Chemoselective phosphine-catalyzed cyanoacylation of α-dicarbonyl compounds: a general method for the synthesis of cyanohydrin esters with one quaternary stereocenter

Article abstract of DOI:10.1039/c8nj04867c

A chemoselective phosphine-catalyzed cyanoacylation of α-dicarbonyl compounds is reported. Under the catalysis of P(NMe₂)₃, the cyanoacylation of α-dicarbonyl compounds such as isatins, α-keto esters, and α-diketones with acyl cyanides exclusively proceeds under very mild conditions, affording a wide range of cyanohydrin esters bearing one quaternary stereocenter in moderate to excellent yields. It represents the first phosphine-catalyzed cyanoacylation of α-dicarbonyl compounds and also provides a general method to prepare fully substituted cyanohydrin esters.

Full text of DOI:10.1039/c8nj04867c

Environmental
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Water Research & Technology
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Feasibility evaluation of the treatment and
recycling of shale gas produced water: a case
study of the first shale gas field in the Eastern
Sichuan Basin, China†
Cite this: DOI: 10.1039/c8ew00760h
a
a Yiling Zhuang,ab Junjie Li,ab Zejun Zhouc and Shaohua Chen
*
*
Zhaoji Zhang,
A cost-effective shale gas produced water (PW) treatment plant is an essential facility for safe wastewater
management in the shale gas field in the Eastern Sichuan Basin, which is the first commercial shale gas field
in China. However, knowledge on PW characteristics and disposal features of this developing shale gas field
is limited. This study evaluated the feasibility of pollutant removal from PW in the Eastern Sichuan Basin by
a sequential physicochemical process through laboratory and on-site pilot-scale experiments. The results
indicated that the CODCr, ammonia nitrogen, total nitrogen, and total phosphorus in the PW are efficiently
removed by Fenton–NaClO oxidation, multi-media filtration, and reverse osmosis (RO) processes. A novel
recycling route was introduced to rationally dispose the RO concentrate, which was recycled as a raw
brine material for a chlor-alkali enterprise through a physicochemical advanced treatment process. A
laboratory-scale co-treatment of PW with domestic wastewater was also conducted. Salinity fluctuation,
chemical pre-treatment toxicity, resistance, and resilience of the bioreactor after fluctuations may heavily
influence the planning, design, and construction of a PW treatment plant, although a coupled chemical–
biological process for treating PW is a potential cost-effective management technique. Overall, the se-
quential physicochemical process that includes RO is a feasible process for the cost-effective treatment of
PW from the shale gas field in the Eastern Sichuan Basin. The recycling of the RO concentrate as a raw
brine material for the chlor-alkali enterprise is a potential and novel resource recycling mode. The detailed
results derived from this study will be considered as a reference for developing the first full-scale central-
ized shale gas PW treatment plant in this field.
Received 25th October 2018,
Accepted 10th December 2018
DOI: 10.1039/c8ew00760h
rsc.li/es-water
Water impact
Hydraulic fracturing technology has been successfully applied and vigorously promoted in China. However, flowback and produced water have gained new
environmental concerns and have required special treatments before final disposal or reuse. Knowledge on the characteristics and disposal feasibility of
produced water in the first shale gas field in the Eastern Sichuan Basin, China, is limited, and challenges in implementing a cost-efficient technology in
the treatment practice exist. We evaluated the feasibility of pollutant removal and desalination from produced water by a sequential physicochemical pro-
cess through laboratory and on-site pilot-scale experiments. This paper is the first systematic report on produced water management in a shale gas field in
China.
Introduction
The immense success of exploration and commercialization
of shale gas in the USA over the past two decades has shown
considerable progress in energy revolution and gained great
concerns around the world.1–3 In China, shale gas has been
commercially developed as an important unconventional nat-
ural gas resource since 2013.14 Although a bright prospect for
developing shale gas industry has been demonstrated, com-
plaints have been raised about the potential negative impacts
of hydraulic fracturing on environmental quality and human
health.5–7 The generally used hydraulic fracturing technology,
a Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment,
Chinese Academy of Sciences, Xiamen 361021, People's Republic of China.
E-mail: zjzhang@iue.ac.cn, shchen@iue.ac.cn
b University of Chinese Academy of Sciences, Beijing 100049, People's Republic of
China
c Sinopec Chongqing Fuling Shale Gas Exploration & Development Co. Ltd.,
Chongqing 408014, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8ew00760h
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which has enabled large-scale commercial development of
shale gas coupled with horizontal drilling, has been applied
extensively.8 Large volumes of freshwater mixed with sand
and other chemicals are injected into wells under high pres-
sure during the fracturing process to create fractures in low-
permeability shale formations.8 At the early stage, that is,
within the first 30 days, approximately 15–25% of water that
returns to the surface is called flowback water, and the water
that continues to flow for the entire life of the wells at a
reduced rate is referred to as produced water (PW). Such
wastewater mainly contains extremely complex dissolved
chemicals, hydrocarbons, heavy metals, naturally occurring
radioactive material (NORM), bacteria and water-soluble salts
(such as potassium, calcium, sodium, carbonate, and chlo-
ride), thereby gaining increasing environmental concerns and
requiring special treatment before final disposal or reuse.9–12
Implementing cost-effective PW management is one of the
most important issues during the entire life cycle of shale
gas fields.13,14 To date, four PW disposal patterns have been
implemented, namely, (1) underground injection, (2) reuse
for subsequent hydraulic fracturing, (3) co-treatment with
sewage in municipal wastewater treatment plants (WWTPs),
and (4) centralized treatment in private industrial WWTPs in
shale gas fields.14,15 The technical feasibility of underground
injection (option 1) is hampered in the shale gas field under
delicate geological conditions and developed groundwater
ecosystems.4,13,16 Reusing PW for continuous hydraulic frac-
turing (option 2) is merely a temporary choice before the end
of well drilling.13 Furthermore, transporting PW to nearby
municipal WWTPs for co-treatment (option 3) may result in
salt accumulation in the sediments downstream of the plant
and cause unstable operational performance of the biological
treatment units.14 On the basis of previous practical experi-
ence, PW management strategies tend to change from waste-
water disposal to treatment to satisfy rapidly upgrading envi-
ronmental requirements. Therefore, transporting PW into
private industrial WWTPs for centralized treatment (option 4)
has been considered as the optimum choice in view of sus-
tainable shale gas environment management.13,16,17
Although a number of studies have examined the effective-
ness of independent technology for removing partial contam-
inants or TDS from PW,17–26 studies on the feasibility of an
integral treatment process in private centralized shale gas
PW treatment plants are limited.13,17,18 Particularly, the effi-
ciency and capacity of each technical unit in the treatment
process, the capital investments and operational costs of the
entire treatment process, and the effluent standard, as well
as environmental risk control status, are urgently needed to
guide the practical engineering design and implementation,
especially in China.27 As a rule, the applied wastewater treat-
ment processes in the oil & gas industry for pollutant reduc-
tion of conventional PW can serve as models or references
for establishing a feasible shale gas PW treatment process.18
Generally, an integral PW treatment process in the conven-
tional onshore oil & gas industry for a high-grade discharge
level usually contains phase separation (e.g., oil–water and
solid–liquid separation), physicochemical primary pre-
treatment (e.g., coagulation), biological treatment (e.g., aera-
tion tank), secondary pre-treatment (e.g., air flotation), mem-
brane filtration (e.g., ultrafiltration, nanofiltration, and re-
verse osmosis), and post-treatment (e.g., softening).18,28–31
The techniques for each treatment phase are varied.32,33 Con-
sequently, the shale gas PW combined treatment process of a
centralized private industrial WWTP can be proposed after
independent technique optimization when the TDS of the
shale gas PW is below the limitation of pressure-driven mem-
brane technology.27,34
Nevertheless, in addition to the well-investigated conven-
tional oil & gas PW, the shale gas PW usually contains more
salts and complex pollutants. Therefore, novel independent
technologies, such as innovative desalination and advanced
oxidation processes (AOPs), are being tested for the advanced
removal of certain group components to meet increasingly
strict environmental regulations.13,17,18 Among them, innova-
tive membrane-based technologies (e.g., forward osmosis
(FO)20 and membrane distillation (MD)21), combined AOPs
(e.g., catalyzed ozone22 and photo-Fenton process23), ad-
vanced thermal technologies,24 and microbial capacitive de-
salination technique25,26 have been successfully implemented
in bench-or pilot-scale research. However, mature and emerg-
ing techniques should be evaluated as constituent parts in a
proposed cost-effective treatment process to satisfy technical
and environmental criteria.
The Eastern Sichuan Basin is the first commercialized
large-scale shale gas field, which is a typical Lower Silurian
Longmaxi marine shale gas block with an estimated reserve
of 1067.5 billion m3. Since 2016, this shale gas field has ac-
cumulated 12.23 billion m3 of natural gas production and
provided a bright prospect for developing unconventional
natural gas in China. The development of centralized private
industrial WWTPs for PW treatment and reuse is an impor-
tant supporting project for the scaling-up of the shale gas
field. Our team was authorized and financed by Sinopec in
2015 to conduct laboratory and on-site pilot-scale experi-
ments to access the feasibility of the combined PW treat-
ment and reuse processes. The technical reliability and oper-
ational cost of the integral process in the proposed full-scale
plants were determined in detail. To the best of our knowl-
edge, our study is the first systematic report on actual PW
treatment and reuse by a combined process in China. The
results of this study could pave the way for optimizing the
PW treatment process in centralized private industrial
WWTPs.
Materials and methods
PW samples
The PW referred to in this study is the freshly stored waste-
water tank in the finished shale gas well station, which has
been devoted to commercial production for 1 year. PW sam-
ples from 15 wells in the shale gas field of the Eastern
Sichuan Basin in 2015 were collected, shipped to the
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laboratory, and stored at 4 °C in the dark prior to detailed
pollutant measurements. The PW from one of the wells,
which was represented as an influent for the planning-
centralized shale gas PW treatment plant, was periodically
collected and used for laboratory-scale experiments.
(2) PW RO concentrate treatment for external reuse:
laboratory-scale of the sequential physicochemical process. A
homologous sequential physicochemical process was
designed to recycle RO concentrates as crude brine materials
for
a chlor-alkali factory, which is near the planning-
centralized PW treatment plant in the shale gas field in the
Eastern Sichuan Basin (Fig. 1). Detailed operational parame-
ters for the laboratory-scale experiment of RO reuse are sum-
marized in Table S2.†
Experimental setup and procedure
(1) PW treatment for discharge: laboratory and on-site pi-
lot-scale physicochemical pre-treatment–reverse osmosis (RO)
process. Desalination is a crucial step for the effective treat-
ment or reuse of PW. To date, emerging desalination pro-
cesses, such as FO, MD, mechanical vapor compression,
humidification–dehumidification, and pervaporation, have
gained increasing popularity in comparison with traditional
pressure-driven membrane processes.13 However, RO still
provides the best energy efficiency for desalination when the
salinity of the saline wastewater is below the oceanographic
range.34 Therefore, regarding the salinity of the PW (26 g
L−1), RO was selected as the core desalination technique in
this study. A sequential Fenton–NaClO oxidation combined
with multi-media filtration was designed for eliminating
suspended and dissolved solids, colloids, and organic and in-
organic compounds to realize a low degree of membrane
fouling (Fig. 1). Laboratory and on-site pilot-scale experi-
ments were conducted. The detailed operational parameters
for the laboratory-scale experiments are summarized in Table
S1.† For the pilot-scale experiments, a 5 m3 d−1 sequential ad-
vanced oxidation and RO process device was established near
a typical well in the shale gas field in the Eastern Sichuan Ba-
sin. The process operational performance was optimized and
compared with the obtained laboratory-scale results.
(3) PW biological co-treatment with sewage in WWTPs:
laboratory-scale of
a sequential chemical pre-treatment–
biological process. A sequential chemical–biological process
was established to evaluate the treatability of PW biologically
to save disposal cost (Fig. S1†). The same Fenton–NaClO oxi-
dation was chosen as the chemical pre-treatment approach. A
moving-bed biofilm reactor (MBBR) process was selected as
the biological treatment. The detailed operational parameters
for the laboratory-scale co-treatment of PW with sewage are
summarized in Table S3.†
Analytical methods
Dissolved oxygen (DO) and pH were measured using a Hach
HQ40d portable analyzer. The turbidity of wastewater was de-
termined by using a Hach 2100AN turbidimeter. CODCr was
monitored by utilizing a Lianhua 5b-3 spectrophotometer.
Total organic carbon (TOC) was determined using Shimadzu
TOC-VCPH analyzers. BOD5 was detected by utilizing a WTW
OxiTop IS12 analyzer. Inorganic anions and cations were
measured by ion chromatography using a Dionex ICS-3000
and a Thermo Fisher Aquion ICS, respectively. Total nitrogen
+
(TN), ammonia nitrogen (NH4 -N), nitrate nitrogen, nitrite ni-
trogen, total phosphorus (TP), total suspended solids (TSS),
Fig. 1 Flow diagram of the laboratory-scale treatment and recycling of PW by a sequential physicochemical process.
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and total dissolved solids (TDS) were determined through
standard methods.35 Volatile organic compounds (VOCs) in a
typical PW sample was measured by performing thermal
desorption-gas chromatography-mass spectroscopy (TD-GC-
MS) using an Agilent HP-5MS capillary column (Agilent
7890A GC with 5975C MSD, Agilent Technologies, USA) with
a headspace autosampler (Tekmar HT3, Teledyne Technolo-
gies, USA). Peaks were identified by using Agilent Technolo-
gies ChemStation Software in accordance with the National
Institute of Standards and Technology mass spectrum library
(NIST08.L).
The water acute toxicity of the PW collected from several
typical shale gas wells in this field was determined by lumi-
nescent bacteria testing. Water samples were filtered with a
0.45 μm pore-diameter membrane, and the toxicity was deter-
mined using a rapid screening kit for water acute toxicity (JQ
TOX-kit IV, J&Q Environmental Technologies Co., Ltd., China)
in accordance with the manufacturer's recommendations and
standard method.35 The results were calculated as the lumi-
nescence inhibition rate (H, 15 min, 15 1 °C) of the lumi-
nescent bacterium Vibrio fischeri NRRL B-11177 according to
eqn (1).
in the Eastern Sichuan Basin varied considerably among the
wells. For example, the TDS concentration exhibited an obvi-
ous fluctuation, which ranged from 4.9 g L−1 to 52.5 g L−1
(Table 1), between two close wells. The mean concentrations
+
of TDS, CODCr, and NH4 -N were 26.5 g L−1, 2356 mg L−1, and
86.27 mg L−1, respectively (Table 1). The contents of phos-
phates, sulfates, and heavy metals in the PW were low. For
the PW centralized treated in private industrial WWTPs, the
TDS present in this brine met the requirements of RO desali-
nation (maximum TDS of 40 g L−1) after effectively removing
of organic compounds and particulates.13 The content of bro-
mide ions in this shale gas field was related to the chloride
content (R2 = 0.8281) (Fig. S2†). Bromate is typically gener-
ated from a bromide ion during an advanced oxidation pro-
cess and has been studied for its high carcinogenicity and
genotoxicity.36,37 Acute toxicity tests of the raw PW samples
among the wells showed that the majority of raw PW samples
were at mid-toxicity levels (with an inhibition ratio between
30% and 40%) except for two samples (Fig. S3†).
In general, pollutant compositions of the PW were derived
from chemical additives in the hydraulic fracturing fluid and
formation brine which were produced with the nature gas af-
ter the shale gas well started production.9–11 Potential toxic
organic compounds, such as quaternary ammonium biocides
and 2-butoxyethanol in the hydraulic fracturing fluid and
polycyclic aromatic hydrocarbons and phthalates in the for-
mation brine, have been regularly determined in the PW.38
The qualitative identification of VOCs by TD-GC-MS also
ascertained a group of complex organic compounds that con-
tain C, H, O, N, S, and Cl (Tables S4†). Therefore, environ-
mental risk assessment of the raw PW and the treatment pro-
cess is important for the degradation process that is
frequently accompanied by a transformation of pollutants
that produce toxic products.11,15,38
H = [( f·IT ) − IT ]/( f·IT ) × 100% = 1 − IT /( f·IT ) × 100%
(1)
0
t
0
t
0
where IT is the initial luminous intensity of the sample; IT is
0
t
the luminous intensity after 15 min of the sample; f is the cor-
rection coefficient, and f = IC /IC ; IC is the luminous intensity
t
0
t
after 15 min of the control, and IC is the initial luminous
0
intensity of the control. The luminous intensity was measured
using a Hitachi F-4600 fluorescence spectrophotometer.
Results and discussion
Characterization of shale gas PW in the shale gas field in the
The water quality and regional environmental regulations
(e.g., regulations and permits that are created by environmen-
tal protection agencies within the jurisdiction of the shale
gas field to restrict the discharges from hydraulic fracturing
into ambient waters) are the two deciding factors for
Eastern Sichuan Basin
Pollutant concentrations of the PW samples among the first
15 shale gas wells in the Eastern Sichuan Basin are shown in
Table 1. The characteristics of the PW in the shale gas field
Table 1 Water quality of the PW in the Eastern Sichuan Basin of China
Parameters
pH
Min.
Max.
Mean
Parameters
As (mg L−1
Min.
Max.
Mean
6.45
5.20
0.50
4900
300
8.29
52.1
26.20
52 500
9900
7.00
)
N.D.
5.03
3.76
29.52
5.58
N.D.
N.D.
0.06
0.21
0.03
0.30
N.D.
0.66
N.D.
N.D.
558.68
86.16
508.69
150.50
N.D.
N.D.
2.13
5.45
0.71
3.36
0.42
N.D.
170.02
33.90
249.50
87.03
N.D.
N.D.
0.34
1.59
0.17
1.38
0.07
TS (g L−1
)
31.13
14.35
26 500
2000
2356
111.59
83.63
3.01
13.68
13.99
240.63
2.70
Ba (mg L−1
Mg (mg L−1
Ca (mg L−1
Fe (mg L−1
Cd (mg L−1
Co (mg L−1
Cr (mg L−1
Mn (mg L−1
Ni (mg L−1
Pb (mg L−1
V (mg L−1
Zn (mg L−1
As (mg L−1
)
TVS (g L−1
TDS (mg L−1
TSS (mg L−1
CODCr (mg L−1
TN (mg L−1
NH4+-N (mg L−1
TP (mg L−1
SO42− (mg L−1
Cl− (g L−1
K (mg L−1
F (mg L−1
Al (mg L−1
)
)
)
)
)
)
)
)
)
556
3767
)
44.27
42.63
0.29
N.D.
2.80
30.73
0.88
N.D.
317.40
170.70
14.56
38.85
24.70
746.84
8.09
)
)
)
)
)
)
)
)
)
)
)
)
)
7.20
N.D.
3.61
N.D.
)
29.04
5.41
N.D. = Not detected
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designing PW treatment processes. To date, the USA has ac-
cumulated rich experience in this aspect. Compared with the
reported water quality of PW in the US shale gas fields, the
PW wastewater in the Eastern Sichuan Basin has a similar
pollutant composition fingerprint.9–11 However, the TDS con-
tent in the PW in this shale gas field (26.5 g L−1) is consider-
ably lower than those in the Marcellus, Barnett–Woodford,
and Bakken shale gas fields in the USA (mean TDS is above
100 g L−1).10–39 This result suggests that pressure-driven
membrane processes are a cost-effective option for PW treat-
ment compared with emerging hypersaline disposal
technologies.34
PW treatment for discharge: laboratory and on-site pilot-scale
physicochemical pre-treatment–RO process
For the suspended and colloidal substance particle removal
from PW, conventional coagulation and advanced oxidation
techniques were comprehensively evaluated. The order of the
turbidity purification performance of raw PW was as follows:
polymeric aluminum sulfate > polymeric ferric sulfate >
Al2IJSO4)3 > Fe2IJSO4)3. The turbidity of raw PW was approxi-
mately 500–1000 NTU and could be lower than 1 NTU after
conventional coagulation. Subsequently, six industrialized ad-
vanced and chemical oxidation techniques were assessed
with a mutual comparison for the removal efficiency of
CODCr (Fig. S4†). The results indicated that the combination
of modified Fenton and NaClO oxidation processes achieved
the highest CODCr elimination efficiency. The combination
of activated potassium persulfate and NaClO oxidation pro-
cesses would be another appropriate option considering the
massive solid waste from the Fenton reaction (Fig. S4†). For
the activated potassium persulfate oxidation, in addition to
its high cost, the formation of bromate, which has high toxic-
ity and carcinogenicity, would be another potential drawback
of this process in practice.40 Overall, the modified Fenton
and NaClO oxidation processes were selected in this study as
a PW pre-treatment approach after coagulation. The follow-
ing multi-media filtration with artificial zeolite was operated
before RO. The commercial artificial zeolite has an inefficient
ammonia removal rate of 24.6% (Fig. 2) on account of the
3.5% salinity of the PW. For an efficient ammonia elimina-
tion and alleviation of the membrane fouling of RO, NaOH-
modified artificial zeolite was utilized. It remarkably en-
hanced the ammonia removal rate by 60% under the same
conditions (Fig. S5 and S6†). The content of contaminants in
the effluents was below the vast majority of the current pol-
lutant discharge or emission standards of industrial wastewa-
ter in China (Fig. 2 and Table S5†). Furthermore, membrane
fouling was controlled in a reasonable range as a result of
the systematic physicochemical pre-treatments.
Fig. 2 Removal profiles of (a) CODCr, (b) NH4+-N, and (c) TN from PW
during the laboratory-scale sequential physicochemical treatment (A.
raw PW; B. coagulation-modified Fenton process; C. NaClO oxidation;
D. zeolite adsorption; E. activated carbon filtration; F. RO).
ated formally after the initial adjustment period. The 15 days
successive operational result is shown in Fig. 4. Organic car-
bon and inorganic nitrogen compounds were efficiently re-
moved. This outcome coincided with the laboratory-scale ex-
A
5
m3 d−1 pilot-scale PW treatment station was
constructed and implemented near a typical well in the shale
gas field in the Eastern Sichuan Basin. The PW disposal ap-
proach was based on the laboratory-scale sequential physico-
chemical treatment route. The pilot-scale apparatus was oper-
+
perimental results. The concentrations of CODCr, NH4 -N,
and TN in the permeate water from RO were far below the
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Fig. 3 Profiles of ammonia and TN removal by laboratory-scale co-treatment of pre-treated PW and domestic wastewater.
limitation level of the emission standard of pollutants for the
petroleum chemistry industry in China.
nation technology, the effectiveness of the pre-treatments to
alleviate the reversible and irreversible fouling during real
PW desalination still needs to be tested. In general, mineral,
particulate, colloidal, and organic fouling result in internal
fouling, cake layer formation, and pore blocking of the RO
membrane.42 Physical and chemical pre-treatments in oil and
conventional natural gas industries are usually applied as ref-
erence in the PW pre-treatment, such as coagulation and
electrocoagulation.18,42 In the present study, Fenton and so-
dium hypochlorite oxidation showed effective organic com-
pound removal performance through laboratory- and pilot-
scale tests (Fig. 4 and S4†). For the chemical degradation of
refractory components (e.g., landfill leachate), combining
Fe2+ and NaClO has a higher COD removal rate of landfill
leachate than individual Fe2+ coagulation or NaClO oxida-
tion.43 Redundant Fe2+ during the second dosing of ferrous
The results from the laboratory- and pilot-scale pre-treat-
ment–RO process show that organic compound mineraliza-
tion and salt desalination of PW in this field were technically
feasible. Currently, RO is the most applied cost-effective tech-
nology to treat PW with low and moderate salinity for direct
discharge or reuse.13 Recently, a combined ultrafiltration
(UF) – RO process was successfully applied for the treatment
of PW with a relatively low TDS content (18.9 g L−1) in the
Weiyuan shale gas field, which is located in the southwestern
Sichuan Basin of China.27 Although improved operational
performance can be achieved by applying advanced UF and
RO membranes,41 effective pre-treatment approaches are ap-
parently the major factors that reduce membrane fouling.42
However, beyond the well-established and tested RO desali-
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concept, where HOCl is generated from chloride ions and
substitutes for H2O2 in the conventional Fenton reaction,
has been developed in recent years and is being studied for
its applicability to dispose wastewater that contains con-
centrated chloride ions.44,45 The new Fe2+/HOCl electro-
chemical Fenton-type process could effectively promote the
reusability of Fe3+ that contains sludge and could lighten
the burden of solid waste disposal during the pre-treatment
process.
Overall, compared with the reported RO pre-treatments for
PW disposal, the present study established a more effective
process for elimination of colloids, mineral elements, and or-
ganic compounds.
PW RO concentrate treatment for external reuse: laboratory
scale of sequential physicochemical processes
The planning-centralized PW treatment plant in the shale
gas field in the Eastern Sichuan Basin is in close proximity
to a large chlor-alkali enterprise, which requires a large
number of brine as raw material to produce sodium hydrox-
ide. The RO concentrates, which have nearly a half volume
of raw PW, were advanced treated and evaporative concen-
trated to a salinity of above 23% and the pollutant concen-
trations below the brine quality standard limit were reached
(Table S2†). Colloids and calcium, and magnesium ions
were effectively removed through coagulation and softening
treatment processes. The vast majority of ammonia (94.8%)
was removed by NaClO oxidation, and the residual ammo-
nia was below 4.0 mg L−1. The following advanced oxidation
for degrading refractory organic compounds was the main
challenge in recycling the PW RO concentrate. Batch tests
of O3–H2O2 oxidation and dimensionally stable anode
(DSA)-type electrochemical oxidation were performed com-
paratively (Table S2†). The results indicated that the O3–
H2O2 process exhibited effective TOC removal rates. The re-
sidual TOC in the concentrate was below 1.6 mg L−1. The
TOC removal rate was unstable in the DSA-type electro-
chemical oxidation, although the experimental parameters
were comprehensively adjusted. Overall, the quality of the
concentrated brine satisfied the requirements of the chlor-
alkali enterprise after bromine and iodine ion exchange and
evaporation.
The internal reuse of PW for subsequent hydraulic fractur-
ing is a conventional practice during the development of well
drilling after preliminary treatments.17 Compared with the
surface discharge standards, the water quality of this internal
reuse is relatively conservative and has usually no strict limi-
tation on TDS.17 The present study focused on the external
reuse of wastewater from shale gas fields when drilling and
hydraulic fracturing are finished. Until recently, the use of
RO concentrate from PW desalination processes has been
rarely reported. Given these factors, final reuse patterns and
advanced treatment technologies are necessary to optimize
design and management. In general, the sustainable treat-
ment, management, and recycling resources from RO rejects
Fig. 4 Removal profiles of (a) CODCr, (b) NH4+-N, and (c) TN from PW
using a pilot scale of the sequential physicochemical treatment in the
shale gas field in the Eastern Sichuan Basin.
salts in the Fenton process might improve the COD re-
moval performance at the NaClO oxidation stage in this ex-
periment. One drawback of the Fenton–NaClO process for
PW treatment is producing considerable ferric hydroxide ex-
cess sludge (3.83 g L−1). A new electrochemical Fenton-type
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have become major challenges worldwide.46 Thermal and
membrane-based technologies are the existing and currently
applied processes, although FO and MD coupled with crystal-
lization have received considerable attention as cost-effective
emerging technologies for RO reject disposal.24,47 In the pres-
ent study, the RO rejects from the PW treatment line were re-
covered as raw brine for the chlor-alkali industrial require-
ment. This technique is less energy intensive than
evaporative crystallization. The key point of the RO rejection
for this recycling process was to eliminate ammonia and TOC
from the concentrate. The water quality of the raw brine re-
ception standard is more stringent than the one reused for
subsequent hydraulic fracturing. According to the process re-
quirements of chlor-alkali industrial companies, the parame-
ters that need to be limited include the following: TOC < 7
mg L−1, ammonia < 2 mg L−1, iodine < 1 mg L−1, iron < 2
mg L−1, strontium < 0.1 mg L−1, silicon < 0.1 mg L−1, and
bromine < 28 mg L−1. Therefore, advanced catalytic oxidation
of refractory organic matters and interception of inorganic
and metal elements in the PW RO concentrate are indispens-
able approaches. In this study, O3/H2O2 oxidation achieved a
mineralization of refractory organic matters in the RO con-
centrate, thereby indicating the potential application of this
combined technique for advanced treatment of high salinity
wastewater. Similar reports have confirmed that the O3/H2O2
process is a promising technology for treating organic com-
pounds in typical wastewater matrices, such as landfill leach-
ate and disinfected by-products.48,49 Besides being reused as
raw brine in chlor-alkali industries, other potential options
for the cycling of RO concentrates include recovery as
industrial-grade sodium chloride for snow melting, textile
printing, well drilling, and raw salt for sodium carbonate pro-
duction. However, the corresponding industrial standards are
inadequate to date.
ratio of the two wastewaters was increased to 65% after 68
days of operation when the salinity reached 26 g L−1; more-
over, the nitrification efficiency was obviously affected
(Fig. 3a). However, an unexpected increment in denitrifica-
tion efficiency was achieved when the mixed ratio was
maintained at as high as 65% (Fig. 3b).
PW co-treatment with sewage in municipal WWTPs has
been a cost-effective PW management strategy in shale gas
fields in the USA.14 However, the negative effects of high TDS
and refractory organic compounds in the PW on conventional
biological sewage disposal processes have limited the long-
term implementation of this strategy.14 Moreover, informa-
tion on modified or advanced co-treatment technologies to
improve the operational stability of biological treatment of
PW, such as necessary pre-treatment processes and biological
reactor configurations, is still lacking.17,50 Riley and co-
workers indicated that seeding functional bacteria present in
the raw PW could improve the removal rates of organic com-
pounds in a biologically active filtration reactor treating
PW.50 The present study provides a new modification process
of the co-treatment of PW with municipal sewage by chemical
pre-treatment of PW before mixing with sewage. The follow-
ing biological process realized a complete ammonia removal
at a mixed ratio of pre-treated PW of 65%. The results re-
vealed that PW with an appropriate pre-treatment can be suc-
cessfully imported into the WWTPs with a limited mixed ra-
tio of 5–15% to realize a cost-effective PW disposal. Although
the importance of biological co-treatment of PW was demon-
strated in the present laboratory-scale study, the sequential
chemical–biological process to treat this industrial wastewa-
ter is usually case-specific due to the uncertainty of the influ-
ent and the complexity of microbial ecology.51 Further stud-
ies must focus on improving PW pre-treatment methods to
reduce the toxicity and enhance the biodegradability of PW.
PW biological co-treatment with sewage in WWTPs: labora-
tory scale of the sequential chemical pre-treatment–biological
process
Economic evaluation of PW desalination and RO concentrate
recycling processed in a full-scale centralized shale gas PW
treatment plant
A laboratory-scale co-treatment of PW with domestic wastewa-
ter testing was performed to evaluate the feasibility of the
biological treatment of PW. In an initial trial, PW without
pre-treatment was mixed with sewage and tested. The results
indicated that raw PW had a significant inhibition effect on
biological process performance, although the PW-to-sewage
ratio was maintained below 15% (data not shown). As such,
PW was pre-treated by modified Fenton and NaClO oxidation
processes which were conducted before co-treatment with
sewage. Fig. 3 illustrates the ammonia and TN removal from
the mixed wastewater when the post-treated PW-to-sewage ra-
tio increased from 10% to 65%. The co-treatment process
The operating cost of a centralized full-scale shale gas PW
treatment plant with a 1500 m3 d−1 capacity is presented in
Table 2. The results were calculated on the basis of the PW
desalination, RO concentrate advanced treatment, and
recycling processes. The total treatment and recycling cost of
1 m3 of PW from the shale gas field in the Eastern Sichuan
Basin was $12.114 per m3. As expected, the most significant
cost of the entire treatment route included the advanced oxi-
dation of raw PW and membrane concentrate, brine evapora-
tion, and sludge disposal (Table 2). The operation cost of the
PW desalination was high, although the PW of this shale gas
field had similar salinity to that of seawater. Previous model
studies have revealed that the least work required to desali-
nate PW is nearly nine times more than desalinating seawa-
ter at 50% recovery.34 High additional cost is required to re-
move hydrophobic organics and colloidal particles in PW to
achieve low membrane fouling during ultrafiltration and RO
+
was operated for 108 days. The NH4 -N removal efficiency of
the mixed wastewater after two weeks of adaptation reached
approximately 85% when the ratio was maintained below
50% (Fig. 3a). However, the denitrification performance
showed an opposite result (below 20%)(Fig. 3b). The mixed
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Table 2 Operating costs of a centralized shale gas PW treatment plant considered on the basis of advanced oxidation, desalination, and membrane
concentrate recycling processes ($ per m3)
Stages
PW desalination
RO concentrate advanced treatment and recycling
Ion exchange- Evaporation Brine
Costa
Coagulation Fenton–NaClO Multi-media Ultrafiltration- Pre-treatment O3-H2O2
($ per m3)
oxidation
0.034
0.854
filtration
0.028
0.050
—
0.034
0.109
RO
oxidation carbon filtration recycling
0.015
Electricity 0.005
Chemicals 0.039
Sludge
Labor
Total
1.110
0.030
—
0.034
1.171
0.011
0.092
0.773
0.034
0.910
7.965
0.612
0.145
—
0.022
0.095
—
5.980
0.050
—
—
—
0.625
0.034
0.700
4.149
1.250
0.034
2.169
0.034
0.791
0.034
0.151
0.034
6.064
0.034
0.049
a
Costs are reported in US dollar for treating 1 m3 of raw PW from the shale gas field in the Eastern Sichuan Basin, and the capacity of the
centralized shale gas PW treatment plant is 1500 m3 d−1. The selling price of the purified brine from the membrane concentrate is excluded.
desalination.34 Further studies should optimize the pre-
treatment efficiency and reduce sludge production provided
that membrane fouling is restricted at a low level. The maxi-
mum operational cost in the membrane concentrate recycling
process is the energy consumption of the MVC (Table 2). A
similar study has indicated that the energy consumption for
single- and dual-effect MVC systems that treat PW at medium
and large scales are 23–42 and 20 kW h m−3, respectively.34
Overall, the RO system showed a cost efficiency advantage
over other desalination techniques when treating PW with a
moderate recovery on the basis of the salinity of PW from the
shale gas field in the Eastern Sichuan Basin. The optimiza-
tion of the operation efficiency of evaporative systems for
brine recycling can significantly reduce the total expense
ratio.34
ondary pre-treatment units before membrane separation,
pressure-driven membrane separation, and the membrane
concentrate treatment or reuse.
Nevertheless, the present study still indicates that the bio-
logical unit is unsuitable for shale gas PW treatment, and in-
creased profound physicochemical pre-treatments are needed
to reduce the TOC before RO desalination. For RO concen-
trate reuse, a conventional disposal by deep well injection
has been investigated in the literature.31 Meanwhile, this
study proposes a new reuse method, that is, the recycling of
shale gas PW RO concentrate as a raw brine material in
chlor-alkali enterprises.
Conclusion
This study assessed the feasibility of an integral treatment
process of sequential pre-treatment and pressure-driven
membrane desalination, which was proposed for the first pri-
vate centralized shale gas PW treatment plant in the Eastern
Sichuan Basin, China. The reuse of the membrane concen-
trate and co-treatment of PW with sewage by biological pro-
cesses were also evaluated. The results included technical
principles and policy implications, discussed as follows.
(1) For the PW treatment of discharge, a sequential pre-
treatment and pressure-driven membrane desalination pro-
cess including Fenton–NaClO oxidation, multi-media filtra-
tion, and RO processes was demonstrated where bulk pollut-
ants in PW were eliminated. The content of contaminants in
the RO effluents was below the vast majority of the current
pollutant discharge standards of oil & gas industrial wastewa-
ter in China.
(2) For the external reuse of the corresponding PW RO
concentrate, a novel recycling route was proposed to ratio-
nally dispose the RO concentrate. The RO concentrate was
recycled as a raw brine material for a chlor-alkali enterprise
through a physicochemical advanced treatment process.
(3) For the PW co-treatment with sewage by biological pro-
cess, salinity fluctuation, chemical pre-treatment toxicity, and
resistance and resilience of bioreactors after fluctuations may
heavily influence the planning, design, and construction of
PW treatment plants, although the coupled chemical–
Comparison of the combined treatment process with the
reported or existing ones in centralized shale gas PW
treatment plants
The research on the shale gas industry has been rapidly de-
veloping since 2008, and the strategy of PW management has
gradually shifted from primary disposal to profound treat-
ment and resource recovery due to the increasing environ-
mental criteria.17,52 However, the feasibility of the integral
technology roadmap of PW treatments for high-grade dis-
charge in certain private centralized shale gas PW treatment
plants has yet to be explored in detail.13,17,18 Only several
integral treatment processes for PW from conventional oil &
gas industries have been proposed.29,31 An applicable treat-
ment process is not merely a simple combination of process-
ing units but is also an optimized result in consideration of
technical, environmental, and economic factors.13
In comparison with the reported centralized treatment
processes of conventional oil & gas PW with similar TDS
levels (<40 g L−1), a consensus had been reached on the
physicochemical methods and RO processes for TDS removal.
The reported pilot process consisted of warm softening, coco-
nut shell filtration, cooling, trickling filtration, ion exchange
and reverse osmosis to remove hardness, silica, boron, TOC
and ammonia.29,31 This study also established a broadly simi-
lar technology roadmap, including pre-treatment units, sec-
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biological process for treating PW is a potential cost-effective
treatment technique.
10 J. S. Shih, J. E. Saiers, S. C. Anisfeld, Z. Y. Chu, L. A.
Muehlenbachs and S. M. Olmstead, Characterization and
analysis of liquid waste from Marcellus shale gas
development, Environ. Sci. Technol., 2015, 49, 9557–9565.
11 W. J. Fan, K. F. Hayes and B. R. Ellis, Quantitative survey
and structural classification of hydraulic fracturing
chemicals reported in unconventional gas production,
Environ. Sci. Technol., 2018, 52, 10839–10847.
12 Y. M. Zhang, Z. S. Yu, H. X. Zhang and I. P. Thompson,
Microbial distribution and variation in produced water from
separators to storage tanks of shale gas wells in Sichuan
Basin, China, Environ. Sci.: Water Res. Technol., 2017, 3,
340–351.
(4) Our results indicate that the current regulations, such
as the Technical Specification for Oil Production Wastewater
Treatment (HJ2041-2014), and the Technical Specification for
Wastewater Treatment in Petroleum Refining Industry
(HJ2045-2014), are unsuitable for shale gas PW treatment.
New technical specifications for the treatment of specific in-
dustrial wastewater, that is, gas- and oil-field PW in China,
must be developed. The toxicity and NORM content should
be included in the standard. Moreover, new standards for in-
dustrial salt or brine, which is recycled from RO concentrate
solutions of shale gas PW, should be developed and adopted.
13 T. L. S. Silva, S. Morales-Torres, S. Castro-Silva, J. L.
Figueiredo and A. M. T. Silva, An overview on exploration
and environmental impact of unconventional gas sources
and treatment options for produced water, J. Environ.
Manage., 2017, 200, 511–529.
14 B. D. Lutz, A. N. Lewis and M. W. Doyle, Generation,
transport, and disposal of wastewater associated with
Marcellus Shale gas development, Water Resour. Res.,
2013, 49, 647–656.
15 A. Vengosh, R. B. Jackson, N. Warner, T. H. Darrah and A.
Kondash, A critical review of the risks to water resources
from unconventional shale gas development and hydraulic
fracturing in the United States, Environ. Sci. Technol.,
2014, 48, 8334–8348.
16 R. B. Jackson, A. Vengosh, J. W. Carey, R. J. Davies, T. H.
Darrah, F. O'Sullivan and G. Petron, The environmental
costs and benefits of fracking, Annu. Rev. Environ. Resour.,
2014, 39, 327–362.
17 J. M. Estrada and R. Bhamidimarri, A review of the issues
and treatment options for wastewater from shale gas
extraction by hydraulic fracturing, Fuel, 2016, 182, 292–303.
18 S. Jiménez, M. M. Micó, M. Arnaldos, F. Medina and S.
Contreras, State of the art of produced water treatment,
Chemosphere, 2018, 192, 186–208.
19 A. B. Schantz, B. Y. Xiong, E. Dees, D. R. Moore, X. J. Yang
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work is supported by the National Science and Technol-
ogy Major Project of China under Grant No. 2016ZX05060-
022. We wish to thank the anonymous reviewers for their
valuable suggestions on revising the work.
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