Research: A new process to remove salt and organic compounds from frack wastewater

PAPER
Pei 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
WaterResearch&Technology
Volume 1 Number 1 January 2015 Pages 1–122

Environmental
Science
Water Research & Technology
PAPER
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 integrated
organic matter and salt removal and energy
production from unconventional natural gas
produced 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.
Introduction
Produced water generated during oil and gas exploration and
production is a significant environmental impediment to the
energy industry. In the United States, the total produced
water volume was estimated to be at 21 billion barrels per
year, and its disposal costs an estimated $5 billion dollars.1
The produced water volume significantly increased in recent
years due to the dramatic expansion of natural gas produc-
tion from unconventional sources such as shale gas, coalbed
methane, and tight sand.2
Natural gas is a low-cost and
cleaner fuel source as compared to coal, but the production
of unconventional gas using hydraulic fracturing requires a
significant amount of water and produces a large amount of
wastewater.3–5
Handling, treatment, disposal, and beneficial reuse of pro-
duced water have been great challenges for the industry and
regulators.6,7
Produced water quality from different sites
varies significantly, and the water generally contains multiple
pollutants that cannot be discharged and reused directly.8,9
Both hydraulic fracturing flowback water and produced water
contain a wide range of hydrocarbons, salts, chemical
Environ. Sci.: Water Res. Technol., 2015, 1, 47–55 | 47This journal is © The Royal Society of Chemistry 2015
a
Department of Civil, Environmental, and Architectural Engineering, University of
Colorado Boulder, Boulder, CO 80309, USA. E-mail: jason.ren@colorado.edu;
Fax: +1(303) 492-7317; Tel: +1(303) 492–4137
b
Department of Civil Engineering, New Mexico State University, Las Cruces, NM,
88003, USA. E-mail: pxu@nmsu.edu; 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 and organic carbon but without
an efficient and sustainable treatment method. Much of such produced water being generated is in remote locations with limited access to water and
energy, which drives up treatment cost and promotes technologies with water reuse and energy generation capabilities. The microbial capacitive
desalination system produces energy and simplifies the treatment process through the integration of organic and salinity removal, which makes field
point-of-source treatment a more viable option.
Publishedon05November2014.Downloadedon23/03/201514:23:03.
View Article Online
View Journal | View Issue

48 | Environ. Sci.: Water Res. Technol., 2015, 1, 47–55 This journal is © The Royal Society of Chemistry 2015
additives, sands, and occasionally naturally occurring radio-
active materials.5
The quality of produced water is highly
variable 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) and
total organic carbon (TOC) ranging up to 2000 mg L−1
.10
Currently, underground injection is the primary disposal
method for onshore wells to maintain formation pressure
and minimize disposal cost.1
Due to shortages of water
resources and the safety and health concerns associated with
underground injection, treatment and reuse of produced
water 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 the
associated traffic and cost are very high, and the high salinity
and hydrocarbon content in such water may negatively affect
the performance of traditional treatment processes.11
For
onsite produced water treatment, multiple technologies are
often combined to target removal of different constituents.
For example, an air flocculation separator is in general the
first step, which removes and recovers suspended oil. Then,
separate processes such as biological treatment and mem-
brane processes are connected to remove organic contents
and salts, respectively. Popular desalination technologies
such as thermal evaporation and reverse osmosis (RO) can be
effective in salt removal but at very high operation and energy
cost. New membrane-based technologies such as forward
osmosis and reverse electrodialysis are currently being investi-
gated as pretreatment options for RO to lower the required
energy demand.12
However, most membrane technologies are
ineffective in organic hydrocarbon removal and require exten-
sive pre-treatment to protect system components.13,14
One approach to accomplish sustainable produced water
management is to develop technologies that remove both
organic contaminants and salts without external energy
consumption 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 the
fundamental work on bioelectrochemical systems (BES),
which employ microorganisms to breakdown organic or inor-
ganic sources of electrons and transfer those electrons to a
terminal electron acceptor such as oxygen through a pair of
electrodes. The internal potential generated between the
anode and the cathode drives additional salt removal, and
the energy can be harvested for electricity and chemical
production.16–18
Different reactor configurations have been
reported, such as a microbial desalination cell (MDC), in
which three chambers were separated by a pair of ion
exchange membranes and salt removal was accomplished by
migrating ions from the middle chamber to the anode and
cathode chambers.19–23
This study used a newly developed microbial capacitive
desalination cell (MCDC) to demonstrate its efficacy in
removing both organic contaminants and salts from pro-
duced water collected from a shale gas field and its energy
recovery during the operation.24,25
MCDC alleviates salt
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 high
surface area electrodes to adsorb charged organic and inor-
ganic species for desalination.24
CDI is a dynamic process of
salt removal and recovery.26
When the electrical potential is
removed, the capacitively desalinated salts can be removed
and captured for beneficial use, and part of the electrical
charge can be recovered.16,24,25,25,27,28
CDI only requires a
small voltage (<1.4 V) to form the electric double layer, so it
can be externally powered by an MFC.28,29
Previous studies
showed that such an MFC–CDI system could achieve a desali-
nation rate of 35.6 mg of TDS per liter per hour, with a
desorption 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 electrodes
for CDI. The use of microbial-generated energy applied to
CDI electrodes allows for an energy-positive desalination sys-
tem. A reactor with integrated capacitive electrodes has also
been studied for energy harvesting from the entropic energy
released from mixing salt water and freshwater, called capaci-
tive mixing.29
This study also compared the performance
with traditional MDC reactors. Compared to previous studies
that employed multistage treatment processes for organic
matter and salt removal, this study introduces a new and
simple approach for energy-positive produced water manage-
ment, and it is one of the few studies that used real produced
water samples instead of artificial mixtures. It is also the first
attempt at using the same raw feed water in all three cham-
bers of the reactors, which greatly simplified the operation
and brings the technology one step further toward application.
Materials and methods
Produced water characteristics and reactor design
The produced water samples were collected from a wastewater
treatment plant (WWTP) in Piceance Basin, Colorado, which
receives wastewater during shale gas exploration and produc-
tion. The water samples were pretreated in the WWTP to
partially remove solid, oil, and volatile compounds through
hydrocyclones, dissolved air flotation, and air stripping. Table S1
in the ESI† lists the main characteristics of the produced
water, which was used as the sole influent for the reactors in
this study. The total dissolved solid (TDS) concentration
was 15 870 ± 290 mg L−1
, the COD concentration ranged
800–1100 mg L−1
, and the pH value ranged from 7.4 to 7.8.
The major ions detected in the produced water include
chloride, alkali, calcium, magnesium and sodium, and the
relative abundance of these ions is consistent with the litera-
ture on produced water.5–8
Two experimental procedures were
designed and operated for produced water treatment and
energy production. The first was a comparison study between
the MCDC using capacitive deionization and the MDC tech-
nology using electrodialysis for desalination. The second only
investigated the MCDC maximum desalination rate, operating
Environmental Science: Water Research & TechnologyPaper
View Article Online

under advanced desalination methods with a regeneration
method similar to capacitive mixing.
The MCDC reactor consisted of three cubic polycarbonate
blocks with a 3 cm diameter hole forming an internal anode
chamber, a desalination chamber, and a cathode chamber
with volumes of 23, 12, and 27 mL, respectively (Fig. 1).24,30
The anode and cathode chambers were 4 cm in length and
the desalination chamber had a length of 1.5 cm. Carbon
brush (Golden Brush, CA) was used as the anode electrode,
and traditional air cathodes were made by applying
0.5 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 three
chambers. The use of two cation exchange membranes allows
for 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 of
0.7 mm, and a 40% w/w ethyl acetate uptake) electrode
assemblies, which were connected to the anode and cathode,
respectively. Each assembly was split into three parts and
placed overlapping. Plastic mesh was used to prevent short
circuiting between the electrodes. Spacing between the
electrodes was 2.1 mm. A total of 0.72 g of ACC was used for
the CDI module. Ni/Cu was used as a current collector
(McMaster-Carr) for ACCs. The MDC reactor had the same
reactor dimensions and used the same anode and cathode
materials as those of MCDC, except that an anion exchange
membrane (AEM) (Astom Corporation, Japan) was placed
between the anode and the desalination chambers and a
CEM was placed between the desalination and the cathode
chambers. No CDI modules were integrated with MDC, and a
1000 Ω resistor was placed between the anode and the
cathode.
Reactor operation and controls
Both MCDC and MDC reactors were initially operated in
microbial fuel cell mode by removing the middle chamber
and inoculating the anode chamber with activated sludge.
Inoculation was only required initially for anode biofilm
acclimation. When a repeatable voltage profile (>500 mV)
was observed for three consecutive cycles, which suggests
stable microbial activity, the reactors were shifted to desali-
nation mode by partially disassembling the reactors and
quickly inserting the membranes and the middle chamber
as described previously.20,32
The anolyte was transitioned from
10 mM sodium acetate-buffered medium to 100% produced
water. In the comparative study, only a 200 mM phosphate
buffer solution was used as the catholyte (pH 7.0 ± 0.1), and
produced water was used in the desalination chamber. Each
of the three chambers was connected to a separate 100 mL
reservoir, and water was continuously recirculated at a rate of
2 mL min−1
between each chamber and reservoir.15
The
MCDC desalination chamber was first recirculated with pro-
duced water to ensure saturated physical adsorption. Full
physical 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 to
measure the capacity of electrical adsorption. Based on the
results from an early feasibility study, the end of a desalina-
tion cycle for the MCDC was determined when the voltage
across the CDI modules became stable, dV/dt ~0.24,25
Maxi-
mum voltage was normally achieved in 2 hours. The end of a
desalination cycle for the MDC was capped at 24 hours. Data
from the MCDC and MDC were normalized per hour. Desali-
nation for both MCDC and MDC would have continued past
the predetermined cycle of operation, 2 hours for MCDC or
24 hours for MDC, but the normalized cycle times allowed
each system to be honestly evaluated for comparison purposes.
MDC operates on a longer time scale because it requires ions
to migrate across a membrane while CDI does not.
Following the MCDC desalination cycle, the CDI module
was regenerated using one of two methods. The CDI module
was either short circuited and regenerated within 1 hour or
connected to an external charge pump (Seiko Instruments,
Japan), which connected to a capacitor (2.5 V and 12 F) for
energy harvesting. Regeneration was completed when the
measured voltage between the CDI modules was less than
5 mV. Energy harvested from MCDC was determined based
on the equation E = ½CV2
, where E is the energy in joules, C is
the capacitance in Farads, and V is the voltage in volts.
To evaluate the influence of the CDI module on the MCDC
desalination performance, two controls were investigated.
The MCDC short circuit control investigated cation migration
when an electrical potential is generated by the anode and
cathode electrodes. The anode and cathode electrodes were
electrically connected, while the CDI module was short
circuited. To evaluate cation migration without an electrical
potential, an MCDC open circuit control was performed by
disconnecting the anode and cathode and short circuiting
the CDI module.
In the advanced desalination study, only the MCDC was
evaluated. The catholyte was transitioned to 100% produced
water. Starting with an unsaturated CDI module, 100 mL of
Fig. 1 Configuration of the MDC (left) and MCDC (right) systems. The
image represents the investigated small cube systems. The cut-out of
the MDC shows where the anion exchange membrane and cation
exchange membrane are located compared to that of the MCDC
system with a blow-up of the capacitive deionization module in the
middle chamber.
Environmental Science: Water Research & Technology Paper
View Article Online

produced water was recirculated in the desalination chamber
for 1 hour while connected to the anode and cathode for
in situ physical and electrical adsorption. Following the salt
removal through adsorption on the CDI module, the desali-
nation chamber was completely removed of solution. For
regeneration, 100 mL of deionized water was recirculated
through 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, and
the previously desalinated solution was reintroduced into the
desalination chamber. A total of three cycles were operated
with the desalination and regeneration solutions. One cycle
is defined as desalination and regeneration. All experiments
were repeated at least 3 times, and the average and standard
deviation were used in data presentation.
Analysis
Conductivity and pH (HACH, CO) were constantly measured
for all chambers of the reactors before, during (20 minute
intervals), and after the experiments. Standard chemical
oxygen demand (COD) (HACH, CO) measurement was used
to determine the change in organic content. Dissolved organic
carbon (DOC) was measured using a Sievers 5310C Series
TOC analyzer. The change in concentration of total dissolved
solids (TDS) and ion concentrations were determined by
Optima 3000 inductively coupled plasma (ICP) spectrometry
and Dionex DC80 ion chromatography (IC). Total alkalinity
was measured using a HACH alkalinity test kit (Loveland,
CO). The changes of different types of organic compounds
during the experiment were characterized by fluorescence
excitation–emission matrix (F-EEM) spectroscopy using an
ultraviolet radiation spectrometer (Beckman Coulter, CA) in
conjunction with a spectrofluorometer (Edison, NJ). Samples
were normalized at 5 mg L−1
DOC; contour lines were set at
50 and a maximum intensity of 1.5 UV-vis suppression. The
voltage of MDC was measured using a Keithley 2300 data
acquisition system, and the electrical potential between the
electrode assemblies in MCDC was monitored by a program-
mable multimeter (Amprobe, WA). Reactor coulombic effi-
ciency was calculated based on the fraction of electrons
removed from the organic electron donor that are recovered
as current through the external circuit.18,19
The energy
harvested by MCDC using the charge pump from the activated
carbon capacitor was defined as the coulombs harvested by
the charge pump versus the coulombs transferred to the
capacitors normalized by the weight of the activated carbon
capacitor.
Results and discussion
Salt and organic removal efficiencies by MCDC and MDC
The treatment efficiencies of salts and organic contaminants
present in the produced water by the MCDC and MDC are
summarized and compared in Fig. 2. It depicts that the
specific ion removal rates from the MCDC were drastically
faster than those from the MDC. For the MCDC, the ion
conductivity in the desalination chamber using only electrical
adsorption decreased on an average of 0.9 mS cm−1
h−1
. The
conductivity in the anode chamber also decreased slightly
by 0.1 mS cm−1
h−1
, while the cathode chamber slightly
increased by 0.2 mS cm−1
h−1
. The standard deviation for the
MCDC fluctuation was hypothesized to be mainly due to the
dynamic system of using raw produced water and microbial
exoelectrogenic activities, which fluctuated by 33% and up to
12% due to the heterogeneous nature. For the MDC configu-
ration, the rate of ion removal was much slower than the
MCDC. Over each 24 hour operational period, the MDC
Fig. 2 Rate changes per hour in salt removal, COD, and pH for all
three chambers of the MCDC and MDC, as well as two control
reactors for 5 data sets. Salt removal rate was based on conductivity
changes, with positive values indicating salt accumulation and negative
values indicating salt removal. For the COD removal rate, positive
values indicate COD increase and negative values indicate COD
removal. Changes in pH are shown, with positive values indicating pH
increase and negative values indicating pH decrease.
View Article Online

desalination chamber showed an average of 0.05 mS cm−1
h−1
ion removal rate, while the conductivity in the anode and
cathode chambers increased by 0.04 mS cm−1
h−1
and
0.07 mS cm−1
h−1
, respectively. The MCDC higher removal
rate is partially attributed to a 15% higher voltage output
than that of the MDC, which may be contributed by the force
charging associated with the capacitive electrodes.29
In terms
of percent salt removal, the MCDC removed an average of
8.6% of salt within its 2 hour cycle period, while the MDC
removed 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 dropped
to 0.1 mS cm−1
h−1
, which is similar to the desalination rate
in the MDC. This indicates that it was the electrical adsorp-
tion that led to the faster desalination rate. The MCDC open
circuit 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 from
reactor configuration on system performance. The MCDC
desalination chamber ion removal rate is the fastest because
of the high surface area and small distance for ion migration.
By comparison, the low desalination rate in the MDC middle
chamber was believed to be due to the low concentration gra-
dient between the three chambers in MDC. Previous studies
showed that high concentration gradients contribute signifi-
cantly in MDC performance, and molecular diffusion across
a membrane in electrodialysis could contribute up to 25% of
the total ion flux.33,34
Because the MCDC uses two cation exchange membranes,
ions migrate from the anode chamber to the desalination
chamber and then to the cathode chamber to balance the
transfer of electrons from the anode to the cathode. This is
believed to be the reason that the cathode chamber conduc-
tivity increased for both MDC and MCDC because it was
found that such increases were proportional to the salt
migration rate from the desalination chamber to the cathode
chamber. The ion increase in the MDC anode chamber was
expected, as the MDC configuration does not completely
remove ions but rather ions migrate away from the middle
chamber to the other chambers.
One of the most interesting findings in this study is illus-
trated in Fig. 2. It was originally hypothesized that organics
would be removed in the anode chamber and salts would be
removed in the desalination chamber. The results indeed
presented that both MDC and MCDC anode chambers
removed about 50–80% of COD at a similar rate of 40 mg of
COD per liter per hour, but more COD was found to be actu-
ally removed in the desalination chamber of the MCDC at a
much faster rate, ranging from 100 to 160 mg of COD per
liter per hour, which is faster than any other chambers inves-
tigated. Unlike the anode chambers which oxidize the
organic faction of the COD to CO2 by microbial activities, the
COD removed in the desalination chamber is electro-
chemically removed through adsorption. The high surface
area CDI module is believed to contribute to the faster
removal rate, but further analysis will need to be conducted
to quantify the contribution. The capacity of the CDI module
to remove organic carbon was previously demonstrated in
produced water treatment,4
and it was suggested that poten-
tially any charged organic molecule could be removed using
capacitive deionization. Almost no COD was removed from
the MDC desalination chamber or the cathode chambers of
both configurations.
The pH changes in the three chambers between the MDC
and MCDC also showed interesting though not significant
differences. As shown in Fig. 2, the anode chamber pH in the
MCDC decreased by 0.01 pH units per hour while the pH in
MDC decreased by 0.08 pH units per hour. However, the
MDC had little change in pH in the desalination chamber,
while the MCDC decreased by 0.08 pH units per hour. In the
cathode chamber, the MDC increased by 0.03 pH units per
hour while the MCDC increased by 0.04 pH units per hour.
These results indicate that MCDC configuration can reduce
pH changes in the anode with a different set of membranes,
while pH change in the MCDC middle chamber was more
significant. Fluctuations in pH in CDI units have been
observed, and the causes were potentially linked to water
hydrolysis.35
Given no microbial functions were expected in
the middle chamber, such pH changes are not of much
concern as they are in the anode chamber.
Power production and MCDC regeneration energy harvesting
Fig. 3 shows the comparison of the operational cycles and
voltage generation between MCDC and MDC during one
MDC cycle period, and the graph inset shows three con-
secutive cycles of the MDC. The voltage was measured on a
30 minute interval and error bars were based on 6 consecu-
tive cycles. While the MDC was operated in longer cycles of
approximately 40–70 hours based on anode COD removal,
the MCDC was operated in much shorter desalination and
regeneration cycles based on ion adsorption in the desalination
Fig. 3 Electrical charges formed on the MCDC and voltage profile of
the MDC. Major graph shows a full cycle of MDC, and for the same
period, multiple cycles of MCDC operation were performed. Graph
inset shows three cycles of MDC operation, with arrows indicating
electrolyte change and the last cycle used in the main graph.
View Article Online

chamber. For each repetitive MCDC cycle, the voltage across
the CDI module reached a maximum value of 558 mV,
with an average of 530 mV. Anode and cathode potentials
versus an Ag/AgCl reference electrode can be seen in the ESI†
(Fig. S2). Regeneration of the MCDC in the experiment was
achieved by either connecting the CDI modules in short
circuit or connecting them with a charge pump for in situ
energy harvesting to a 12 F external capacitor. The charge
pump is a simple electrical circuit which passively collects
energy.18
Energy harvested with the charge pump was not
due to capacitive mixing because the energy was harvested
without use of a low-salinity electrolyte. Energy harvesting
was not conducted for the advanced study or for the MDC.
For the same normalized desalination period, the energy
stored on the MCDC CDI module was 1789 mJ g−1
ACC,
which was calculated based on the coulombs transferred to
the CDI module during middle chamber desalination.36,37
The energy harvesting coulombic efficiency was calculated at
0.94%, which indicates that the use of a charge pump is not
sufficient for energy harvesting during the regeneration step.
Further research will need to be conducted to improve this
energy extraction efficiency.
For the MDC configuration, a typical voltage profile was
generated across the 1000 Ω resistor with a maximum voltage
of 450 mV. The voltage increased rather fast in the MDC
within the first 3–5 hours then stabilized until organic avail-
ability became limited in the anode chamber, which caused
the voltage decline. The maximum current produced in the
MDC reactor was 450 μA, which is comparable to previous
studies that used synthetic media and artificial salt
water.18,19,38
The maximum current calculated for the MCDC
was 1089 μA, and both current profiles can be found in the
ESI† (Fig. S1). More fluctuation in power output was observed
compared to previous lab studies, which is believed to be due
to the use of produced water with complex constituents. This
fluctuation in power output is the main reason for the large
standard deviation error bars in Fig. 2.
Advanced desalination capacity and rate of the MCDC system
Following the comparative feasibility study, a more thorough
investigation into the capacity of MCDC was performed. It
was observed in the comparative study that faster cycles in
MCDC can result in higher desalination efficiency. Shorter
1 hour desalination and 20 minute regeneration cycles were
performed based on the results from the comparative study.
Fig. 4 shows the TDS and COD removal in three consecutive
cycles of operation. Compared with Fig. 2, where removal
rates were compared in one cycle, Fig. 4 shows the average
ion removal of a typical 4 hour operation with three consecu-
tive cycles. It was determined from control tests that physical
adsorption accounted for approximately 40% of the TDS
removed and electrical adsorption contributed to 60% of the
removal. This distribution is similar to previous results using
synthetic salt water containing 10 g L−1
NaCl.24
The majority
of 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 TDS
per liter per hour), which is dramatically faster than MDC.
The adsorption capacity decreased over successive cycles
mainly due to the reduced regeneration efficiency that cannot
restore all adsorption capacities. This loss in capacity has
been observed in most CDI studies.39
With improvements in
reactor design and operation, the loss of capacitance can be
minimized; if the applied electrical potential is reversed, the
Fig. 4 The removal and recovery of TDS, COD, and four predominant
ions in the three MCDC chambers in three successive cycles under
advanced operating conditions. The concentration difference between
each blue bar and red bar indicates the level of removal of a specific
parameter, and the green bar shows the percentage of recovery. The
data presented here are from three consecutive runs.
View Article Online

electrodes can be fully recovered.37
The change in pH for the
anode and desalination chambers was similar to the pH
change in the comparative study. The change in pH for the
cathode chamber when it switched to 100% produced water
had a larger increase in pH (0.2 ± 0.1 pH units per hour) than
that in the comparative study (0.003–0.04 pH units per hour),
but the change was not significant due to the natural buffer
capability of the produced water.
The removal of COD in successive MCDC cycles is shown
in Fig. 4. During the three cycles, the anode chamber showed
a COD removal of 20 mg L−1
during 4 hours, while the desali-
nation chamber showed a COD removal of 660 mg L−1
with
83% removal efficiency during the same period. As shown in
Fig. 4, after MCDC regeneration, 75% of the COD that was
removed during desalination was fully recovered. It is quite
clear that the organic removal in the desalination chamber
was due to physical and electrical adsorption, which is much
faster 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 electrode
after regeneration, and it was hypothesized that they were
lost due to electrochemical oxidation. Even though COD
adsorption on the CDI is not a permanent removal method,
it fulfills the goal of quickly removing organic carbon from
the desalinated solution and allows faster produced water
treatment, which is considered very important in the indus-
try. The adsorbed COD can then be concentrated during
regeneration and served as electron donors for microbial
metabolisms. This is beneficial to the anode because low
electron donor availability has been considered an issue for
MDC applications.
Fig. 4 also shows the four predominant ion concentrations
for all three chambers, initial, final or after desalination, and
after regeneration, to illustrate the ion balance over the
course of desalination. While the anode and cathode showed
limited 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 water
and were predominantly removed. More than 66.3% of the
sodium was removed, and then 98.9% of the removed
sodium could be recovered during regeneration. Similar rates
were observed for chloride (69.5% in removal and 66.7% for
recovery). An average of 82% of the calcium and 69% of the
bromide were removed, and the recovery for calcium was
42% and for bromide was 81%. The lower recovery rate for
calcium may be due to its hardness nature, which has been
reported 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 fully
regenerated, and the MCDC had been operated for more than
300 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 that
used real industrial wastewater.15,22,24
The reactor CE will be
highly dependent upon the organic carbon characteristics of
the specific produced water.
Additionally, fluorescence excitation–emission matrix
(F-EEM) spectroscopy was used to investigate the changes of
fluorescing compounds in the MCDC desalination chamber
from the starting point to the end of the three desalination
cycles. The excitation and emission contour map is generated
by plotting each value against each other. Generally the
F-EEM is classified into five different regions. Zones 1 and 2
are for aromatic proteins, zone 3 is for fulvic acid-like com-
pounds, zone 4 is for soluble microbial products, and zone 5
is for humic acid-like organics. Fig. 5A and B show distinct
differences in fluorescence intensity, which indicate that after
the cycles, a majority of aromatic proteins and soluble microbial
byproduct-like materials have been removed. A separate study
focusing on using F-EEM and other tools on produced water
organic removal can be found in the study by Stoll et al.40
Outlook
Due to the pressing health and safety concerns associated
with unconventional natural gas exploration and production,
Fig. 5 Fluorescence excitation–emission matrix (F-EEM) shows
fluorescing organic removal from produced water by the MCDC
desalination chamber. (A) F-EEM matrix before treatment and (B)
F-EEM matrix after treatment cycles. Images were generated from a
prewritten MATLAB program. Contour lines were set at 50 and a
maximum intensity of 10.
View Article Online

the oil and gas industry is searching for on-site treatment
and reuse options for produced water to replace deep well
injection, open pond evaporation, or truck transportation
and treatment. In this context, simple, effective, and low-cost
treatment processes are urgently needed. For removing both
organic pollutants and salts from produced water, current
solutions require a long chain of biological and membrane
processes with high energy demand. This study presents that
microbial desalination systems can be a new approach to
solve such problems, also with the benefit of energy output.
Traditional microbial fuel cell-based MDC processes could
accomplish organic matter and salt removal, but the rates
have been slow, and the amount of concentrates generated
can be larger than the desalinated water. The MCDC system
largely overcame these challenges by integrating capacitive
deionization into the system, which increased the desalina-
tion rate 18 times and the COD removal rate 5 times faster
than the traditional MDC.
Both MDC and MCDC require a certain amount of
organics in the produced water, so microorganisms can oxi-
dize these electron donors to drive desalination. This require-
ment may limit their applications for certain produced
waters, such as those from formations with very high salinity
but low organics, but because MCDC can accumulate and
recover organics from the CDI modules, the recovered
COD can be recycled back to the anode chamber to alleviate
potential shortage of organics. Because the rate of microbial
desalination is correlated with the initial salt and organic
concentrations, it may not be as efficient as the membrane
processes in producing directly reusable water (TDS
<500 mg L−1
). However, it can be an efficient pretreatment
for membrane systems to reduce organic and salinity
loadings and prevent membrane fouling, as well as provide
renewable power sources. Additionally, the deionized water
can be replaced with treated effluent; the production of a
concentrated brine solution from the MCDC system may be a
value-added product, as opposed to cost, with industries
and municipalities requiring salt solutions for product pro-
cessing. Overall, with further research and development, the
unique feature of simultaneous organic matter and salt removal
and 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 Research
under Award N000141310901 and US National Science
Foundation under Award CBET-1235848. We appreciate
Dr. Tzahi Y. Cath, Colorado School of Mines, for proving
produced water samples.
Notes and references
1 C. E. Clark and J. A. Veil, Produced Water Volumes and
Management Practices in the United States, National Energy
Technology Laboratory, 2009.
2 EIA, Annual Energy Outlook 2013 with Projections to 2040.
Report #:DOE/EIA-0383(2013). Prepared by the U.S. Energy
Information Administration (EIA). Release Date: April,
2013. Available at: <http://www.eia.gov/forecasts/aeo/pdf/
0383(2013).pdf>.
3 P. Xu, J. E. Drewes and D. Heil, Desalination, 2008, 225,
139–155.
4 P. Xu, J. E. Drewes, D. Heil and G. Wang, Water Res.,
2008, 42, 2605–2617.
5 GWPC and ALL, 2009.
6 K. L. Hickenbottom, N. T. Hancock, N. R. Hutchings,
E. W. Appleton, E. G. Beaudry, P. Xu and T. Y. Cath,
Desalination, 2013, 312, 60–66.
7 P. Xu and J. E. Drewes, Sep. Purif. Technol., 2006, 52, 67–76.
8 K. G. Dahm, K. L. Guerra, P. Xu and J. R. E. Drewes, Environ.
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.
10 D. M. Kargbo, R. G. Wilhelm and D. J. Campbell, Environ.
Sci. Technol., 2010, 44, 5679–5684.
11 S. Yoshie, N. Noda, S. Tsuneda, A. Hirata and Y. Inamori,
Appl. Environ. Microbiol., 2004, 70, 3152–3157.
12 K. L. Hickenbottom, N. T. Hancock, N. R. Hutchings,
E. W. Appleton, E. G. Beaudry, P. Xu and T. Y. Cath,
Desalination, 2013, 312, 60–66.
13 J. E. Drewes, N. T. Hancock, K. L. Benko, K. Dahm, P. Xu,
D. Heil and T. Y. Cath, Treatment of coalbed methane produced
water. 126–128, Colorado School of Mines, Golden, CO., 2009.
14 S. Mondal and S. R. Wickramasinghe, J. Membr. Sci.,
2008, 322, 162–170.
15 H. Wang and Z. J. Ren, Biotechnol. Adv., 2013, 31, 1796–1807.
16 P. Liang, L. Yuan, X. Yang, S. Zhou and X. Huang, Water
Res., 2013, 47, 2523–2530.
17 H. Luo, P. E. Jenkins and Z. Ren, Environ. Sci. Technol.,
2011, 45, 340–344.
18 H. Wang, J.-D. Park and Z. Ren, Environ. Sci. Technol.,
2012, 46, 5247–5252.
19 X. X. Cao, X. Huang, P. Liang, K. Xiao, Y. J. Zhou, X. Y. Zhang
and B. E. Logan, Environ. Sci. Technol., 2009, 43, 7148–7152.
20 H. Luo, P. Xu, P. E. Jenkins and Z. Ren, J. Membr. Sci.,
2012, 409, 16–23.
21 H. Luo, P. Xu and Z. Ren, Bioresour. Technol., 2012, 120, 187–193.
22 K. S. Jacobson, D. M. Drew and Z. He, Environ. Sci. Technol.,
2011, 45, 4652–4657.
23 Y. Kim and B. E. Logan, Desalination, 2013, 308, 122–130.
24 C. Forrestal, P. Xu and Z. Ren, Energy Environ. Sci., 2012, 5,
7161–7167.
25 C. Forrestal, P. Xu, P. E. Jenkins and Z. Ren, Bioresour.
Technol., 2012, 120, 332–336.
26 P. M. Biesheuvel and M. Z. Bazant, Phys. Rev. E: Stat.,
Nonlinear, Soft Matter Phys., 2010, 81, 031502.
27 L. Yuan, X. Yang, P. Liang, L. Wang, Z.-H. Huang, J. Wei and
X. Huang, Bioresour. Technol., 2012, 110, 735–738.
28 C. Feng, C.-H. Hou, S. Chen and C.-P. Yu, Chemosphere,
2013, 91, 623–628.
View Article Online

29 M. C. Hatzell, R. D. Cusick and B. E. Logan, Energy Environ.
Sci., 2014, 7, 1159–1165.
30 H. Wang, M. Davidson, Y. Zuo and Z. Ren, J. Power Sources,
2011, 196, 5863–5866.
31 J.-D. Park and Z. Ren, J. Power Sources, 2012, 205, 151–156.
32 H. Luo, P. Xu, T. M. Roane, P. E. Jenkins and Z. Ren,
Bioresour. Technol., 2012, 105, 60–66.
33 M. P. Mier, R. Ibañez and I. Ortiz, Sep. Purif. Technol.,
2008, 59, 197–205.
34 Y. Kim and B. E. Logan, Environ. Sci. Technol., 2011, 45,
5840–5845.
35 S. J. Seo, H. Jeon, J. K. Lee, G. Y. Kim, D. Park,
H. Nojima, J. Lee and S. H. Moon, Water Res., 2010, 44,
2267–2275.
36 P. M. Biesheuvel, J. Colloid Interface Sci., 2009, 332, 258–264.
37 M. A. Anderson, A. L. Cudero and J. Palma, Electrochim.
Acta, 2010, 55, 3845–3856.
38 K. S. Jacobson, D. M. Drew and Z. He, Bioresour. Technol.,
2011, 102, 376–380.
39 Y. Oren, Desalination, 2008, 228, 10–29.
40 Z. A. Stoll, C. Forrestal, Z. Ren and P. Xu, J. Hazard. Mater.,
2014. (Pending).
View Article Online

Research: A new process to remove salt and organic compounds from frack wastewater

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (6)

Similar to Research: A new process to remove salt and organic compounds from frack wastewater

Similar to Research: A new process to remove salt and organic compounds from frack wastewater (20)

More from Marcellus Drilling News

More from Marcellus Drilling News (20)

Recently uploaded

Recently uploaded (20)

Research: A new process to remove salt and organic compounds from frack wastewater