Coal-bed-methane (CBM) gas recovery techniques are unique compared to other production methods.  Formation water must be removed, or “dewatered” as it holds the methane gas in the coal seam by hydrostatic pressure.  Removing the formation water de-pressurizes the formation, thus releasing the gas to production.  Initial water volumes are very high, but decrease rapidly to allow for the release of the methane gas.  Producers must manage these considerable volumes of water generated during the dewatering process.  Much of the water can be disposed of by direct discharge given the high quality of the CBM produced water in the Powder River Basin.  Produced water of a lower quality, however, must be managed depending on environmental compliance and economic objectives.  This would include volume of produced water, proximity to surface water, rights-of-way, influent chemistry, discharge quality requirements, land use provisions (public or private) and recycle objectives.

This article will compare two treatment plant designs configured to process produced water from the Powder River Basin.  Both designs incorporate reverse osmosis (RO) and recovery reverse osmosis (RRO), as this configuration has proven effective for meeting produced water treatment objectives.  The reverse osmosis RO/ RRO process has been permitted through the Wyoming Department of Environmental Quality/Water Quality Division (WDEQ) Chapter 3 process, requiring review and monitoring by the department’s Water and Wastewater Division engineers.  Both plants minimize waste by maximizing system recovery, and use an aeration pond for evaporating and concentrating the brine.  The plants are designed with bypass and blend provisions so the produced water can be blended to a wide range of discharge specifications.  Both plants maximize membrane performance with filtration and scale control, but differ in the approach to controlling scale.  It is the nature of the scale control that is the primary focus of this article.  The article will also discuss selected components of the treatment process for each plant and lessons learned as the plant design matured with treatment experience and the producer’s needs.

Influent and Effluent Criteria

Feedwater characteristics must be clearly understood in order to properly design the treatment plant.  This includes seasonal variability that may identify influent extremes or complex chemistries.  Waste and product stream characteristics must also be understood so that service factor, redundancy and compliance can be addressed in the plant system design.  By its inherent nature, CBM water is high in sodium and bicarbonate and low in hardness, and may also include suspended solids, iron, silica and barium.   Sodium is a closely monitored aspect of the treatment plant effluent.   Soils with an excess of sodium ions, as compared to calcium and magnesium ions, can impact the way plants adsorb water.  The ratio of the sodium to calcium and magnesium is referred to as the Sodium Adsorption Ratio (SAR).

The plants also require Wyoming Pollutant Discharge Elimination System (WYPDES) permits issued by the Wyoming DEQ for construction, operation and discharge of the produced water.  The plants are designed to discharge greater than 95% of the influent water into the Powder River.  The state permitting authority defines effluent standards to protect aquatic life and downstream uses of the water.  The treatment systems are designed with sufficient flexibility to meet the defined effluent recipe as it changes on a monthly basis. 

Application engineers use solubility indices to understand the relationship of the dissolved ions as they move through the treatment process.  For instance, one technique for predicting calcium carbonate solubility considers the bicarbonate/ carbonate and calcium concentration to access the potential for hardness scale formation.  This is the concept behind the Langelier Saturation Index (LSI).  A positive LSI denotes an increased potential for calcium carbonate scale formation while a negative LSI denotes that calcium carbonate may dissolve in the solution.  LSI is one of the many solubility indices that facilitate designer engineers’ understanding of ion interactions as water chemistries change through a process.  This information helps designers control the severity of the process and apply appropriate equipment and chemistries to moderate the behavior of the water as it progresses through the process.

Another constituent common in CBM water is silica.  Because of its unique chemistry, silica poses special treatment challenges to design engineers.  While the silica concentration in Powder River Basin produced water is moderate, the high recovery rate of the membrane system creates ideal conditions for silica to scale membrane surfaces.  Silica precipitation control is further complicated, since control techniques for other ions conflict with methods for controlling silica.

Geographical and Environmental Concerns

The Powder River Basin is a sparsely populated region of the country, and unlike water treatment plants in industrial or municipal applications, manpower coverage is intermittent.  This must be considered when designing the system to ensure sufficient redundancy to address uptime and reliability objectives.  Key considerations include redundancy, call-out features, response time and safety.

The two treatment plants discussed in this article are in the Powder River Basin.  Given the potential for inclement weather, inventory controls must incorporate the possibility for restricted site access during seasonal extremes.

Acid feed systems must be carefully designed to minimize risks to personnel and facility.   The volume of hydrochloric acid needed to neutralize the alkalinity inherent in the CBM water is considerable.  The acid is delivered in tanker trucks, often down lease roads and potentially during severe weather.  The acid should be stored outdoors in double contained tanks.  Feed lines and valves should also be double contained.  Tanks should be located as close as possible to injection points to minimize the length of the feed lines.

Another key criterion in system design is meeting discharge specifications to comply with WDEQ specifications for protecting aquatic life from toxicity.  The test commonly used to confirm compliance is referred to as Whole Effluent Toxicity Test, or WET test.  Effluent samples are collected at appropriate outfalls and analyzed to determine the impact of the discharge water on aquatic life in the receiving waters.  The acute WET test is a 48-hour static test using Daphnia magna (water flea) and an acute 96-hour static test using Pimephales promelas (fathead minnow), as collected from designated outfalls.  Toxicity occurs if mortality exceeds 50% for either species at the effluent concentrations.  Chronic WET testing is a 7-day test using Pimephales promelas.  A series of composite samples are collected over a number of days.  The subject water is diluted with synthetic lab water to evaluate the degree of toxicity as compared to the lab control sample.

Case Studies of Two CBM Treatment Plants

The first CBM treatment plant, which we will refer to as Plant A, was commissioned in 2006.  It was designed to process 120,000 bpd (795 m3/h or 3500 gpm) of produced water at peak production and discharge treated water to a tributary of the Powder River. The plant was designed to discharge to a blended sodium level and to meet WET standards.  The flow schematic for Plant A is shown in Figure 1.



Since the customer was eager to start processing the produced water as soon as possible, a temporary mobile treatment system was placed online while the permanent system was being installed. The mobile system components included media filtration and RO skids.  The skids were contained in fully automated trailers that included instrumentation and climate control.  Siemens personnel located onsite operated and maintained the application with support and critical spares sourced from the Siemens Colorado Springs, Colorado branch office.

Plant “B”

In 2008, Siemens was awarded a second operating contract for the treatment of PRB CBM water, which we will refer to as Plant B.   As of the date of this writing, the plant has not been commissioned.  The plant was designed to treat 72,000 bpd (477 m3/h or 2,100 gpm).  The Siemens engineers wanted to advance the Plant “A” design by focusing on hardness and silica scale formation and acid feed.

Siemens added ion exchange softening into the process flow as a key innovation over the Plant A design.  Ion exchange removes polyvalent cations from the feed water.   Constituents like calcium, magnesium, barium, and soluble iron are removed to very low levels as they are exchanged for sodium on the ion exchange resin. 

On first review, adding a sodium-form softener may not be an obvious addition as sodium is a strictly controlled effluent contaminant.  However, the amount of calcium and magnesium in the CBM water relative to the amount of sodium is low.  So the percentage increase in the amount of sodium is low.

The softener provides several advantages.  First the potential for scale formation is reduced by removing dissolved cations such as calcium, magnesium and barium.  This reduces the antiscalant and acid chemical requirement typically used to control solubility when solubility limits are challenged by influent concentration or by system design. 

Plant B operates at a higher pH than Plant A.  As stated previously, acid is typically fed to a neutral or slightly acidic pH range to control hardness scale.  Without acid feed, the bicarbonate alkalinity concentration increases, resulting in an alkaline feedwater condition.  The higher pH offers preferred operating parameters that increase the solubility of residual organics, thus reducing the potential for organic fouling on the membrane surface.  The higher pH shifts the boric acid to borate equilibrium so that the boron is more easily rejected by the membrane, resulting in lower boron concentrations in the effluent water.  The higher pH increases silica solubility, thus lowering the potential for silica fouling. 

A flow schematic of Plant B is shown in Figure 2.



Plant A - Lessons Learned

The CBM treatment system at Plant A was a confirmed success, as it provided the intended recovery rate, was reliable, and handled changing effluent water standards.  The system design for Plant A followed a conventional approach including influent settling, media filtration/iron removal, acid feed and RO.  As disposal cost is a primary driver for produced water projects, this objective was met by maximizing system recovery to minimize brine disposal costs. 

Managing the soluble iron was one of the challenges addressed by the design engineers.  Iron and other soluble metals can oxidize within the membrane modules and foul the membranes, resulting in reduced performance.  Plant A-produced water averaged greater than 10 ppm; however, it is not uncommon for Powder River Basin CBM water to contain 20-30 ppm of dissolved iron.  On initial inspection, iron-laden produced water may appear clear, but as the water is exposed to air and the iron oxidizes, the water takes on a rust color and becomes more turbid, representing increased loading for media filters and potential fouling of the membrane systems.

The client addressed the soluble iron problem by installing a “riprap” system, which is commonly installed to control bank erosion.  In this application, the influent was allowed to cascade over coarse stones to oxygenate the water and thus oxidize the iron prior to entering the influent equalization basin.  As a precautionary step, chlorine was added to further oxidize the soluble iron.  This process was successful in converting the iron to an insoluble form that could be removed by iron removal filtration media. 

Plant A was configured to address dynamic operating conditions as produced water systems do not operate at steady state conditions.  As defined by the discharge permit, effluent standards change monthly so as to limit the sodium loading into the receiving water.  A second challenge was the changing influent silica concentration.  Silica is a primary factor affecting system recovery for membrane systems.  In Plant A, influent silica fluctuated between 8 and 12 ppm, which resulted in a corresponding fluctuation in RO recovery, between 92 and 96%.



The following factors affected system productivity:

• When influent silica concentration was above design peak, RO system recovery was reduced,
• Silica concentration in brine needs to be limited for sustainable RO membrane performance,
• Antiscalant may not protect against silica scaling above a certain level,
• Silica levels appear to vary over time, either seasonally and/or with extended production from the CBM wells.

The Plant A design was able to address dynamic influent and effluent conditions.  Key to this capability was the excess capacity on the RO system and control strategies that optimized the system configuration. 

Acid feed control was vital to reliable operation of the Plant A produced water treatment system.  Failure to properly control acid addition can result in scale formation on membrane systems.   For instance, there was a period of time in which changing influent conditions resulted in scale formation on piping and the valves between the primary RO and the recovery RO.  Increasing the acid dosage to reduce the pH reversed the scaling process. 

Plant B – Design Improvements

Plant “B” has not been commissioned as of the date of this paper.  However, Siemens expects the following benefits from the Plant B design. 

CBM water is characteristically high in sodium and low in calcium and magnesium hardness.  The low hardness concentration makes it an ideal application for sodium-form ion exchange.  By removing the hardness ions, the risk of hardness scale formation is reduced and the need for acid also is reduced. 

CBM water contains relatively high concentrations of bicarbonate alkalinity.  Given the concentration effect across a membrane system, the pH of the feed water will increase to increase the solubility of silica and residual organics and improve the rejection of boron.

Acid has been eliminated for scale control, resulting in improved plant safety and system reliability.  Trucks hauling acid have been removed from the public highways and the risk to plant and personnel significantly reduced.  Acid feed equipment and controls have been removed from the system design as pH control is no longer a primary concern to system operation.   System reliability is improved as inventory control is less vulnerable to delivery interruptions.

Water Analysis Comparison of Plants A and B

Table 1 illustrates the changes to feed water quality while applying each scale control technique.  The feed water data is approximated from actual Powder River Basin produced water analyses.  The analyses are then modeled using the Dow Chemical Company’s Reverse Osmosis System Analysis (ROSA) program to illustrate the changing conditions. 

In the pH-adjusted feed water case (Plant A), the pH is reduced from 8.0 to 7.0 using hydrochloric acid as opposed to sulfuric acid, since the sulfate in sulfuric acid can cause scaling as it bonds with divalent cations present in the feed water.  Acid addition changes the characteristics of the water as hydrogen neutralizes bicarbonate to form carbon dioxide and chloride. There is a marked increase in chloride concentration from the contribution of hydrochloric acid.  The LSI is reduced from 1.2 to -0.1 in the raw water.

In the softened feed water case (Plant B), the divalent cations calcium, magnesium, strontium and barium are reduced to less than 0.3 ppm in the adjusted feed water.  There is a corresponding increase in sodium content as an equivalent amount of sodium is exchanged into the water. Unlike the acid feed case (Plant A), the pH and TDS stay about the same.  The LSI scaling calculation shows -1.2 in the adjusted feed water.  Furthermore, the barium sulfate and calcium fluoride scaling calculations also show reduced scaling potential with the softened feed water case (Plant B).

Conclusion

The Plant A design confirmed a high-rate design to economically treat produced water to a discharge standard.  The Plant B designed leveraged the inherent characteristics of CBM water to incorporate ion exchange and eliminate acid feed.

In 2006, Siemens designed, constructed, installed and began operation of a produced water treatment plant in Wyoming’s Powder River Basin (Plant A).  The plant used hydrochloric acid to control scale formation of hardness salts in the membrane system.  Influent silica presented operational challenges given seasonal variability and the concentration effect on membrane surfaces versus discharge effluent standards.  Siemens designed a second plant that has not been commissioned as of this writing.  Plant “B” eliminated hydrochloric acid feed from the design and replaced it with sodium- form ion exchange.  This design offers improved safety to personnel and the facility.   The design change also offers preferred operating characteristics for organic solubility, boron rejection and silica solubility.

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