can exit the pressure
vessel, usually at near ambient pressures, from either end of the pressurevessel.
In a single pressure vessel with six elements, between 40 and 60% of the feed-
water to the pressure vessel is recovered as product water from a brackish water
feed and from 25 to 35% is recovered from a seawater feed.
The advantages of the spiral wound elements are the high packing density
and high flux which makes it one of the most cost effective elements. The dis-
advantage of the element is that a moderate amount of pretreatment is required
for some feedwaters to prevent fouling of the mesh brine spacers.
Tubular
The tubular reverse osmosis device is shown in Figure 4.10. The tube serves
as the pressure vessel and the membrane is installed inside the tube. Tubes with
inside diameters of % and 1 inch have been used. Uniformly porous fiberglass
reinforced plastic tubes have been used and nonporous but perforated copper,
stainless steel and fiberglass tubes have also been successfully used. The mem-
brane can be bonded to the tube in which case it is cast in situ or the membrane
can be loose. The loose membrane is cast in sheet form and a cylindrical section
is formed and placed in the tube. Packing densities for the %-inch diameter tube
are about 100 square feet per cubic foot and about 50 square feet per cubic foot
for the l-inch diameter tube.
Pressurized feedwater enters the tube through an end fitting which sealsthe
membrane to the tube and prevents cross contamination of the product water.
The feed water flows down the length of the tube and product water permeates
through the membrane and weeps through the tubular pressure vesselinto a col-
lection basin. The reject flows through an end fitting and is routed to additional
tubes in series or to waste.
The major advantages of the tubular reverse osmosis configuration are the
ability to tolerate high suspended solids concentrations in the feed and the pos-
sibility of mechanical membrane cleaning. The disadvantages are the excessive
number of tube end fittings in proportion to the active membrane area in each
pressure vessel, the bulkiness of the reverse osmosis plant and the high cost.

End Cap Retaining Ring O-Ring Seal Interconnector
I
5
g
8
k
0
FRP Pressure Vessel Brine Seal
Figure 4.9: Spiral wound element pressure vessel assembly.

Reverse Osmosis 279
Practical Unit
Salt
Water
In
Fresh Water
Figure 4.10: Tubular packaging.
There have been a number of attempts to commercialize tubular reverse osmosis
systems in industrial applications. As of the end of 1987, there were no large
scale tubular reverse osmosis manufacturers in the United States, although there
are some in Europe and Japan.
Hollow Fine Fiber
Hollow fine fiber synthetic filaments have been prepared for use in the tex-
tile industry for a long time. This well-known technology was adapted to prepare
polymers, which are suitable for reverse osmosis desalination, into hollow fine
fibers. The most readily available polymers in hollow fine fiber elements are
aromatic polyamide, cellulose diacetate and cellulose triacetate. The fibers are
indeed very fine in that they approach the diameter of a single human hair. The
aromatic polyamide hollow fine fiber has an outside diameter of 85 micro-
meters for the brackish water fiber and an outside diameter of 95 micrometers
for the seawater fiber. Both of the fibers have an internal diameter of 42 micro-
meters.” The ratio of outside to inside diameter exceeds two. The fibers are
thick walled cylinders that have the compressive strength necessary to with-
stand the operating pressures. This concept is capable of achieving a high pack-
aging density of 3,000 square feet per cubic foot but the flux is considerably
lower than with sheet membrane.
Figure 4.11 is a schematic of a typical hollow fine fiber element. A con-
tinuous fiber is looped into a bundle around a central feed tube. The central
feed tube is sealed at the product water end and is perforated within the active
membrane area. Both ends of the fiber bundle are potted in an adhesive. When
cured, the adhesive and protruding fibers in the product water end are machined

EPW
Nub Shell
HollowFiber
Membrane Sheet Support
Bktck
Figure 4.11: Hollow fine fiber packaging.

Reverse Osmosis 281
to open the fibers at that end. The fiber bundle is then installed in a pressure
vessel and an O-ring in the product water end tube sheet seals against the pres-
sure vessel wail to prevent the high pressure reject stream from contaminating
the low pressure product stream. The element resembles a tube and shell heat
exchanger. The pressure vessels are fiberglass reinforced plastic and they are
designed and fabricated to accommodate operating pressures from 250 to
1,000 psig. Hollow fine fiber elements are available with nominal diameters
of 4,6 and 8 inches.
Pressurized feedwater enters the element through the central feed tube
which is connected to the high pressure feed manifold. The feed flows through
perforations in the tube and then radially between the spacesin the fiber toward
the outer diameter of the bundle. Longitudinal feed flow is minimized by the re-
ject end being potted. Product water permeates the membrane, enters the capil-
laries of the fiber bundle and flows to the tube sheet where it is discharged into
the product water plenum and then into the product water manifold. The reject
flows from the outer diameter of the fiber bund!es, through the end cap and to
waste via the high pressure reject manifolding. Normally, each hollow fine fiber
element is capable of recovering from 40 to 75% of the feed water from brack-
ish water and from 25 to 35% from seawater.
The advantages of the hollow fine fiber element are the high packaging
density and the elimination of membrane support materials. The prime disad-
vantage is the need for an efficient feedwater pretreatment to remove suspended
and colloidal solids.
Dynamic Membranes
Dynamic membranes are formed by flowing a feed solution containing 50
to 100 mg/!?of membrane-forming material tangential to a clean porous surface
at velocities from 5 to 50 feet per second under pressures from 500 to 1,200
psi.” The dynamic membrane is formed within minutes and the optimum
performance is usually achieved within an hour. The membrane-forming ma-
terials have been hydrous thorium, natural humic and fulvic acids, pulp mill
wastes, municipal wastes, synthetic polyelectrolytes, zirconium oxide, and poly-
acrylic acid. One of the most successful membrane-forming materials has been a
layer of polyacrylic acid deposited on top of a layer of zirconium oxide. The
porous surfaces used have been carbon, ceramic and stainless steel tubes. Use
of a stainless steel tube which had been pretreated with a filter aid and coated
with a layer of polyacrylic acid over a layer of zirconium oxide achieved a so-
dium chloride rejection of 90% with a flux of about 60 gfd. Recently, several
plants using dynamic membranes have been installed and there has been a re-
newed interest for such applications as orange juice concentration. These mem-
brane promise a high flux with a lower rejection and a low membrane cost.
However, the capital and operating costs have not always been commercially
competitive.
PLANT DESIGN
The first step in the design of an industrial reverse osmosis plant is to de-
termine the amount of water to be treated, peak demand, product water quality,

source of feedwater and reject discharge requirements. The next step is to ob-
tain an accurate and representative analysis of the feedwater. This analysis
should include determination of the concentration of the feedwater constituents
shown in Table 4.6. Knowledge of the feedwater constituents in Table 4.6 will
provide sufficient information for an experienced design engineer to successfully
design a reverseosmosis plant in the majority of applications.
Table 4.6: Feedwater Analysis
Cations Anions Other
Calcium Carbonate Silica
Magnesium Bicarbonate Total DissolvedSolids
Sodium Sulfate Total SuspendedSolids
Potassium Chloride Turbidity
Iron Nitrate Silt DensityIndex
Manganese Phosphate TemperatureRange
Ammonia Fluoride Total OrganicCarbon
PH
The above determinations with the exception of the silt density index should be
made in accordance with Standard Methods for the Examination of Water and
Wastewater. l3 A schematic diagram of the silt density index apparatus is shown
in Figure 4.12 and the silt density index (SDI) is determined as follows:
(I)
(2)
(3)
Measure the amount of time required for 500 ml of feedwater to
flow through a 0.45 micrometer Millipore filter (47 mm in diam-
eter) at a pressure of 30 psig.
Allow the feedwater to continue flowing at 30 psig applied pres-
sure and measure the time required for 500 ml to flow through the
filter after 5, 10 and 15 minutes.
After completion of the test, calculate the SDI by using the equa-
tion below.
where SDI =
Tt =
Ti =
Tf =
SDI = 100 (1 - Ti/Tf)
‘t
Silt Density Index
Total elapsed test time (either 5, IO or 15 min-
utes)
Initial time in seconds required to collect the 500
ml sample
Time in seconds required to collect the second 500
ml sample after test time Tt (normally after 15
minutes).”

Reverse Osmosis 283
Qauge
at 3Opsig)
~illip~re Filter Holder
(0.46 mloron -47mm dla. Mlllipore FIlterI
Graduate Cylinder
(Measure Rate With Stop Watch)
Figure 4.12: Silt density index apparatus.
Manufacturers of hollow fine fiber elements usually require that the pre-
treated feedwater have an SDI of 3.0 or below in order for the element war-
ranty to be effective. One manufacturer of spiral wound element requires that
the pretreated feedwater have a turbidity of lessthan 1.O turbidity unit to main-
tain the element warranty, As a general rule, if hollow fine fiber elements are
to be used, the pretreated feedwater should have an SDI of 3.0 or less and, if
spiral wound elements are to be used, an SDI of 5.0 or less. However, spiral
wound elements have been used to recover municipal wastes with an SDI in
excessof 5.0 after pretreatment.
Pretreatment Section
The pretreatment section of a reverse osmosis plant is designed:
(1) to reduce suspended solids (1 micrometer or larger) to zero and to
minimize the effects of colloids in membrane fouling,
(2) to adjust the pH of the feedwater,
(3) to add a threshold inhibitor to the feedwater and
(4) to remove oxidizing compounds in the feedwater if required.

In some applications, such as the desalination of a well water, suspended
solids and colloids may be negligible and the pretreatment section may consist
of PH adjustment and addition of a threshold inhibitor. At the other extreme
is the desalination of surface waters, municipal wastes and industrial wastes
which may require all of the steps outlined above.
The following techniques can be used to remove suspended solids or to miti-
gate the effects of colloids:
(1) If the feedwater is a municipal water supply or well water with an
SDI of 6.0 or below it may only be necessary to filter the feed.
There are a number of filters available that have been successfully
employed and these are: diatomaceous earth, single media, dual
media and mixed media. A properly designed filter in these applica-
tions should be able to reduce the suspended solids load to zero.
(2) In-line coagulation may be used to reduce colloidal membrane
fouling. In this technique, coagulant aids such as alum, ferric chlo-
ride and/or a polyelectrolyte are added to the feed stream prior to
filtration. It is necessary to conduct on-site jar tests in order to
determine which coagulant aid is effective and what is the proper
dose for that coagulant aid.
(3) In the case of a feedwater with a high suspended solids, turbidity
and/or SDI, it may be necessary to use conventional coagulation
followed by sedimentation prior to filtration. Again, on-site jar
tests are required to determine the proper coagulant aid, dose and
settling rate.
The feedwater pH is usually adjusted to a pH of between 4 and 6 for the
following reasons:
(1) It is required (usually as a condition of warranty) to minimize the
rate of hydrolysis of the cellulose acetate ester. Cellulose acetate
hydrolysis reduces the useful life of the membrane by increasing
the flux and reducing the rejection of the membrane.
(2) The flux and rejection of some composite membranes are a func-
tion of pH and the optimum pH is between 4 and 6.
(3) Many natural waters are saturated in calcium carbonate which is
highly rejected by the membrane. Consequently, it is concentrated
in the feed/reject stream during the reverse osmosis process and it
will precipitate on the membrane decreasing the flux and rejec-
tion. Lowering the pH of the feedwater to between 4 and 6 con-
verts some of the carbonate or bicarbonate ions to carbon dioxide
and this prevents carbonate precipitation.
Some feedwaters contain compounds in addition to calcium carbonate that
may become saturated in the feed/reject stream and when that stream is concen-
trated in the reverse osmosis process, these compounds will precipitate on the
membrane with a resultant loss of membrane performance. Some of the more
sparingly soluble compounds of concern are:

Reverse Osmosis 285
Calcium sulfate Barium sulfate
Calcium phosphate Strontium sulfate
Calcium fluoride Silica
One method of preventing precipitation is to operate the reverse osmosis
unit at a recovery which will not concentrate the feed/reject stream to the com-
pound saturation level. Another method of preventing precipitation is to add a
threshold inhibitor, such as sodium hexametaphosphate, certain polyacrylates,
organophosphates or phosphonates, to the feedwater. The threshold inhibitors
are added at the rate of 1 to 5 mg/Q of feedwater, and they serve to disrupt the
formation of a crystalline precipitate during the residence time of the feedwater
in the reverse osmosis unit. In doing this, they broaden the solubility limits of
the sparingly soluble compounds.
As noted above, the polyamide and polyurea membranes cannot tolerate an
oxidizing agent, such as residual chlorine in the feed. Consequently, if these
membranes are used and the feed has a residual chlorine content, then it is
necessary to remove it. This is usually done by adding a stoichiometric excess of
sodium bisulfite or sodium thiosulfate in accordance with recommendations
from the membrane manufacturer.
Even with the best of pretreatment schemes, membrane elements will foul
over a long period of time and they must be cleaned. Each membrane packaging
configuration has a different degree of susceptibility to fouling and an ease of
cleaning.
The tubular element is the least susceptible to fouling and the easiest to
clean. This element is widely used in ultrafiltration applications where the proc-
ess streams contain suspended solids. The tubular element can be cleaned not
only by chemical action but also by mechanical means. A sponge rubber ball is
pumped through the tubular element with the chemical cleaning solution to
scour the membrane surface.
The spiral wound element requires less pretreatment than the hollow fine
fiber element or, stated in another manner, it is less susceptible to fouling. For
instance, the hollow fine fiber has been tested and found to foul excessively on
municipal wastewater reclamation applications while the spiral wound element
has been operated successfully. The spiral wound element must be cleaned with
chemical solutions, but there are no mechanical means available to clean this
element.
The hollow fine fiber element, with the great number of close packed fibers,
is an effective filter in itself. Consequently, it is the most easily fouled mem-
brane configuration and requires the most pretreatment. The hollow fine fiber
element can be cleaned with chemical cleaning solutions, but it is not amenable
to mechanical cleaning. It is also more difficult to clean than the spiral wound
element.
Element manufacturers have developed cleaning solutions for their specific
membranes and elements. Generally, there are two types of cleaning solutions-
one for removal of organic foulants and another for removal of metal hydrox-
ides. A cleaning system should be provided with each reverse osmosis system and
the cleaning system should consist of a tank to mix the cleaning solution and a
pump with associated piping, valves and instruments to circulate the cleaning
solution through the reverse osmosis elements.

Reverse Osmosis Section
Once the pretreatment study had been completed, it will be possible to de-
cide on the type of elements to be used in the reverse osmosis unit. If the SDI
of the pretreated feed is 3.0 or less, then either the spiral wound or hollow fine
fiber elements can be used. The choice will depend on economics (element
price) and desalination characteristics (flux and rejection). If the pretreated feed
SDI is more than 3.0, then the spiral wound element should be used. When the
decision as to element type is made, then it is appropriate to forward a copy of
the pretreated feed water analysis to reverse osmosis element manufacturers to
obtain a prediction of product water quality, recommended type of element,
total number of elements required, possible problems with sparingly soluble
compounds in the feedwater, allowable recovery, and price and delivery.
Figure 4.13 shows a flow diagram for a reverse osmosis unit with 75% re-
covery on a brackish feed. The pretreated feed is routed to the high pressure
pump where the feed pressure is raised to between 250 and 400 psig as required
for brackish water desalination. The pressurized feed is then pumped to the first
pass pressure vessels where about 50% of the feed .is recovered as product and
50% is reject. The reject from the first passpressure vesselsis then routed to the
second pass pressure vessels where, again, about 50% of the first pass reject is
recovered as product and 50% is reject which is sent to waste. Thus, the over-
all recovery of the unit is 75% as product. As can be seen, a normal array for a
75% recovery unit is two first pass pressure vessels feeding one second pass
pressure vessel or a 2-I array. If the system recovery were from 40 to 60%,
all of the pressure vesselswould be in parallel. However, if the system recovery
were raised to between 85 and 90%. the pressure vesselswould be arranged in
a 4-2-l array.
pace”cT WATER - 250.000 QPO
+-A
Typl.Xl
SECOND PASS
REJECT
83.000 GPD
Figure 4.13: Reverse osmosis system.
The quality of the product water is a function of the rejection asshown in
the following equation:

Reverse Osmosis 287
cp = Cf * (1 - R)
where Cp = Concentration of the product
Cf = Concentration of the feed
R = Rejection
This equation produces accurate results for a membrane sample or a small ele-
ment with a low recovery, e.g., 2% or less. However, a practical reverse osmosis
system is designed to recover from 25 to 90% of the feedwater. This means that
the concentration of the feed varies throughout the membrane system. At 90%
recovery, the initial membranes will have a feed which is about IO times less
concentrated than the feed to the final membranes and the quality of the prod-
uct water will vary incrementally throughout the system. The product water
from the first membrane elements will be less concentrated than the product
water from the last elements. The product water from the practical reverse os-
mosis system is combined in the product water manifold and its concentration
is usually represented as the average product water concentration. The average
product water concentration is determined by the following formula:
i?$ = E”(l-R)
where cp = Average or total product water concentration
B = Average concentration of the feed and reject streams.
The average concentration of the feed and reject streams as represented by the
following equation is not truly representative:
7=~ = feed concentration + reject concentration
2
As Figure 4.13 shows, there are more elements in the first passof a 75% re-
covery reverse osmosis system than in the second passand the first passelements
produce more product. Thus, a greater percentage of the total product water is
derived from the first pass and the total product water will be nearer in concen-
tration to the first passaverage than to the second passaverage.
Saltonstal114 has derived an equation to determine the average concentration
of the feed and reject streams accurately and this equation is as follows:
_ (1 - Y)(l - R)
(1 - R) Y 1
where m, Cf and R are defined above and Y = system recovery.
The concentration of the total product then becomes:
cp = Cf *
C
1 - (1 - Y)(’ - R, + (1 _ R)
(I - RI Y 1

Thus, the product water concentration is dependent on the feed concentra-
tion, membrane rejection and system recovery. Figure 4.14 shows the dependence
of product concentration on these variables. It is possible to use a lower rejec-
tion membrane and a high recovery with a low TDS feedwater. On the other
hand, a high TDS feedwater, such as seawater is limited to a low recovery and
requires a high rejection membrane.
500
_ 400
.
ol
E
;
t-_ 300
;:
z
z 200
3
:
a
a 100
I I I I I
20 40 60 60 100
RECOVERY, %
Figure 4.14: Product water vs. recovery.
The membrane flux is also dependent on recovery. The average feed/reject
concentration increases with pressure and, since the osmotic pressure is propor-
tional to concentration, the average osmotic pressure will increase with increased
recovery. At constant applied pressure, the membrane flux will decrease with in-
creased recovery. Figure 4.15 shows the increase in average osmotic pressure and
the decrease in flux for a membrane which would produce 20 gfd at zero recov-
ery operating on a feed of 5,000 mg/Q at 450 psi applied pressure. Concentration
polarization is assumed to be constant at all recoveries.

26
20
16
10
6
0
I-
O
Reverse Osmosis
FEED - 6.000; m2ll
MEMBRANE REJECTION - 96%
NET PRESSURE - 400 PSI
I I I I
20 40 60 SO 1
RECOVERY, %
‘160
-120
*QO
-80
-30
*o
0
Figure 4.15: Flux and average osmotic pressure vs. recovery.
It is noted that the osmotic pressure of seawater is about 400 psig and an
operating pressure of between 800 and 1,000 psig is required to obtain optimum
flux and rejection. If a recovery of only 50% were used, the reject osmotic pres-
sure would be in excess of 700 psig and the average osmotic pressurewould be
about 550 psig. This would present problems in obtaining a potable product of
500 mg/Q or less. Consequently, most seawater desalination reverse osmosis units
are operated at a recovery of from 25 to 35%.
Since lowering of the feed pH usually means that the feed contains signifi-
cant quantities of carbon dioxide, the feed and reject streams are extremely cor-
rosive. In addition, the membrane does not reject carbon dioxide and, conse-
quently, the product stream contains about the same concentration of carbon
dioxide as the feed and reject streams, i.e., all three process streams are corro-
sive. General practice is to use PVC pipe and valves for low pressure piping (less
than 100 psig) and 316 stainless steel pipe and valves for the high pressure
(above 100 psig) process streams. The high pressure pump wetted parts should
be either 316 stainless steel, corrosion resistant plastic or other corrosion resis-
tant materials. The pressure vessels are fabricated with fiberglass reinforced plas-
tic which is corrosion resistant.
The reverse osmosis process is relatively simple and instrumentation re-
quirements are minimal. Following is a list of the minimum recommended para-
meters to be measured in a reverse osmosis system:

Feed
FlOW
Temperature
Pressure
Conductivity
PH
Product Other
Flow Differential Pressure
Conductivity across each array
A variety of control schemes can be incorporated in the design of a reverse
osmosis plant. However, this subject is beyond the scope of this manual.
Posttreatment Section
In most plants that use reverse osmosis in the preparation of processwater,
the reject stream is routed directly to waste discharge without any additional
posttreatment. In industrial plants that use reverse osmosis to treat industrial
wastes, the reject stream may contain valuable materials and this stream would
be sent back to the process. In other applications of industrial waste treatment
by reverse osmosis, the reject stream may require additional treatment prior to
ultimate discharge. In this case, the reverse osmosis unit will have provided a
large volume of water that is disposable or can be reused (the product) and a
smaller volume of reject which can be treated more economically.
The product water from a reverse osmosis unit will have a low pH and most
probably a high concentration of carbon dioxide. The carbon dioxide can be
removed and the pH of the product increased by use of a decarbonator. A de-
carbonator is a packed column in which product water is introduced at the top
while either forced or induced air is introduced at the bottom. The air and water
flow countercurrently over and around the column packing. The carbon dioxide
is stripped from the water and exits from the decarbonator at the top in the air
stream. In a well-designed decarbonator, the carbon dioxide content can be re-
duced to about 5 mg/Q in the water effluent.
INDUSTRIAL REVERSE OSMOSIS AT A REFINERY
One of the most innovative industrial uses of reverse osmosis is at the
Petromin Refinery in Riyadh, Saudi Arabia. The refinery takes an unusable
municipal wastewater, secondary effluent from the Riyadh sewage treatment
plant, and by using lime clarification, filtration, reverse osmosis and ion ex-
change demineralization, it converts that useless waste into the entire process
water requirements for the refinery. Figure 4.16 is the process flow schematic
for the refinery water treatment plant.
Chlorinated secondary effluent arrives at the pumping station, which is
adjacent to the sewage treatment plant in Riyadh, through an open channel
which flows to the pump station inlet basins. The 4.63 MGD of effluent is then
pumped 19 kilometers to the refinery by either of two full capacity pumps
which take suction from the inlet basins. The effluent is pumped through a

2
2
t
Sludge Thickener Overflow
:
t
Slaked Lime
2 9
Ion Exchange Regenerant
Iii d 0.35 MOD
c
r I I
I
: 4 SURGE PONDS ’
RAPID MIX, COAGULATION
t
Eo,_ ,
8.04 MGD + 6 CLARIFICATION 5.02 MGD.
“E
1.500 mgli 1,100 mgli
$z Plant Use Return
0 m)o” 4
Sludge to Thickener
= ?a
1.08 MOD D
: 0 d
Backwash 0.37 MOD
I
rz
0.13 MOD
c
PRIMARY
Reject
1.12 MGD-
REVERSE OSMOSIS
SYSTEM
I
4.27 MOD
FILTRATION
* 5.48 MOD
h
1
z A
2
:
P
4
i ‘g’p;ke
: z
General Plant Use’
L
; gz
SO mg/iv 0 : d 1.08 MOD
0 I;
SECONDARY
Regenerant
+
z0
REVERSE OSMOSIS.
0.02 MGD + :
IL
(0
z, .
SYSTEM
1.13 MOD )
z
10 mgli 0 ;
=
:
B ION EXCHANGE
=
mo
or
D
1.12 MOD SYSTEM
0
0
I-
- 10 mg/i
Figure 4.16: Industrial reverse osmosis at a refinery.

24:inch diameter cement lined carbon steel pipeline. The pipeline is equipped
with vacuum relief valves and air-release safety valves at the two highest Points
ln the line plus air-release safety valves at the third highest point in the line.
The pipeline is also equipped with pigging facilities (for pipeline cleaning) which
consist of 2 pigs, a pig launcher and pig receiver.
On arrival at the refinery, the effluent is discharged into a 2 compartment
splitting box which divides the flow evenly for discharge into the two on-site
surge ponds. The effluent flows from the splitting box by gravity into the surge
ponds. At the same time, miscellaneous refinery plant streams and backwash
from the refinery water treatment plant filter are returned to the surge ponds
and mixed with the secondary effluent. The return stream flow is estimated to
be 1.19 MGD with a quality similar to the secondary effluent. Each of the surge
ponds is a concrete lined, earth basin with a capacity of 5.3 million gallons. The
surge ponds have a static air diffusion system to prevent sewage septicity and im-
prove homogeneity in the secondary effluent. Chlorine can be added to the sec-
ondary effluent either at the sewage treatment plant pump station or at the
surge ponds at the refinery, if required.
All surface waters and municipal effluents contain suspended solids as well
as dissolved solids and the presence of suspended solids dictates the need for a
pretreatment section. Experience has shown that effective removal of the sus-
pended solids in pretreatment is a prerequisite to efficient reverse osmosis mem-
brane performance. Suspended solids in secondary effluent are primarily organic
in nature and, due to their small size, it is difficult to remove them by settling.
Therefore, it is necessary to aggregate the smaller particles into larger particles
which can more easily be removed by settling and filtration.
For some time, it has been known that suspended solids or colloids in water
possessan electrical charge which is predominantly negative. As a result of this
electrical charge, the colloidal particles tend to repel each other and thereby
limit the potential for aggregation and the colloidal suspension is said to be sta-
bilized. Before the SLQended particles can coalesce, the stabilizing forces must
be neutralized and this is usually done by adding chemicals. Trivalent aluminum,
ferric hydroxides, and/or lime have been used to destabilize colloidal suspensions
and allow suspended particles to grow in size. Another means of particle destabi-
lization is the use of natural or synthetic polymers which are long chain, high
molecular weight polyelectrolytes with many active sites. The negatively charged
colloids are adsorbed on the active sites with the resultant coagulation or growth
of the particles.
The refinery clarification equipment has the capability of adding any of the
chemicals mentioned above. However, lime clarification was chosen asthe method
to be used. The combined secondary effluent and the plant return streams (5.64
MGD) are pumped from the surge ponds to the rapid mix basin in the clarifier.
The rapid mix basin has two compartments in series and each compartment has
a high speed mixer. Lime and sodium hydroxide are added to the first compart-
ment and these are vigorously mixed with the secondary effluent in both com-
partments. As a result, the pH of the effluent from the rapid mix basin is raised
to between 10.8 to 11 .O. At this pH, much of the bicarbonate in the water re-
acts with the lime and forms an insoluble calcium carbonate and the magnesium
in the water reacts with hydroxyl ions to form insoluble magnesium hydroxide.
The water flows from the rapid mix basin to the flocculation basin which
has four compartments. Each compartment is further divided into three sec-

Reverse Osmosis 293
tions and all twelve sections are equipped with a slow speed turbine mechanism.
The gentle agitation provided in the flocculation basin promotes contact be-
tween the calcium carbonate, magnesium hydroxide and suspended matter
which results in the formation of larger particles.
The water then flows by gravity from the flocculation basin to the sedimen-
or clarification basin. The enlarged floe particles settle to the bottom of this
basin and they are removed as a 1% sludge by three circular rakes into a sludge
basin or hopper. This amounts to about 0.37 MGD of sludge which is pumped to
the sludge thickener tank. The sludge is thickened to about 6% in the sludge
thickener and then filtered in a plate and frame filter pressto about 50% solids.
The filtered sludge may then be transported to a a landfill for disposal or sent to
the on-site multiple hearth lime kiln for regeneration. Operation of the lime kiln
is economically marginal. The overflow from the sludge thickener and the filter
press filtrate are returned to the rapid mix basin for reclamation. In addition,
part of the water used in the regeneration of the ion exchange demineralizers is
also returned to the rapid mix basin for recovery. The total amount of water
from these three sources is 0.35 MGD with variable quality.
Effluent from the clarifier is saturated in calcium carbonate and this would
precipitate on the filter media to clog the filter which is the next step of pre-
treatment. Consequently, the clarifier effluent flows by gravity to the serpentine
recarbonator basin where carbon dioxide is added to reduce the pH to between
7.5 and 8.0. The insoluble calcium carbonate and magnesium hydroxide are con-
verted to soluble calcium and magnesium bicarbonate in the recarbonator basin.
The 5.62 MGD of effluent from this basin has a suspended solids concentration
of about 2 mg/R and a TDS of about 1,100 mg/Q.
Following lime clarification and recarbonation, the treated water is pumped
to the fire water storage tanks (0.14 MGD) and the remainder (5.48 MGD) is
pumped to the water treatment plant cooling tower. The cooling tower is used
to reduce reverse osmosis feedwater temperature when required. For a large
part of the year, the cooling tower is inoperative. A static mixer is installed in
the line upstream of the cooling tower. At this point, sulfuric acid can be added
to the process stream to reduce the pH of the water in the event that carbon di-
oxide is unavailable to the recarbonation system. Chlorine can be added to the
process water at the cooling tower influent line or at the chlorine diffuser of the
filter aid basin. The water flows by gravity from the cooling tower to the filter
aid mix station where it is possible to add coagulant aids to assist in the removal
of suspended solids during filtration. Coagulant aids are not being added at the
filter aid mix basin at this time. The water then flows by gravity to the dual
media filters.
There are four separate filter basins with filter media as described in Table
4.7 below.
Table 4.7: Filter Media Description
Media
Coal
Height
mm Inches Size, mm
so0 31.5 0.6 - 1.6
Sand 400 15.7 0.6 - 1.0
Gravel Support 400 3 - 35

The process water flows through the filter media by gravity to remove the re-
maining suspended solids. It then flows through a flow control valve to the fil-
tered water reservoir by gravity. Backwash water overflows from the filtered
water reservoir to the wash water storage tank. The filter effluent of 5.48 MGD
provides 1.06 MGD of general use water to the refinery, 0.13 MGD of filter
backwash water, 0.02 MGD of filtered water to the lime slakers at the rapid mix
basin and the remaining 4.27 MGD flows by gravity to the reverse osmosis de-
chlorination basin.
Chlorine has been added to the feedwater upstream of reverse osmosis pre-
treatment. However, since chlorine will depolymerize the polyurea membrane
barrier layer in the spiral wound element, with subsequent loss of desalination
properties, the chlorine is removed in the pretreatment system dechlorination
basin. This removal is chemically accomplished by the addition of sodium bi-
sulfite. The chlorine level in the influent and effluent to the dechlorination basin
is continuously monitored. The feedwater is then transferred from the dechlor-
ination basin to the cartridge filter feed pumping station by gravity flow and it
is then pumped to the cartridge filters.
Sulfuric acid and sodium hexametaphosphate (SHMP) are injected in the
feedwater line upstream of the cartridge filters. The sulfuric acid is injected to
adjust the feedwater pH to a level of between 4 and 6. The purpose of acid in-
jection is twofold. The primary purpose is to mitigate the possibility of calcium
carbonate deposition by conversion of bicarbonate to carbon dioxide. Coinci-
dentally, the rejection performance of the thin film composite membrane is pH
sensitive and the optimum performance is at the operating pH level. SHMP is
added to the feedwater as a threshold inhibitor to inhibit the crystalline growth
of sparingly soluble salts such as calcium sulfate.
After chemical addition, the feedwater is routed to cartridge filters which
serve to mix the chemicals which have been added upstream and to insure that
any particles that may have escaped the gravity filters, such as sand or other par-
ticulate matter is removed. In general, the cartridge filters do not improve the
quality of the reverse osmosis feedwater to a large degree and they are not in-
tended as continuous duty filters. The effluent from the cartridge filters is
routed to the primary reverse osmosis feed pump wet well.
The primary reverse osmosis system contains five trains of equal capacity.
Each train is operated independently and there is a vertical turbine high pressure
pump for each train mounted on the primary feed pump wet well. The high pres-
sure feedwater is pumped to the reverse osmosis train where the operating pres-
sure of each train is adjusted by the pressure control valve upstream of the spiral
wound element pressure vessels.The elements are located in fiberglass reinforced
plastic pressure vesselswith six elements per pressure vessel. The elements have
nominal dimensions of 40 inches in length by eight inches in diameter and they
contain composite membrane with a polyurea membrane barrier layer. Each pri-
mary train has 180elementsin 30 pressurevesselswhichare manifolded in a20-10
array. The pressurized water is fed to 20 pressure vesselsin parallel where about
50% of the feed is recovered as product and the reject from these pressure ves-
sels is manifolded and fed to 10 pressure vesselsin parallel. About 50% of the
feed to these latter pressure vesselsis recovered as product and the reject is man-
ifolded through the flow control valve to waste. The product water from the
primary reverse osmosis system is transferred to the cooling tower makeup and

Reverse Osmosis 295
desalt water storage tanks or to the secondary reverse osmosis system feed pump
wet well. The overall recovery for each train is 75% and the design capacity of
each train is 0.83 MGD of product for a total product water capacity of 4.10
MGD in the first stage. The daily requirements for product water are 3.35 MGD
so there is an installed spare capacity of about 20%. The product water require-
ments are 2.02 MGD for the refinery cooling towers and the desalter and 1.33
MGD for the second stage reverse osmosis system. The reject (1.12 MGD) from
the first stage reverse osmosis system is sent to the on-site solar evaporation
pond for disposal.
The first stage product water concentration of 80 mg/Q is adequate for cool-
ing tower makeup and desalter process water but it is not pure enough for mod-
erate pressure boiler feed. Consequently, the first stage product must be further
treated with ion exchange demineralization to achieve the desired purity. A sec-
ond stage of reverse osmosis is used at the refinery to pretreat the first stage
product prior to ion exchange demineralization. An improved quality feed to the
ion exchangers will improve not only the quality of ion exchanger effluent but it
will reduce the quantity of regenerative chemicals required by the ion exchangers.
The secondary reverse osmosis system contains three trains of equal capac-
ity. As in the primary reverse osmosis system, each train is operatred independ-
ently and there is a vertical turbine high pressure pump associated with each
train. The pressurized feedwater is pumped to the secondary trains where the
operating pressure of each train is adjusted by the pressure control valve up
stream of the element pressure vesselswhich are fabricated from fiberglass rein-
forced plastic. Each secondary train has 21 pressure vessels (126 elements)
which are manifolded in a 12-6-3 array. The spiral wound elements in the sec-
ondary trains are identical to the elements in the primary stage. The pressurized
feedwater is fed to the first 12 pressure vesselsin parallel where about 50% of
the feed is recovered as product and the remaining 50% is manifolded and fed
to the next six pressure vessels in parallel. Product recovery in these pressure
vesselsis again about 50% and the reject is manifolded and fed to the last three
pressure vesselswhere about 40% of the feed is recovered as product. The re-
ject from the last three pressure vesselsis manifolded through the flow control
valve in each train to the dechlorination basin. The reject flow of 0.20 MGD
from the secondary reverse osmosis trains is of better quality than the feed to
the primary reverse osmosis system and, thus, it is used as feed to the primary
system. The overall design recovery of each of the secondary trains is 85%. Each
of the trains is rated at 0.57 MGD for a total capacity of 1.71 MGD. The total
required capacity is 1.31 MGD which can be produced by two trains with one
train in standby. The recovered product is routed to the forced air decarbonators
in the ion exchange demineralization system.
The product water from the secondary reverse osmosis system contains a
high concentration of carbon dioxide as a result of pH adjustment in the pri-
mary reverse osmosis system which converts bicarbonate and carbonate alkalin-
ity to carbon dioxide. Since the membrane is “transparent” to carbon dioxide,
it passesthrough both the primary and secondary reverse osmosis systems into
the secondary system product water. Although the ion exchange resin would
remove the carbon dioxide, it is more economical to do so in the decarbonator.
There are two degasifiers (decarbonators) in the refinery water treatment
plant and each of these are in parallel and normally in operation, i.e., one de-

gasifier is not used as a standby. The secondary product is fed to the top of the
degasifiers where it is allowed to cascade over the degasifier packing. Air is
blown through the packed section from the bottom and it rises in the packed
column countercurrent to the water. It is estimated that 0.01 MGD of product
water are lost to evaporation in the decarbonator. Since the water is at a low pH,
carbon dioxide is transferred from the liquid phase to the gas phase. The result
is that the carbon dioxide concentration in the water is reduced to about 5 mg/ll.
Two transfer pumps, both full capacity, are installed to pump the decarbonated
water through the ion exchange system.
There are two full capacity ion exchange trains in the system and each train
consists of a cation tank, an anion tank, instrumentation, and the necessary
valves and piping for process control. The ion exchange demineralization is ac-
complished in a two-step process involving treatment with both cation and anion
resins in separate process vessels. The water is first passed through the strong
acid cation exchange resin (Amberlite IR-20) to exchange the cations for the hy-
drogen ion. The effluent is then passed through a Type I, strong base anion ex-
change resin (Amberlite IRA-410) where the anions are exchanged for the hy-
droxide ion. The result is that cations and anions are substituted by water mol-
ecules and a high purity effluent is available for use as boiler feedwater. When
the ion exchange capacity of one train is depleted, this train is removed from
service, the standby train is placed in operation and the depleted train is regen-
erated with acid and caustic. About 0.02 MGD of decarbonator effluent are used
for regeneration of the resins and much of this is returned to the rapid mix ba-
sins for recovery.
REVERSE OSMOSIS AND ION EXCHANGE
The preceding example of a reverse osmosis industrial application at a re-
finery showed that the process is capable of:
(1) treating a feedwater with high suspended solids and dissolved sol-
ids concentrations;
(2) reclaiming a water that is considered by most as unusable; and
(3) developing a number of process streams with different quality
requirements.
Reverse osmosis also has been used to treat municipal water supplies for in-
dustrial purposes even though these supplies are generally low in turbidity, sus-
pended solids and dissolved solids. A large number of reverse osmosis systems
have been installed in industrial plants to prepare industrial process water with
municipal water as the feed source. A significant number of these industrial ap-
plications are to either replace ion exchange demineralization or to pretreat
municipal supplies prior to ion exchange demineralization.
Reverse osmosis systems now commercially available will remove 95% or
more of the dissolved solids normally removed in ion exchange, and as will be
discussed later, a few that are not. The ionized solutes are not all removed to
the same degree by reverse osmosis any more than ion exchange resins have the
same effect on all solutes. Divalent and multivalent ions, such as calcium, mag
nesium, sulfate, iron and manganese, can be rejected to greater than 99%. So-

Reverse Osmosis 297
dium, potassium and chloride are normally rejected to the 95% level or better.
The net effect is to reduce the number of regenerations required of the ion ex-
change columns by a factor of 20 or more. This results in a significant reduc-
tion in the amount of waste regenerant solutions that must be disposed of and
a material reduction in the dissolved solids that might normally be discharged
to the environment. A concentrate or reject is produced by reverse osmosis,
but there is little change in the environmental salt budget. Concommitant re-
sults are a major decrease in the space and equipment necessary for regenerant
storage and an extension of the useful life of the resins, owing to reduced resin
attrition. In addition, substances difficult to remove from the resin, which also
affect its performance, are greatly reduced or are removed by reverse osmosis
pretreatment.
When reverse osmosis is used for preliminary demineralization, the variation
in the quality of the ion exchange demineralized water is reduced. The amount
of dissolved solids in the feed to the ion exchange beds is 5% or lessthan when
the raw water is fed directly. It is, therefore, obvious that the variation in solids
in the finished water will also be less when breakthrough occurs. As a result,
the resins may be utilized more efficiently. Additional reliability and control
can be gained by measuring the solids concentration following reverse osmosis
treatment. This precaution virtually eliminates shock loadings on mixed bed
polishing columns. Where small point of use polishing columns are used, such
as in microelectronics manufacturing, the danger of a rapid breakthrough be-
comes considerably reduced, and there is an improvement in production and
product quality.
An important factor to be remembered is that in some caseswater supplies
unsatisfactory for processing to high purity water may be the only sources avail-
able. Preliminary demineralization by reverse osmosis will make this water suit-
able for subsequent demineralization by ion exchange. It is thus apparent that
such an economically important factor as plant site location, which may be de-
pendent on the availability of suitable water, can be made more flexible through
the use of reverse osmosis. It may be possible now to utilize seawater as a source
of industrial processwater.
During the 1960’s, reverse osmosis was compared with other methods of
demineralization. It was indicated in these comparisons that reverse osmosis
could not compete favorably with ion exchange at dissolved solids concentra-
tions below 700 mg/P and that its most favorable area of use would be from
about 1,200 to 5,000 mg/Q dissolved solids. This idea has been totally refuted
because some of the most successful applications of reverse osmosis, particularly
as part of the process to produce high purity water, have been in treating low
dissolved solids water. Water containing 200 mg/Q dissolved solids or less has
been treated at costs equal to or lower than those of ion exchange alone.
Two substances that are frequently of concern in ion exchange demineraliz-
ation are silica and organics. The organics are frequently present in natural wa-
ters as aromatic polycarboxylic acid derivatives known as humic and fulvic acids.
Silica may be the limiting factor in the efficiency of the anionic resins, and (par-
ticularly in boiler feedwater applications) the lower the concentration before
ion exchange demineralization, the better. Reverse osmosis will frequently pro-
duce 90% or greater reductions in total silica concentrations. However, perform-
ance should be tested on the specific water to be treated since me results can be
variable and the reason for differences between waters is not yet understood.

298 Handbook of industrial Membrane Technology
Where the silica concentrations in the raw water are high, reverse osmosis has
been most effective. Even in trace quantities, humic and fulvic acids have been
responsible for impairing the life of anionic resins and affecting performance of
ion exchange columns. These organics are readily removed by reverse osmosis
membranes.
REVERSE OSMOSIS AND POLLUTION CONTROL
Pollution control legislation has made industry aware of the economic pen-
alty for inefficient use of raw materials. The plating industry was one of the
first industries to experience this enforced awareness. Loss of raw materials in
this industry” can result in five distinct costs:
(1) replacement of the plated material that was discharged to waste
prior to initiation of pollution control practices;
(2) removal of that material from the wastewater after pollution con-
trol enforcement;
(3) disposal of the residue from item 2;
(4) replacing of the process water (at times quite expensive) lost in
wastewater; and
(5) treatment of the wastewater in a publicly owned treatment works
after discharge into a sewage system.
In response to the plating industry’s increased awareness of the above costs,
plating shops have modified their practices to reduce their material and eco-
nomic losses. While pollution control was greeted by many with coolness, the
resultant cost savings have shown that pollution control is not as onerous as ex-
petted .
Reverse osmosis has been installed in many plating shops as a way to re-
source recovery and minimizing the size of waste treatment equipment and vol-
ume. Reverse osmosis is particularly suited to waste treatment in the plating in-
dustry because most of the toxic ions in the plating solutions are well rejected
by commercially available membranes. Table 4.8 shows the rejection ranges of
some of the more common toxic materials in the plating industry.16
Table 4.8: Reverse Osmosis Rejection Range
Ion-
Nickel
Cower
Cadmium
Chromate
Cyanide
Zinc
RejectionRange,X
98 - 99
98 - 99
96 - 98
90 - 98
90 - 95
98 - 99

Reverse Osmosis 299
The nickel plating industry is a typical candidate for the use of reverse os-
mosis in pollution control. Figure 4.17 shows a schematic of this industrial ap-
plication. The workpiece travels from the plating bath with a concentration of
270,000 mg/Q to the rinse tanks. There are three rinse tanks in series and rinse
water flows countercurrent to the workpiece. The work piece drags out plating
bath to the first rinse tank, first rinse tank solution to the second rinse tank and
second rinse tank solution to the third rinse tank. Consequently, the first, sec-
ond and third rinse tanks have concentrations of 3,000 mg/Q, 333 mg/ll and 37
mg/ll, respectively.
About 100 gallons/hour (GPH) are pumped from the first rinse tank through
a cartridge filter and into a reverse osmosis unit. The reject stream contains 99%
(59,400 mg/Q) of the nickel in the feed stream with 1% (32 mg/Q) remaining in
the product stream, The reject stream is routed through an activated carbon col-
umn to the plating bath. The reverse osmosis product stream is combined with
5 GPH of tap water makeup, which is added to compensate for surface evapora-
tion in the plating tank, and the combined stream is returned to second rinse
tank. The waste stream (10 GPH) is sent to waste treatment which is a precipi-
tation process.
WORKPIECE
Figure 4.17: Reverse osmosis in nickel plating.
REVERSE OSMOSIS AND SEAWATER DESALINATION
It has been estimated that the oceans cover about 70% of the earth’s area
and contain about 80% of the water on or in the earth. Seawater concentra-
tion varies from 25,000 mg/!.?of total dissolved solids up to over 50,000 mg/ll
in the Arabian Gulf. The average composition of seawater is about 35,000 mg/lZ
with the major ions being shown in Table 4.9.

Table 4.9: Major Constituents of Seawater
Constituent Concentration, mg/l
Sodium 10,500
Magnesium 1,350
Calcium 400
Potassium 380
Chloride 19,000
Sulfate 2,700
Bicarbonate 142
Bromide 65
Other Ions 33
Total 34,570
While seawater is abundant, it is to a large degree unusable and it is the con-
stant dream of the desalination industry to desalt seawater in an economical
manner that would allow the product to be used for agricultural purposes. This
goal is a long way from reality. On the other hand, distillation and now reverse
osmosis are desalting seawater efficiently enough to be used for preparing pot-
able waters in affluent areas. Seawater, with a concentration of 35,000 mg/P, has
an osmotic pressure of almost 400 psig and this mandates a membrane element
with high pressure capabilities. If the reverse osmosis seawater plant were re-
quired to deliver a product water of potable quality (500 mg/Q or less), then the
membrane element would require a minimum rejection of 98.6% to attain that
product quality at 0% recovery. At higher recoveries, both pressure and rejec-
tion must increase to obtain potable quality. Spiral wound and hollow fine fiber
elements have been developed with the capability of operating at 800 to 1,000
psig and sodium chloride rejections up to 99.5%. This capability has placed re-
verse osmosis in the seawater desalination business and reverse osmosis is be-
ginning to replace distillation as the result of the clear cut economic advantage
of reverse osmosis.
An example of reverse osmosis seawater desalination for industrial purposes
is the system installed in a thermoelectric power plant in Venezuela in 1980.”
The original segment of the plant is designed to produce 800,000 GPO of boiler
feedwater and potable water. A process flow diagram for this system is shown in
Figure 4.18.
Seawater is pumped from an intake channel to the roughing filters which
are the first part of the pretreatment process. Ferrous chloride and chlorine
are added to the filter influent line. The chlorine disinfects the seawater and oxi-
dizes the ferrous ion to the ferric ion which forms insoluble ferric hydroxide.
The ferric hydroxide acts as a coagulant aid. A large Percentage of the coagu-
lated particles are removed in the roughing filter which reduces the feedwater
Sol from 15 to about 3. There are five gravity roughing filters which have a dual
media of sand and anthracite. Four of the filters are in operation while one iS

Reverse Osmosis 301
in standby to be placed in operation when one of the operating filters is back-
washed.
The effluent from the roughing filter clearwell is pumped to the pressurized
polishing filters which further reduce the SDI to less than 3. There are four op-
erating polishing filters and one standby filter. The filter media is anthracite
and green sand.
Sodium bisulfite is added to the effluent from the polishing filters to re-
move residual chlorine which would be harmful to the polyamide reverse os-
mosis membrane. Sulfuric acid is then added to the filter effluent to adjust the
pH to about 6.5 and the pretreated water is routed to the primary reverse os-
mosis system.
The first stage reverse osmosis systems consist of four trains which are cap-
able of producing 200,000 GPD each of product water. Each train consists of a
5-g cartridge filter with a high pressure pump feeding two subunits of 25 DuPont
B-10 permeators each plus the necessary piping, valves and instrumentation. The
first stage system reverse osmosis units are operated at 30% recovery with a
pump discharge pressure of 900 psig. The total dissolved solids content of the
feed was reported to be 37,000 mglR and the product concentration from this
stage was 490 mg/Q. This product water is sent to a 262,000-gallon storage
tank which provides feed to the two second stage reverse osmosis trains and it
is the source of potable water for the power plant. The reject from the first
stage is routed to waste. A high pressure recovery turbine is not used and a con-
siderable amount of energy is wasted.
From Seawater Intake
I FILTERS
I FIRST STAGE
I
REVERSE OSMOSIS ti
3.407 kgpd
30% RECOVERY
37.000mg11
; 800 kgpd
:
0
4gOmgll
-
a.
SECOND STAGE
I
Potable Water
REVERSE OSMOStS c;
85% RECOVERY
867 kgpd
133 kggd
4gOmgtt HOLDING 490 mgll
TANK
Figure 4.18: Power plant process water from seawater.

The first stage product quality meets potable standards but it is far from
the quality required for boiler feedwater. Preparation of boiler feedwater is done
in an ion exchange system and, with an influent of 490 mg/Q, the operating cycle
would be short and chemical regenerant cost high. In this case, it is economical
to further treat the first stage reverse osmosis product in a second stage reverse
osmosis system to further decrease the load on the ion exchange resins.
About 85% of the first stage reverse osmosis system product water ispumped
to the second stage reverse osmosis system which consists of two trains. Each
train consists of a 5-p cartridge filter, a high pressure pump, DuPont 8-9 per-
meators, piping, valves and instrumentation. Each train has 22 permeators in a
12-7-3 array and it is operated at 85% recovery. The product from the second
stage has a TDS of 28 mg/ll and this is sent to the ion exchange system. The re-
ject from the second stage is of much better quality than the incoming sea-
water and this is routed to the roughing filter clearwell.
The reverse osmosis system has operated well and the capacity of the plant
has been increased to twice that of the initial segment described above.
GENERAL APPLICATIONS OF REVERSE OSMOSIS
The above applications were specific examples that were chosen to demon-
strate the versatility of the reverse osmosis process. The following list enumer-
ates a wide variety of applications for which the reverse osmosis process may be
considered:
(1) Municipal Potable Water
General quality improvement of present supplies
Upgrade total municipal supply
Potable water from degraded supplies
Brackish water desalination
Seawater desalination
Removal of nitrates, fluorides, heavy metals, etc.
Bottled water production
(2) Industrial Water
Provide usable water where none available
Brackish water desalination
Seawater desalination
Pure water production
Industrial rinse waters
Food industry
Electroplating
Power plant boiler feed
Beverage production
Medical
Ultrapure water production
Pharmaceutical

Reverse Osmosis 303
Electronics
Medical
(3) Municipal Wastes
Reclaim municipal wastewaters (sewage) for
Ground water recharging
Agricultural or landscape irrigation
Industrial process water
Improve effluent quality to meet discharge requirements
(4) industrial Wastes
Reclaim industrial wastewaters for
Reuse within industrial plant
Zero discharge
Agricultural or landscape irrigation
Removal of toxic substances prior to discharge
Resource recovery
Separate or concentrate valuable materials
Electroplating industry
Dairy industry
(5) Miscellaneous
Production of pure water for high value crops
Recovery of agricultural irrigation drainage
Production of shipboard drinking water
COSTS OF REVERSE OSMOSIS
There is no such thing as the typical cost of a reverse osmosis system or
of the product water from that system as these costs depend on a number of
things:
Economic conditions Power costs
Market conditions Site preparation
Plant size Product quality requirements
Local labor rates Feedwater availability
Chemical costs Accounting procedures
The DuPont Permasep Engineering Manual” has published a “guide” for the
capital, operating and maintenance costs for both a brackish water system and a
seawater system. The brackish water system costs are shown in Table 4.10. They
are based on a large brackish water system built in the southern United States in
1982. The estimated capital cost of the plant is $1.25 per gallon per day of prod-
uct water installed. This cost includes the cost of wells, a reverse osmosis system
with pretreatment, a building for the reverse osmosis systems and office. The
above installed capacity cost does not include the cost of land nor an independ-
ent power source.

Table 4.10: Total Water Cost for Brackish Water RO
Cost/l,000 Gals. Product
Energy (O.O6/KWH) $0.36
Chemicals 0.09
Labor 0.12
Maintenance and Repair 0.05
Membrane Replacement 0.10
Amortization (12%/20 years) 0.48
Total $1.20
Data are provided for a seawater system that will produce IO million gallons
per day of product water. The system is a two stage system similar to the one
shown in Figure 4.18 where the product water from the first stage high pressure
seawater system is treated in a second stage lower pressure brackish water sys-
tem. The product water TDS is 200 mg/g. The estimated 1982 cost for such a
system was $45 million which includes the RO system with pretreatment and a
building for the RO system, controls and office. It does not include the cost of
land or an independent energy source. The total water cost for such a plant is
shown in Table 4.11.
Table 4.11: Seawater RO System
Cost/l,000 Gals.Product
Energy ($O.O6/KWH) $1.80
Chemicals 0.14
Labor 0.19
Maintenance 0.22
Membrane Replacement 0.90
Amortization (12X/20 years) 1 75d
Total $5.00
Both the brackish and seawater reverse osmosis product water costs are
based on 1982 costs and they are indicative of specific plants in an assumed
location in the southern United States. The cost of energy in the seawater sys-
tem assumes that the reject from the first stage high pressure reverse osmosis
system is sent to an energy recovery system which reduces the overall energy
requirements for the total system by 31%.

Reverse Osmosis 305
FUTURE PROJECTIONS
The initial projections of 20 years ago have proven to be unrealistic in that
reverse osmosis has not caused deserts to bloom, nor does every household con-
tain a reverse osmosis unit to improve the tap water. Yet, the process has been of
economic value in providing process water to industry, potable water to high in-
come arid regions and a method of reclaiming municipal and industrial wastes.
As of 1985, it was estimated that the worldwide market for reverse osmosis
membrane elements (not total systems) was about $50 million.
The future of the process rests in the research that will result in the develop-
ment of a product that will compete with and win over other separation proc-
esses that can do the same thing. Reverse osmosis has gone a long way toward
becoming more economical than multistage flash distillation in the production
of potable water from seawater. The development of membranes with higher
fluxes and improved rejections will be incorporated into single stage seawater
plants with lower operating pressures. This will not only permit reverse osmosis
to win the competition for new seawater desalination capacity but it will allow
the RO process to replace distillation plants that are being retired in the Middle
East where the enormous capacity of these plants exists. The low pressure mem-
branes which are being developed for brackish applications will also further po-
tential municipal wastewater reclamation as an alternate source of potable and
industrial water. The reduced costs which will result from lower pressure/higher
rejection systems will effectively compete with pipelines, dams and other water
supply schemes that proliferate in the western United States.
While not totally essential to progress, it is suggested that oxidizing-agent-
resistant membranes will be developed in a thin film composite membrane.
This will broaden the applications available to RO and, at the same time, re-
duce the cost and complexity of existing plants which presently use chlorine
sensitive membranes.
An elusive goal has been the development of ion specific membranes. Based
on limited knowledge of worldwide research programs, it appears that this goal
will remain elusive for the forseeable future.
Finally, as the world becomes more aware of the environmental damage
caused by indiscriminate waste disposal it is apparent the RO process will play
a key role in mitigating that problem. It appears that the next market for the RO
process will be in industrial waste treatment in the United States to be followed
by application in other countries. Eventually, the world will be forced to use
reverse osmosis to reclaim municipal wastewater on a large scale and to put the
reclaimed water to a number of already demonstrated beneficial uses.
REFERENCES
1. El-Ramly, N.A. and Congdon. C.F., Desalting Plants Inventory, Report No.
6, United States Department of Interior (1977).
2. El-Ramly, N.A. and Congdon. CF., Desalting Plants Inventory, Report No.
7, National Water Supply Improvement Association (1981).
3. Wangnick, K., Desalting Plants inventory, Report No. 8, Water Supply Im-
provement Association (1984).

4. Wafer Desalination Report (M.C. Smith, Ed.), Vol. XXI, No. 37 (September
19, 1985).
5. Soltanieh, M. and Gill, W.N., Chemical Engineering Communications, Vol.
12, pp. 279-363.
6. Fluid Systems Division of UOP, Inc., Product Catalog (1984).
7. Beasley, J.K., “The Evaluation and Selection of Polymeric Materials for
Reverse Osmosis Membranes,” Proceedings of the International Con-
gress on Desalination and Water Reuse, International Desalination and
Environmental Association (1977).
8. Sudak, R.G. and McKee, M.E., “Low Pressure Composite Membranes,”
Proceedings of the International Congress on Desalination and Water
Reuse, International Desalination and Environmental Association (1977).
9. Orange County Water District, Evaluation of Membrane Processesand their
Role in Wastewater Reclamation, Vol. I (1979).
IO. Orange County Water District, Evaluation of Membrane Processesand their
Role in Wastewater Reclamation, Vol. I I (1980).
11. E.I. DuPont de Nemours & Co., Permasep Engineering Manual (1982).
12. Sourrajan, S., Editor, Reverse Osmosis and Synthetic Membranes, Nat-
ional Research Council of Canada (1977).
13. American Public Health Association, American Water Works Association
and Water Pollution Control Federation, Standard Methods for the
Examination of Water and Wastewater, 16th Edition (1985).
14. Saltonstall, C.W., Jr., Proceedings of the 10th Annual Conference of NWSIA
(1982).
15. Environmental Protection Agency, Environmental Regulations and Tech-
nology- The Electroplating Industry (1980).
16. Environmental Protection Agency, Economics of Wastewater Treatment
Alternatives for the Electroplating Industry (1979).
17. Vera, I., “Operating Experiences on a Reverse Osmosis Plant Which Con-
verts Seawater to Boiler Feedwater,” Industrial Water Engineering (No-
vember 1981).

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