¶
THE GROWTH OF MEMBRANE
TECHNOLOGY
Membrane systems havebeen used in
specialized applications for more than 30
years, largely for water treatment,
including desalination of seawater and
brackish water. With technical advances
and correspopding cost reductions,
membrane systems are nowcapable of
decontaminating nonsaline waters
(including treated wastewaters) in singlestep processes atcompetitive costs.
The demand for membranesin the
water and wastewaterindustry is projected to increase at a 9% annual rate
and reach $540 million by year 2000.
About two-thirds of the market will be for
water, and one-third for wastewater.
Membrane technologies are receiving
special recognition as alternatives to
conventional water treatment and as a
means of polishing treated wastewater
effluent for reuse applications. Membrane
technologies are energy intensive. New
membrane technologies feature the use
of low pressure systems that significantly
reduce energy use and operation and
maintenance costs.
Membranes are commonly used for
the removal of dissolved solids, color, and
hardness in drinking water. Membrane
technologies have also been proposed by
the USEPA as a means of: (1) complying
with current and anticipated regulations
for particle removal: (2) reducing disinfection by-products such as trihalomethanes
(THMs) and haloacetic acids (HAAs); and
The reverse osmosis facility for brackish
water desalination at Wellington,
Florida processes1.8 million gallons perday.
(3) eliminating illness-causing microorganisms such as Giardia and
Cryptosporidium in drinking water
applications.
In wastewater reclamation and reuse,
water quality requirements may call for
Table 1. Comparison of Membrane Features
reductions in suspended solids, total
dissolved solids, and selected constituents
such as nitrates, chlorides, and natural
and synthetic organic compounds.
Membrane treatment, applied to the end of
conventional wastewater treatment
systems, is a viable method of achieving
desired effluent quality levels at reasonable costs.
I
WHAT IS NfElinBFiANE TECHNOLOGY?
Matter crossing membrane
Water
Ions
Matter removed from water
Inorganics, most organics,
silica, suspended solids,
and microorganisms
Ions
only
Matter not removed
Gases
Gases, silica, organics. and
suspended solids
.
Membrane technology utilizes a
semipermeable membrane for the
separation of suspended and dissolved
solids from water. There are two basic
types of membrane separation processes;
pressure-driven and electrically-driven.
Each type is described on the next page,
and a comparison of their features is given
in Table 1.
Techcommentary 1
~~~~~~
Micrometers
(log scale)
o.Ooo1
Approximate RAW
0.01
0.001
loo
200
0.1
1.o
10
100
~~
lo00
500,000
20,000
-
01' 1
Dissolved organics
Sand
Giardia
C yptospordium
Typical size
range of
selected water
constituents
I
Bacteria
Viruses
I
Reverse
Membrane
processes
osmosis
.
Conventional media filtration
Ultrafittration
Nanofiltration
Microfiltration
I
I
Note: MW = Molecular
Weight
Conventional
media
filtration
comparison
is shown
foronly
Figure 1. The Filtration Spectrum
sion inthis TechCommentarywill focus
on them.
In the electrically-driven membrane
process, electric current is used to move
ions across the membrane, leaving
purified water behind. In thisprocess, the
ions are collected in the concentrate
stream for disposal. The product water is
the purified feedwater.
Pressure-driven membrane
technologies. Pressure-driventechnolo-
Pressure-driven processes use
hydraulic pressure to force water molecules through the membranes. Impurities are retained and concentrate in the
feedwater, which becomes the reject
water or concentrate stream. The
permeate, the water that passes through
the membrane, is recovered as product or
pure water. Since pressure-driven
systems are the most commonlyused
membrane systems, mostof the discus-
.
(-)
Cathode
I
(+)
Anode
I
I
1
I
C
A
C
A
I
Membranes
figure 2. The Electrodialysis Process Diagram
2 Techcommentary
I
C
Concentrate
gies include, in order of decreasing
permeability: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and
reverse osmosis (RO). The range of sizes
of selected constituents in water and
wastewater and the performance capabilities of the different membranes are
illustrated in Figure 1.
MF and UF often serve to remove
large organic molecules, large colloidal
particles, and many microorganisms (see
Table 2).MF performs as a porous barrier
to reduce turbidity and m e types of
colloidal suspensions. UF offers higher
removals than MF, but operates at higher
pressures. In wastewater reclamation, MF
or UF might provide a suitable level of
treatment. In drinking-water treatment, MF
or UF might be used intandem with NF or
RO to remove coarser material so that
fouling of the less permeable membranes
is minimized.
The most commonly used process for
the production of drinking water is RO, but
NF is now emerging as a viable alternative to conventional water treatment
because it can operate at lower pressures
and higher recovery rates than RO
systems. NF is also cost-effective in many
groundwater softening applications where
the incoming turbidity is low.
Electricaliy-drivenmembrane
technology. Electrodialysis reversal
(EDR) is an improvement over the original
electrodialysis process. In EDR, the
direct-current driving force is periodically
reversed to prevent scaling andfouling of
-
Table 2. Comparison of Pressuredriven Membrane Systems
Product particle
size, pm
Retained
compounds
0.08to 2.0
I
Organics >lo00MW,
pyrogens, viruses,
bacteria, colloids
Very small
suspended
particles, some
colloids, most
bacteria
Operating
pressure, psi
1 to 15
10 to 100
Maximum
temperature, "F ("C)
80 (27)
80 (27)
Recovery rate, %
I
100
o.Ooo1 to 0.001
0.001 to 0.01
0.005 to 0.2
75
I
85
I
50 to 85
Note: Recovery rate ISthe percent of product recovered from
me feedwater.
the membrane surface. This innovation
improves bottithe efficiency and the
operating life of membranes.
Ion exchange membranes are the
heart of the process. Cation-selective and
anion-selective membranes are alternately placed in a membrane "stack (see
Figure 2).
Water flows between the membranes,
and when direct current is applied across
the stack, positive Ions move toward the
cathode and negative ions move toward
the anode. Due to the alternating membranes, salt is removed from every other
compartment and collected in intervening
compartments. The salt-laden water is
then discharged as a brine concentrate;
'
Concentrate
water
Product
water
desalted water is discharged to the
purified-water collection system.
TYPES OF PRESSURE-DRIVEN
MEMBRANES
Membranes are typically made from
polymeric materials, although ceramic
and metal oxide membranes are also
available. Cellulose polymers are
inexpensive and widely used. More recent
polyamide thin-film composite membranes are more chemically robust, have
longer life, possess greater rejection of
dissolved salts and organics, and operate
at lower pressures. They are, however,
more expensive than cellulose mem-
branes. Ceramic and metal oxide
membranes are traditionally used for UF
and are commonly available in tubular
form. Although ceramic and metal oxide
membranes are more costly than other
types, they are used for many industrial
processes because they can withstand
very high temperatures.
Two types of membrane configuration
used extensively for water and wastewater treatment are hollow-fiber and
spiral-wound. In a hollow-fiber element
(Figure 3a), fibers made of porous
polymer material are bundled together
and sealed in a pressure vessel. For
some UF designs, feedwater enters
through a perforated central tube and
Concentrate
water
Product
water
t
Concentrate
A
F
Y
Product
water
t
Feedwater
"
\ Membrane
spacer
Feedwater
3a
Figure 3. Hollow-fiber (3a) and Spiral-wound(3b) Modules
TechCommentary 3
A cartridge filter is nearly always
provided by the membranemanufacturer-usually for the removal of
particles 5 microns and larger in size.
The filter provides protection against an
upset in the pretreatment step that could
cause fouling of the membrane.
The membranes arethe heart of the
treatment system. They maybe
hydraulically connected in series or
parallel configurations, depending upon
the feedwatercomposition or desired
water recovery.
Post-treatment may include: (1 ) a
degasifier to remove carbon dioxide and
hydrogen sulfide; and (2) the addition of
lime or caustic to prevent corrosion of
the subsequent piping or distribution
system.
1
2
3
4
5
6
7
Membrane Process
Process
EDR
MF (0.1 kWh/1,000 gal [3,785 liters])
UF
NF and ultra low pressure RO
(1 25 psi [862 kPa])
code
5 = Low pressure RO (225 psi [I,550kPa])
6 = Standard pressure RO (400 psi 12,760 kPa1)
7 = High pressure RO (1,000psi [6.890 kPa])
Figure 4. Typical Energy Consumptionfor Various Membrane Processes
flows radially outward through the fiber
bundle. Under pressure, water is forced
through the hollow-fiber bores and exits
through one or more ports. Water that
does not penetrate the membrane
continues through the fiber bores and
exits at the opposite end. For RO,
feedwater enters from the outside
surface of the fiber and product water is
removed from the bores.
Spiral-wound elements (Figure 3b),
usually range from 2 to 10 inches (5 to
25 cm) indiameter and 10 to 60 inches
(25 to 152 cm) inlength. They consist of
two flat membrane sheets separated by
a thin, mesh-like porous support or
spacer and are sealed on three sides
like an envelope. The fourth side is fixed
onto a perforated plastic center tube that
collects the product water. The membranes are rolled up around the tube in
the form of a spiral. Feedwater is
pumped through the layers, and product
water passes through the membranes
and follows the spiral configuration to
the central perforated tube. Water that
does not penetrate the membrane exits
the element as concentrate. Spiralwound elements are used for MF, UF,
and RO.
4 TechCommenfary
COMPONENTS OF A MEMBRANE
SYSTEM
Membrane processes use a significant amount of energy. Even low
pressure membranes use approximately
100 k w h per million gallons (3.785
million liters) of water produced. The
development of new composite membranes has reduced the operating
pressures considerably. Lower pressure
operation means lower energy consumption. Whereas 400 pounds per
square inch (psi) (2,760 kPa) pressure
was considered normal for RO as
recently as ten years ago, today's ultralow pressure RO membranes function
efficiently at pressures as low as 125 psi
(862 kPa); the norm for brackish water
desalination is 225 psi (1,550 kPa). A
comparison of energy consumption per
1,000 gallons (3,785liters) of water
produced is illustrated inFigure 4 for
EDR and various types of pressuredriven membranes.
Typical membrane systemsconsist of:
(1) pretreatment; (2) pumping; (3)
CONSIDERATIONS FOR THE DESIGN
cartridge filtration; (4) membranes; and
OF MEMBRANE SYSTENlS
(5) post-treatment.
In addition to levels of constituent
Pretreatment is required to remove
removal required, factors to be considexcessive suspended solids and other
ered in the design of membrane
constituents that would foul the memsystems include membrane life,
brane surface. For most municipal
membrane fouling, and disposal of
surface water supplies, filtration with
concentrate. Typical membrane life is
granular media filters is adequate
three to five years depending upon the
pretreatment. For groundwater, pretype of service and type of membrane
treatment is usually not needed, except
used. Membranes used in municipal
perhaps for chemical addition. For
water treatment may last five years or
wastewater systems, secondarytreatmore before they require replacement.
ment followed by chemical coagulation,
sedimentation, and filtration is customar- Membranes used in wastewater
treatment typically have a life of four to
ily employed for pretreatment. Pretreatfive years. For seawater desalination,
ment may include the addition of
chemicals to prevent organic materials or the normal life of a membrane is five
soluble salts from fouling the membrane. years, and many have been in service
for more than six years.
Pumping is required to raise the
Two principal types of membrane
pressure to the desired operating level
fouling may occur: (1 ) precipitation of
and to maintain sufficient velocity across
the membranes. The ranges of pressures soluble salts such as strontium sulfate,
barium sulfate, and/or calcium sulfate;
required for various types of pressureor (2) organic fouling. The former is
driven membranes are given in Table 2.
~~
~
-
eastlv rnltigated by addingcommerclal
innlbrtors and by operatrng the system
withln safe operating parameters.
Organc fouling may be mitigated by
employing good pretreatmentpractices.
maintalntng satisfactory veloclty across
the membranes, and perrodic cleaning
wlth chemicals.
Membrane processes producea
concentrate or brine In the case of
seawater. The method of disposal of
concentrate must be carefully considered, smce waste streams con!aining
high concentrations of solids may be
difficult to dispose of. Optlons available
include disposal to a sanitary sewer.
direct ocean disposal. surface water
disposal, land application, evaporation
ponds, or deep well injection. Sanitary
sewer disposal is typically the easiest
method, but its ease mustbe balanced
against pretreatment requtrements and
fees that may be imposed bythe
sewering agency.
CASE STUDIES
'
I
.
Two case studies (see adjoining
boxes) are presented to exemplify the
use of membrane systems for waterand
wastewater treatment. In Fort Myers,
Florida, membranes were selected over
conventional water treatment systems
for improving the quality of their groundwater supply [Ref. 31. For Harlingen,
Texas, membrane treatmentof treated
wastewater provides the necessary
source for process water to attract a
new industry to the area [Ref. 41. Both
examples illustrate how membranes can
provide high quality treatment and
extend the capacity of existing water
resources.
Fort Myers, located on the southwest
coast of Florida, has experienced steady
population growth and commercial
development. As the freshwater
resources of southwest Florida have
become more scarce,innovative
resource management has been
exercised to meet the increasing needs
of the area. Membranetechnology is
one of the principal components of Fort
Myers' long range water resources plan.
Harlingen, located in the Rio Grande
Valley, selected RO for treating wastewater effluent. The treated effluent is
high in totaldissolved solids (TDS) and
hardness, and the alkalinity and
chlorides are 50-percent higher than
that needed by the textile plant. A 0 has
achieved the following reductions:
.
.
.
=
TDS are reduced from 1,200 mg/L to
less than 200 mg/L.
Hardness is reduced from 375 mg/L
to less than 15 mg/L.
Alkalinity is reduced from 150 mg/L
to less than 10 mg/L.
Chlorides are reduced from 300 mg/L
to less than 100 mg/L.
l m
J
!l
-~
WATER TREATMENT CASE STUDY: Fort Myers, Florida
he City ofFort Myers Installeda 12-mgd membrane water treatment plant in 1992 for treat
ing their groundwater supply. The plant was installed to meet regulatoty requirements for
reducing trihalomethanes (maxlmum level 100
of micrograms per liter in the distribution
system), while softening the water and reducing total dissolved solids. (Trihalomethanes are
potential cancer precursors).
The plant consists of three identical 4-mgd process trains. The membranes are configured
in a three stage system with the reject water from stage
1 feeding stage 2 and the reject water
from stage 2 feeding stage
3.This configuration results in a 90-percent recovery rate. In other
words, nine gallonsof product water are produced for every
10 gallons of feedwater.
Pretreatment chemicals are added to reduce potential scaling, and post-treatment chemicals
are added for disinfection,corrosion protection, and stabilization. Fluoride is also added
as a
deterrent to tooth decay.
The construction cost of the facility was $14.2 million. The membrane system accounted for
about 37% of the total cost. Total operation and maintenance cost
is per 1.000 gallons;
$0.55
the Dower cost averaaes $0.1
5 w r 1.000 aallons.
T
figure 5. Fort Myers Membrane System Schematic
r
TO
Indutrl.l
Iu
n
II
WASTEWATER REUSE CASE STUDY: Harlingen, Texas
hen Fruitof the Loom expressed interest in locating a major facility in the Harlingen area
3,000new jobs, the newly formed Harlingen Development
that would result in
Corporation (HDC) investigated alternatives for supplying
1.6 mgdof high quality water to meet
industrial needs.The most economical water supply alternative available from the water-short
ocal wastewater treatment plant.
Hartingen area was effluent from lthe
To meet Fruit of the Loom's water quality requirements, the existing wastewater treatment
process was upgraded and an
RO system was added. Solids contactlclarification
and twostage gravity filterswith in-line coagulation were added to reduce suspended solids
to a level
acceptable forRO (turbidity 4 unit). The plant has met the water quality requirements since
startup in 1990, producing excellent water at economicalThe
cost.
total annual process water
cost averages $0.87 per
1,000 gallons produced. Power
costs account for about 30 percent of
the total operating cost. Plans are undemay to double the plant capacity.
W
figure 6. Process Flow Diagram for Wastewater Reuse, Harlingen, Texas
TechCommentary 5
m
SUMMARY
In the past, the useof membranes has
been limited bytwo factors: (1) other
technologies were capableof meeting
less stringent treatment or disposal
requirements; and (2) membrane systems
had higher capital and operating
Costs
than other technologies offering similar
performance. Becauseof the improvements madein membrane technology
and the impositionof new water quality
requirements that exceedthe capabilities
of existing treatment processes,membranes are now cost-competitive alternatives for many treatment applications.
With the new low pressure membranes,
the energy requirementsand operating
costs have been significantly reduced.
For each individual application where
membranes are being considered, the
characteristics of the water to be treated
and the performance requirements have
to be carefully evaluated. To aid in this
evaluation, the advantages and disadvantages of membrane systems are summarized in Table 3.
The membrane market is entering an
era of rapid growth. Many communities
that have relied on conventional technologies in the past to solve their water quality
problems are now turningto membranes.
Membrane applications offer solutionsto
many difficult water quality applications,
now at an affordable cost.
References
1. Mueller. Floyd H.. ed. Electrodialysis (ED) and
Electrodlalysis Reversal (EDR) Technology,
lonics, March 1984.
2. Jacangelo, J.G.. N.L. Patanla. and R.R. Trussel.
"Membranes In Water Treatment," Civil
Englneering, May 1989. pp 68-71.
3. Cannarella. R.A., and T.M. Curran. "Membrane
Softening Meets the Needs
of the Cityof Fort
Myers," AWWA Proceedings, 1993 Membrane
Technology Conference, August 1993,
Baltimore, MD.
Table 3. Advantages and Disadvantages
of Membrane Separation
Uses more electricity; high
pressure systems can be energy
intensive
Reduces the amount of treatment
chemicals
Uses smaller space requirements
(footprint); membrane equipment
requires 90 to 95% less space than
conventional plants
May need pretreatment to prevent
fouling; pretreatment facilities
increase space needs
Requires disposal of concentrate
Eliminates residuals handling and
disposal
Requires replacement of
membranes about every five years
Reduces labor requirements; can
be automated easily
Works best on groundwater or low
solids surface water
Removes natural organic matter(a
disinfection by-product precursor)
and inorganic matter
New membrane design allows use
Flux rate (the rateof feedwater flow
through the membrane) gradually
declines over time
of lower pressures; system cost
may be competitive with
Recovery rates may be less than
100%
conventional water treatment
processes
Lack of a reliable low-cost method
of monitoringthe integrity of low
Removes bacteria and viruses;
may also removeCrypfosporidium
pressure membrane processes
4. Filteau, G . , C. Whitney. and I. Watson. "Water
Use Fuels Economlc Growthin Harlingen, TX."
American DesaltingAssmiatlon Conference in
Monterey. CA. August 1994.
Basic funding for this Techcommentary
is provided by
the Electric Power Research Institute (EPRI). a
nonprofit institute that conducts appllcations and
development on behalf of the UnitedStates electric
utllity industry. Techcommentaryis one way that the
EPRl Industrial and Agricultural business area assists
in communicating information concerning energyefflctent, electric-based technologles.
This issueof Techcommentary was written by
David H. Furukawa of Separatlon Consultants, Inc.
and Franklin L. Burton of Burton Environmental
Engineering and was edited by Melissa Blanton of
Black & Veatch.
This issueof TechCommentary was produced by
EPRI-CEC and ProWrite Inc.
Figures 1 and 2 were adapted from Ref.1. Figure 3 is
used with perrnisslonof the American Society of Civtl
Engmeers.
Applicable SIC Codes:
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