# P roduced waters can contain significantlevels of inorganic and organic constituents. The waters usually contain elevated levels of total dissolved solids (TDS) concentrations between 2,000 mg/L and100,000 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 (perfield and per unit of energy produced) are significantly increasing, while re-injection capacities are remaining the same or decliningbecause 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 meetwater 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
S tates, 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, adesalinization 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.

So me 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 initiallyproduced, 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 hydrocarbonsincluding 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 

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by Tom Hayes, Gas Technology Institute 

# The Electrodialysis Alternative for Produced Water Management

T

wenty 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 largevolumes 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
HCO 3 mg/L - - - 150 – 2,000 2,000 - 10,000 T able 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 

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problems are largely responsible for the lack of deployment success of reverse osmosis in theoil 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. Mostof 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 beenpaid to the use of electrodialysis for the treatment of produced waters of modest concentrations of dissolved solids.

Many produced waters, including most ofthe same streams in CBM areas, contain modest TDS levels – less than 20,000 mg/L.Thesewaters 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 demineralizationapproach 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 alternativeElectrodialysis is an electrically driven membrane separation process capable of separating, concentrating and purifying selected ions fromaqueous 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
S tates 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 electrodialy sis processing also represented a cost-effective way to desalt brackish water.

F igures 1 and 2 show the differencesbetween 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 underpressure 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 anelectrically 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 connectedto an outside source of direct current, an electrical current is passed through the water andthe 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

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

U sing 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 

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The conventional electrodialysis process consists of several hundred cell pairs and is calleda membrane stack. In practice, the electrodialysis system is composed of a series of stacks.

Pe r iodically, 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

F igure 4. An electrodialysis processing train usually includes the following components and steps:

• p retreatment;

• membrane stack;
• l o w-pressure circulating pump;

• p ow er supply for delivering direct

current; and

• post-treatment for water conditioning.

Du r ing the past 30 years, a number of features have been developed to improve theelectrodialysis process. In the 1970s, the electrodialysis reversal process was introducedinto the commercial sector. This process operates on a similar principle as the conventionalelectrodialysis unit. First, the brine and product channels are constructed identically.
S econd, the polarity of the electrodes is

re versed 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 beforethey can foul the membranes.The electrodialy sis reversal process configuration, however,does add some complexity in design and control. For example, for many water treatmentapplications, effective clean-in-place operations can be accomplished with conventional
automated chemical treatments.

In practical operation, conventional electrodialysis and electrodialysis reversal process can

re duce 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.Thisprocess, commercially known as electrodeionization (EDI), achieves low salt concentrations (below 10 mg/L of TDS) of salts at reducedpower inputs and is widely used in the industry where high-quality water streams are

required for manufacturing.

Du r ing 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

ED I. Although valuable for many industrialapplications requiring the generation of ultrahigh quality water, the performance of EDI usually exceeds the levels of treatment required for upgrading brackish water or for thedesalinization of produced waters, where effluents containing less than 500 mg/L of TDS would be environmentally acceptable.

Electrodialysis has the following potentialcharacteristics 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 streamthrough adjustments of flow and electric al energy inputs;

• higher tolerance for higher levels of suspended solids compared with reverseosmosis (requiring less rigorous pretreatment for particulate removal);

• l ow 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 

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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 associatedwith 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,

c alcium and bicarbonate. These ions
accounted for about 93% of the dissolvedinorganic 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 (BOD 5 ) – 70% of which was made up of
acetate and other volatile acids. The BOD 5parameter 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 (T O C). The goal was to treat the produced
water to meet the NPDES criteria for surface discharge and meet specifications for use ofthe 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

r emove 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

r eductions 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 daysto 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

r ecommendation for maintenance. Duringthe operational period, samples were collected throughout the pilot system to determine the effectiveness of each unit process in the treatment of Lysite produced water. Atthe 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

r educe the oil and grease to 4 mg/L or less, which offered sufficient protection to avoidfouling 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 unitwas 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

r each 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 BOD 5 * 330 51 84.5

BTEX** 11 0.1 99.1
T otal Dissolved Solids (TDS) 9,100 1,000*** 88.9 T able 2. Performance results for the integrated electrodialysis pilot unit.

* BOD 5 = 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 L ysite, Wyo.Summer 2004 • GasTIPS 19 

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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-usegoals 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 solutionto the partial demineralization of CBM produced waters, which tend to be lower in TDSconcentration 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

r equire about 16,000 sq ft of electrodialysiscell surface area. The power cost for electrodialysis demineralization would be less than $0.01/bbl. Total processing cost would be lessthan $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. Pr eprocessing of CBM produced waterslikely would not require the rigorous biologic al 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.

Pr eprocessing, 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 thatincluded 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 ofproduced 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 than92%, 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 theoil 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 reversalprocess and EDI, has potential for the management of growing produced water problemsin 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 containless 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.

Wi thin its economical niche of applicationto 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 topretreatment problems. Less pretreatment results in substantially lower preprocessing cost for the produced water demineralization system;

• i n the processing of produced waters

with a high bicarbonate alkalinity, the formation of precipitates, such as metal

carb onates, that is a problem with reverse osmosis is not as much of a problem withelectrodialysis 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 

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deep well-disposal vs. 0.3 gal generated
with reverse osmosis.



### T

echnology development needs

F

uture development of electrodialysis for produced water applications should emphasize the design of protective front-end processing and automated clean-in-place technology thatensures a 5-year to 7-year life for electrodialysis membranes with high dependability of performance, even with wide perturbations inproduced water quality that can be encountered in the field. Initial development shouldcontinue 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 tominimize 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

mu ltiple 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 aroundthe world. The results of recent GTI pilotscale tests in Wyoming on conventional gaswell produced water – using de-oiling andbiological pretreatment followed by electrodialysis – show
that such treatment is technically possible and economicallyattractive 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 reducing10-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.

F

or 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

F

oundation, U.S. Department of Energy

N

ational 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
W

ater 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

Pr oduced Water and Creating a ‘New’ Water Resource, U. S. Department of Energy report DOE/MT/95008-4 (2000).

5. AWWA, Wa ter Desalting Planning
G uide 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.