___________________________________________ 

##   

## IWC-10-54



# HIGH RECOVERY REVERSE OSMOSIS FOR TREATMENT OF

PRODUCED WATER

BOB KIMBALL Camp Dresser & McKee, Inc.
Helena, Montana___________________________________________ 

##   Key Words: Coalbed Methane, Produced Water Treatment, Reverse Os mosis, Powder River Basin, Silica, High Recovery, Brine Minimization, Zero Discharge

Abstract: As the need for produced water treatment associated with natural gas production increases, the ability of reverse osmosis (RO) systems to operate at high-efficiencies becomes more im portant. Recent studies have focused on developing methods for maximizing the recovery of RO systems for the treatment of gas field produced water. Being able to operate at maximum water recovery minimizes the volume of waste brine requiring disposal and therefore significantly reduces the overall cost of produced water management. This pr esentation will discuss tested methods of treatment that result in process water recoveries as high as 98% with minimal potential for membrane fouling. This paper will provide a comparison of a near zero-liquid-discharge (ZLD) process with more conventional brine disposal methods for produced water

residuals. All systems employed to treat produced water have a residual stream to manage. The cost-effective disposal of the concentrated residual stream from a produced water treatment fac ility is one of the primary factors affecting the feasibility o f well development projects in areas that are limited by injec tion well disposal capacity. This presentation will explore the limiting factors and costs for new and existing RO systems for th e purposes of reducing the cost of brine disposal. Case studies will be presented using a range of water chemistries typical of those seen in the industry. Results of tests run on typical water will also be presented.

INTRODUCTION

Natural gas production , especially from unconventional resources such as coal seams, tight
sands and shale formations, is one of the fastestgrowing energy sources in the United States and across the world. A primary factor considered for development of these resources is the management of water produced in association with the gas. Table 1 shows a summary of the typical quality of various produced waters in the U.S. While the water volume and quality of gas field produced water vary considerably between basins, a common characteristic of almost all produced water sources is the presence of total dissolved salts (TDS) at concentrations that, typically, are well above the secondary drinking water standard of 500 mg/L.

TABLE 1. CBM PRODUCED WATER QUALITY Methods of disposing of produced water require a range of management and treatment approaches and depend on several site-specific factors, including water quality, flow, landowner lease agreements, economics, discharge options and a variety of other influences. Industry currently uses several different water management tools, such as direct discharge, managed irrigation, storage and infiltration, and deep well injection. In some cases, these options are not sufficient and the only feasible option is direct “end-of-pipe” treatment. As development of the resources in the oil and gas industry increases, so too does the regulatory and public pr essure to apply directtreatment in the field. Unfortunately, all end- ofpipe technologies currently in use in the oil and gas industry are limited by an array of technical challenges and a substantially higher cost in comparison to traditional, non-treatment

alternatives. Much of the cost is attributed to the cost of brine disposal. In fact, in some instances, it has been shown that brine disposal accounts for nearly 50% of the cost of produced water treatment systems. Therefore, reducing the volume of brine requiring disposal is a key objective of most produced water treatment

systems. To help address these issues, CDMdeveloped and field tested an innovative, costeffective water treatment solution for the

removal of dissolved salts that maximizes treated water recovery for discharge or beneficial reuse, while minimizing chemical consumption and the volume of waste requiring disposal. The technology was originally developed for the

treatment of coal-bed methane sources in the Powder River Basin of Wyoming and Montana. Max Min Max Min Max Min

TDS 3,119 402 11,867 7,252 23,464 10,434

Cations

Na 800 110 2,673 1,976 5,939 3,674

Ca 69 6 1,839 13 119 7

Mg 46 1.6 7.77 0 39 2.88

Anions

As 0.0026 < 0.0002 NA NA NA NA

Fe 4.9 0.2 NA NA NA NA

K 18 3.8 NA NA NA NA

HCO3 - 2,100 275 6,612 5,250 14,701 6,083

Cl- 64 5.2 600 4 2499 668

SO4 2- 17 0.01 135 8.32 166.8 0 Power River

Basin, WY Piceance Basin,

CO San Juan Basin,

CO, NM, AZ, UT Parameter___________________________________________ 

##   However, with a ppropriate modifications to the pretreatment system, the technology has much broader application for the treatment of produced waters in other operating basins in the U.S., Canada and overseas. This report provides a description of the treatment process, the results of a three-month pilot test and case studies for the full-scale treatment of various water qualities.

EXISTING TREATMENT TECHNOLOGIES

Treatment of produced water from unconventional gas resources is a rapidly expanding science and technology. Much of the original development and application of these technologies began in the late 1990’s with the rapid growth of coal-bed methane production in the Powder River Basin (PRB) of Wyoming. In recent years, many of these same technologies are being expanded into
other basins. In order to be successful in most basins, the treatment technology must meet the following primary objectives: 1) it must be capable of removing dissolved salts and other contaminants from water to very low levels, 2) the system must produce a minimal brine stream requiring disposal, and 3) it must be cost-effective. To accomplish these objectives, there are essentially three treatment technologies in commercial use today. They include various proprietary and non-proprietary configurations involving ion exchange (IX), reverse osmosis (RO), and mechanical evaporation. Figure 1 illustrates the general range of application versus cost for the primary TDS removal technologies.

FIGURE 1. RANGE OF APPLICABILTIY VERSUS

COST FOR TDS REMOVAL TECHNOLOGIES

Ion exchange is typically the most cost effective technology for TDS levels less than about 750 mg/L. The exception to this is for treatment of
bicarbonate-based water, such as in the Powder River Basin, where the technology is cost effective up to about 3,000 mg/L TDS. The reason for this is discussed below. Reverse osmosis is generally the most cost effective technology for treating produced waters having TDS levels ranging from about 750 mg/L to 40,000 mg/L. Consequently, RO has the broadest range of application of any of the TDS removal technologies for treatment of produced waters. Above 40,000 mg/L, relatively expensive evaporation technologies are required. Evaporation and crystallization technologies are generally reserved for low volume, high TDS source, such as those produced in an IX and RO system. As a result, the objective of IX and RO systems is to operate at maximum water recoveries to produce the lowest volume, highest TDS brine possible.

ION EXCHANGE - Ion exchange (IX) technology is currently the most prevalent treatment technology used in the Powder River Basin, but limited application elsewhere. Produced water from CBM deposits in the PRB typically contains 1,000 to 4,000 mg/L of total dissolved solids (TDS). As shown in Table 1, over 90 percent of the TDS in produced water in the PRB is sodium bicarbonate and contains very little hard ness, chloride or sulfate. Because of this unique water chemistry, all of the ion exchange systems in the PRB are a single resin system consisting of either strong acid cation (SAC) or weak acid cation (WAC) resin operated in the hydrogen form. When produced water is passed through the cation resin, the hydrogen on the resin is exchanged for sodium (and other cations). The hydrogen released into the water then reacts with bicarbonate to produce carbon dioxide gas according to the following equation:

H-R + Na + + HCO 3 - ---> Na-R + CO 2 + H 2 O (1)

where R = resin

The carbon dioxide is subsequently stripped in a decarbonator, resulting in a net decrease in TDS.
When the resin becomes loaded with sodium, it will require regeneration with either hydrochloric acid or sulfuric acid to restore the resin to the hydrogen

from. The amount of acid re quired on a daily basis is directly proportional to the cation load (namely sodium) removed. With some of the proprietary IX technologies, the resulting brine stream is generally___________________________________________ 

##   very smal l (less than 2% of the feed water), resulting in very high water recoveries. More conventional IX systems have had limited success in achieving high

water recoveries due to the inherent inefficiencies associated with the regeneration process. Regardless of which IX technologies are used, this technology requires large volumes of acid and produces a brine stream that is acidic (due to the excess acid required for regeneration), resulting in the need to neutralize the brine prior to disposal. Outside of the PRB, ion exchange technology has limited application, due to the presence of much higher concentrations of mixed
salts, such as sodium ch loride and sodium sulfate (compared to bicarbonate-ba sed water). In this case a two stage treatment process consisting of a cation resin and an anion resin is required in order to achieve a net reduction in TDS. The two step process is illustrated by the following general equations:

Cations: H-R + Na + + Cl

- ---> Na-R + H + + Cl

- (2)

Anions:: OH-R + H + + Cl

- ---> Cl-R + H 2 O (3)

where R = resin

To operate a dual resin system, an additional chemical is required. In addition to the use of acid for regeneration of the cation resin, sodium hydroxide (caustic) is also needed to regenerate the anion resin, increasing the amount of brine requiring neutralization and disposal. The amount of caustic required is directly proportional to the amount of chloride (and other anions) removed. Because of the high chemical demand, treatment of produced waters containing mixed salt sources is generally cost prohibitive. In summary, while ion exchange technology has been proven to be effective in the Powder River
Basin, there are a number of important limitations that need to be considered: They include:

 Large Chemical Requirements and Potential

Liabilities. Ion exchange systems are chemically driven technologies that require large quantities of sulfuric or hydrochlor ic acid for regeneration

of cation resins and caustic for anion resins. The volume of acid required is directly proportional

to the cation load. The volume of caustic required is proportional to the non-bicarbonateanion load. The higher the cation and nonbicarbonate anion loads, the greater the chemical consumption and cost. In addition, the safety and liabilities associated with hauling large quantities of chem icals on roadways, even

in remote areas, is a co ncern for many operators.

 Brine and Effluent Neutralization Required. The waste brine produced from the regeneration of cation and anion ion exchange resins generally requires neutralization prior to disposal.

Neutralization increases treatment cost as well as disposal volumes. In addition, when only a
cation resin is used, the treated effluent is slightly acidic and requires neutralization, degasification, or blending with raw, untreated water to raise the pH prior to discharge.

 Limited Application Outside of Powder River

Basin: Ion exchange technology used in the Powder River Basin has limited application in other basins where the produced water often
contains anions besides bicarbonate, such as

chloride and sulfate. In order to treat waters containing these anions, an additional treatment train using anion resin is necessary to remove TDS. Anion resins requir e sodium hydroxide for regeneration, resulting in another chemical and additional waste for disposal. Consequently, use of ion exchange technology is generally cost prohibitive for treatment of produced waters containing mixed sodium salts.

REVERSE OSMOSIS – Reverse osmosis is a membrane separation te chnology that removes

99+% of all dissolved salts in solution. RO is nonselective in that it removes both cations and anions simultaneously. System s are constructed as modular, skid-mounted systems that can achieve relatively high capacities with a small footprint. As with IX, RO systems generate a high quality effluent for discharge and a brine stream requiring disposal. Reverse osmosis is a “pressure driven” technology operating on electricity rather than “chemically driven” as with IX technology; in most cases, this results in lower operating costs than IX due to the lost cost of energy in most producing fields. In
addition, because RO membranes remove both cations and anions simultaneously and is cost effective up to 40,000 mg/L TDS, it has much broader application than IX for treatment of produced waters across various basins. Several proprietary and non-proprietary configurations of RO involving varying levels of pretreatment have been tested and applied in various basins for the treatment of produced water. The goal of most produced water treatment systems___________________________________________ 

##   is to implement the most effici ent pretreatment system possible to allow maximum water recovery without membrane fouling. To help illustrate the challenges of achieving this objective, Figure 2 shows the relationship between the influent TDS and maximum theoretical water recovery and associated concentration factor (CF) of all salts in the water. Water recovery is defined as the treated water flow (or permeate) divided by the feed water

flow.

Since 99+% of all salts in the feed water are rejected by the membrane and end up in the concentrated brine stream, the higher the water recovery, the higher the concentration factor of salts that end up in the concentrated brine. For example, at 75% water recovery, the RO concentrate is 4 times as concentrated as the feed; at 95% water recovery,

FIGURE 2. WATER RECOVERY VS SALT

CONCENTATION FACTOR

the concentrate flow is 20 times as concentrated as
the feed. As a result, an increase of 20% in water recovery from 75% to 95% results in a five-fold increase in concentrate brine salinity. Any colloidal material or debris that passes through the RO pretreatment steps will also be concentrated in the RO system. As we approach recoveries near 98% (50 times concentration factor), this effect becomes more problematic for the system design and operation, and the potential effects of even minor feed stream constituents must be analyzed in order to optimize

the membrane-based brine minimization process.

As a result of the ve ry high concentration factors required for achieving high water recovery, most of the RO systems used to date for thetreatment of produced waters have not been costeffective. This is primarily due to inadequate pretreatment of the feed water that caused premature membrane fouling. Membrane fouling
results in reduced membrane flux rates, low water recoveries and increased cleaning frequency, all of which increases capital and operating costs. A detailed review of some of the produced water RO systems, primarily in the PRB, indicates that most of

the issues that have limited their success were a

result of:

 Poor filtration efficiencies. The presence of 1 to 3 micron clay particulates and coal fines in the raw produced water has resulted in difficult and costly filtration ahead of RO.

 Scaling from sparin gly soluble salts. In most cases, produced water contains iron, barium,

calcium and bicarbonate. Insufficient attention to these constituents has lead to precipitation and scaling of sparingly soluble compounds such as iron hydroxide, calcium carbonate, and barium sulfate, as the water recovery increases and their solubility limits are reached.

 Costly chemical additions. Many RO systems rely on pH adjustment using acid ahead of the RO system to limit scaling from calcium carbonate. Because of the high buffering capacity of many of the produced waters containing bicarbonate, a significant amount of acid is required resulting in an increase in TDS, lower water recoveries and a higher cost of operation.

 Silica precipitation at high water recoveries. Silica is often the limiting factor in achieving high water recoveries with existing RO systems. Based on the typical concentrations of silica in produced water, the maximum practical recovery of existing RO systems is generally limited to about 90%. Achieving recoveries beyond 90% requires additional considerations related to silica chemistry.

To address the issues, CDM has developed a new patent-pending approach that uses an efficient combination of treatment technologies, including advanced filtration, ion exchange, reverse osmosis and evaporation, arranged in an optimized sequence

that maximizes overall water recoveries and minimizes cost. For some produced water sources, the system requires very little chemical addition (only periodic regeneration of the IX softener) and

eliminates the need to haul brine off-site. This approach has been demonstrated on the bench and pilot-scale in the PRB to achieve a recovery greater than 97% and produce an effluent that is better than drinking water standards.

PROCESS DESCRIPTION 4 6.7 10 20 50

0 10 20 30 40 50 60

75% 80% 85% 90% 95% 100%Salt Concentration Factor

% RO Water Recovery___________________________________________ 

##   In order to develop a cost-effective treatment approach for produced waters, it is necessary to overcome the many challenges and

limitations associated with current RO technologies. Studies have shown that brine disposal represents the largest cost of treatment due to the cost of trucking, limited number of disposal wells and limited injection capacity where unconventional gas resources are being developed. Chemical consumption is also a key

consideration due to wide fluctuations of chemical prices and the need to haul large quantities hazardous chemicals on public highways and rural roads, especially in areas that are difficult to access year round. Consequently, CDM’s goal of developing a cost-effective treatment approach was focused on maximizing the overall recovery of the water treatment process while minimizing chemical requirements. This was accomplished by applying efficient RO pret reatment technologies to eliminate water quality limitations that cause membrane fouling so that the RO system would be limited only by osmotic pressure. As discussed previously, one of the primary, historical problems with reverse osmosis technology for treating produced water is membrane fouling. Fouling can occur from bacterial growth on membrane surfaces, scaling from hardness compounds (e.g., barium sulfate and calcium carbonate), plugging from very fine particulates (e.g., clay fines) present in the water, and from silica precipitation at water re coveries exceeding about 90%. CDM has developed solutions to these issuesby efficiently combining commercially available pretreatment technologies in the following order (Figure 3):

 Filtration to remove suspended particulates, including clay particles (1 to 3 micron range)

 Efficient hardness and metals removal using a

weak acid cation resin or sodium zeolite

softener.

 UV disinfection to minimize the potential for biological fouling

 High recovery reverse osmosis at maximum operating pressure (currently limited to 1200 psig)

 Efficient and cost-effective on-site evaporation of the RO brine concentrate, if necessary.

FIGURE 3. CMD TREATMENT PROCESS This pretreatment approach effectively removes all constituents that can cause membrane fouling. The only remaining constituent that prevents operating the RO system at the maximum recovery is silica. Until recently, silica management options were limited to the following:

 High pH precipitation with magnesium at elevated pH . This is costly due to the high reagent demand and volume of sludge produced.

 Ion Exchange using anion resin. While this is possible, it is cost prohibitive for this type of application.

 Increase pH to 10.5 where silica is more soluble. This requires significant and costly chemical additions to raise the pH beyond 10.5, especially for highly buffered water containing

bicarbonate.

Another alternative, which is the basis of CDM’s patent-pending approach, is to keep silica in solution in a supersaturated, or meta-stable state, long enough for the water to leave the RO system.

A review of numerous literature references indicates that silica can be supersaturated well beyond its solubility limit for relatively long periods of time at neutral pH when the following conditions persist:

 Hardness and metal ions, especially iron and aluminum, are not present. If not removed,

metal ions will form insoluble silicate compounds which will result in precipitation of

silica onto the membrane surface.

 Particulates are not present. If not removed, particulates, including bacteria, will provide the necessary seed crystals that provide nucleation___________________________________________ 

##   sites and subs equent rapid polymerization of

silica from solution. The high recovery RO treatment process includes the necessary pret reatment steps to allow super-saturation of silica by first removing hardness, metal ions and particulates without the continuous
addition of chemicals. In addition, because the bicarbonate is not destroyed through the addition of
chemicals, the final RO conc entrate, in the case of water in the PRB, will be high quality sodium
bicarbonate. As a result, sodium bicarbonate can be converted on-site to sodium carbonate decahydrate through mechanical evaporation and disposed of as a solid – eliminating the need to haul brine off site for disposal. To demonstrate the efficacy of the treatment process, CDM conducted bench- and pilot-scale testing. The results of th ese tests are summarizes in the following sections

BENCH SCALE TESTING

CDM completed a bench-scale test of its high recovery reverse osmosis process for treating coalbed natural gas (CBNG) produced water collected from gas fields in the Powder River Basin. Treatment is required prior to discharge via an NPDES permit to a nearby river. This water contained approximately 1,900 mg/L total dissolved solids (TDS), consisting mostly of sodium

bicarbonate but also contained trace level constituents, including si lica, hardness and other sparingly soluble salts (e .g., barium, strontium and iron). The primary objectives of the bench-tests were

to:

 Determine the potential for membrane fouling to occur, primarily from silica.

 Determine the maximum water recovery of the treatment process without observing membrane fouling. The target water recovery was 98%.

 Evaluate the effectiveness for meeting the SC discharge standards of 1,000  S/cm.

For the bench-scale test, the following treatment steps were completed:

 Raw water was softened through a weak acid cation (WAC) resin operated in the sodium

form.

 The softened water was filtered through a one micron cartridge filter.  The softened, filtered water was then treated to 85% recovery in a bench-scale RO test system.

 The reject from the low pressure RO system was then injected with 3.5 mg/L of an antiscalant and treated in the high pressure RO system to a final RO recovery of about 98%.

The results of the bench test are shown in Table

3 and are as follows:

 Silt density index (SDI) of the initial raw feed water was found to be too high to measure (> 5).

 The WAC treatment step successfully reduced

the hardness of the raw feed water to less than 1 mg/L as CaCO 3

.

 After softening and cartridge filtration, the SDI

of the RO feed water was reduced to less than

1.0.

 The final water recovery from the RO process was determined to be 98% (by volume) at a final pressure of about 1,100 psig.

TABLE 3. BENCH TEST RESULTS

Description Raw

Feed Soft

Feed RO

Perm RO

Reject

Commons

pH 8.1 7.5 6.8 8.4

SC, uS/cm 3,020 57 57,400

TDS 1,970 38 79,300
Total Alkalinity 1,700 1,600 26 68,000

Chloride 18 <1.0 825

Fluoride 3.3 0.2 175

Sulfate 138 <1.0 7,290

Nutrients

Ammonia, as N 3.0 0.4 <0.1 8.6

Phosphorus 0.09 <0.01 4.17

TOC <1.0 <1.0 100

Total Metals

Sodium 836 808 12 39,200

Calcium 7 <1.0 <1.0 10

Magnesium 5 <1.0 <1.0 1

Manganese <0.01 <0.01 <0.01 0.02

Potassium 7 2.00 <1.0 8

Barium 0.4 <0.1 <0.1 <0.1

Iron 0.50 0.04 <0.03 0.32

Strontium 0.50 <0.1 <0.1 <0.1

Silica 11.20 <0.2 482
(1) Overall recovery is 98%___________________________________________ 

##    There was no evidence of membrane fouling throughout the process, event at 98% recovery,

where the salt concentration was about 50 times

the feed concentration.

 The silica concentration was determined to be 482 mg/L, which is significantly above the solubility limit of about 120 mg/L. There was no silica visually present either in the final reject stream or in the equipment once it was

dismantled.

 A sample of the final reject stream was collected and maintained at room temperature overnight. After approximately 12 to 14 hours, precipitate began to form in the sample bottle, indicating that the silica remained at supersaturated conditions for extended periods of time before finally precipitating from solution.

 A sample of the final reject was also placed in an ice bath and left overnight. After being cooled, a significant volume of precipitate was formed at

the bottom of the container. This was

determined to be sodium bicarbonate salts.

The overall results from this test indicated that 98% recovery is possible for this water source without membrane fouling from ions such as silica

PILOT SCALE TESTING

To confirm bench-scale test results and demonstrate the efficacy of the treatment process, CDM conducted a full-scale continuous flow pilot test at a feed rate of 100-gpm or 3,500 barrels of water per day (BWPD). The test was conducted from September through November of 2007. The objectives of the pilot test were to:

 Verify the necessary treatment steps to meet treatment and recovery objectives.

 Determine the maximum possible recovery achievable for full-scale operation.

 Obtain site-specific information that will be used for design of the full-scale system.

 Evaluate the pre-filtration step for reducing the silt density index (SDI) to less than 3.0.

 Evaluate the RO membrane fouling potential and methods for minimizing fouling.

 Evaluate the quality of the RO permeate (product water) and reject streams.

 Confirm reagent consumption rates and other

consumables.

 Optimize the effectiveness of each treatment step, as deemed appropriate in the field. Figure 4 shows a picture of the pilot-scale treatment system operated using feed water supplied from an operator’s pipeline at a feed rate of 3,500 BWPD (100 gpm), producing a final concentrated brine flow of 2 to 3 gpm.

FIGURE 4. PILOT SCALE SETUP

Treatment steps consisted of:

1. Storing feed water in a 5,000 gallon tank

2. Filtration of the feed water through a 2 micron cartridge filter. Various cartridge filter sizes

were also evaluated.

3. Removing all polyvalent cations in a weak acid cation ion exchange softener. The ion exchange vessels were designed for off-site regeneration for purposes of the pilot test.

4. Disinfection of water using a UV light

5. Treatment of 85% of the filtered, softened and disinfected feed water in a low pressure RO (LPRO) system. This sy stem produced 85 gpm of permeate for discharge back into the produced water pipeline and 15 gpm of RO

concentrate. The LPRO concentrate was routed

to a small brine tank for further treatment.

6. Treatment of the LPRO concentrate in a high pressure RO (HPRO) system. Antisclant was injected ahead of a 5 micron cartridge filter prior to entering the HPRO system. The HPRO system was operated at various water recoveries with and without brine recycle.

7. Final concentrate was collected in a tank and reinjected back into the produced water pipeline.___________________________________________ 

##   Following ar e the major conclusions from the pilot test:

 The pilot successfully treated more than 500,000 gallons of feed at 98% recovery on an

intermittent basis and more than 3.5 million gallons of water at an average recovery of 97%.

 The combination of the LPRO unit and the first HPRO stage achieved an overall permeate water recovery rate of about 95%. This section of the pilot system exhibited stable operation with essentially no deterioration in performance over the entire test period.

 Adding brine recycle in the HPRO system was found to accelerate the rate of membrane fouling in the last high pressure RO stage. Adding brine recycle increases the residence time of supersaturated silica in the RO system and likely resulted in the precipitation of silica on the last stage membranes during early testing.

 The product quality had a TDS of less than 500 mg/L, which will easily meet effluent limits for NPDES discharge.

 The raw feed water contained higher than expected TSS concentrations resulting in frequent cartridge filter change-outs. Analysis

of raw feed water indicated that the TSS consists primarily of clay particles in the 1 to 3 micron size range. Consequently, the 2-micron cartridge filter proved to have the longest run life and lowest SDI (typically less than 3.0).

 The weak acid cation ion exchanger performed better than expected, with one vessel treating more than 4 million gallons with no change-out or regeneration and a hardness leakage below the target value of 5.0 mg/L as CaCO3.

 Given the limited data collected, the UV light appeared to provide effective control of microbiological organisms throughout each of the downstream pilot treatment system units.

CASE STUDY 1. POWDER RIVER BASIN

CDM completed an evaluation of its high recovery reverse osmosis technology for treatment of 1,000 gpm (35,000 BWPD) of coal-bed methane produced water in the Powder River Basin. This technology was being considered as an alternative to IX technology. Table 4 shows the design feed water quality and effluent targets. The feed water consists primarily of sodium bicarbonate with trace levels of ammonia. The water is also very low in hardness. The primary goals of treatment were to reduce the conductivity from about 3,000  S/cm to less than

1,000  S/cm. In addition, the water is discharged to a receiving stream which is used for irrigation purposes. Consequently, the sodium adsorption

ratio must be also be reduced from 61 to less than 3.

TABLE 4. POWDER RIVER BASIN TREATMENT

DESIGN BASIS

Parameter Feed Effluent Targets

Conductivity 2,933 <1,000

Hardness (CaCO 3 ) 33

Sodium 787

Bicarbonate 2,047

Total Nitrogen 2 1.1

SAR 61 <3

Based on the results of pilot scale testing, CDM completed a performance evaluation of the high recovery RO treatment process. To minimize the hauling of brine off site, this evaluation assumed that an evaporator would be installed to convert thesodium bicarbonate to a dry sludge suitable for offsite landfill disposal or resale. The results of this evaluation along with a comparison to the current IX technology are shown in Table 5.

TABLE 5. RO PERFORMANCE RESULTS VS

CURRENT IX TECHNOLOGY

Performance

Parameter CDM RO

Process with

ZLD Current IX Technology

Overall Recovery 100% 97+%

Effluent Electrical Conductivity (EC) 235 mS/cm 1,000 mS/cm

Off-Site Brine Disposal 0 bbl/wk 4,800 bbl/wk

Brine Composition Sodium

Bicarbonate (97% Pure) Sodium

Chloride (acidic)

Gypsum Addition for SAR Adjustment 0 tons/yr 865 tons/yr

Acid Consumption (32% HCl) <750 gal/wk 40,000 gal/wk

As shown, the major benefits of the high recovery RO process compared to IX technology for this application include:___________________________________________ 

##    High Overall System Recovery: With the on-site evaporator, the overall system recovery is 100% for the RO process where as the IX system is

97%.

 High Quality Effluent: Because of the high membrane rejection rate, the effluent quality from the RO system easily meets effluent conductivity standards. The IX system produces higher TDS due to need to neutralize the acidic effluent with lime prior to discharge.

 No Off-site Hauling of Brine: The RO concentrate is treated in an evaporator to produce carbonate sludge for on-site or off-site disposal or potentially resale, effectively eliminating the number of truck hauls of brine off-site. The IX system produces a sodium chloride brine which requires a crystallizer to convert it to a dry salt. Crystallizers are more costly to construct and maintain than an

evaporator.

 No Need to Add Gypsum. The RO treatment process reclaims hardness ions present in the feed water for adjustment of the SAR prior to discharge. Consequently, the RO approach does not require addition of gypsum, which is necessary for the IX process.

 Much Lower Chemical Requirements: The RO system requires a relatively small amount (<750 gallons/wk) of HCl to regenerate the WAC softener as compared to the IX system which uses an estimated 40,000 gallons per week.

The RO treatment process achieves high effluent quality, low volumes of waste requiring disposal, and significantly less chemical addition than the IX process. This results in a safer, more reliable process to operate with a reduced liability of hauling brine and hazardous chemicals on roadways. In addition, because of the low chemical requirements, the RO system has a much lower and more stable O&M cost structure that is less susceptible to commodity chemical price fluctuations.

CASE STUDY 2. PICEANCE BASIN

CDM completed another evaluation of its high recovery reverse osmosis technology for treatment of tight gas in the Piceance Basin in Colorado. As shown in Table 6, the quality of this

water is much different than CBM water in the PRB. Key differences include:  Higher TDS. The TDS of the feed water is greater than 11,000 mg/L. As shown in Figure 1, this level of TDS is in the cost-effective range for RO technology.

 High Concentrations of Mixed Sodium Salts.

This water contains a mixture of both sodium

bicarbonate and sodium chloride salts. This is one of the key differences as compared to CBM water in the PRB. Because of the high
concentration of non-bicarbonate salts, the application of IX technology would require a dual IX train consisting of both cation and anion resins and two different chemicals (an acid and a base) for regeneration. As a comparison, both sodium salts are removed simultaneously in a single RO system.

 Higher Hardness. The hardness of the feed water is 213 mg/L.

 Higher Silica. The feed water contains silica at or slightly above saturation. Consequently, pretreatment for silica removal is required ahead of the RO system.

 Contains Petroleum Hydrocarbons. The feed water also contains hydrocarbons that must be removed prior to RO treatment and discharge, including oil and grease and light petroleum hydrocarbons.

 Lower Feed Water Volumes. Finally, the volumes of produced water in the Piceance Basin are much lower than in the PRB. Typical treatment capacities rang e from 2,500 barrels of water per day (BWPD) to 10,000 BWPD versus
more than 30,000 BWPD in the PRB.

TABLE 5. PICEANCE BASIN TREATMENT

DESIGN BASIS

Parameter Feed Effluent Targets

TDS 11,215 <500

Hardness (CaCO 3 ) 213

Sodium 4,036

Bicarbonate 2,059

Chloride 5,202

Sulfate 21

Oil/Grease 438 <10

Silica 119

The treatment approach for this application requires the addition of a hydrocarbon removal step and a softening step ahead of the RO system. This is___________________________________________ 

##   necessary in order to maximize overall water recovery by minimizing the potential for membrane fouling from organics, silica and harness ions. There are a wide range of organic removal steps available on the market. For purposes of this study, CDM proposed the following treatment process steps (Figure 5).

 Dissolved Gas Flotation. This step involves the

addition of sulfuric acid ahead of the DGF unit to remove bicarbonate and help break any emulsions present in the feed water. A petroleum byproduct stream would be produced for potential sale or off-site disposal.

 Lime Softening. A lime softening step involves adding lime and magnesium sulfate (or magnesium chloride) to precipitate calcium carbonate and magnesium silicate. The system would be operated at high pH, which would also kill bacteria present in the feed water.

 High Recovery RO Treatment Process. Following pretreatment, the water would be fed to the same RO process described above. Some of the pretreatment steps ahead of the RO system could potentially be eliminated based on the degree of pretreatment achieved in the lime softening clarifier. Using this treatment approach, the following results are expected for this application:

 Effluent TDS: 478 mg/L

 Estimated Water Recovery: 90% (with evaporator)

 NaCl Brine Requiring Disposal: 10% of feed

volume

 Chemical Usage (for added steps):

– Sulfuric Acid: 0.4 lbs/bbl

– Magnesium Sulfate: 0.15 lbs/bbl

– Lime: 0.084 lbs/bbl

 By-products:

– Oil: 10 bbl/d

– Lime Sludge: 4.5 wet tons/d FIGURE 5. TREATMENT PROCESS FOR

PICEANCE BASIN

In summary, the CDM high recovery RO treatment process is a field tested and proven treatment system that is adaptable and cost-effective for the treatment of a wide range of gas-field produced waters. The RO process requires very little chemical addition and produces a minimal brine stream requiring off-site disposal. A full-scale system is currently being designed and constructed for treatment of 1,000 gpm (35,000 BWPD) of CBM produced water in the Powder River Basin. Startup is expected in the Spring of 2011.

### Discussion by Francis Boodoo of:

High Recovery Reverse Osmosis for Treatment of Produced Water

By Bob Kimball, Camp Dresser & McKee Inc. Helena, Montana

Discussion by Francis Boodoo (Purolite):

Effective control over silica scaling is essential if high recovery ROs are to be used for treating
produced water from CBM applications. This paper pushes the envelope with the introduction

of a bold treatment ap proach that relies on the virtual elimination of divalent cations and

colloidal particulate from the feedwater to the RO. The paper recognizes that the presence of

hardness, iron, aluminium and similar metals can co

‐ precipitate with silica as concentrations

increase during the desalination process. Equally important is the conclusion that any particulate present in the RO concentrate can act as nucleation sites for precipitation of silica

from a supersaturated solution; thus pre

‐ treatment of the feed for removal of colloidal
particulate such as clay and bacteria is an essential prerequisite for the proposed recovery rates

of 98% or higher. The technology eliminates the use of expensive acid for pH control of the feed

water, thus creating the conditions for zero liquid discharge by concentrate evaporation,

converting the largely sodium bicarbonate solution into solid sodium carbonate decahydrate.

While the some of the unit operations proposed for the process are well established, such as softening, UV and filtration, the innovative part of the process is really derived from the new understanding that supersaturated silica under neutral pH conditions can remain in solution
long enough to allow operation of RO systems with silica concentrations as high as 450 mg/l.

While the author correctly identifies pre

‐ treatment issues that have limited the efficiency an d

the recovery rates of RO systems treating produced waters, such as poor filtration, costly

chemical dosing, scaling and silica precipitation, the author fails to point out that new technology introduced in the last few years has significantly helped in resolving many of these

issues, allowing RO recovery rates higher than the 90% limit indicated by th e author for

conventional treatment. For example, James Welch, author of an article published 5 th 

### October

2009 in International Petroleum News and Technology entitled “Reverse osmosis treatment of

CBM produced water continues to evolve” clearly advises that sodium zeolite softening can be

cost effectively used to remove divalent cations from produced water since the impact of the

sodium in the brine regenerant is manageable from a discharge standpoint due to low hardness

content of th e water. As a result, costly acid for pH control can be largely eliminated, as well as

the consequent increase in chloride it produces.

The benefit of eliminating acid is the higher pH in the RO concentrate which helps to increase

the solubility of silica and TOC. Mr. Welch reports recovery rates rangin g from 92% to 96% for

waters with similar concentrations of silica of about 9 to 11 mg/l. Overall, the High Recovery RO

process for produced water proposed by the author is definitely worth further consideration as

it offers the industry a technology that promises to significantly reduce overall treatment cost

for produced water.