Read Membrane Technologies for Water and Wastewater treatment text version

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 now capable of decontaminating nonsaline waters (including treated wastewaters) in singlestep processes atcompetitive costs. The demand for membranes the in 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 per day.

(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

I

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.

WHAT I NfElinBFiANE TECHNOLOGY? S

Matter crossing membrane Matter removed from water Water Inorganics, most organics, silica, suspended solids, and microorganisms Gases Ions Ions only 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

.

Matter not removed

Gases, silica, organics. and suspended solids

~~~~~~

~~

Micrometers (log scale) Approximate R W A

o.Ooo1

0.001

0.01

0.1

1.o

10

100

lo00

loo

200

20,000

Dissolved organics

Typical size range of selected water constituents

Viruses

500,000

Giardia C yptospordium

0 '1 1

Sand

I

Bacteria

I

Membrane processes

Reverse

osmosis

.

Ultrafittration

Conventional media filtration

Nanofiltration Note: M = Molecular media W Weight filtration Conventional

Microfiltration

I I

comparison is shown for only

Figure 1. The Filtration Spectrum

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-

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-

.

(-)

Cathode

I

(+)

Anode

I

C

I A

1

I

I

C

I

A

C

Membranes

Concentrate

figure 2 The Electrodialysis Process Diagram .

2 Techcommentary

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

0.08t 2 0 o .

Very small suspended particles, some colloids, most bacteria

1 to 15

I

0.005 t 0 2 o .

0 0 1 to 0.01 .0

o.Ooo1 to 0.001

Organics >lo00MW, pyrogens, viruses, bacteria, colloids

Operating pressure, psi Maximum temperature, "F ("C)

Recovery rate, %

10 to 100

80 (27)

100

80 (27)

I

75

I

85

I

50 to 85

Note: Recovery rate ISthe percent of product recovered from feedwater. me

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;

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

'

Concentrate water

Product

water

t

Concentrate

A

F

Feedwater

3a

Y

Product water

Feedwater

t

"

\ Membrane

spacer

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

code

5 = Low pressure RO (225 psi [I ,550 kPa]) 6 = Standard pressure RO (400 psi 12,760 kPa1)

7 = High pressure RO (1,000 [6.890 kPa]) psi

EDR MF (0.1 kWh/1,000 gal [3,785 liters]) UF NF and ultra low pressure RO (1 25 psi [862 kPa])

Figure 4. Typical Energy Consumption Various Membrane Processes for

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 (3,785 liters) of water produced is illustrated in Figure 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 must balanced be against pretreatment requtrements and fees that may be imposed bythe sewering agency.

l m

J ! l

T

-~

CASE STUDIES

Two case studies (see adjoining boxes) are presented to exemplify the use of membrane systems for water and 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:

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 micrograms per liter in the distribution of 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 This configuration results in a 90-percent recovery rate. In other 3. words, nine gallonsof product water are produced for everygallons of feedwater. 1 0 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 $0.1w r 1.000 aallons. 5

'

I

figure 5. Fort Myers Membrane System Schematic

.

r

TO Indutrl.l

Iu

n

II

.

=

. .

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.

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 mgdof high quality water to meet 1.6 industrial needs. most economical water supply alternative available from the water-short The Hartingen area was effluent from local wastewater treatment plant. the To meet Fruit of the Loom's water quality requirements, the existing wastewater treatment process was upgraded and an system was added. Solids contactlclarification twoRO and stage gravity filters with in-line coagulation were added to reduce suspended solidslevel to a acceptable forRO (turbidity 4 unit). The plant has met the water quality requirements since startup in 1990, producing excellent water at economicalThe total annual process water cost. cost averages $0.87 per,000 gallons produced. Power 1 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

SUMMARY In the past, the use membranes has of been limited bytwo factors: (1) other of technologies were capable 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 of and the imposition new water quality requirements that exceed capabilities the 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 turning membranes. to Membrane applications offer solutions to many difficult water quality applications, now at an affordable cost.

Table 3 Advantages and Disadvantages Membrane Separation . of

Reduces the amount of treatment chemicals Uses smaller space requirements (footprint); membrane equipment requires 90 to 95% less space than conventional plants Eliminates residuals handling and disposal Reduces labor requirements; can be automated easily Removes natural organic matter (a disinfection by-product precursor) and inorganic matter New membrane design allows use

of lower pressures; system cost may be competitive with

Uses more electricity; high pressure systems can be energy intensive May need pretreatment to prevent fouling; pretreatment facilities increase space needs Requires disposal of concentrate Requires replacement of membranes about every five years

Works best on groundwater or low solids surface water

Flux rate (the rateof feedwater flow through the membrane) gradually declines over time

Recovery rates may be less than

100%

conventional water treatment processes Removes bacteria and viruses; may also removeCrypfosporidium

Lack of a reliable low-cost method of monitoringthe integrity of low pressure membrane processes

References

1. Mueller. Floyd H.. ed. Electrodialysis (ED) and Electrodlalysis Reversal (EDR) Technology, lonics, March 1984.

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 provided by is 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 perrnisslon the American Society of Civtl of Engmeers. Applicable SIC Codes: 4 9 4 1 , 52

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 the Cityof Fort of Myers," AWWA Proceedings, 1993 Membrane Technology Conference, August 1993, Baltimore, MD.

EPRI Municipal Water & Wastewater Program

For technical information call EPRl CommunityEnvironmentalCenter Washington University Campus Box 11 50 One Brookings Drive Cupples II, Room 11 St. Louis, MO 63130-4899

314-935-8590

b -

"I .-

..

w

Fax:314-935-8599

or EPRI Northeast Regional Community EnvironmentalCenter Manhattan College, RLC204 3840 Corlear Avenue Riverdale, NY 10463

1-800-424-EPRI

Prfntad on recycled paperfn the United States of Amenca Copynght 0 1997 Electric Power Research ln.st!lule (EPRI) Palo Alto. Californla

Fax:71

8-862-7234

For additional copies this publication call of EPRiAMP CustomerAssistance Center

1-800-4320-AMP

6 Techcommentary

m

TC - 107698

Information

Membrane Technologies for Water and Wastewater treatment

6 pages

Find more like this

Report File (DMCA)

Our content is added by our users. We aim to remove reported files within 1 working day. Please use this link to notify us:

Report this file as copyright or inappropriate

693


You might also be interested in

BETA
April 2005 JAM
Microsoft Word - FRWA-OMPreventiveManual-022106.doc
MATERIAL SAFETY DATA SHEET
Water Treatment Technology | Water Treatment | Reverse Osmosis Water Treatment
Membrane Technologies for Water and Wastewater treatment