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IWC-10-54 

HIGH RECOVERY REVERSE OSMOSIS FOR TREATMENT OF 

PRODUCED WATER 

BOB KIMBALL Camp Dresser & McKee, Inc. 

Helena, Montana 

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Key Words: Coalbed Methane, Produced Water Treatment, Reverse Osmosis, 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 important. 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 presentation 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 facility is one of the primary factors affecting the feasibility of 

well development projects in areas that are limited by injection well disposal capacity. This presentation will explore the 

limiting factors and costs for new and existing RO systems for the 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 pressure to apply direct 

treatment 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, CDM 

developed 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

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However, with appropriate 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 hardness, 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+ + HCO3

- ---> Na-R + CO2 + H2O (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 required 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 

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very small (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 chloride and sodium sulfate 

(compared to bicarbonate-based 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 + H2O (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 hydrochloric 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-bicarbonate 

anion 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 chemicals on roadways, even 

in remote areas, is a concern 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 require 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 technology that removes 

99+% of all dissolved salts in solution. RO is nonselective in that it removes both cations and anions 

simultaneously. Systems 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 

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is to implement the most efficient 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 very high concentration 

factors required for achieving high water recovery, 

most of the RO systems used to date for the 

treatment 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 sparingly 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

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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 pretreatment 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 recoveries exceeding about 

90%. CDM has developed solutions to these issues 

by 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 

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sites and subsequent rapid polymerization of 

silica from solution. 

The high recovery RO treatment process 

includes the necessary pretreatment 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 concentrate, 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 these 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 silica, 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 CaCO3. 

 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%

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 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 system 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. 

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Following are 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 (CaCO3) 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 the 

sodium 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: 

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 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 range 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 (CaCO3) 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 

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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. 

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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 approach 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 and

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 the author for

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

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