research: a new process to remove salt and organic compounds from frack wastewater

$: Research: A new process to remove salt and organic compounds from frack wastewater$
PAPERPei Xu, Zhiyong Jason Ren et al.Microbial capacitive desalination for integrated organic matter and salt removal and energy production from unconventional natural gas produced water

ISSN 2053-1400

rsc.li/es-water

Environmental Science Water Research & Technology

Volume 1 Number 1 January 2015 Pages 1–122

EnvironmentalScienceWater Research & Technology

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PAPER View Article OnlineView Journal | View Issue

Environ. Sci.: Water Res.This journal is © The Royal Society of Chemistry 2015

aDepartment of Civil, Environmental, and Architectural Engineering, University of

Colorado Boulder, Boulder, CO 80309, USA. E-mail: [email protected];

Fax: +1(303) 492-7317; Tel: +1(303) 492–4137bDepartment of Civil Engineering, New Mexico State University, Las Cruces, NM,

88003, USA. E-mail: [email protected]; Tel: +1(575) 646-5870

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ew00050a

Water impact

The natural gas boom through hydraulic fracking generates a tremendous amount of wastewater containing high salinity andan efficient and sustainable treatment method. Much of such produced water being generated is in remote locations withenergy, which drives up treatment cost and promotes technologies with water reuse and energy generation capabilitiedesalination system produces energy and simplifies the treatment process through the integration of organic and salinitypoint-of-source treatment a more viable option.

Cite this: Environ. Sci.: Water Res.

Technol., 2015, 1, 47

Received 31st August 2014,Accepted 24th September 2014

DOI: 10.1039/c4ew00050a

rsc.li/es-water

Microbial capacitive desalination for integratedorganic matter and salt removal and energyproduction from unconventional natural gasproduced water†

Casey Forrestal,a Zachary Stoll,b Pei Xu*b and Zhiyong Jason Ren*a

The rapid development of unconventional oil and gas generates a large amount of contaminated produced

water. The treatment and reuse of such water generally require multiple treatment processes to remove

different constituents from the waste, such as salts and organic matter. This study characterized the

performance of a new microbial capacitive desalination technology for simultaneous removal of organic

pollutants and salts from shale gas produced water as well as its energy production during the process.

The microbial capacitive desalination cell (MCDC) was able to remove total dissolved solids (TDS) at a rate

of 2760 mg of TDS per liter per hour and chemical oxygen demand (COD) at a combined rate of 170 mg

of COD per liter per hour, which was 18 times and 5 times faster than the traditional microbial desalination

cell (MDC), respectively. The MCDC had a coulombic efficiency of 21.3%, and during capacitive

deionization regeneration, 1789 mJ g−1 activated carbon cloth (ACC) was harvested. One advantage of

MCDC is that all three chambers could be used to remove both organic and inorganic contaminants. The

reactor removed greater than 65% of the TDS and 85% of the COD in 4 hours of operation in the

desalination chamber, and more than 98% of the salts and 75% of the organics were recovered during the

regeneration process. This technology provides a new integrated process to complement current systems

for organic matter and salt removal as well as energy recovery from real produced water.

organic carbon but withoutlimited access to water ands. The microbial capacitiveremoval, which makes field

Introduction

Produced water generated during oil and gas exploration andproduction is a significant environmental impediment to theenergy industry. In the United States, the total producedwater volume was estimated to be at 21 billion barrels peryear, and its disposal costs an estimated $5 billion dollars.1

The produced water volume significantly increased in recentyears due to the dramatic expansion of natural gas produc-tion from unconventional sources such as shale gas, coalbedmethane, and tight sand.2 Natural gas is a low-cost andcleaner fuel source as compared to coal, but the productionof unconventional gas using hydraulic fracturing requires asignificant amount of water and produces a large amount ofwastewater.3–5

Handling, treatment, disposal, and beneficial reuse of pro-duced water have been great challenges for the industry andregulators.6,7 Produced water quality from different sitesvaries significantly, and the water generally contains multiplepollutants that cannot be discharged and reused directly.8,9

Both hydraulic fracturing flowback water and produced watercontain a wide range of hydrocarbons, salts, chemical

Technol., 2015, 1, 47–55 | 47

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http://dx.doi.org/10.1039/c4ew00050a

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http://pubs.rsc.org/en/journals/journal/EW?issueid=EW001001

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additives, sands, and occasionally naturally occurring radio-active materials.5 The quality of produced water is highlyvariable depending on the geochemistry of the formation,with total dissolved solids (TDS) ranging from 8000 mg L−1

(Greater Green River) to 180 000 mg L−1 (Marcellus Shale) andtotal organic carbon (TOC) ranging up to 2000 mg L−1.10

Currently, underground injection is the primary disposalmethod for onshore wells to maintain formation pressureand minimize disposal cost.1 Due to shortages of waterresources and the safety and health concerns associated withunderground injection, treatment and reuse of producedwater have become emerging needs. However, the transporta-tion of a large volume of water to existing wastewater treat-ment facilities is prohibitive in many areas because theassociated traffic and cost are very high, and the high salinityand hydrocarbon content in such water may negatively affectthe performance of traditional treatment processes.11 Foronsite produced water treatment, multiple technologies areoften combined to target removal of different constituents.For example, an air flocculation separator is in general thefirst step, which removes and recovers suspended oil. Then,separate processes such as biological treatment and mem-brane processes are connected to remove organic contentsand salts, respectively. Popular desalination technologiessuch as thermal evaporation and reverse osmosis (RO) can beeffective in salt removal but at very high operation and energycost. New membrane-based technologies such as forwardosmosis and reverse electrodialysis are currently being investi-gated as pretreatment options for RO to lower the requiredenergy demand.12 However, most membrane technologies areineffective in organic hydrocarbon removal and require exten-sive pre-treatment to protect system components.13,14

One approach to accomplish sustainable produced watermanagement is to develop technologies that remove bothorganic contaminants and salts without external energyconsumption or potential net energy gain. In this context,recently developed microbial desalination systems (MDS)may provide a niche in the market.15 MDS is based on thefundamental work on bioelectrochemical systems (BES),which employ microorganisms to breakdown organic or inor-ganic sources of electrons and transfer those electrons to aterminal electron acceptor such as oxygen through a pair ofelectrodes. The internal potential generated between theanode and the cathode drives additional salt removal, andthe energy can be harvested for electricity and chemicalproduction.16–18 Different reactor configurations have beenreported, such as a microbial desalination cell (MDC), inwhich three chambers were separated by a pair of ionexchange membranes and salt removal was accomplished bymigrating ions from the middle chamber to the anode andcathode chambers.19–23

This study used a newly developed microbial capacitivedesalination cell (MCDC) to demonstrate its efficacy inremoving both organic contaminants and salts from pro-duced water collected from a shale gas field and its energyrecovery during the operation.24,25 MCDC alleviates salt

48 | Environ. Sci.: Water Res. Technol., 2015, 1, 47–55

migration problems associated with MDC through the inte-gration with capacitive deionization (CDI). CDI is a desalina-tion method where an electrical potential is applied to highsurface area electrodes to adsorb charged organic and inor-ganic species for desalination.24 CDI is a dynamic process ofsalt removal and recovery.26 When the electrical potential isremoved, the capacitively desalinated salts can be removedand captured for beneficial use, and part of the electricalcharge can be recovered.16,24,25,25,27,28 CDI only requires asmall voltage (<1.4 V) to form the electric double layer, so itcan be externally powered by an MFC.28,29 Previous studiesshowed that such an MFC–CDI system could achieve a desali-nation rate of 35.6 mg of TDS per liter per hour, with adesorption rate up to 200.6 mg of TDS per liter per hour.27

The MCDC system consists of three chambers, the anode,cathode and middle chambers which contain the electrodesfor CDI. The use of microbial-generated energy applied toCDI electrodes allows for an energy-positive desalination sys-tem. A reactor with integrated capacitive electrodes has alsobeen studied for energy harvesting from the entropic energyreleased from mixing salt water and freshwater, called capaci-tive mixing.29 This study also compared the performancewith traditional MDC reactors. Compared to previous studiesthat employed multistage treatment processes for organicmatter and salt removal, this study introduces a new andsimple approach for energy-positive produced water manage-ment, and it is one of the few studies that used real producedwater samples instead of artificial mixtures. It is also the firstattempt at using the same raw feed water in all three cham-bers of the reactors, which greatly simplified the operationand brings the technology one step further toward application.

Materials and methodsProduced water characteristics and reactor design

The produced water samples were collected from a wastewatertreatment plant (WWTP) in Piceance Basin, Colorado, whichreceives wastewater during shale gas exploration and produc-tion. The water samples were pretreated in the WWTP topartially remove solid, oil, and volatile compounds throughhydrocyclones, dissolved air flotation, and air stripping. Table S1in the ESI† lists the main characteristics of the producedwater, which was used as the sole influent for the reactors inthis study. The total dissolved solid (TDS) concentrationwas 15 870 ± 290 mg L−1, the COD concentration ranged800–1100 mg L−1, and the pH value ranged from 7.4 to 7.8.The major ions detected in the produced water includechloride, alkali, calcium, magnesium and sodium, and therelative abundance of these ions is consistent with the litera-ture on produced water.5–8 Two experimental procedures weredesigned and operated for produced water treatment andenergy production. The first was a comparison study betweenthe MCDC using capacitive deionization and the MDC tech-nology using electrodialysis for desalination. The second onlyinvestigated the MCDC maximum desalination rate, operating

This journal is © The Royal Society of Chemistry 2015


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under advanced desalination methods with a regenerationmethod similar to capacitive mixing.

The MCDC reactor consisted of three cubic polycarbonateblocks with a 3 cm diameter hole forming an internal anodechamber, a desalination chamber, and a cathode chamberwith volumes of 23, 12, and 27 mL, respectively (Fig. 1).24,30

The anode and cathode chambers were 4 cm in length andthe desalination chamber had a length of 1.5 cm. Carbonbrush (Golden Brush, CA) was used as the anode electrode,and traditional air cathodes were made by applying0.5 mg cm−1 Pt/C and four PTFE diffusion layers on 30% wet-proofed carbon cloth.21,31 Two cation exchange membranes(Astom Corporation, Japan) were used to separate the threechambers. The use of two cation exchange membranes allowsfor cations to freely move throughout the entire reactor.24

Inside the desalination chamber, a capacitive deionization(CDI) module consisted of two activated carbon cloths (ACC)(with a BET surface area of 1019.8 m2 g−1, a thickness of0.7 mm, and a 40% w/w ethyl acetate uptake) electrodeassemblies, which were connected to the anode and cathode,respectively. Each assembly was split into three parts andplaced overlapping. Plastic mesh was used to prevent shortcircuiting between the electrodes. Spacing between theelectrodes was 2.1 mm. A total of 0.72 g of ACC was used forthe CDI module. Ni/Cu was used as a current collector(McMaster-Carr) for ACCs. The MDC reactor had the samereactor dimensions and used the same anode and cathodematerials as those of MCDC, except that an anion exchangemembrane (AEM) (Astom Corporation, Japan) was placedbetween the anode and the desalination chambers and aCEM was placed between the desalination and the cathodechambers. No CDI modules were integrated with MDC, and a1000 Ω resistor was placed between the anode and thecathode.

Reactor operation and controls

Both MCDC and MDC reactors were initially operated inmicrobial fuel cell mode by removing the middle chamber


Fig. 1 Configuration of the MDC (left) and MCDC (right) systems. Theimage represents the investigated small cube systems. The cut-out ofthe MDC shows where the anion exchange membrane and cationexchange membrane are located compared to that of the MCDCsystem with a blow-up of the capacitive deionization module in themiddle chamber.

and inoculating the anode chamber with activated sludge.Inoculation was only required initially for anode biofilmacclimation. When a repeatable voltage profile (>500 mV)was observed for three consecutive cycles, which suggestsstable microbial activity, the reactors were shifted to desali-nation mode by partially disassembling the reactors andquickly inserting the membranes and the middle chamberas described previously.20,32 The anolyte was transitioned from10 mM sodium acetate-buffered medium to 100% producedwater. In the comparative study, only a 200 mM phosphatebuffer solution was used as the catholyte (pH 7.0 ± 0.1), andproduced water was used in the desalination chamber. Eachof the three chambers was connected to a separate 100 mLreservoir, and water was continuously recirculated at a rate of2 mL min−1 between each chamber and reservoir.15 TheMCDC desalination chamber was first recirculated with pro-duced water to ensure saturated physical adsorption. Fullphysical adsorption was determined when the change in elec-trical conductivity was ~0 in the recirculation reservoir. Fol-lowing full physical adsorption the circuit was connected tomeasure the capacity of electrical adsorption. Based on theresults from an early feasibility study, the end of a desalina-tion cycle for the MCDC was determined when the voltageacross the CDI modules became stable, dV/dt ~0.24,25 Maxi-mum voltage was normally achieved in 2 hours. The end of adesalination cycle for the MDC was capped at 24 hours. Datafrom the MCDC and MDC were normalized per hour. Desali-nation for both MCDC and MDC would have continued pastthe predetermined cycle of operation, 2 hours for MCDC or24 hours for MDC, but the normalized cycle times allowedeach system to be honestly evaluated for comparison purposes.MDC operates on a longer time scale because it requires ionsto migrate across a membrane while CDI does not.

Following the MCDC desalination cycle, the CDI modulewas regenerated using one of two methods. The CDI modulewas either short circuited and regenerated within 1 hour orconnected to an external charge pump (Seiko Instruments,Japan), which connected to a capacitor (2.5 V and 12 F) forenergy harvesting. Regeneration was completed when themeasured voltage between the CDI modules was less than5 mV. Energy harvested from MCDC was determined basedon the equation E = ½CV2, where E is the energy in joules, C isthe capacitance in Farads, and V is the voltage in volts.

To evaluate the influence of the CDI module on the MCDCdesalination performance, two controls were investigated.The MCDC short circuit control investigated cation migrationwhen an electrical potential is generated by the anode andcathode electrodes. The anode and cathode electrodes wereelectrically connected, while the CDI module was shortcircuited. To evaluate cation migration without an electricalpotential, an MCDC open circuit control was performed bydisconnecting the anode and cathode and short circuitingthe CDI module.

In the advanced desalination study, only the MCDC wasevaluated. The catholyte was transitioned to 100% producedwater. Starting with an unsaturated CDI module, 100 mL of

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Fig. 2 Rate changes per hour in salt removal, COD, and pH for allthree chambers of the MCDC and MDC, as well as two controlreactors for 5 data sets. Salt removal rate was based on conductivitychanges, with positive values indicating salt accumulation and negativevalues indicating salt removal. For the COD removal rate, positivevalues indicate COD increase and negative values indicate CODremoval. Changes in pH are shown, with positive values indicating pHincrease and negative values indicating pH decrease.


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produced water was recirculated in the desalination chamberfor 1 hour while connected to the anode and cathode forin situ physical and electrical adsorption. Following the saltremoval through adsorption on the CDI module, the desali-nation chamber was completely removed of solution. Forregeneration, 100 mL of deionized water was recirculatedthrough the desalination chamber at a rate of 50 mL min−1

for 20 minutes while the CDI module was short circuited.Following regeneration, all of the solution was removed, andthe previously desalinated solution was reintroduced into thedesalination chamber. A total of three cycles were operatedwith the desalination and regeneration solutions. One cycleis defined as desalination and regeneration. All experimentswere repeated at least 3 times, and the average and standarddeviation were used in data presentation.

Analysis

Conductivity and pH (HACH, CO) were constantly measuredfor all chambers of the reactors before, during (20 minuteintervals), and after the experiments. Standard chemicaloxygen demand (COD) (HACH, CO) measurement was usedto determine the change in organic content. Dissolved organiccarbon (DOC) was measured using a Sievers 5310C SeriesTOC analyzer. The change in concentration of total dissolvedsolids (TDS) and ion concentrations were determined byOptima 3000 inductively coupled plasma (ICP) spectrometryand Dionex DC80 ion chromatography (IC). Total alkalinitywas measured using a HACH alkalinity test kit (Loveland,CO). The changes of different types of organic compoundsduring the experiment were characterized by fluorescenceexcitation–emission matrix (F-EEM) spectroscopy using anultraviolet radiation spectrometer (Beckman Coulter, CA) inconjunction with a spectrofluorometer (Edison, NJ). Sampleswere normalized at 5 mg L−1 DOC; contour lines were set at50 and a maximum intensity of 1.5 UV-vis suppression. Thevoltage of MDC was measured using a Keithley 2300 dataacquisition system, and the electrical potential between theelectrode assemblies in MCDC was monitored by a program-mable multimeter (Amprobe, WA). Reactor coulombic effi-ciency was calculated based on the fraction of electronsremoved from the organic electron donor that are recoveredas current through the external circuit.18,19 The energyharvested by MCDC using the charge pump from the activatedcarbon capacitor was defined as the coulombs harvested bythe charge pump versus the coulombs transferred to thecapacitors normalized by the weight of the activated carboncapacitor.

Results and discussionSalt and organic removal efficiencies by MCDC and MDC

The treatment efficiencies of salts and organic contaminantspresent in the produced water by the MCDC and MDC aresummarized and compared in Fig. 2. It depicts that thespecific ion removal rates from the MCDC were drasticallyfaster than those from the MDC. For the MCDC, the ion


conductivity in the desalination chamber using only electricaladsorption decreased on an average of 0.9 mS cm−1 h−1. Theconductivity in the anode chamber also decreased slightlyby 0.1 mS cm−1 h−1, while the cathode chamber slightlyincreased by 0.2 mS cm−1 h−1. The standard deviation for theMCDC fluctuation was hypothesized to be mainly due to thedynamic system of using raw produced water and microbialexoelectrogenic activities, which fluctuated by 33% and up to12% due to the heterogeneous nature. For the MDC configu-ration, the rate of ion removal was much slower than theMCDC. Over each 24 hour operational period, the MDC



Fig. 3 Electrical charges formed on the MCDC and voltage profile ofthe MDC. Major graph shows a full cycle of MDC, and for the sameperiod, multiple cycles of MCDC operation were performed. Graphinset shows three cycles of MDC operation, with arrows indicatingelectrolyte change and the last cycle used in the main graph.


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desalination chamber showed an average of 0.05 mS cm−1 h−1

ion removal rate, while the conductivity in the anode andcathode chambers increased by 0.04 mS cm−1 h−1 and0.07 mS cm−1 h−1, respectively. The MCDC higher removalrate is partially attributed to a 15% higher voltage outputthan that of the MDC, which may be contributed by the forcecharging associated with the capacitive electrodes.29 In termsof percent salt removal, the MCDC removed an average of8.6% of salt within its 2 hour cycle period, while the MDCremoved an average of 7.4% over a 48 hour cycle period.

When MCDC was operated in negative control mode,where the electrode assemblies were connected in short cir-cuit, the performance of the desalination chamber droppedto 0.1 mS cm−1 h−1, which is similar to the desalination ratein the MDC. This indicates that it was the electrical adsorp-tion that led to the faster desalination rate. The MCDC opencircuit control showed similar results to the negative control,with a removal of 0.15 mS cm−1 h−1. Both controls were oper-ated to elucidate the effects of reactor configuration on over-all performance. Results indicate a trivial contribution fromreactor configuration on system performance. The MCDCdesalination chamber ion removal rate is the fastest becauseof the high surface area and small distance for ion migration.By comparison, the low desalination rate in the MDC middlechamber was believed to be due to the low concentration gra-dient between the three chambers in MDC. Previous studiesshowed that high concentration gradients contribute signifi-cantly in MDC performance, and molecular diffusion acrossa membrane in electrodialysis could contribute up to 25% ofthe total ion flux.33,34

Because the MCDC uses two cation exchange membranes,ions migrate from the anode chamber to the desalinationchamber and then to the cathode chamber to balance thetransfer of electrons from the anode to the cathode. This isbelieved to be the reason that the cathode chamber conduc-tivity increased for both MDC and MCDC because it wasfound that such increases were proportional to the saltmigration rate from the desalination chamber to the cathodechamber. The ion increase in the MDC anode chamber wasexpected, as the MDC configuration does not completelyremove ions but rather ions migrate away from the middlechamber to the other chambers.

One of the most interesting findings in this study is illus-trated in Fig. 2. It was originally hypothesized that organicswould be removed in the anode chamber and salts would beremoved in the desalination chamber. The results indeedpresented that both MDC and MCDC anode chambersremoved about 50–80% of COD at a similar rate of 40 mg ofCOD per liter per hour, but more COD was found to be actu-ally removed in the desalination chamber of the MCDC at amuch faster rate, ranging from 100 to 160 mg of COD perliter per hour, which is faster than any other chambers inves-tigated. Unlike the anode chambers which oxidize theorganic faction of the COD to CO2 by microbial activities, theCOD removed in the desalination chamber is electro-chemically removed through adsorption. The high surface


area CDI module is believed to contribute to the fasterremoval rate, but further analysis will need to be conductedto quantify the contribution. The capacity of the CDI moduleto remove organic carbon was previously demonstrated inproduced water treatment,4 and it was suggested that poten-tially any charged organic molecule could be removed usingcapacitive deionization. Almost no COD was removed fromthe MDC desalination chamber or the cathode chambers ofboth configurations.

The pH changes in the three chambers between the MDCand MCDC also showed interesting though not significantdifferences. As shown in Fig. 2, the anode chamber pH in theMCDC decreased by 0.01 pH units per hour while the pH inMDC decreased by 0.08 pH units per hour. However, theMDC had little change in pH in the desalination chamber,while the MCDC decreased by 0.08 pH units per hour. In thecathode chamber, the MDC increased by 0.03 pH units perhour while the MCDC increased by 0.04 pH units per hour.These results indicate that MCDC configuration can reducepH changes in the anode with a different set of membranes,while pH change in the MCDC middle chamber was moresignificant. Fluctuations in pH in CDI units have beenobserved, and the causes were potentially linked to waterhydrolysis.35 Given no microbial functions were expected inthe middle chamber, such pH changes are not of muchconcern as they are in the anode chamber.

Power production and MCDC regeneration energy harvesting

Fig. 3 shows the comparison of the operational cycles andvoltage generation between MCDC and MDC during oneMDC cycle period, and the graph inset shows three con-secutive cycles of the MDC. The voltage was measured on a30 minute interval and error bars were based on 6 consecu-tive cycles. While the MDC was operated in longer cycles ofapproximately 40–70 hours based on anode COD removal,the MCDC was operated in much shorter desalination andregeneration cycles based on ion adsorption in the desalination




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chamber. For each repetitive MCDC cycle, the voltage acrossthe CDI module reached a maximum value of 558 mV,with an average of 530 mV. Anode and cathode potentialsversus an Ag/AgCl reference electrode can be seen in the ESI†(Fig. S2). Regeneration of the MCDC in the experiment wasachieved by either connecting the CDI modules in shortcircuit or connecting them with a charge pump for in situenergy harvesting to a 12 F external capacitor. The chargepump is a simple electrical circuit which passively collectsenergy.18 Energy harvested with the charge pump was notdue to capacitive mixing because the energy was harvestedwithout use of a low-salinity electrolyte. Energy harvestingwas not conducted for the advanced study or for the MDC.For the same normalized desalination period, the energystored on the MCDC CDI module was 1789 mJ g−1 ACC,which was calculated based on the coulombs transferred tothe CDI module during middle chamber desalination.36,37

The energy harvesting coulombic efficiency was calculated at0.94%, which indicates that the use of a charge pump is notsufficient for energy harvesting during the regeneration step.Further research will need to be conducted to improve thisenergy extraction efficiency.

For the MDC configuration, a typical voltage profile wasgenerated across the 1000 Ω resistor with a maximum voltageof 450 mV. The voltage increased rather fast in the MDCwithin the first 3–5 hours then stabilized until organic avail-ability became limited in the anode chamber, which causedthe voltage decline. The maximum current produced in theMDC reactor was 450 μA, which is comparable to previousstudies that used synthetic media and artificial saltwater.18,19,38 The maximum current calculated for the MCDCwas 1089 μA, and both current profiles can be found in theESI† (Fig. S1). More fluctuation in power output was observedcompared to previous lab studies, which is believed to be dueto the use of produced water with complex constituents. Thisfluctuation in power output is the main reason for the largestandard deviation error bars in Fig. 2.

Fig. 4 The removal and recovery of TDS, COD, and four predominantions in the three MCDC chambers in three successive cycles underadvanced operating conditions. The concentration difference betweeneach blue bar and red bar indicates the level of removal of a specificparameter, and the green bar shows the percentage of recovery. Thedata presented here are from three consecutive runs.

Advanced desalination capacity and rate of the MCDC system

Following the comparative feasibility study, a more thoroughinvestigation into the capacity of MCDC was performed. Itwas observed in the comparative study that faster cycles inMCDC can result in higher desalination efficiency. Shorter1 hour desalination and 20 minute regeneration cycles wereperformed based on the results from the comparative study.Fig. 4 shows the TDS and COD removal in three consecutivecycles of operation. Compared with Fig. 2, where removalrates were compared in one cycle, Fig. 4 shows the averageion removal of a typical 4 hour operation with three consecu-tive cycles. It was determined from control tests that physicaladsorption accounted for approximately 40% of the TDSremoved and electrical adsorption contributed to 60% of theremoval. This distribution is similar to previous results usingsynthetic salt water containing 10 g L−1 NaCl.24 The majorityof the salts were removed in the desalination chamber, with


65% of the TDS being removed in three cycles. This repre-sents a 10 600 mg L−1 TDS drop in 4 hours (2650 mg of TDSper liter per hour), which is dramatically faster than MDC.The adsorption capacity decreased over successive cyclesmainly due to the reduced regeneration efficiency that cannotrestore all adsorption capacities. This loss in capacity hasbeen observed in most CDI studies.39 With improvements inreactor design and operation, the loss of capacitance can beminimized; if the applied electrical potential is reversed, the



Fig. 5 Fluorescence excitation–emission matrix (F-EEM) showsfluorescing organic removal from produced water by the MCDCdesalination chamber. (A) F-EEM matrix before treatment and (B)F-EEM matrix after treatment cycles. Images were generated from aprewritten MATLAB program. Contour lines were set at 50 and amaximum intensity of 10.


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electrodes can be fully recovered.37 The change in pH for theanode and desalination chambers was similar to the pHchange in the comparative study. The change in pH for thecathode chamber when it switched to 100% produced waterhad a larger increase in pH (0.2 ± 0.1 pH units per hour) thanthat in the comparative study (0.003–0.04 pH units per hour),but the change was not significant due to the natural buffercapability of the produced water.

The removal of COD in successive MCDC cycles is shownin Fig. 4. During the three cycles, the anode chamber showeda COD removal of 20 mg L−1 during 4 hours, while the desali-nation chamber showed a COD removal of 660 mg L−1 with83% removal efficiency during the same period. As shown inFig. 4, after MCDC regeneration, 75% of the COD that wasremoved during desalination was fully recovered. It is quiteclear that the organic removal in the desalination chamberwas due to physical and electrical adsorption, which is muchfaster than the microbial oxidation in the anode chamber.There was approximately 25% of the COD that was not recov-ered, but no detectable COD was found on the ACC electrodeafter regeneration, and it was hypothesized that they werelost due to electrochemical oxidation. Even though CODadsorption on the CDI is not a permanent removal method,it fulfills the goal of quickly removing organic carbon fromthe desalinated solution and allows faster produced watertreatment, which is considered very important in the indus-try. The adsorbed COD can then be concentrated duringregeneration and served as electron donors for microbialmetabolisms. This is beneficial to the anode because lowelectron donor availability has been considered an issue forMDC applications.

Fig. 4 also shows the four predominant ion concentrationsfor all three chambers, initial, final or after desalination, andafter regeneration, to illustrate the ion balance over thecourse of desalination. While the anode and cathode showedlimited ion removal, the desalination chamber showed signif-icant removal of all species. Sodium and chloride concentra-tions make up a majority of the TDS in the produced waterand were predominantly removed. More than 66.3% of thesodium was removed, and then 98.9% of the removedsodium could be recovered during regeneration. Similar rateswere observed for chloride (69.5% in removal and 66.7% forrecovery). An average of 82% of the calcium and 69% of thebromide were removed, and the recovery for calcium was42% and for bromide was 81%. The lower recovery rate forcalcium may be due to its hardness nature, which has beenreported more difficult to desorb from the media.20 However,by reversing the polarity of the applied potential to the CDI,numerous studies have shown that the electrodes can be fullyregenerated, and the MCDC had been operated for more than300 cycles without showing significant decrease in regenera-tion performance. The reactor CE was calculated to be 21.4%,which is similar to previous studies, especially those thatused real industrial wastewater.15,22,24 The reactor CE will behighly dependent upon the organic carbon characteristics ofthe specific produced water.


Additionally, fluorescence excitation–emission matrix(F-EEM) spectroscopy was used to investigate the changes offluorescing compounds in the MCDC desalination chamberfrom the starting point to the end of the three desalinationcycles. The excitation and emission contour map is generatedby plotting each value against each other. Generally theF-EEM is classified into five different regions. Zones 1 and 2are for aromatic proteins, zone 3 is for fulvic acid-like com-pounds, zone 4 is for soluble microbial products, and zone 5is for humic acid-like organics. Fig. 5A and B show distinctdifferences in fluorescence intensity, which indicate that afterthe cycles, a majority of aromatic proteins and soluble microbialbyproduct-like materials have been removed. A separate studyfocusing on using F-EEM and other tools on produced waterorganic removal can be found in the study by Stoll et al.40

Outlook

Due to the pressing health and safety concerns associatedwith unconventional natural gas exploration and production,




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the oil and gas industry is searching for on-site treatmentand reuse options for produced water to replace deep wellinjection, open pond evaporation, or truck transportationand treatment. In this context, simple, effective, and low-costtreatment processes are urgently needed. For removing bothorganic pollutants and salts from produced water, currentsolutions require a long chain of biological and membraneprocesses with high energy demand. This study presents thatmicrobial desalination systems can be a new approach tosolve such problems, also with the benefit of energy output.Traditional microbial fuel cell-based MDC processes couldaccomplish organic matter and salt removal, but the rateshave been slow, and the amount of concentrates generatedcan be larger than the desalinated water. The MCDC systemlargely overcame these challenges by integrating capacitivedeionization into the system, which increased the desalina-tion rate 18 times and the COD removal rate 5 times fasterthan the traditional MDC.

Both MDC and MCDC require a certain amount oforganics in the produced water, so microorganisms can oxi-dize these electron donors to drive desalination. This require-ment may limit their applications for certain producedwaters, such as those from formations with very high salinitybut low organics, but because MCDC can accumulate andrecover organics from the CDI modules, the recoveredCOD can be recycled back to the anode chamber to alleviatepotential shortage of organics. Because the rate of microbialdesalination is correlated with the initial salt and organicconcentrations, it may not be as efficient as the membraneprocesses in producing directly reusable water (TDS<500 mg L−1). However, it can be an efficient pretreatmentfor membrane systems to reduce organic and salinityloadings and prevent membrane fouling, as well as providerenewable power sources. Additionally, the deionized watercan be replaced with treated effluent; the production of aconcentrated brine solution from the MCDC system may be avalue-added product, as opposed to cost, with industriesand municipalities requiring salt solutions for product pro-cessing. Overall, with further research and development, theunique feature of simultaneous organic matter and salt removaland energy production will help microbial desalination sys-tems play an important role in produced water management.

Acknowledgements

This work was supported by the US Office of Naval Researchunder Award N000141310901 and US National ScienceFoundation under Award CBET-1235848. We appreciateDr. Tzahi Y. Cath, Colorado School of Mines, for provingproduced water samples.

Notes and references

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2 EIA, Annual Energy Outlook 2013 with Projections to 2040.
Report #:DOE/EIA-0383(2013). Prepared by the U.S. EnergyInformation Administration (EIA). Release Date: April,2013. Available at: <http://www.eia.gov/forecasts/aeo/pdf/0383(2013).pdf>.
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7 P. Xu and J. E. Drewes, Sep. Purif. Technol., 2006, 52, 67–76.
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Sci. Technol., 2011, 45, 7655–7663.9 D. L. Shaffer, L. H. Arias Chavez, M. Ben-Sasson,

S. Romero-Vargas Castrillón, N. Y. Yip and M. Elimelech,Environ. Sci. Technol., 2013, 47, 9569–9583.

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research: a new process to remove salt and organic compounds from frack wastewater

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