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Application of coagulation-UF hybrid process for shale gas fracturing flowback water recycling: Performance and fouling analysis
Journal of Membrane Science 524 (2017) 460–469
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Application of coagulation-UF hybrid process for shale gas fracturing flowback water recycling: Performance and fouling analysis
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Fan-xin Konga, Jin-fu Chena, , He-ming Wanga, Xiao-ning Liub, Xiao-mao Wangc, Xia Wend, Chun-mao Chena, Yuefeng F. Xiec,e a State Key Laboratory of Heavy Oil Processing, Beijing Key Laboratory of Oil & Gas Pollution Control, China University of Petroleum, Beijing 102249, China b Key Laboratory of Microorganism Application and Risk Control of Shenzhen, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China c State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China d Institute of Nuclear and NewEnergy Technology, Tsinghua University, Beijing 100084, China e Environmental Engineering Programs, The Pennsylvania State University, Middletown, PA 17057, USA
A R T I C L E I N F O
A BS T RAC T
Keywords: Shale gas fracturing flowback wastewater (SGFFW) Coagulation Ultrafiltration Resue
Shale gas fracturing flowback water (SGFFW) generated during shale gas extraction is of great concern and recycling for another fracking is the common disposal way. In this study, the feasibility of coagulation–UF hybrid process in assisting SGFFW reuse was systematically evaluated. Organics in SGFFW of Fuling were comparable with that reported in Marcellus. Poly aluminium chloride (PAC) dosage of 1500 mg/L may be preferred due to relatively low TOC (16.02 mg/L) and turbidity (3.03 NTU) in permeate and similar water flux (4.0×10−4 m/s) with that under the dosage of 2000 mg/L. With increase dosage of PAC, fouling mechanism changed from complete blocking to intermediate-blocking or cake standard. SEM-EDS indicated foulant was rich in carbon and oxygen with iron oxide and sulfate precipitates. According to volumetric integration method, overall rejection ratio for organics in different region in hybrid process decreased in the order of V (89.0%), IV 86.2%, III (80.3%), II (77.7%), I (55.2%) and VI (49.3%). LC-OCD illustrated coagulation mainly removed the organics with molecular weight of 20 kDa, while UF could remove a fraction of low molecular weight components (i.e., 200 Da). Fouling was reversible by backwashing and thus hybrid process without sedimentation can be potentially used for SGFFW treatment.
1. Introduction With accelerated production of shale gas using horizontal drilling and hydraulic fracturing, there is a growing concern over shale gas fracturing flowback water (SGFFW) around the world [1]. Flowback water generated by shale gas exploration was not only estimated to be 10 times more than that generated by conventional oil and gas extraction, but also can pose potential environmental risk due to its high total dissolved solids (TDS), organic matter and radioactive elements [1,2]. Deep injection, reuse for hydraulic fracturing and discharged to surface water are the three common disposal ways for SGFFW [3]. Backed by the largest proven shale gas reserve worldwide in China, Fuling shale gas play with the capacity of 380.6 billion cubic meters was discovered in 2014 and annual capacity has reached 1.08 billion cubic meters by the end of 2014 and 5 billion cubic meters by 2015, which is the largest shale gas play in China and the second largest in the world following the Marcellus shale gas play in the US
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[4,5]. Currently, most of the SGFFW in Fuling and Marcellus shale gas play is reused for another fracking event, but high efficiency treatment technology is still lacking. Therefore, developing the reliable method for SGFFW reclamation is on the agenda for shale gas exploration in China and the world. Prior to reuse, SGFFW is typically treated on-site to remove suspended solids or specific constituents that may not be compatible with fracturing fluid chemistry [6,7]. Due to the low porosity and permeability of the shale gas reservoirs, methods for the conventional oil & gas wastewater treatment (i.e., sand filtration and multiple agent filtration) could not meet the standard for the fracking reuse due to potential risk of pore blocking in shale gas reservoirs. Membrane process is regarded as a promising and high efficiency technology for SGFFW recycling due to its high efficiency and excellent permeate quality [8]. Until now, some studies adopted MF for the direct treatment of SGFFW (mainly form Marcellus shale gas play). Jiang et al. [9] used ceramic MF membrane (0.2 µm and 0.8 µm) to filter the
Corresponding author. E-mail address: [email protected] (J.-f. Chen).
http://dx.doi.org/10.1016/j.memsci.2016.11.039 Received 2 September 2016; Received in revised form 11 November 2016; Accepted 11 November 2016 Available online 23 November 2016 0376-7388/ © 2016 Elsevier B.V. All rights reserved.
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2.2. Jar test
SGFFW and severe fouling was observed for both membranes. He et al. [10] evaluated the mechanisms that are responsible for severe fouling of polyvinylidene fluoride (PVDF) membrane for SGFFW and revealed that the submicron particles present in the early flowback water caused rapid flux decline. Recently, He et al. [11] also found that intermediate pore blocking was the dominant fouling mechanism of two ceramic MF membranes for Marcellus SGFFW treatment. These above studies indicated that MF fouling caused by SGFFW was complex and severe with no pretreatment applied. Therefore, pretreatment for the membrane process should be adopted to alleviate the membrane fouling for SGFFW treatment. Pre-treatment involving coagulation or coagulation-hydraulic flocculation has been shown to be an effective and low-cost approach for improving overall water quality and reducing membrane fouling, among which, chemical coagulation is an effective approach [12–16]. A hybrid process with the combination of the coagulation and low pressure membrane filtration has been widely used to alleviate membrane fouling and improve the water quality in terms of particle, colloid and DOC compared with either coagulation or membrane filtration process alone [12,13,17]. Therefore, a possibly feasible pathway for the treatment of SGFFW with the effective removal of colloid particle and mitigation of membrane fouling is the combination of coagulation and membrane filtration, which has not been reported yet for SGFFW treatment. Compared with MF, UF membrane with pore size of 0.01–0.1 µm has been widely adopted in both municipal and industrial water treatment to effectively remove suspended solids, colloidal material, inorganic particulates, and microorganisms [12,18]. Most recently, Xiong et al. [19] found that colloids and organics in the SGFFW in Marcellus shale gas play were only partially removed by MF and caused substantial fouling during a subsequent membrane process, indicating that MF alone couldn’t provide an effective treatment for SGFFW due to the high concentrations of small colloidal particles and organic matter. Thus, coagulation-UF can effectively separate out these small colloidal particles and the removal efficiency might outperform MF and coagulation-MF. A case as an example is that the integration of coagulation and UF has attracted considerable attention in recent years [12,13,17,20]. However, the efficacy of coagulation to alleviate UF fouling and the applicability of this hybrid process for SGFFW treatment still need to be investigated. The objective of this study was to evaluate the feasibility of coagulation–UF hybrid process in assisting SGFFW reuse in Fuling shale gas play. The performance of the hybrid process, the effect of coagulation on membrane fouling and the dissolved organics removal in this process were systematically elucidated. The results of this study allowed to examine the efficacy of coagulation to alleviate membrane fouling and the applicability of coagulation–UF process for treatment of SGFFW, provide a better understanding of UF fouling in this hybrid process and develop solutions that enable the use of membrane filtration in recycling of wastewater produced during shale gas exploration.
To obtain the optimal pretreatment condition, separate jar tests were conducted prior to the UF tests. PAC, a commonly used coagulant was selected in the coagulation experiment. The raw water (1 L) was stirred at 50 rpm for 60 s and then PAC were added (from 500 to 2000 mg/L), with a simultaneous increase of stirring speed to 300 rmp. The rapid mix speed of 300 rpm was maintained for 2 min and then reduced to 100 rpm for 15 min to allow floc growth to occur. Finally, precipitation lasted for 30 min as experiment needed. Then the supernatant or the mud mixture was introduced to the reservoir of the membrane filtration system. 2.3. Membrane filtration producers A bench-scale dead-end filtration system in constant pressure mode was followed by coagulation process, which is ideal for evaluating the fouling potential when only limited volumes of feed are available [19]. The schematic diagram of the experimental setup was shown in Fig. S1 of the supporting information. The system consisted of a 2 L feed reservoir, a 200 mL customer-made stirred filtration cell, a nitrogen gas cylinder for pressure generation, two pressure sensor for pressure measurement, a digital balance (Mettler Toledo, Germany) for the measurement of water flux, a computer for data logging and necessary tubing. The stirred dead-end filtration cell with the effective area of 20.5 cm2 was made of stainless steel. A magnetic stirring bar identical to that used in the Amicon 8400 (Millipore, US) unit was installed in the filtration cell and the feed reservoir leaving a gap of 3.0 mm to the membrane's upper surface. The shear intensity was varied by controlling the rotation speed of the magnetic bar. The PVDF UF membrane (FF4002, SEM image can be seen in Fig. S2 of the supporting information) was purchased from Minglie Chemical Co., Ltd.(Shanghai, China) with the pore size of 0.02 µm (nominal molecular weight cutoof 30 kDa) and water permeability of 5.53×10−3 m/(s Pa), corresponding to the membrane resistance of 1.8×1010 m−1. All membrane coupons were soaked in ultrapure water for at least 12 h at room temperature prior to use. The membrane was placed in the bottom of the stirred cell on top of a mesh to minimize deformation of the membrane into the support structure. In every filtration experiment, the fresh membrane in the filtration cell was first compacted by filtering ultrapure water at 150 kPa until a stabilized water flux was reached, and then continued to filter for about 30 min with the pressure adjusted to 100 kPa to obtain the pure water flux. Finally, the reservoir was filled with the water sample of interest (effluent of the coagulation process) with the stirring speed of approximately 500 rpm, and the system was re-pressurized under the constant pressure of 100 kPa until quasis-steady flux was reached. All the experiments were conducted in an air-conditioned room with the temperature set at 25 ± 1 °C and repeated at least once. Permeates were sampled when the quasis-steady state was reached. All samples were stored at 4 °C for one day at maximum. As for the backwashing, a peristaltic pump (Longer, USA) was connected to the outlet of the filtration cell with the inlet of the filtration cell open to the atmosphere. Ultrapure water was used for backwashing. A pressure transducer (Universal CB1020, Labom) was installed between the filtration cell and the pump in order to monitor the variation in applied pressure during backwashing. The pressure profile was logged as a function of operation time using Labview (National Instruments) software. The rotation speed of the peristaltic pump was maintained constant at 0.1 L/min during each experiment and, as such, the backwashing flux should be constant provided the transmembrane pressure was not too high [21].
2. Materials and methods 2.1. Raw water The characteristic of the SGFFW was reported to be complicated, which was mainly consisted of dissolved salts, chemical additives and solid particles. The raw water in this study was sampled from one of the reservoir in Fuling shale gas play, Chongqing, China. It was turbid with a light yellow color with some small floccules, which is similar to the previous report on Marchellus shale gas play [9]. The detailed water quality will be comprehensively discussed in the following session.
2.4. Fouling model and SEM observation The fouling mechanism identification approach in dead-end mode 461
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with constant pressure reported by Bowen [22] was adopted in this study to interpret the water flux decrease of UF, in which, there were four classical types of flux decline mechanisms describing characteristic change in flux over time under constant pressure including complete blocking, standard blocking, intermediate blocking and cake standard. The experimental data was fitted based on the equations (More details can be seen in session S3 of the supporting information) with R squats of difference between experimental data and fitted values to indicate the goodness of the model. The top surface morphology of the membranes and main elemental composition before and after the experiments were acquired by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) (Hitachi S-5500, Japan). Samples were freezedried for 24 h, followed by drying under vacuum overnight at 30 °C and gold coating.
Table 1 Comparison of characteristics of SGFFW with Marcellus and criteria.
2.5. Fluorescence EEM measurements A fluorescence spectrophotometer (F-7000, Hitachi, Japan) was used to characterize the type of dissolved organic matters (DOM). The raw water and samples after coagulation were filtered by 0.45 µm nylon filter (VWR Int., PA, USA) and diluted for 10 times before measurement. To obtain fluorescence excitation emission matrix (EEM), excitation wavelengths were incrementally increased from 200 to 400 nm at 5-nm steps; for each excitation wavelength, the emission at longer wavelengths was detected at 5-nm steps. For each excitation wavelength, the emission at longer wavelengths was detected.
Constituents
Fuling shale gas play
Nearby shale gas play [8]
Marcellus shale gas play [9]
Criteria for surface water quality (Ⅴ)
TOC(mg/L) CODcr(mg/L) Turbidity（NTU）
78 472.6 37.1
_ 358.5 _
_ 40 _
TDS (g/L) Conductivity （ms/ cm） Na+(mg/L) + K (mg/L) NH4+(mg/L) Ca2+(mg/L) Mg2+(mg/L) Fe3+(mg/L) Mn2+(mg/L) Al3+(mg/L) Ba2+(mg/L) Cl- (mg/L) Br-(mg/L) NO3- (mg/L) SO42- (mg/L) HCO3- (mg/L) SiO2 (mg/L) Boron（mg/L）
13.5 2.36
6.91 11.29
720 ,96–390a _ 770,150– 3000a 48,31–130a 67,54–140a
3980 110 25 164 16.0 6.78 0.479 3.10 30.7 6930 25. < 0.1 72.9 1010 72.3 10.6
2109 393 11.5 140.2 18.05 _ _ _ _ 4202.2 _ _ 2.2 149 19.2 16.9
12200 363 _ 2935 104 <1 <2 105 697 28500 19 _ 12.9 205 _ _
_ _ _ _ _ _ _ _ _ 250 _ _ 250 _ _ _
_ _
∑ ici (+) = 188.04 mmol/L; ∑ ici (−) = 213.61 mmol/L. a
Obtained from reference [19].
2.6. Molecular weight distribution measurement play. The water sample contained a high concentration of organic materials, at 78 mg/L TOC, which was approximately the same order of magnitudes with that reported in Marcellus shale gas play [10]. The turbidity of the sample was 37.1 NTU. The TDS of the SGFFW is 13500 mg/L, about 3 times lower than the TDS specified for Marcellus shale gas paly (48000 mg/L) and about 6 times higher than the criteria for surface water quality. The conductivity of the SGFFW also exhibited a high value of 2360 mS/cm, which is about 30 times lower than that reported of Marche shale gas play [9]. Sodium and chloride were the dominant ions in this SGFFW sample followed by HCO3-, Ca2+ and K+. Specifically, the content of Na+ and Cl- is predominantly high with the concentration Na+ of 3980 mg/L and Cl- of 6930 mg/L, 27 times higher than the criteria for surface water quality. Compared to Na+ and Cl-, HCO3-, Ca2+ and K+ showed much lower concentration, which were respective present at 1010, 164 and 110 mg/L. The concentrations of all other ions were insignificant. In general, the organics in the SGFFW is comparable with that reported in Marcellus shale gas play, while TDS and salt ions in the Fuling shale gas play is much lower than that in Marcellus shale gas play. The much lower TDS and salts concentrations was mainly attributed to difference in the rock formation of the geographic location and geologic basin from which the produced water originated [1].
The MW distribution of samples was determined by using a highpressure size exclusion chromatography (HPSEC, LC-20AT, Shimadzu, Japan) system equipped with an on-line organic carbon detector (OCD, Sievers 900 Turbo TOC, GE, USA), where a TSKgelG3000PWXL column (0.78 cm×30 cm, TOSOH, Japan) and a subsequent TSKgelG2500PWXL column (0.78 cm×30 cm, TOSOH, Japan) were used. More details can be found in previous literatures [23,24]. 2.7. Other analytical methods Dissolved organic carbon (DOC) was measured by filtering samples through a 0.45 µm nylon filter （VWR Int., PA, USA）followed by dilution and analyses using a TOC-500 A analyzer (SHIMADZU corporation, Japan), the detection limit of which was approximately 0.2 mg/L. Chemical oxygen demand (CODcr) was measured using HACH (Loveland, CO) COD kits and a DR5000 spectrophotometer, after dilution to avoid chloride interferences. The TDS of the sample was determined according to the standard methods for the examination of water and wastewater [25]. The turbidity was determined by the turbidity meter (1720e, HaCH, USA). The cationic analysis of the samples was obtained using an inductively coupled plasma spectrometer (Thermal PQ3 ICP System), the detection limits of which were approximately 0.1–0.5 mg/L for respective cationic ion. The anionic ion was measured using the ion chromatography (Metrohm, Switzerland), the detection limits of which were approximately 5– 80 μg/L, depending on the anionic ion measured.
3.2. Performance of the hybrid process under the different dosage of PAC 3.2.1. Water quality Removal of TOC and turbidity is critical for SGFFW treatment due to its detrimental effects on shale gas recovery or for other beneficial reuse, including as a pretreatment step prior to desalination. Preparation experiment indicated that the water flux was negligible when directly filtering the raw water without coagulation (data not shown). The variation of TOC and turbidity after coagulation and UF filtration was shown in Fig. 1. The results demonstrated that the rejection ratio for TOC typically showed a monotonous decrease with the increase dosage of PAC (Fig. 1). In other words, the more PAC was
3. Results and discussion 3.1. Characteristics of SGFFW The main constituents of SGFFW in Fuling shale gas play and the comparison with Marcellus shale gas play and the criteria for surface water quality (GB3838-2002, China) was listed in Table 1. As shown in Table 1, all the index was comparable to that reported in the shale gas play nearby [8], but notably differed from that in Marcellus shale gas 462
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Fig. 1. Variation of TOC and turbidity after coagulation and membrane filtration with the dosage of different concentration of PAC. Fig. 2. Effect of different dosage of PAC on membrane flux (a) Water flux (b) Normalized flux.
added, the more particles and colloids were removed. Under the PAC dosage of 500–2000 mg/L, the TOC decreased by 70–80%, while there was minor difference for TOC with the PAC dosage of 1500 and 2000 mg/L. In contrast, the turbidity of the permeate could reach their minimum under a medium PAC dosage of 1500 mg/L. Turbidity of the coagulated samples increased from 33 to 42.5 NTU initially with the dosage of 500 mg/L PAC due to the fine grains and an increase in aggregation size due to the increase of PAC dosage resulted in a further decrease in turbidity, while negligible change of TOC was observed after UF. It is evident that the turbidity was significantly decreased from 63.2% to 86.2% after UF owing to sieving effect. More water quality indexes (i.e., CODCr, pH and conductivity) of permeate can also be seen in the Table S2 of supporting information. Based on the above study, the dosage of 1500 mg/L PAC may be preferred in this hybrid process with relatively low TOC (16.02 mg/L) and turbidity (3.03 NTU) of the permeate. However, the dosage of coagulant is relatively high compared with that in the surface water treatment and sewage reclamation. Thus, the appropriate disposal way for sludge or new types of coagulant should be explored.
under the dosage of 500 mg/L PAC with the fouling resistance up to 2.55×1013 m−1, which is about three orders of magnitude higher than the intrinsic membrane resistance (1.8×1010 m−1). When the PAC dosage was 1000 mg/L, the stable water flux was 50% of the initial flux and the resistance of the fouling layer was calculated to be 6.36×1012 m−1. When the PAC dosage was increased to 1500 mg/L and 2000 mg/L, the stable water flux increased to approximately 4.0×10−4 m/s, which was one order of magnitude higher than that under the PAC dosage of 1000 mg/L. There was minor difference in stable water flux between PAC dosage of 1500 and 2000 mg/L, albeit the higher initial water flux under the PAC dosage of 2000 mg/L. The fouling resistance after the pretreatment of coagulation by the PAC dosage of 1500 mg/L and 2000 mg/L was calculated to be 2.25×1011 and 2.03×1011 m−1, respectively. Normalized water flux of the membrane followed the order of 1000 mg/L PAC > 1500 mg/L PAC > 2000 mg/L PAC > 500 mg/L PAC (Fig. 2b). Normalized water flux depended strongly on the initial permeate flux and foulant characterization, the decline of which was the synergistic effect of the initial water flux and water quality [27,28]. The poor water quality (i.e., high TOC and turbidity) after coagulation may dominate the flux decline under the dosage of 500 mg/L despite relatively low water flux. The initial water flux under the dosage of 1000 mg/L PAC was approximately 1/6 higher than that of under the dosage of 500 mg/L PAC but was only 1/3 of that under the dosage of 1000 and 2000 mg/L PAC, while TOC concentration was 33.2% lower than that under the dosage of 500 mg/L PAC but was only 15.2% and 10.4% higher than that under the dosage of 1500 and 2000 mg/L PAC (Fig. 1), respectively. Therefore, the slightly higher initial water flux and much lower TOC under the PAC dosage of 1000 mg/L than that of 500 mg/L contributed to much slower flux decline. Meanwhile, the much lower initial water
3.2.2. Water flux Fig. 2 showed the water flux and the normalized water flux by filtering the SGFFW after the dosage of 500, 1000, 1500 and 2000 mg/ L PAC. Two aspects should be taken into account for PAC dosage on UF fouling control: one is the removal of colloids; the other is the in-situ formed particulate alum flocs, which may exert adverse influence on membrane fouling [26]. With the increase of PAC dosage, the surfactant and polyacrylamide in the raw water might be greatly removed, and the viscosity of the coagulated raw water decreased which resulted in much higher initial water flux with the PAC dosage of 1500 and 2000 mg/L compared to that of 500 and 1000 mg/L. It can be seen the stable water flux was only 20% of the initial water flux 463
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among which the poor removal of colloids and fine particles contributed for the highest R2 value under the dosage of 500 mg/L. Intermediate blocking phenomenon caused by occlusion of pores by particles with particle superimposition due to the heterogeneity of the particle size, can be observed under the different dosage of PAC (R2 > 0.85) and fitted fairly well under the PAC dosage of 1000 and 2000 mg/ L. Cake block model fitted the results reasonably well under the dosage of 1000 (R2=0.95) and 1500 mg/L (R2=0.98) PAC. However, under the dosage of 2000 mg/L, the R2 for complete blocking and intermediatestandard blocking model is higher than 0.97, while cake filtration model was relatively low mainly due to the rapid initial flux decline, which is plausible that the number of larger size folcs decreased and there was still room for particle to directly obstruct the membrane pores. The different behaviors of flux decline reflected the different fouling mechanism of the foulant. Though the individual model might be insufficient to elucidate the membrane fouling mechanism as membrane fouling may be contributed simultaneously by several individual fouling models, In general, this modeling results indicated that the fouling mechanism changed from complete blocking to intermediate-standard blocking or cake blocking model with the increase of PAC dosage. SEM images of membrane were taken at the end of the filtration period to further investigate the fouling behaviors ( Fig. 4). It can be seen that fine flocs and colloids were closely packed and deposited on the membrane with the white spot scattering on the surface under the dosage of 500 mg/L PAC compared to the virgin membrane (Fig. S2 in the supporting information). Although the fouling layer were cracked when sample preparation by freezing-drying, it was obvious that the membrane fouling layer were more serious under the PAC dosage of
flux and comparable TOC for PAC dosage of 1000 mg/L compared with that of 1500 and 2000 mg/L is a likely reason for the observed slower flux decline rate. A little higher initial water flux and TOC under the dosage of 2000 mg/L PAC might result in the higher permeation drag and deposition rate for the foulant, and therefore a slightly higher flux decline rate than the PAC dosage of 1500 mg/L. 3.2.3. Membrane fouling analysis Serious fouling of the membrane can be observed under the dosage of 500 mg/L and 1000 mg/L PAC, while fouling was tremendously mitigated with PAC dosage of 1500 and 2000 mg/L (Fig. S3 in the supporting information) plausibly due to the increase removal of surfactant and polyacrylamide by coagulation. The fouling model under constant pressure developed by Bowen [22] was adopted to better understand UF fouling mechanism (More details in Table S1 of the supporting information), which was initially developed to describe the protein fouling of the membrane, under which condition, a single fouling mechanism dominated in a given system. However, not all the cases of interest can be fit to such one single equation and the individual model might be insufficient to elucidate the membrane fouling mechanism, as membrane fouling may be contributed simultaneously by several individual fouling models [29]. According to the modeling result ( Fig. 3), standard blocking where particles smaller than the membrane pore size deposited onto the pore walls thus reducing the pore size which played negligible role for the membrane fouling, indicating heterogeneity and irregularity of foulants for all the cases. The complete blocking model caused by occlusion of pores by particles with no particle superimposition fitted the fouling scenario fairly well under the dosage of 500, 1000, 1500 and 2000 mg/L PAC,
Fig. 3. Water flux fitted by UF fouling model after the pretreatment by coagulation with different PAC dosage (a) Complete blocking (b) Standard blocking (c) Intermediate blocking (d) Cake standard.
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Fig. 4. Surface topography of the membrane after the pretreatment by coagulation with different PAC dosage (a) 500 mg/L (b) 1000 mg/L (c) 1500 mg/L (d) 2000 mg/L.
layer on the surface of the UF membrane with the increase of PAC dosage.
1000 mg/L than that of 1500 and 2000 mg/L. It should be noticing that white spot with small size can also be seen on the surface of the fouling layer (Fig. 4). In order to identify elemental composition of the fouling layer, the fouled membrane was analyzed using EDS (Table S3 in the supporting information). It can be observed that C and O is the dominant elements in the fouling layer. The weight proportion of C was 46.9%, 43.13%， 38.11% and 35.67%, while the weight proportion of O was 39.64%, 39.96% ,41.31% and 37.58% under the dosage of 500, 1000, 1500 and 2000 mg/L PAC, respectively. Except for Na, Al and Si, the proportion of other inorganic elements was relatively low (lower than 1%), which indicated the organic matters mainly contributed to the membrane fouling and removing the particles or organic matters by coagulation can effectively mitigate membrane fouling. The elemental analysis by SEM-EDS demonstrated the foulant had very low levels of calcium but was rich in carbon and oxygen, similar with the foulant in Marcellus SGFFW [10]. However, the iron was relatively low in this foulant, which was much lower than that reported in the particles of Marcellus flowback water [10]. As mentioned above, there were white spots on the surface of fouling layer. As for the elemental composition of white spot on the membrane surface, the weight proportion of C decreased, while that of Fe, S and O increased with the increase dosage of PAC ( Fig. 5). Under the dosage of 500 mg/L, the weight proportion of Na and Cl of the white spot is relatively high compared to other areas, indicating that the white spot on the surface might be the crystallization of NaCl after drying, which rejected by the dense fouling layer under the dosage of 500 mg/L PAC. This was evidenced by the much lower TDS compared with other conditions (Table S2 in the supporting information). Under the dosage of 1000, 1500 and 2000 mg/L, the weight proportion of Fe, S and O in the fouling layer was relatively high compared to other areas (Fig. 5), which indicated that the white spot on the surface might be iron oxide and oxysulfide. He et al. [11] reported that low concentration of organics in Marcellus SGFFW had minimal impact on MF membrane fouling and that densely packed cake layer of fine organic-coated iron oxide particles was the key reason for severe fouling of polymeric MF membrane. It is plausible that the packed cake layer of the fine organic metal oxide particles contribute to the cake
3.3. Dissolved organics removal under the optimal dosage of PAC 3.3.1. Fluorescence EEM spectra As mentioned above, SGFFW contained a wide range of organic compounds including these present in the initial fracking fluid (i.e., guar gum, polyacrylamide, glutaraldehyde, isopropanol and various surfactants) and biocides as well as hydrocarbons released directly from the shale gas formation, which was qualitatively characterized by fluorescence spectra for this hybrid process under the dosage of 1500 mg/L PAC. The widely used classification method, fluorescence regional integration (FRI) could divide the map into five different zones including regions I and II, Ex/Em: 220–250/280–330 nm, 220–250/ 330–380 nm (aromatic protein, which might be guar gum for SGFFW), region III, Ex/Em: 220–250/380–480 nm (fulvic acid-like components), region IV, Ex/Em: 250–440/280–380 nm (soluble microbial products (SMPs) including tyrosine-, tryptophan-, and protein-like components), and region V, Ex/Em: 250–400/380–540 nm (humic acid-like components) [24,30]. Lester et. al. [31] found that the fluorescence signature observed in region IV stems from phenolic compounds (i.e., phenol and 2,4-dimethyphenol) not associated with proteins for SGFFW. It should be noticing that the spectral component with Ex/Em: 200–250/220–280 nm was observed in this water but not identified, which was marked as region VI in this study for comparison purpose. As shown in Fig. 6, the dissolved organics in raw water mainly belonged to region I (aromatic protein), II (aromatic protein) and IV (i.e., phenolic compounds). Pre-treatment by coagulation can effectively intercept aromatic protein (region II), fulvic acid-like components (region III), and phenolic compounds (region IV) (Fig. 6). According to FRI under the EEM within each region volume, the dissolved organics removed by coagulation in different region decreased in the order of V (83.3%), IV(78.6%), III(72.8%), II (64.9%), I(39.1%), VI(32.5%), while the overall rejection ratio of hybrid process decreased in the order of V (89.0%), IV 86.2%, III(80.3%), II(77.7%), 465
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Fig. 5. Elemental composition analysis of the spot on the surface by EDS after the pretreatment by coagulation with different PAC dosage (a) 500 mg/L (b) 1000 mg/L (c) 1500 mg/L (d) 2000 mg/L.
30 kDa, which is much higher than the molecular weight of most organics in SGFFW. However, UF removed a fraction of organics with molecular weight 20 kDa and low molecular weight components (i.e., 100–120 Da), indicating fouling layer was composed of guar gum, polyacrylamide and various surfactants. The increasing rejection of small molecular weight organics can be explained by an accumulation of organic compounds in the retentate and/or the foulant layer of cake/ gel formation, acting as a secondary membrane, which was found to be an effective filtration layer for organics [32]. As a result, a higher percent removal of small molecular weight organics was achieved. Moreover, He et al. [10] hypothesized that small molecular size compounds (i.e., scaling inhibitors and friction reducers) in the SGFFW might be coated on the stabilized colloidal or nano-sized particles and thus the compounds with small molecular weight might be removed with these coated colloids or particles.
I(55.2%), VI(49.3%) (Fig. 6). The retained organics was able to block membrane pores and form a cake/gel layer on the membrane surface. In general, the dissolved organics was further removed by 10–20% by UF filtration and the average percent organic removal of fulvic acid-like components, humic acid-like components and phenolic compounds was higher than 80% in this hybrid process.
3.3.2. Molecular weight distribution The removal of organic compounds including these present in the initial fracking fluid (i.e., guar gum, polyacrylamide, glutaraldehyde, isopropanol and various surfactants) and biocides as well as hydrocarbons was comprehensively indicated by the change of molecular weight distribution for this hybrid process under the PAC dosage of 1500 mg/L (optimal conditions). Fig. 7 indicated that the raw water mainly contained a large portion of high MW biopolymers (20 kDa) which might be guar gum or polyacrylamide and large fraction of low MW substances (150 Da) such as small neutrals and acids, while a small fraction of medium MW components (1500 Da) which might be surfactant can be observed (Fig. 7). The difference in MW distribution between the feed and the coagulated samples showed that the organics with MW20 kDa were greatly removed (Fig. 7), indicating the coagulation mainly intercepted this portion of the organics such as guar gum, polyacrylamide and various surfactants. The nominal molecular weight cutoff of the UF membrane is approximately
3.4. Is the sedimentation for membrane is needed? Though the coagulation-UF process can well reject TOC, turbidity and the dissolved organics, it is still desirable whether the sedimentation in coagulation process can be omitted. The potential to shorten the process will improve the process economics and expand applications. The mixture of water and flocs was filtered directly by the membrane, immediately after slow mixing. Backwashing was con466
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Fig. 5. (continued)
play. The performance and membrane fouling were systematically investigated:
ducted to examine the reversibly of the foulant. The comparison between the two modes was shown in Fig. 8. After pretreatment by coagulation, the flux decline almost followed the same pattern in the first 4 min, but then the water flux without sedimentation decreased sustainably into zero. The limited water flux and the severe fouling are among the biggest impediments to the viability of the coagulation-UF process. However, backwashing can effectively remove the foulant deposited on the membrane surface with the flux recovery of 77.9% of the initial water flux which is comparable with that the case of water after sedimentation (81.9%), indicating the foulant was reversible. In this regard, it is possible that optimizing hydrodynamic conditions in cross-flow system might result in a lower buildup of foulants near the membrane surface. Therefore, coagulation-UF process can potentially be used for SGFFW treatment without sedimentation which will not only promote treatment efficiency but will also save the space for the on-site and cost-effective reclamation of SGFFWs.
• • •
4. Conclusion
•
In this study, a hybrid process with the combination of coagulation and UF membrane filtration was applied to evaluate the efficacy of coagulation in alleviating UF fouling and the feasibility of this hybrid process in assisting SGFFW recycling and reuse in Fuling shale gas 467
The organics in the SGFFW is comparable with that reported in Marcellus shale gas play, while TDS and salt ions in the Fuling shale gas play is much lower than that in Marcellus shale gas play. The rejection for TOC showed a monotonous decrease with the increase dosage of PAC, while turbidity of permeate could reach their minimum under a medium PAC dosage of 1500 mg/L. The dosage of 1500 mg/L PAC may be preferred in this hybrid process with relatively low TOC and turbidity. Normalized water flux of the membrane followed the order of 1000 mg/L PAC > 1500 mg/L PAC > 2000 mg/L PAC > 500 mg/L PAC. The modeling results indicated that the fouling mechanism changed from the complete blocking to intermediate-standard blocking or cake blocking model with the increase of PAC dosage. The foulant had very low levels of calcium but was rich in carbon and oxygen, similar with the foulant in Marcellus SGFFW. However, the white spot on the surface might be iron oxide and oxysulfide. According to the volumetric integration, the dissolved organics removed by coagulation in different region was in the order of V (83.3%) > IV(78.6%) > III (72.8%) > II (64.9%) > I(39.1%) > VI(32.5%), while the overall rejection ratio of hybrid process
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Fig. 7. The variation of molecular weight distribution in the hybrid process.
Fig. 8. Effect of settlement on UF flux and recovery rate of membrane flux after backwashing.
thus coagulation-UF process without sedimentation can be potentially used in SGFFW treatment, which will not only promote treatment efficiency but will save the space for the on-site and cost-effective reclamation of SGFFWs. Acknowledgments The authors acknowledge the financial support provided by Science Foundation of China University of Petroleum, Beijing (No. 2462015YJRC030). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2016.11.039.
Fig. 6. Fluorescence EEM spectra (a) Raw water (b) Coagulated by 1500 mg/L PAC (c) Permeate of coagulation-UF hybrid process ((I) Ex/Em=220–250/280–330 nm; (II) Ex/ Em=220–250/330–380 nm; (III) Ex/Em=220–250/380–480 nm; (IV) Ex/Em=250– 440/280–380 nm; (V) Ex/Em=250–400/380–540 nm; (VI) Ex/Em=200–250/200– 280 nm.
•
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decreased in the order of V (89.0%), IV 86.2%, III(80.3%), II(77.7%), I(55.2%), VI(49.3%).. The difference in MW distribution between the feed and the coagulated samples demonstrated that the coagulation mainly removed the organics with molecular weight of 20 kDa, while UF membrane removed a fraction of low molecular weight components (i.e., 200 Da). The deposited foulants might enhance the removal of low molecular weight components. The foulants in the membrane was reversible by backwashing and 468
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Application of coagulation-UF hybrid process for shale gas fracturing flowback water recycling: Performance and fouling analysis
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Fan-xin Konga, Jin-fu Chena, , He-ming Wanga, Xiao-ning Liub, Xiao-mao Wangc, Xia Wend, Chun-mao Chena, Yuefeng F. Xiec,e a State Key Laboratory of Heavy Oil Processing, Beijing Key Laboratory of Oil & Gas Pollution Control, China University of Petroleum, Beijing 102249, China b Key Laboratory of Microorganism Application and Risk Control of Shenzhen, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China c State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China d Institute of Nuclear and NewEnergy Technology, Tsinghua University, Beijing 100084, China e Environmental Engineering Programs, The Pennsylvania State University, Middletown, PA 17057, USA
A R T I C L E I N F O
A BS T RAC T
Keywords: Shale gas fracturing flowback wastewater (SGFFW) Coagulation Ultrafiltration Resue
Shale gas fracturing flowback water (SGFFW) generated during shale gas extraction is of great concern and recycling for another fracking is the common disposal way. In this study, the feasibility of coagulation–UF hybrid process in assisting SGFFW reuse was systematically evaluated. Organics in SGFFW of Fuling were comparable with that reported in Marcellus. Poly aluminium chloride (PAC) dosage of 1500 mg/L may be preferred due to relatively low TOC (16.02 mg/L) and turbidity (3.03 NTU) in permeate and similar water flux (4.0×10−4 m/s) with that under the dosage of 2000 mg/L. With increase dosage of PAC, fouling mechanism changed from complete blocking to intermediate-blocking or cake standard. SEM-EDS indicated foulant was rich in carbon and oxygen with iron oxide and sulfate precipitates. According to volumetric integration method, overall rejection ratio for organics in different region in hybrid process decreased in the order of V (89.0%), IV 86.2%, III (80.3%), II (77.7%), I (55.2%) and VI (49.3%). LC-OCD illustrated coagulation mainly removed the organics with molecular weight of 20 kDa, while UF could remove a fraction of low molecular weight components (i.e., 200 Da). Fouling was reversible by backwashing and thus hybrid process without sedimentation can be potentially used for SGFFW treatment.
1. Introduction With accelerated production of shale gas using horizontal drilling and hydraulic fracturing, there is a growing concern over shale gas fracturing flowback water (SGFFW) around the world [1]. Flowback water generated by shale gas exploration was not only estimated to be 10 times more than that generated by conventional oil and gas extraction, but also can pose potential environmental risk due to its high total dissolved solids (TDS), organic matter and radioactive elements [1,2]. Deep injection, reuse for hydraulic fracturing and discharged to surface water are the three common disposal ways for SGFFW [3]. Backed by the largest proven shale gas reserve worldwide in China, Fuling shale gas play with the capacity of 380.6 billion cubic meters was discovered in 2014 and annual capacity has reached 1.08 billion cubic meters by the end of 2014 and 5 billion cubic meters by 2015, which is the largest shale gas play in China and the second largest in the world following the Marcellus shale gas play in the US
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[4,5]. Currently, most of the SGFFW in Fuling and Marcellus shale gas play is reused for another fracking event, but high efficiency treatment technology is still lacking. Therefore, developing the reliable method for SGFFW reclamation is on the agenda for shale gas exploration in China and the world. Prior to reuse, SGFFW is typically treated on-site to remove suspended solids or specific constituents that may not be compatible with fracturing fluid chemistry [6,7]. Due to the low porosity and permeability of the shale gas reservoirs, methods for the conventional oil & gas wastewater treatment (i.e., sand filtration and multiple agent filtration) could not meet the standard for the fracking reuse due to potential risk of pore blocking in shale gas reservoirs. Membrane process is regarded as a promising and high efficiency technology for SGFFW recycling due to its high efficiency and excellent permeate quality [8]. Until now, some studies adopted MF for the direct treatment of SGFFW (mainly form Marcellus shale gas play). Jiang et al. [9] used ceramic MF membrane (0.2 µm and 0.8 µm) to filter the
Corresponding author. E-mail address: [email protected] (J.-f. Chen).
http://dx.doi.org/10.1016/j.memsci.2016.11.039 Received 2 September 2016; Received in revised form 11 November 2016; Accepted 11 November 2016 Available online 23 November 2016 0376-7388/ © 2016 Elsevier B.V. All rights reserved.
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2.2. Jar test
SGFFW and severe fouling was observed for both membranes. He et al. [10] evaluated the mechanisms that are responsible for severe fouling of polyvinylidene fluoride (PVDF) membrane for SGFFW and revealed that the submicron particles present in the early flowback water caused rapid flux decline. Recently, He et al. [11] also found that intermediate pore blocking was the dominant fouling mechanism of two ceramic MF membranes for Marcellus SGFFW treatment. These above studies indicated that MF fouling caused by SGFFW was complex and severe with no pretreatment applied. Therefore, pretreatment for the membrane process should be adopted to alleviate the membrane fouling for SGFFW treatment. Pre-treatment involving coagulation or coagulation-hydraulic flocculation has been shown to be an effective and low-cost approach for improving overall water quality and reducing membrane fouling, among which, chemical coagulation is an effective approach [12–16]. A hybrid process with the combination of the coagulation and low pressure membrane filtration has been widely used to alleviate membrane fouling and improve the water quality in terms of particle, colloid and DOC compared with either coagulation or membrane filtration process alone [12,13,17]. Therefore, a possibly feasible pathway for the treatment of SGFFW with the effective removal of colloid particle and mitigation of membrane fouling is the combination of coagulation and membrane filtration, which has not been reported yet for SGFFW treatment. Compared with MF, UF membrane with pore size of 0.01–0.1 µm has been widely adopted in both municipal and industrial water treatment to effectively remove suspended solids, colloidal material, inorganic particulates, and microorganisms [12,18]. Most recently, Xiong et al. [19] found that colloids and organics in the SGFFW in Marcellus shale gas play were only partially removed by MF and caused substantial fouling during a subsequent membrane process, indicating that MF alone couldn’t provide an effective treatment for SGFFW due to the high concentrations of small colloidal particles and organic matter. Thus, coagulation-UF can effectively separate out these small colloidal particles and the removal efficiency might outperform MF and coagulation-MF. A case as an example is that the integration of coagulation and UF has attracted considerable attention in recent years [12,13,17,20]. However, the efficacy of coagulation to alleviate UF fouling and the applicability of this hybrid process for SGFFW treatment still need to be investigated. The objective of this study was to evaluate the feasibility of coagulation–UF hybrid process in assisting SGFFW reuse in Fuling shale gas play. The performance of the hybrid process, the effect of coagulation on membrane fouling and the dissolved organics removal in this process were systematically elucidated. The results of this study allowed to examine the efficacy of coagulation to alleviate membrane fouling and the applicability of coagulation–UF process for treatment of SGFFW, provide a better understanding of UF fouling in this hybrid process and develop solutions that enable the use of membrane filtration in recycling of wastewater produced during shale gas exploration.
To obtain the optimal pretreatment condition, separate jar tests were conducted prior to the UF tests. PAC, a commonly used coagulant was selected in the coagulation experiment. The raw water (1 L) was stirred at 50 rpm for 60 s and then PAC were added (from 500 to 2000 mg/L), with a simultaneous increase of stirring speed to 300 rmp. The rapid mix speed of 300 rpm was maintained for 2 min and then reduced to 100 rpm for 15 min to allow floc growth to occur. Finally, precipitation lasted for 30 min as experiment needed. Then the supernatant or the mud mixture was introduced to the reservoir of the membrane filtration system. 2.3. Membrane filtration producers A bench-scale dead-end filtration system in constant pressure mode was followed by coagulation process, which is ideal for evaluating the fouling potential when only limited volumes of feed are available [19]. The schematic diagram of the experimental setup was shown in Fig. S1 of the supporting information. The system consisted of a 2 L feed reservoir, a 200 mL customer-made stirred filtration cell, a nitrogen gas cylinder for pressure generation, two pressure sensor for pressure measurement, a digital balance (Mettler Toledo, Germany) for the measurement of water flux, a computer for data logging and necessary tubing. The stirred dead-end filtration cell with the effective area of 20.5 cm2 was made of stainless steel. A magnetic stirring bar identical to that used in the Amicon 8400 (Millipore, US) unit was installed in the filtration cell and the feed reservoir leaving a gap of 3.0 mm to the membrane's upper surface. The shear intensity was varied by controlling the rotation speed of the magnetic bar. The PVDF UF membrane (FF4002, SEM image can be seen in Fig. S2 of the supporting information) was purchased from Minglie Chemical Co., Ltd.(Shanghai, China) with the pore size of 0.02 µm (nominal molecular weight cutoof 30 kDa) and water permeability of 5.53×10−3 m/(s Pa), corresponding to the membrane resistance of 1.8×1010 m−1. All membrane coupons were soaked in ultrapure water for at least 12 h at room temperature prior to use. The membrane was placed in the bottom of the stirred cell on top of a mesh to minimize deformation of the membrane into the support structure. In every filtration experiment, the fresh membrane in the filtration cell was first compacted by filtering ultrapure water at 150 kPa until a stabilized water flux was reached, and then continued to filter for about 30 min with the pressure adjusted to 100 kPa to obtain the pure water flux. Finally, the reservoir was filled with the water sample of interest (effluent of the coagulation process) with the stirring speed of approximately 500 rpm, and the system was re-pressurized under the constant pressure of 100 kPa until quasis-steady flux was reached. All the experiments were conducted in an air-conditioned room with the temperature set at 25 ± 1 °C and repeated at least once. Permeates were sampled when the quasis-steady state was reached. All samples were stored at 4 °C for one day at maximum. As for the backwashing, a peristaltic pump (Longer, USA) was connected to the outlet of the filtration cell with the inlet of the filtration cell open to the atmosphere. Ultrapure water was used for backwashing. A pressure transducer (Universal CB1020, Labom) was installed between the filtration cell and the pump in order to monitor the variation in applied pressure during backwashing. The pressure profile was logged as a function of operation time using Labview (National Instruments) software. The rotation speed of the peristaltic pump was maintained constant at 0.1 L/min during each experiment and, as such, the backwashing flux should be constant provided the transmembrane pressure was not too high [21].
2. Materials and methods 2.1. Raw water The characteristic of the SGFFW was reported to be complicated, which was mainly consisted of dissolved salts, chemical additives and solid particles. The raw water in this study was sampled from one of the reservoir in Fuling shale gas play, Chongqing, China. It was turbid with a light yellow color with some small floccules, which is similar to the previous report on Marchellus shale gas play [9]. The detailed water quality will be comprehensively discussed in the following session.
2.4. Fouling model and SEM observation The fouling mechanism identification approach in dead-end mode 461
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with constant pressure reported by Bowen [22] was adopted in this study to interpret the water flux decrease of UF, in which, there were four classical types of flux decline mechanisms describing characteristic change in flux over time under constant pressure including complete blocking, standard blocking, intermediate blocking and cake standard. The experimental data was fitted based on the equations (More details can be seen in session S3 of the supporting information) with R squats of difference between experimental data and fitted values to indicate the goodness of the model. The top surface morphology of the membranes and main elemental composition before and after the experiments were acquired by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) (Hitachi S-5500, Japan). Samples were freezedried for 24 h, followed by drying under vacuum overnight at 30 °C and gold coating.
Table 1 Comparison of characteristics of SGFFW with Marcellus and criteria.
2.5. Fluorescence EEM measurements A fluorescence spectrophotometer (F-7000, Hitachi, Japan) was used to characterize the type of dissolved organic matters (DOM). The raw water and samples after coagulation were filtered by 0.45 µm nylon filter (VWR Int., PA, USA) and diluted for 10 times before measurement. To obtain fluorescence excitation emission matrix (EEM), excitation wavelengths were incrementally increased from 200 to 400 nm at 5-nm steps; for each excitation wavelength, the emission at longer wavelengths was detected at 5-nm steps. For each excitation wavelength, the emission at longer wavelengths was detected.
Constituents
Fuling shale gas play
Nearby shale gas play [8]
Marcellus shale gas play [9]
Criteria for surface water quality (Ⅴ)
TOC(mg/L) CODcr(mg/L) Turbidity（NTU）
78 472.6 37.1
_ 358.5 _
_ 40 _
TDS (g/L) Conductivity （ms/ cm） Na+(mg/L) + K (mg/L) NH4+(mg/L) Ca2+(mg/L) Mg2+(mg/L) Fe3+(mg/L) Mn2+(mg/L) Al3+(mg/L) Ba2+(mg/L) Cl- (mg/L) Br-(mg/L) NO3- (mg/L) SO42- (mg/L) HCO3- (mg/L) SiO2 (mg/L) Boron（mg/L）
13.5 2.36
6.91 11.29
720 ,96–390a _ 770,150– 3000a 48,31–130a 67,54–140a
3980 110 25 164 16.0 6.78 0.479 3.10 30.7 6930 25. < 0.1 72.9 1010 72.3 10.6
2109 393 11.5 140.2 18.05 _ _ _ _ 4202.2 _ _ 2.2 149 19.2 16.9
12200 363 _ 2935 104 <1 <2 105 697 28500 19 _ 12.9 205 _ _
_ _ _ _ _ _ _ _ _ 250 _ _ 250 _ _ _
_ _
∑ ici (+) = 188.04 mmol/L; ∑ ici (−) = 213.61 mmol/L. a
Obtained from reference [19].
2.6. Molecular weight distribution measurement play. The water sample contained a high concentration of organic materials, at 78 mg/L TOC, which was approximately the same order of magnitudes with that reported in Marcellus shale gas play [10]. The turbidity of the sample was 37.1 NTU. The TDS of the SGFFW is 13500 mg/L, about 3 times lower than the TDS specified for Marcellus shale gas paly (48000 mg/L) and about 6 times higher than the criteria for surface water quality. The conductivity of the SGFFW also exhibited a high value of 2360 mS/cm, which is about 30 times lower than that reported of Marche shale gas play [9]. Sodium and chloride were the dominant ions in this SGFFW sample followed by HCO3-, Ca2+ and K+. Specifically, the content of Na+ and Cl- is predominantly high with the concentration Na+ of 3980 mg/L and Cl- of 6930 mg/L, 27 times higher than the criteria for surface water quality. Compared to Na+ and Cl-, HCO3-, Ca2+ and K+ showed much lower concentration, which were respective present at 1010, 164 and 110 mg/L. The concentrations of all other ions were insignificant. In general, the organics in the SGFFW is comparable with that reported in Marcellus shale gas play, while TDS and salt ions in the Fuling shale gas play is much lower than that in Marcellus shale gas play. The much lower TDS and salts concentrations was mainly attributed to difference in the rock formation of the geographic location and geologic basin from which the produced water originated [1].
The MW distribution of samples was determined by using a highpressure size exclusion chromatography (HPSEC, LC-20AT, Shimadzu, Japan) system equipped with an on-line organic carbon detector (OCD, Sievers 900 Turbo TOC, GE, USA), where a TSKgelG3000PWXL column (0.78 cm×30 cm, TOSOH, Japan) and a subsequent TSKgelG2500PWXL column (0.78 cm×30 cm, TOSOH, Japan) were used. More details can be found in previous literatures [23,24]. 2.7. Other analytical methods Dissolved organic carbon (DOC) was measured by filtering samples through a 0.45 µm nylon filter （VWR Int., PA, USA）followed by dilution and analyses using a TOC-500 A analyzer (SHIMADZU corporation, Japan), the detection limit of which was approximately 0.2 mg/L. Chemical oxygen demand (CODcr) was measured using HACH (Loveland, CO) COD kits and a DR5000 spectrophotometer, after dilution to avoid chloride interferences. The TDS of the sample was determined according to the standard methods for the examination of water and wastewater [25]. The turbidity was determined by the turbidity meter (1720e, HaCH, USA). The cationic analysis of the samples was obtained using an inductively coupled plasma spectrometer (Thermal PQ3 ICP System), the detection limits of which were approximately 0.1–0.5 mg/L for respective cationic ion. The anionic ion was measured using the ion chromatography (Metrohm, Switzerland), the detection limits of which were approximately 5– 80 μg/L, depending on the anionic ion measured.
3.2. Performance of the hybrid process under the different dosage of PAC 3.2.1. Water quality Removal of TOC and turbidity is critical for SGFFW treatment due to its detrimental effects on shale gas recovery or for other beneficial reuse, including as a pretreatment step prior to desalination. Preparation experiment indicated that the water flux was negligible when directly filtering the raw water without coagulation (data not shown). The variation of TOC and turbidity after coagulation and UF filtration was shown in Fig. 1. The results demonstrated that the rejection ratio for TOC typically showed a monotonous decrease with the increase dosage of PAC (Fig. 1). In other words, the more PAC was
3. Results and discussion 3.1. Characteristics of SGFFW The main constituents of SGFFW in Fuling shale gas play and the comparison with Marcellus shale gas play and the criteria for surface water quality (GB3838-2002, China) was listed in Table 1. As shown in Table 1, all the index was comparable to that reported in the shale gas play nearby [8], but notably differed from that in Marcellus shale gas 462
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Fig. 1. Variation of TOC and turbidity after coagulation and membrane filtration with the dosage of different concentration of PAC. Fig. 2. Effect of different dosage of PAC on membrane flux (a) Water flux (b) Normalized flux.
added, the more particles and colloids were removed. Under the PAC dosage of 500–2000 mg/L, the TOC decreased by 70–80%, while there was minor difference for TOC with the PAC dosage of 1500 and 2000 mg/L. In contrast, the turbidity of the permeate could reach their minimum under a medium PAC dosage of 1500 mg/L. Turbidity of the coagulated samples increased from 33 to 42.5 NTU initially with the dosage of 500 mg/L PAC due to the fine grains and an increase in aggregation size due to the increase of PAC dosage resulted in a further decrease in turbidity, while negligible change of TOC was observed after UF. It is evident that the turbidity was significantly decreased from 63.2% to 86.2% after UF owing to sieving effect. More water quality indexes (i.e., CODCr, pH and conductivity) of permeate can also be seen in the Table S2 of supporting information. Based on the above study, the dosage of 1500 mg/L PAC may be preferred in this hybrid process with relatively low TOC (16.02 mg/L) and turbidity (3.03 NTU) of the permeate. However, the dosage of coagulant is relatively high compared with that in the surface water treatment and sewage reclamation. Thus, the appropriate disposal way for sludge or new types of coagulant should be explored.
under the dosage of 500 mg/L PAC with the fouling resistance up to 2.55×1013 m−1, which is about three orders of magnitude higher than the intrinsic membrane resistance (1.8×1010 m−1). When the PAC dosage was 1000 mg/L, the stable water flux was 50% of the initial flux and the resistance of the fouling layer was calculated to be 6.36×1012 m−1. When the PAC dosage was increased to 1500 mg/L and 2000 mg/L, the stable water flux increased to approximately 4.0×10−4 m/s, which was one order of magnitude higher than that under the PAC dosage of 1000 mg/L. There was minor difference in stable water flux between PAC dosage of 1500 and 2000 mg/L, albeit the higher initial water flux under the PAC dosage of 2000 mg/L. The fouling resistance after the pretreatment of coagulation by the PAC dosage of 1500 mg/L and 2000 mg/L was calculated to be 2.25×1011 and 2.03×1011 m−1, respectively. Normalized water flux of the membrane followed the order of 1000 mg/L PAC > 1500 mg/L PAC > 2000 mg/L PAC > 500 mg/L PAC (Fig. 2b). Normalized water flux depended strongly on the initial permeate flux and foulant characterization, the decline of which was the synergistic effect of the initial water flux and water quality [27,28]. The poor water quality (i.e., high TOC and turbidity) after coagulation may dominate the flux decline under the dosage of 500 mg/L despite relatively low water flux. The initial water flux under the dosage of 1000 mg/L PAC was approximately 1/6 higher than that of under the dosage of 500 mg/L PAC but was only 1/3 of that under the dosage of 1000 and 2000 mg/L PAC, while TOC concentration was 33.2% lower than that under the dosage of 500 mg/L PAC but was only 15.2% and 10.4% higher than that under the dosage of 1500 and 2000 mg/L PAC (Fig. 1), respectively. Therefore, the slightly higher initial water flux and much lower TOC under the PAC dosage of 1000 mg/L than that of 500 mg/L contributed to much slower flux decline. Meanwhile, the much lower initial water
3.2.2. Water flux Fig. 2 showed the water flux and the normalized water flux by filtering the SGFFW after the dosage of 500, 1000, 1500 and 2000 mg/ L PAC. Two aspects should be taken into account for PAC dosage on UF fouling control: one is the removal of colloids; the other is the in-situ formed particulate alum flocs, which may exert adverse influence on membrane fouling [26]. With the increase of PAC dosage, the surfactant and polyacrylamide in the raw water might be greatly removed, and the viscosity of the coagulated raw water decreased which resulted in much higher initial water flux with the PAC dosage of 1500 and 2000 mg/L compared to that of 500 and 1000 mg/L. It can be seen the stable water flux was only 20% of the initial water flux 463
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among which the poor removal of colloids and fine particles contributed for the highest R2 value under the dosage of 500 mg/L. Intermediate blocking phenomenon caused by occlusion of pores by particles with particle superimposition due to the heterogeneity of the particle size, can be observed under the different dosage of PAC (R2 > 0.85) and fitted fairly well under the PAC dosage of 1000 and 2000 mg/ L. Cake block model fitted the results reasonably well under the dosage of 1000 (R2=0.95) and 1500 mg/L (R2=0.98) PAC. However, under the dosage of 2000 mg/L, the R2 for complete blocking and intermediatestandard blocking model is higher than 0.97, while cake filtration model was relatively low mainly due to the rapid initial flux decline, which is plausible that the number of larger size folcs decreased and there was still room for particle to directly obstruct the membrane pores. The different behaviors of flux decline reflected the different fouling mechanism of the foulant. Though the individual model might be insufficient to elucidate the membrane fouling mechanism as membrane fouling may be contributed simultaneously by several individual fouling models, In general, this modeling results indicated that the fouling mechanism changed from complete blocking to intermediate-standard blocking or cake blocking model with the increase of PAC dosage. SEM images of membrane were taken at the end of the filtration period to further investigate the fouling behaviors ( Fig. 4). It can be seen that fine flocs and colloids were closely packed and deposited on the membrane with the white spot scattering on the surface under the dosage of 500 mg/L PAC compared to the virgin membrane (Fig. S2 in the supporting information). Although the fouling layer were cracked when sample preparation by freezing-drying, it was obvious that the membrane fouling layer were more serious under the PAC dosage of
flux and comparable TOC for PAC dosage of 1000 mg/L compared with that of 1500 and 2000 mg/L is a likely reason for the observed slower flux decline rate. A little higher initial water flux and TOC under the dosage of 2000 mg/L PAC might result in the higher permeation drag and deposition rate for the foulant, and therefore a slightly higher flux decline rate than the PAC dosage of 1500 mg/L. 3.2.3. Membrane fouling analysis Serious fouling of the membrane can be observed under the dosage of 500 mg/L and 1000 mg/L PAC, while fouling was tremendously mitigated with PAC dosage of 1500 and 2000 mg/L (Fig. S3 in the supporting information) plausibly due to the increase removal of surfactant and polyacrylamide by coagulation. The fouling model under constant pressure developed by Bowen [22] was adopted to better understand UF fouling mechanism (More details in Table S1 of the supporting information), which was initially developed to describe the protein fouling of the membrane, under which condition, a single fouling mechanism dominated in a given system. However, not all the cases of interest can be fit to such one single equation and the individual model might be insufficient to elucidate the membrane fouling mechanism, as membrane fouling may be contributed simultaneously by several individual fouling models [29]. According to the modeling result ( Fig. 3), standard blocking where particles smaller than the membrane pore size deposited onto the pore walls thus reducing the pore size which played negligible role for the membrane fouling, indicating heterogeneity and irregularity of foulants for all the cases. The complete blocking model caused by occlusion of pores by particles with no particle superimposition fitted the fouling scenario fairly well under the dosage of 500, 1000, 1500 and 2000 mg/L PAC,
Fig. 3. Water flux fitted by UF fouling model after the pretreatment by coagulation with different PAC dosage (a) Complete blocking (b) Standard blocking (c) Intermediate blocking (d) Cake standard.
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Fig. 4. Surface topography of the membrane after the pretreatment by coagulation with different PAC dosage (a) 500 mg/L (b) 1000 mg/L (c) 1500 mg/L (d) 2000 mg/L.
layer on the surface of the UF membrane with the increase of PAC dosage.
1000 mg/L than that of 1500 and 2000 mg/L. It should be noticing that white spot with small size can also be seen on the surface of the fouling layer (Fig. 4). In order to identify elemental composition of the fouling layer, the fouled membrane was analyzed using EDS (Table S3 in the supporting information). It can be observed that C and O is the dominant elements in the fouling layer. The weight proportion of C was 46.9%, 43.13%， 38.11% and 35.67%, while the weight proportion of O was 39.64%, 39.96% ,41.31% and 37.58% under the dosage of 500, 1000, 1500 and 2000 mg/L PAC, respectively. Except for Na, Al and Si, the proportion of other inorganic elements was relatively low (lower than 1%), which indicated the organic matters mainly contributed to the membrane fouling and removing the particles or organic matters by coagulation can effectively mitigate membrane fouling. The elemental analysis by SEM-EDS demonstrated the foulant had very low levels of calcium but was rich in carbon and oxygen, similar with the foulant in Marcellus SGFFW [10]. However, the iron was relatively low in this foulant, which was much lower than that reported in the particles of Marcellus flowback water [10]. As mentioned above, there were white spots on the surface of fouling layer. As for the elemental composition of white spot on the membrane surface, the weight proportion of C decreased, while that of Fe, S and O increased with the increase dosage of PAC ( Fig. 5). Under the dosage of 500 mg/L, the weight proportion of Na and Cl of the white spot is relatively high compared to other areas, indicating that the white spot on the surface might be the crystallization of NaCl after drying, which rejected by the dense fouling layer under the dosage of 500 mg/L PAC. This was evidenced by the much lower TDS compared with other conditions (Table S2 in the supporting information). Under the dosage of 1000, 1500 and 2000 mg/L, the weight proportion of Fe, S and O in the fouling layer was relatively high compared to other areas (Fig. 5), which indicated that the white spot on the surface might be iron oxide and oxysulfide. He et al. [11] reported that low concentration of organics in Marcellus SGFFW had minimal impact on MF membrane fouling and that densely packed cake layer of fine organic-coated iron oxide particles was the key reason for severe fouling of polymeric MF membrane. It is plausible that the packed cake layer of the fine organic metal oxide particles contribute to the cake
3.3. Dissolved organics removal under the optimal dosage of PAC 3.3.1. Fluorescence EEM spectra As mentioned above, SGFFW contained a wide range of organic compounds including these present in the initial fracking fluid (i.e., guar gum, polyacrylamide, glutaraldehyde, isopropanol and various surfactants) and biocides as well as hydrocarbons released directly from the shale gas formation, which was qualitatively characterized by fluorescence spectra for this hybrid process under the dosage of 1500 mg/L PAC. The widely used classification method, fluorescence regional integration (FRI) could divide the map into five different zones including regions I and II, Ex/Em: 220–250/280–330 nm, 220–250/ 330–380 nm (aromatic protein, which might be guar gum for SGFFW), region III, Ex/Em: 220–250/380–480 nm (fulvic acid-like components), region IV, Ex/Em: 250–440/280–380 nm (soluble microbial products (SMPs) including tyrosine-, tryptophan-, and protein-like components), and region V, Ex/Em: 250–400/380–540 nm (humic acid-like components) [24,30]. Lester et. al. [31] found that the fluorescence signature observed in region IV stems from phenolic compounds (i.e., phenol and 2,4-dimethyphenol) not associated with proteins for SGFFW. It should be noticing that the spectral component with Ex/Em: 200–250/220–280 nm was observed in this water but not identified, which was marked as region VI in this study for comparison purpose. As shown in Fig. 6, the dissolved organics in raw water mainly belonged to region I (aromatic protein), II (aromatic protein) and IV (i.e., phenolic compounds). Pre-treatment by coagulation can effectively intercept aromatic protein (region II), fulvic acid-like components (region III), and phenolic compounds (region IV) (Fig. 6). According to FRI under the EEM within each region volume, the dissolved organics removed by coagulation in different region decreased in the order of V (83.3%), IV(78.6%), III(72.8%), II (64.9%), I(39.1%), VI(32.5%), while the overall rejection ratio of hybrid process decreased in the order of V (89.0%), IV 86.2%, III(80.3%), II(77.7%), 465
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Fig. 5. Elemental composition analysis of the spot on the surface by EDS after the pretreatment by coagulation with different PAC dosage (a) 500 mg/L (b) 1000 mg/L (c) 1500 mg/L (d) 2000 mg/L.
30 kDa, which is much higher than the molecular weight of most organics in SGFFW. However, UF removed a fraction of organics with molecular weight 20 kDa and low molecular weight components (i.e., 100–120 Da), indicating fouling layer was composed of guar gum, polyacrylamide and various surfactants. The increasing rejection of small molecular weight organics can be explained by an accumulation of organic compounds in the retentate and/or the foulant layer of cake/ gel formation, acting as a secondary membrane, which was found to be an effective filtration layer for organics [32]. As a result, a higher percent removal of small molecular weight organics was achieved. Moreover, He et al. [10] hypothesized that small molecular size compounds (i.e., scaling inhibitors and friction reducers) in the SGFFW might be coated on the stabilized colloidal or nano-sized particles and thus the compounds with small molecular weight might be removed with these coated colloids or particles.
I(55.2%), VI(49.3%) (Fig. 6). The retained organics was able to block membrane pores and form a cake/gel layer on the membrane surface. In general, the dissolved organics was further removed by 10–20% by UF filtration and the average percent organic removal of fulvic acid-like components, humic acid-like components and phenolic compounds was higher than 80% in this hybrid process.
3.3.2. Molecular weight distribution The removal of organic compounds including these present in the initial fracking fluid (i.e., guar gum, polyacrylamide, glutaraldehyde, isopropanol and various surfactants) and biocides as well as hydrocarbons was comprehensively indicated by the change of molecular weight distribution for this hybrid process under the PAC dosage of 1500 mg/L (optimal conditions). Fig. 7 indicated that the raw water mainly contained a large portion of high MW biopolymers (20 kDa) which might be guar gum or polyacrylamide and large fraction of low MW substances (150 Da) such as small neutrals and acids, while a small fraction of medium MW components (1500 Da) which might be surfactant can be observed (Fig. 7). The difference in MW distribution between the feed and the coagulated samples showed that the organics with MW20 kDa were greatly removed (Fig. 7), indicating the coagulation mainly intercepted this portion of the organics such as guar gum, polyacrylamide and various surfactants. The nominal molecular weight cutoff of the UF membrane is approximately
3.4. Is the sedimentation for membrane is needed? Though the coagulation-UF process can well reject TOC, turbidity and the dissolved organics, it is still desirable whether the sedimentation in coagulation process can be omitted. The potential to shorten the process will improve the process economics and expand applications. The mixture of water and flocs was filtered directly by the membrane, immediately after slow mixing. Backwashing was con466
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Fig. 5. (continued)
play. The performance and membrane fouling were systematically investigated:
ducted to examine the reversibly of the foulant. The comparison between the two modes was shown in Fig. 8. After pretreatment by coagulation, the flux decline almost followed the same pattern in the first 4 min, but then the water flux without sedimentation decreased sustainably into zero. The limited water flux and the severe fouling are among the biggest impediments to the viability of the coagulation-UF process. However, backwashing can effectively remove the foulant deposited on the membrane surface with the flux recovery of 77.9% of the initial water flux which is comparable with that the case of water after sedimentation (81.9%), indicating the foulant was reversible. In this regard, it is possible that optimizing hydrodynamic conditions in cross-flow system might result in a lower buildup of foulants near the membrane surface. Therefore, coagulation-UF process can potentially be used for SGFFW treatment without sedimentation which will not only promote treatment efficiency but will also save the space for the on-site and cost-effective reclamation of SGFFWs.
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4. Conclusion
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In this study, a hybrid process with the combination of coagulation and UF membrane filtration was applied to evaluate the efficacy of coagulation in alleviating UF fouling and the feasibility of this hybrid process in assisting SGFFW recycling and reuse in Fuling shale gas 467
The organics in the SGFFW is comparable with that reported in Marcellus shale gas play, while TDS and salt ions in the Fuling shale gas play is much lower than that in Marcellus shale gas play. The rejection for TOC showed a monotonous decrease with the increase dosage of PAC, while turbidity of permeate could reach their minimum under a medium PAC dosage of 1500 mg/L. The dosage of 1500 mg/L PAC may be preferred in this hybrid process with relatively low TOC and turbidity. Normalized water flux of the membrane followed the order of 1000 mg/L PAC > 1500 mg/L PAC > 2000 mg/L PAC > 500 mg/L PAC. The modeling results indicated that the fouling mechanism changed from the complete blocking to intermediate-standard blocking or cake blocking model with the increase of PAC dosage. The foulant had very low levels of calcium but was rich in carbon and oxygen, similar with the foulant in Marcellus SGFFW. However, the white spot on the surface might be iron oxide and oxysulfide. According to the volumetric integration, the dissolved organics removed by coagulation in different region was in the order of V (83.3%) > IV(78.6%) > III (72.8%) > II (64.9%) > I(39.1%) > VI(32.5%), while the overall rejection ratio of hybrid process
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Fig. 7. The variation of molecular weight distribution in the hybrid process.
Fig. 8. Effect of settlement on UF flux and recovery rate of membrane flux after backwashing.
thus coagulation-UF process without sedimentation can be potentially used in SGFFW treatment, which will not only promote treatment efficiency but will save the space for the on-site and cost-effective reclamation of SGFFWs. Acknowledgments The authors acknowledge the financial support provided by Science Foundation of China University of Petroleum, Beijing (No. 2462015YJRC030). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2016.11.039.
Fig. 6. Fluorescence EEM spectra (a) Raw water (b) Coagulated by 1500 mg/L PAC (c) Permeate of coagulation-UF hybrid process ((I) Ex/Em=220–250/280–330 nm; (II) Ex/ Em=220–250/330–380 nm; (III) Ex/Em=220–250/380–480 nm; (IV) Ex/Em=250– 440/280–380 nm; (V) Ex/Em=250–400/380–540 nm; (VI) Ex/Em=200–250/200– 280 nm.
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