Produced waters can contain significant

levels of inorganic and organic constituents. The waters usually contain

elevated levels of total dissolved solids (TDS)

concentrations between 2,000 mg/L and

100,000 mg/L. Produced waters also may contain significant concentrations of hydrocarbon

compounds including free oil; dissolved oils;

volatile acids; and benzene, toluene, methylbenzene and xylene (BTEX). Typical compositions of produced waters are shown in Table

1. Regulations that apply include the Federal

Clean Water Act, the Resource Conservation

and Recovery Act, and a tapestry of State and

Regional environmental, natural resource and

water management laws.

More than 90% of produced water generated from oil and gas fields is disposed of using

Class II salt water disposal (SWD) wells,

which are deemed geologically isolated from

potential sources of drinking water. But in a

number of regions, such as parts of the Rocky

Mountain states, produced water outputs (per

field and per unit of energy produced) are significantly increasing, while re-injection capacities are remaining the same or declining

because of permitting and wellsite field injection constraints. Discharge of produced waters

to surface waters would, in most cases, require

control of oil and grease, removal of soluble

organics, reduction of suspended solids and

demineralization prior to surface discharge

under a National Pollutant Discharge

Elimination System (NPDES) permit to meet

water quality standards. This regional challenge has created concern among producers

that if new disposal capacity is not found, the

future energy production potentials of certain

basins of the Rocky Mountains could be

adversely impacted.

An example of how produced water output is

beginning to constrain energy production is

found in the development of coalbed natural

gas in various basins around the United States,

such as the Powder River Basin. An extensive

review of issues related to coalbed methane

(CBM) produced water is provided by ALL

Consulting’s 2003 handbook and has been 

the subject of a three-part series in GasTIPS

(Arthur, et al, 2003). As these sources note,

management costs for treatment and disposal of

produced water severely impact the economics

of CBM development in the western United

States, where high volumes of produced water

and limited SWD injection capacities exist.

Desalinization to reduce brine volumes requiring SWD disposal is one of the broad options

under consideration. Using this approach, a

desalinization process would process each gallon of produced water to generate a small volume of concentrated brine (less than 0.25 gal)

and a larger volume (greater than 0.75 gal) of

demineralized water that could be discharged to

surface waters or used for beneficial purposes,

such as irrigation, groundwater recharge, livestock watering and industrial utilization.

Some energy companies have explored

using reverse osmosis (Figure 1) to reduce

brine volume for CBM waters and conventional gas-well produced water. For both types

of produced water, pilot tests have shown that

although a 3:1 reduction of brine volume could

be achieved, and although a deionized product

stream of good quality water could be initially

produced, many operational problems involving membrane fouling surfaced in the initial

attempts in the field to deploy the technology.

These problems arise from the complex composition of the produced water and the effects

of certain constituents on the membrane

material. Some of these inorganic constituents

consist of divalent cations such as calcium and

magnesium that form precipitates abrasive to

sensitive membranes used in reverse osmosis

systems. Free and dissolved oils collect on

reverse osmosis membranes, causing them to

lose their permeability. Soluble hydrocarbons

including volatile acids and BTEX can promote the growth of microbial films on reverse

osmosis membrane surfaces, causing them to

lose separation performance. These fouling

Summer 2004 • GasTIPS 15

PRODUCED WATER

by Tom Hayes, The Electrodialysis Alternative Gas Technology Institute 

for Produced Water Management

Twenty billion to 30 billion bbl of produced water are generated each year with the production of oil and 

gas in the United States. The management of this water, however, produces a costly problem, as large 

volumes are produced in dispersed locations and must be managed in an environmentally and healthconscious manner.

Parameter End Use Criteria (ppm) CBM Water Non-CBM 

Drinking Irrigation Livestock (Conventional Gas Well) Water

pH 6.5 - 8 - 6.5 – 8 7 - 8 6.5 - 8

TDS, mg/L 500 2,000 5,000 4,000 – 20,000* 20,000 – 100,000

Benzene, ppb 5 5 5 < 100 1,000 – 4,000

SAR** 1.5-5 6 5-8 Highly Varied Highly Varied

Na+ , mg/L 200 See SAR 2,000 500 – 2,000 6,000 – 35,000

Barium, mg/L 0.01 – 0.1 0.1 – 40

Cl - , mg/L 250 - 1,500 1,000 – 2,000 13,000 – 65,000

HCO3 mg/L - - - 150 – 2,000 2,000 - 10,000

Table 1. Typical values for produced water quality compared with end-use criteria.

* Total Dissolved Solids (TDS) range estimated for the lower 50th percentile.

** SAR = ratio of Na to Ca and Mg levels.
16 GasTIPS • Summer 2004

PRODUCED WATER

problems are largely responsible for the lack of

deployment success of reverse osmosis in the

oil and gas industry. Recent advances in applying rigorous preprocessing of produced water

show potential for improving the reliability of

reverse osmosis in achieving economical brine

reduction; however, successful preprocessing is

often complex and site-specific.

The choice of reverse osmosis for demineralization of produced waters is understandable,

given that it is the predominant desalting technology in the United States. In 1997, there

were about 2,000 U.S. reverse osmosis plants,

with a total capacity of 800 million gal/d. Most

of these plants process brackish waters and seawater to supplement water supplies for municipalities and industry. The second most-used

process was electrodialysis, with 250 plants and

a total capacity of about 40,000 gal/d. Though

there have been numerous reverse osmosis tests

on produced waters, little attention has been

paid to the use of electrodialysis for the treatment of produced waters of modest concentrations of dissolved solids.

Many produced waters, including most of

the same streams in CBM areas, contain modest TDS levels – less than 20,000 mg/L.These

waters may require only partial demineralization to achieve good brine volume reductions

and yield a demineralized steam suitable for

surface discharge or beneficial use. In recent

years, Gas Technology Institute (GTI) has

evaluated an electrodialysis demineralization

approach that may offer a cost-effective solution for the economical management of many

produced water streams with low to moderate

levels of contaminants.

The electrodialysis alternative

Electrodialysis is an electrically driven membrane separation process capable of separating,

concentrating and purifying selected ions from

aqueous solutions (as well as some organic solvents). In this process (Figure 2), ions are

transferred through ion-selective membranes

by means of a direct-current voltage.

Electrodialysis has been used in the United

States for decades. It was commercially available in the 1960s, about 10 years before the

introduction of reverse osmosis. During the

past 40 years, electrodialysis has provided

separations for the manufacturing of many

products, including dairy foods, beverages,

pharmaceuticals, metals and commodity biochemicals (such as organic acids). The development of the first generation of electrodialysis processing also represented a cost-effective way to desalt brackish water.

Figures 1 and 2 show the differences

between reverse osmosis and the electrodialysis process. With reverse osmosis, separation is

achieved by size exclusion of molecules larger

than water. A saline solution is passed under

pressure through the membrane and is separated from the solutes (the dissolved material).

Pressure drops across the membrane usually

range from 250psi to 400psi for the treatment

of brackish water and from 800psi to 1,180psi

in the treatment of concentrated salt water and

seawater (above 30,000 mg/L of TDS). The

major energy required for this process is

applied to the pressurization of the water.

Electrodialysis, on the other hand, is an

electrically driven separation process conducted at low pressure drops across the process

(usually less than 25psi). Electrodialysis for the

treatment of salt water depends on a number

of principles. Soluble salts exist in water as

ions. This includes positively charged ions

(cations) such as sodium, calcium, magnesium

and metals, as well as negatively charged ions

(anions) such as chloride, sulfide, sulfate and

bicarbonate. When electrodes are connected

to an outside source of direct current, an electrical current is passed through the water and

the ions migrate to the electrode of the opposite charge.

To achieve good separation, the movement

of ions (TDS) is controlled by the addition of

selectively permeable membranes that form

watertight compartments (Figure 3). Each

anion transfer membrane allows only the

transfer of negatively charged anions, and the

cation transfer membrane allows only the passage of positively charged cations. The membranes are electrically conductive and impermeable to water flow, even under pressure.

Using this arrangement, concentrated and

diluted solutions are produced in the spaces

between the alternating membranes. The

spaces between the membranes are called cells,

and two adjacent cells are called a cell pair.

Figure 1. The reverse osmosis (RO) process. Figure 2. The electrodialysis process.
Summer 2004 • GasTIPS 17

PRODUCED WATER

The conventional electrodialysis process consists of several hundred cell pairs and is called

a membrane stack. In practice, the electrodialysis system is composed of a series of stacks.

Periodically, chemicals can be passed through

the stack to achieve a clean-in-place operation.

A commercial electrodialysis system used in

the demineralization of water is shown in

Figure 4. An electrodialysis processing train

usually includes the following components

and steps:

• pretreatment;

• membrane stack;

• low-pressure circulating pump;

• power supply for delivering direct 

current; and

• post-treatment for water conditioning.

During the past 30 years, a number of features have been developed to improve the

electrodialysis process. In the 1970s, the electrodialysis reversal process was introduced

into the commercial sector. This process operates on a similar principle as the conventional

electrodialysis unit. First, the brine and product channels are constructed identically.

Second, the polarity of the electrodes is

reversed several times per hour, and the flows

are simultaneously switched so the brine

channel becomes the product water channel

and vice versa. The reversal feature is useful in

breaking up films, scales and other deposits,

and flushing them out of the process before

they can foul the membranes.The electrodialysis reversal process configuration, however,

does add some complexity in design and control. For example, for many water treatment

applications, effective clean-in-place operations can be accomplished with conventional

automated chemical treatments.

In practical operation, conventional electrodialysis and electrodialysis reversal process can

reduce salt concentrations to less than 200

mg/L of TDS before the internal resistance of

the solution to current flow – and, therefore,

power demand – rapidly increases. To maintain electrical conductivity (low resistance) at

low salt concentrations, a further improvement

was made in the process by placing ion

exchange media between the membranes.This

process, commercially known as electrodeionization (EDI), achieves low salt concentrations

(below 10 mg/L of TDS) of salts at reduced

power inputs and is widely used in the industry where high-quality water streams are

required for manufacturing.

During the years, electrodialysis has been

applied to achieve partial demineralization of

brackish saline waters to meet criteria for surface discharge or water supply uses. Most of

these applications did not require or utilize

EDI. Although valuable for many industrial

applications requiring the generation of ultrahigh quality water, the performance of EDI

usually exceeds the levels of treatment required

for upgrading brackish water or for the

desalinization of produced waters, where effluents containing less than 500 mg/L of TDS

would be environmentally acceptable.

Electrodialysis has the following potential

characteristics that would facilitate its application to the treatment of produced waters:

• the ability to simultaneously separate a

wide range of ionic constituents – not

only sodium and chloride ions, but also

barium, heavy metal cations, and volatile

acids (at neutral pH levels) – that may

comprise the majority of soluble organics

in most produced waters;

• applicability to treating produced waters

of moderate strength (up to 10,000

mg/L of TDS);

• the capability for high recovery of demineralized water where the product

stream is more than 90% of the flow of

the influent stream (compared with 50%

to 70% with reverse osmosis);

• the possibility to reduce brine volume by

a factor of 10:1 or more (compared with

3:1 with reverse osmosis);

• good control over the degree to which

salts are removed from the water stream

through adjustments of flow and electrical energy inputs;

• higher tolerance for higher levels of suspended solids compared with reverse

osmosis (requiring less rigorous pretreatment for particulate removal);

• low chemical use for pretreatment; and 

• membrane life expectancy of 8 years to

Figure 3. A schematic of industrial electrodialysis.

Figure 4. A commercial electrodialysis system for water treatment.

(Photo courtesy of Ameridia, a division of Eurodia Industrie S.A.)
18 GasTIPS • Summer 2004

PRODUCED WATER

10 years for electrodialysis (compared

with 1 year to 2 years for reverse osmosis).

Preliminary field evaluation 

of electrodialysis 

In recent years, GTI initiated an effort to

examine the performance and costs associated

with the application of electrodialysis to produced water management. The first electrodialysis test on such water was accomplished

in a pilot unit at a conventional-well field site

in the Wind River Basin near Lysite, Wyo.,

(Figure 5). Produced water at this site contained about 8,300 mg/L to 10,000 mg/L of

TDS, comprised mainly of sodium, chloride,

calcium and bicarbonate. These ions

accounted for about 93% of the dissolved

inorganic salts. The produced water also contained 65 mg/L of oils and greases, and more

than 330 ppm of biological oxygen demand as

measured with the standard 5-day test

(BOD5) – 70% of which was made up of

acetate and other volatile acids. The BOD5

parameter is a predictor of the amount of oxygen required in natural streams to oxidize the

organic carbon in a sample and is proportional

to the biodegradable total organic carbon

(TOC). The goal was to treat the produced

water to meet the NPDES criteria for surface

discharge and meet specifications for use of

the discharge water for irrigation and livestock watering.

The following unit processes were used in

the pilot treatment train:

• de-oiling via induced gas flotation to

remove oils and greases to achieve

NPDES oil and gas limits and protect

downstream unit processes;

• dissolved organics removal using two

biological fluidized bed reactor 

(FBR) processes in series: 1) an anoxic,

nitrate-consuming FBR to achieve large

reductions in soluble TOC; and 2) an

aerobic FBR to ensure efficient oxidation of soluble TOC and BTEX to low

levels; and 

• partial demineralization to achieve

NPDES criteria for TDS, chloride 

and sulfate.

The pilot system was operated for 65 days

to obtain water-quality data and assess treatment economics. A schematic of the pilot

process train is shown in Figure 6. Produced

water flow to the induced gas flotation

process and the FBR processes was 15 gpm.

A split stream of treated water from the aerobic FBR was fed to the on-site electrodialysis unit, which was designed and operated for

batch recirculation flows of about 0.5 gpm to

4 gpm, depending on the final TDS target for

the effluent. The electrodialysis unit was

operated in continuous mode for most of the

65-day period.The operation was interrupted

by 12 brief batch tests to determine the

energy inputs necessary to achieve 5,000

mg/L, 2,500 mg/L and 1,000 mg/L of TDS.

An acid wash was applied to the membranes

daily, in accordance with the manufacturer’s

recommendation for maintenance. During

the operational period, samples were collected throughout the pilot system to determine the effectiveness of each unit process in

the treatment of Lysite produced water. At

the conclusion of the 65-day test, the electrodialysis stack was dismantled and key electrode and membrane components were sent

to the manufacturer for inspection.

Results from the electrodialysis field pilot

system are summarized in Table 2. Overall,

the integrated treatment system performed

well throughout the test period. Consistent

with the nature of conventional natural gas

wells, the produced water contained elevated

levels of oils and greases, approaching 100

mg/L. However, the combination of induced

gas flotation – using a coagulant dose of 5

mg/L – and biological treatment was able to

reduce the oil and grease to 4 mg/L or less,

which offered sufficient protection to avoid

fouling of the electrodialysis unit. The “frontend” preprocessing was also able to achieve

nearly 90% removal of soluble organics before

the produced water was introduced to the

electrodialysis stage. The electrodialysis unit

was able to decrease the salinity of the product water from 9,000 mg/L to the desired target levels of 5,000 mg/L, 2,500 mg/L and

1,000 mg/L TDS. Power costs required for

each of these levels is shown in Figure 7

(based on an electricity price of $0.6/kwh),

based on actual electrical energy needed to

reach the target TDS level. These results

showed the Lysite electrodialysis pilot system

was capable of demineralizing a conventional

gas produced water stream from 9,000 mg/L

to as low as 1,000 mg/L for only $0.03/42-gal

bbl, or about $0.01/bbl to reach 2,500 mg/L.

Parameter Influent, mg/L Effluent, mg/L Overall Removal, %

Oil and Grease 90 4 95.5

BOD5 * 330 51 84.5

BTEX** 11 0.1 99.1

Total Dissolved 

Solids (TDS) 9,100 1,000*** 88.9

Table 2. Performance results for the integrated electrodialysis pilot unit.

* BOD5 = Biological Oxygen Demand measured in a 5-day test.

** BTEX = Sum of benzene, toluene, ethylbenzene and xylenes.

*** Generated at peak performance of the electrodialysis unit. 

Figure 5. A granular activated carbon-fluidized bed reactor and induced gas flotation unit is shown at the Gas Technology

Institute electrodialysis pilot site near

Lysite, Wyo.
Summer 2004 • GasTIPS 19

PRODUCED WATER

Projected economics 

Compared with current costs for SWD well

disposal, which can exceed $0.50/bbl, the

pilot results show that power costs ($0.01/bbl

to $0.03/bbl) for electrodialysis treatment to

achieve NPDES criteria or beneficial end-use

goals can be reasonable for processing produced waters containing up to 10,000 mg/L

of TDS. Above this concentration, however,

the power costs increase exponentially. For

example, the cost to treat a 15,000 mg/L TDS

influent to below 2,000 mg/L are projected to

be about $0.40/bbl, which may be prohibitive

for most field applications.

Based on these power cost relationships, it

appears electrodialysis may be a good solution

to the partial demineralization of CBM produced waters, which tend to be lower in TDS

concentration and very low in organic content. For example, the processing of 35,000

bbl of produced water per day to decrease the

TDS from 3,000 mg/L to 1,700 mg/L would

require about 16,000 sq ft of electrodialysis

cell surface area. The power cost for electrodialysis demineralization would be less than

$0.01/bbl. Total processing cost would be less

than $0.10/bbl, including preprocessing, electrodialysis, adsorption polishing and calcium

addition for adjustment of the sodium

absorption ratio – amortized capital, labor,

power cost and labor included.

Preprocessing of CBM produced waters

likely would not require the rigorous biological treatment needed for conventional gaswell produced water. Soluble organics probably would be less than 50 mg/L, and oil 

and grease levels would be less than 10 mg/L.

Preprocessing, therefore, would only require

optional de-oiling, suspended solids filtering

and surfactant-enhanced ultrafiltration to

provide adequate pretreatment (Figure 8).

This would yield savings of more than

$0.06/bbl compared with a system that

included biological treatment in the preprocessing. In addition, any soluble organics 

present in the CBM produced water, in the

form of volatile acids (such as acetic), would

be efficiently removed in the electrodialysis

process because these acids predominate in

the ionic form at the normal pH range of 

produced waters (above pH 6.5). The resulting concentrated produced water brine that

would require SWD well disposal is projected

at 0.08 gal for every gallon of produced 

water introduced into the electrodialysis unit.

This represents a brine reduction greater than

92%, which is based on previous electrodialysis performance levels observed with brackish

water treatment.

Potential for implementation 

Electrodialysis is not commercially applied to

produced water treatment due to the dominance of reverse osmosis and the lack of operational experience with electrodialysis in the

oil and gas industry. In view of numerous successful electrodialysis applications in manufacturing processes and in the commercial treatment of brackish waters for community water

service worldwide, electrodialysis, including its

processing variants of electrodialysis reversal

process and EDI, has potential for the management of growing produced water problems

in areas of the United States where re-injection capacities are constraining energy development. Electrodialysis may be a good fit for

CBM basins where produced waters contain

less than 15,000 mg/L TDS, and where interest is growing in the use of produced waters for

irrigation, watering of livestock and surface

discharge for maintaining fish habitat.

Within its economical niche of application

to produced water demineralization, electrodialysis has a number of potential advantages

compared with reverse osmosis:

• electrodialysis does not require high

feedwater quality and is less sensitive to

pretreatment problems. Less pretreatment results in substantially lower preprocessing cost for the produced water

demineralization system;

• in the processing of produced waters

with a high bicarbonate alkalinity, the

formation of precipitates, such as metal

carbonates, that is a problem with reverse

osmosis is not as much of a problem with

electrodialysis because of its lower operational pressures. This means pretreatment using acidification and carbon

dioxide degassing would not be needed

for electrodialysis; and 

• in single-pass processing, electrodialysis

has the potential to achieve higher recoveries of demineralized water with a concomitant generation of lower volumes of

brine requiring deep-well disposal. For

each gallon of produced water processed,

less than 0.1 gal of concentrated brine

would be generated that would require

Figure 6. Schematic of the Gas Technology Institute electrodialysis pilot unit.
20 GasTIPS • Summer 2004

PRODUCED WATER

deep well-disposal vs. 0.3 gal generated

with reverse osmosis.

Technology development needs 

Future development of electrodialysis for produced water applications should emphasize

the design of protective front-end processing

and automated clean-in-place technology that

ensures a 5-year to 7-year life for electrodialysis membranes with high dependability of performance, even with wide perturbations in

produced water quality that can be encountered in the field. Initial development should

continue to emphasize low- to moderatestrength produced water conversion to beneficial-use water streams to control electricity

costs in the operation of the electrodialysis

unit. Lastly, future development should aim to

minimize solid-waste generation and maximize process-train automation. If the process

train required little attention from on-site

operators, water-treatment companies and

other entities could participate in managing

multiple treatment systems in each producing

basin, thereby improving the economy of 

scale of operating such systems throughout 

the Rocky Mountain region. This business

model may be particularly beneficial for 

the CBM industry because produced water

output from each well is very high in the 

first 3 years, and tapers off during the next 

10 years to minimal levels; and substantial 

savings would be realized through the strategic

use of leased equipment. Electrodialysis 

treatment systems are highly modular by

nature. The maximum number of modules

would be in operation in the first years of

CBM well operation, but up to 80% of the

modules could be removed as produced water

output substantially decreased.

Summary

Electrodialysis is a proven economical 

separations process in a number of industries

and in the treatment of brackish waters around

the world. The results of recent GTI pilotscale tests in Wyoming on conventional gaswell produced water

– using de-oiling and

biological pretreatment followed by

electrodialysis – show

that such treatment is

technically possible

and economically

attractive for the processing of produced

waters with weak to

moderate levels of

TDS. Electrodialysis

is at the pilot-ready

stage of development

for produced water treatment applications. A

successful, cost-effective electrodialysis system

could yield substantial benefits for the public,

industry, agriculture and other stakeholders.

Electrodialysis could help substantially expand

the development of CBM reserves by reducing

10-fold the volume of brines requiring subsurface disposal, and also would generate substantial quantities of demineralized water for use as

a supplemental source of water for agriculture

and industry.

For more information about this research,

contact Tom Hayes, associate director,

environmental engineering, Gas Technology

Institute, at tom.hayes@gastechnology.org or

(847) 768-0722. ✧

References

1. ALL Consulting, Handbook on Coalbed

Methane Produced Water: Management 

and Beneficial Use Alternatives, prepared 

for Ground Water Protection Research

Foundation, U.S. Department of Energy

National Petroleum Technology Office and

Bureau of Land Management, available at

www.all-llc.com/CBM/BU/index.htm

(2003).

2. Arthur, D. et al., Coalbed Natural Gas

Resources: Beneficial Use Alternatives, Parts

1-3, GasTIPS (2003).

3. Lawrence, A.W. et al, Evaluation of

Results from Lysite Wyoming Produced

Water Treatment System Field Experiment,

GRI Report No. GRI-97/0287 (1998).

4. Doran, G. and Leong, L.Y.C., Developing

a Cost-Effective Environmental Solution for

Produced Water and Creating a ‘New’ Water

Resource, U.S. Department of Energy report

DOE/MT/95008-4 (2000).

5. AWWA, Water Desalting Planning 

Guide for Water Utilities, John Wiley & Sons

Inc. (2004).

Figure 8. The electrodialysis processing scheme for the conversion of coalbed methaneproduced water to beneficial-use water is shown above.

Figure 7. The power cost of electrodialysis.