Fouling of microfiltration membranes by flowback and produced waters from the Marcellus shale gas play

Improving membrane distillation performance by Fe(II) activated sodium percarbonate oxidation during the treatment of shale gas produced water

Membrane distillation (MD) offers promise for recycling shale gas produced water (SGPW), while membrane fouling is still a major obstacle in standalone MD. Herein, sodium percarbonate (SPC) oxidation was proposed as MD pretreatment, and the performance of the single MD, SPC-MD hybrid process and Fe(II)/SPC-MD hybrid process for SGPW treatment were systematically evaluated. Results showed that compared to raw SGPW, the application of SPC and Fe(II)/SPC led to the decrease of the fluorescent organics by 28.54 % and 54.52 %, respectively. The hydrophobic fraction decreased from 52.75 % in raw SGPW to 37.70 % and 27.20 % for SPC and Fe(II)/SPC, respectively, and the MD normalized flux increased from 0.19 in treating raw SGPW to 0.65 and 0.81, respectively. The superiority of SPC oxidation in reducing the deposited membrane foulants and restoring membrane properties was further confirmed through scanning electron microscopy observation, attenuated total reflection fourier transform infrared, water contact angle and surface tension analyses of fouled membranes. Correlation analysis revealed that hydrophobic/hydrophilic matters and fluorescent organics in SGPW took a crucial role in MD fouling. The mechanism of MD fouling mitigation by Fe(II)/SPC oxidation was attributed to the decrease in concentrations and hydrophobicity of organic by synergistic oxidation, coagulation and adsorption.

Enhancing microfiltration membrane performance by sodium percarbonate-based oxidation for hydraulic fracturing wastewater treatment

Low pressure membrane takes a great role in hydraulic fracturing wastewater (HFW), while membrane fouling is a critical issue for the stable operation of microfiltration (MF). This study focused on fouling mitigation by sodium percarbonate (SPC) oxidation, activated by ultraviolet (UV) and ferrous ion (Fe(II)). The higher the concentration of oxidizer, the better the anti-fouling performance of MF membrane. Unlike severe MF fouling without oxidation (17.26 L/(m2·h)), UV/SPC and Fe(II)/SPC under optimized dosage improved the final flux to 740 and 1553 L/(m2·h), respectively, and the latter generated Fe(III) which acted as a coagulant. Fe(II)/SPC oxidation enabled a shift in fouling mechanism from complete blocking to cake filtration, while UV/SPC oxidation changed it to standard blockage. UV/SPC oxidation was stronger than Fe(II)/SPC oxidation in removing UV254 and fluorescent organics for higher oxidizing capacity, but the opposite was noted for DOC removal. The deposited foulants on membrane surface after oxidation decreased by at least 88% compared to untreated HFW. Correlation analysis showed that UV254, DOC and organic fraction were key parameters responsible for membrane fouling (correlation coefficient＞0.80), oxidizing capacity and turbidity after oxidation were also important parameters. These results provide new insights for fouling control during the HFW treatment.

The combination of precipitative softening and ozonation as a pretreatment of ultrafiltration in flowback water treatment: performance and fouling analysis

Ultrafiltration (UF) technology is an efficient shale gas flowback water treatment method. However, severe membrane fouling is the primary restriction on the application of UF technology. Here, we studied the impact of three pretreatments: precipitative softening (PS), precipitative softening, followed by ozonation (PS-O) and ozonation, followed by precipitative softening (O-PS), on pollutants' removal efficiencies and membrane fouling. The results showed that (1) the hardness, bacteria, scaling trend and compatibility with formation water exceeded the requirements for water reuse; (2) pretreatments effectively increased water flux and prolonged ultrafiltration membrane life, and both of them followed the order of PS-O process > O-PS process > PS process; (3) the fouling mechanism was changed from the complete blocking model to the standard blocking model by the PS process and the addition of ozonation enhanced the correlation of standard blocking model; (4) the quality of fracturing liquid prepared by the effluent treated by the PS-O-UF process was the best and satisfied the requirements of slick water. This paper indicated that the PS-O-UF process was suitable for the treatment of Changning shale gas flowback water for reuse.

Simultaneous electrocoagulation and E-peroxone coupled with ultrafiltration membrane for shale gas produced water treatment

Membrane fouling caused by the organics-coated particles was the main obstacle for the highly efficient shale gas produced water (SGPW) treatment and recycling. In this study, a novel hybrid electrocoagulation (EC) and E-peroxone process coupled with UF (ECP-UF) process was proposed to examine the efficacy and elucidate the mechanism for UF fouling mitigation in assisting SGPW reuse. Compared to the TMP (transmembrane pressure) increase of -15 kPa in the EC-UF process, TMP in ECP-UF system marginally increased to -1.4 kPa for 3 filtration cycles under the current density of 15 mA/cm2. Both the total fouling index and hydraulically irreversible fouling index of the ECP-UF process were significantly lower than those of EC-UF process. According to the extended Derjaguin-Landau-Verwey-Overbeek theory, the potential barriers was the highest for ECP-UF processes due to the substantial increase of the acid-base interaction energy in ECP-UF process, which was well consistent with the TMP and SEM results. Turbidity and TOC of ECP-UF process were 63.6% and 45.8% lower than those of EC-UF process, respectively. According to the MW distribution, the variations of compounds and their relative contents were probably due to the oxidation and decomposing products of the macromolecular organics. The number of aromatic compound decreased, while the number of open-chain compounds (i.e., alkenes, alkanes and alcohols) increased in the permeate of ECP-UF process. Notably, the substantial decrease in the relative abundance of di-phthalate compounds was attributed to the high reactivity of these compounds with ·OH. Mechanism study indicated that ECP could realize the simultaneous coagulation, H2O2 generation and activation by O3, facilitating the enhancement of ·OH and Alb production and therefore beneficial for the improved water quality and UF fouling mitigation. Therefore, the ECP-UF process emerges as a high-efficient and space-saving approach, yielding a synergistic effect in mitigating UF fouling for SGPW recycling.

Removal of organic pollutants in shale gas fracturing flowback and produced water: A review

Shale gas has been developed as an alternative to conventional energy worldwide, resulting in a large amount of shale gas fracturing flowback and produced water (FPW). Previous studies focus on total dissolved solids reduction using membrane desalination. However, there is a lack of efficient and stable techniques to remove organic pollutants, resulting in severe membrane fouling in downstream processes. This review focuses on the concentration and chemical composition of organic matter in shale gas FPW in China, as well as the hazards of organic pollutants. Organic removal techniques, including advanced oxidation processes, coagulation, sorption, microbial degradation, and membrane treatment are systematically reviewed. In particular, the influences of high salt on each technique are highlighted. Finally, different treatment techniques are evaluated in terms of energy consumption, cost, and organic removal efficiency. It is concluded that integrated coagulation-sorption-Fenton-membrane filtration represents a promising treatment process for FPW. This review provides valuable information for the feasible design, practical operation, and optimization of FPW treatment.

Oxidation treatment of shale gas produced water: Molecular changes in dissolved organic matter composition and toxicity evaluation

Produced water (PW) is the largest waste stream generated by hydraulic fracturing in an unconventional shale gas reservoir. Oxidation processes (OPs) are frequently used as advanced treatment method in highly complicated water matrix treatments. However, the degradation efficiency is the main focus of research, organic compounds and their toxicity have not been properly explored. Here, we obtained the characterization and transformation of dissolved organic matters of PW samples from the first shale gas field of China by two selected OPs using FT-ICR MS. CHO, CHON, CHOS, and CHONS heterocyclic compounds associated with lignins/CRAM-like, aliphatic/proteins, and carbohydrates compounds were the major organic compounds identified. Electrochemical Fe²⁺/HClO oxidation preferentially removed aromatic structures, unsaturated hydrocarbons, and tannin compounds with a double-bond equivalence (DBE) value below 7 to more saturated compounds. Nevertheless, Fe (VI) degradation manifested in CHOS compounds with low DBE values, especially single bond compounds. Oxygen- and Sulfur-containing substances, primarily O_4-11, S₁O₃-S₁O₁₂, N₁S₁O₄, and N₂S₁O₁₀ classes, were the main recalcitrant components in OPs. The toxicity assessment showed that the free-radical-formed Fe²⁺/HClO oxidation could cause significant DNA damage. Therefore, the toxicity response byproducts need spcial attention when conducting OPs. Our results led to discussions on designing appropriate treatment strategies and the development of PW discharge or reuse standards.

Polymer-flooding produced water treatment using an electro-hybrid ozonation-coagulation system with novel cathode membranes targeting alternating filtration and in situ self-cleaning

Polymer-flooding produced water is more difficult to treat for reinjection compared with normal produced water because of the presence of residual hydrolyzed polyacrylamide (HPAM). A novel cathode membrane integrated electro-hybrid ozonation-coagulation (CM-E-HOC) process was proposed for the treatment of polymer-flooding produced water. This process achieved in situ self-cleaning by generated microbubbles in the cathode membrane. The CM-E-HOC process achieved a higher suspended solid (SS), turbidity and PAM removal efficiency than the CM-EC process. The SS in the CM-E-HOC effluent was ≤ 20 mg/L SS, which met the reinjection requirements of Longdong, Changqing Oilfield, China (Q/SYCQ 08,011-2019) at different current densities (3, 5 and 10 mA/cm²). The CM-E-HOC process greatly mitigated both reversible and irreversible membrane fouling. Therefore, excellent flux recovery was obtained at different in situ self-cleaning intervals during the CM-E-HOC process. Furthermore, alternating filtration achieved continuous water production during the CM-E-HOC process. On one hand, the effective removal of aromatic protein-like substances and an increase in oxygen-containing functional groups were achieved due to the enhanced oxidation ability of the CM-E-HOC process, which decreased membrane fouling. On the other hand, the CM-E-HOC process showed improved coagulation performance because of the increased oxygen-containing functional groups and polymeric Fe species. Therefore, larger flocs with higher fractal dimensions were generated, and a looser and more porous cake layer was formed on the membrane surface during the CM-E-HOC process. Consequently, the CM-E-HOC process exhibited better in situ self-cleaning performance and lower filtration resistance than the CM-EC process.

Coupling iron-carbon micro-electrolysis with persulfate advanced oxidation for hydraulic fracturing return fluid treatment

Improving the sustainability of the hydraulic fracturing water cycle of unconventional oil and gas development needs an advanced water treatment that can efferently treat flowback and produced water (FPW). In this study, we developed a robust two-stage process that combines flocculation, and iron-carbon micro-electrolysis plus sodium persulfate (ICEPS) advanced oxidation to treat field-based FPW from the Sulige tight gas field, China. Influencing factors and optimal conditions of the flocculation-ICEPS process were investigated. The flocculation-ICEPS system at optimal conditions sufficiently removed the total organic contents (95.71%), suspended solids (92.4%), and chroma (97.5%), but the reaction stoichiometric efficiency (RSE) value was generally less than 5%. The particles and chroma were effectively removed by flocculation, and the organic contents was mainly removed by the ICEPS system. Fourier-transform infrared spectroscopy (FTIR) analysis was performed to track the changes in FPW chemical compositions through the oxidation of the ICEPS process. Multiple analyses demonstrated that PS was involved in the activation of Fe oxides and hydroxides accreted on the surface of the ICE system for FPW treatment, which led to increasing organics removal rate of the ICEPS system compared to the conventional ICE system. Our study suggests that the flocculation-ICEPS system is a promising FPW treatment process, which provides technical and mechanistic foundations for further field application.

Fouling and chemically enhanced backwashing performance of low-pressure membranes during the treatment of shale gas produced water

The treatment of shale gas produced water (SGPW) for beneficial reuse is currently the most dominant and economical option. Membrane filtration is one preferred method to deal with SGPW, but membrane fouling is an unavoidable problem. In this study, two types of ultrafiltration (UF) membranes and one type of microfiltration (MF) membrane were investigated to treat SGPW from Sichuan basin. Results showed that increased total dissolved solid (31-40 g/L) and UV₂₅₄ (10-42.9 m^-1) were observed for the same shale gas plays, and the primary fluorescent organic substances were humic acid-like components. Compared to UF membranes with the flux decline by 2% to 60%, MF membranes with larger pore size were more likely to be fouled with the flux decline by 43% to 95%. Cake layer filtration was verified to be the primary membrane fouling mechanism. Statistical analysis showed that UV₂₅₄ played the most significant role in membrane fouling which had the highest correlation (0.76 to 0.93). Compared to permeate backwashing (13%), deionized water backwashing and chemically enhanced backwashing (CEB) using NaClO, H₂O₂ and citric acid improved the cleaning efficiencies (31%-95%). CEB using NaOH prepared by deionized water aggravated membrane fouling, while excellent cleaning efficiencies (39%-79%) were observed for CEB using NaOH prepared by permeate. The difference in cleaning behaviors for fouled membranes by SGPW was verified by morphology observation and element composition analysis.

Mechanism of the Production Impact in Shale Gas Wells Caused by Water Invasion during Interwell Interference

Interwell interference is a universal problem in shale gas development and can cause severe reductions in the productivity of producing wells. Studies have attempted to identify the root cause of interference in producing wells, but the mechanisms of production reduction and recovery in impacted wells are still not clear. Thus, an effective preventive strategy is needed for producing wells when fracturing is performed in adjacent wells. According to the mechanism of spontaneous imbibition and water drainage in shale mico- and nanoscale pores, this paper introduces the water-gas distribution during fracturing and production and reveals that water drainage in micro- and nanoscale pores is mainly controlled by the amount of stored gas and follows the order of pore size. Based on this analysis, the mechanism by which interwell interference impacts the production of producing wells is explained for the first time. It is concluded that the secondary water invasion caused by interwell interference completely blocks the pores associated with long-term gas production but has little influence on the pores that have not yet drained or have produced only a small amount of gas, and smaller pores face a greater risk of water blockage. The proportion of drained pores formed during long-term gas production determines the effect of interwell interference on production; when more pores are drained by long-term gas production, greater damage occurs to the productivity of the producing well. The suggestion for preventing interwell interference is to reduce the time interval between fracturing operations at two adjacent wells, thereby diminishing the reduction in production.

Comparison of the Hydraulic Fracturing Water Cycle in China and North America: A Critical Review

There is considerable debate about the sustainability of the hydraulic fracturing (HF) water cycle in North America. Recently, this debate has expanded to China, where HF activities continue to grow. Here, we provide a critical review of the HF water cycle in China, including water withdrawal practices and flowback and produced water (FPW) management and their environmental impacts, with a comprehensive comparison to the U.S. and Canada (North America). Water stress in arid regions, as well as water management challenges, FPW contamination of aquatic and soil systems, and induced seismicity are all impacts of the HF water cycle in China, the U.S., and Canada. In light of experience gained in North America, standardized practices for analyzing and reporting FPW chemistry and microbiology in China are needed to inform its efficient and safe treatment, discharge and reuse, and identification of potential contaminants. Additionally, conducting ecotoxicological studies is an essential next step to fully reveal the impacts of accidental FPW releases into aquatic and soil ecosystems in China. From a policy perspective, the development of China's unconventional resources lags behind North America's in terms of overall regulation, especially with regard to water withdrawal, FPW management, and routine monitoring. Our study suggests that common environmental risks exist within the world's two largest HF regions, and practices used in North America may help prevent or mitigate adverse effects in China.

Protocol for Preparing Synthetic Solutions Mimicking Produced Water from Oil and Gas Operations

Produced water (PW) is the water associated with hydrocarbons during the extraction of oil and gas (O&G) from either conventional or unconventional resources. Existing efforts to enhance PW management systems include the development of novel membrane materials for oil-water separation. In attempting to evaluate these emerging physical separation technologies, researchers develop various formulations of test solutions aiming to represent actual PW. However, there is no clear scientific guideline published in the literature about how such a recipe should be prepared. This article develops a protocol for preparing synthetic solutions representing the characteristics and behavior of actual PW and enabling the performance comparisons of different oil-water separation membranes at the bench scale level. In this study, two different brine recipes were prepared based on salts present in actual PW, crude oil was used as the hydrocarbon source, and a surfactant was added to disperse the oil into the aqueous phase. The recipe is accessible to the wider scientific community and was proven to be reproduceable, homogenous, stable, and comparable to actual PW field samples through analytical monitoring measurements and bench scale evaluations.

Highly Efficient Bromide Removal from Shale Gas Produced Water by Unactivated Peroxymonosulfate for Controlling Disinfection Byproduct Formation in Impacted Water Supplies

Shale gas extraction processes generate a large amount of hypersaline wastewater, whose spills or discharges may significantly increase the bromide levels in downstream water supplies and result in the formation of brominated disinfection byproducts (DBPs) upon chlorination. Although a few studies have investigated selective bromide removal from produced water, the low removal efficiencies and complex system setups are not desirable. In this study, we examined a simple cost-effective approach for selective bromide removal from produced water relying on the oxidation by unactivated peroxymonosulfate. More than 95% of bromide was removed as Br₂(g) in less than 10 min under weakly acidic conditions without significant formation of Cl₂(g) even when the chloride concentration was more than 2 orders of magnitude higher. A kinetic model considering the involved reactions was then developed to describe the process well under various reaction conditions. The organic compounds in the produced water neither noticeably lowered the bromide removal efficiency nor reacted with the halogen species to form halogenated byproducts. The tests in batch and continuously stirred tank reactor systems suggested that it was feasible to achieve both high bromide removal and neutral effluent pH such that further pH adjustment was not necessary before discharge. After the treatment, the effect of the produced water on DBP formation was largely eliminated.

Reuse of shale gas flowback and produced water: Effects of coagulation and adsorption on ultrafiltration, reverse osmosis combined process

The shale gas flowback and produced water (FPW) from hydraulic fracturing in the Sichuan province of China has relatively low to moderate levels of total dissolved solids (<20 g/L) and organics (<50 mg/L of dissolved organic carbon). As such, a combined ultrafiltration (UF), reverse osmosis (RO) system can be successfully applied to desalinate this feed water with the goal of reuse. However, the concentration of influent organic matter and particulates in the UF and RO stage is high, and the overall ionic and organics composition is highly complex, so that the membrane processes do not perform well, also due to fouling. To ensure the long-term and efficient operation of the UF-RO stages, a combined pretreatment of the FPW with coagulation and adsorption was investigated. The effect of different parameters on the performance on the system was studied in detail. Overall, the coagulation-adsorption pre-treatment greatly reduced fouling of the membrane processes, thanks to the high removal rate of turbidity (98.8%) and dissolved organic carbon (86.3%). The adsorption of organic matter by powdered activated carbon was best described by the Freundlich equilibrium model, with a pseudo second-order model representing the adsorption kinetics. Also, the various ions had competitive removal rates during the adsorption step, a phenomenon reported for the first time for FPW treatment. Also, an optimal dose of activated carbon existed to maximize fouling reduction and effluent quality. The overall treatment system produced a high-quality water streams, suitable for reuse.

Smart ultrafiltration membrane fouling control as desalination pretreatment of shale gas fracturing wastewater: The effects of backwash water

BACKGROUND

Increasing attention is being paid to the treatment of shale gas fracturing wastewater, including flowback and produced water (FPW). Energy-efficient pretreatment technologies suitable for desalinating and reusing FPW are of paramount importance.

OBJECTIVES

This work focused on enhanced fouling alleviation of ultrafiltration (UF) as a pretreatment for desalinating shale gas FPW in Sichuan Basin, China. The UF fouling behaviors under various backwash water sources or coagulant dosages were evaluated, and membrane surface characteristics were correlated with UF fouling. The feasibility of Fourier transform infrared (FTIR) microscope mapping technique in quantifying UF fouling was also assessed.

METHODS

Various backwash water sources, including UF permeate, ultrapure water, nanofiltration (NF) permeate, reverse osmosis (RO) permeate, RO concentrate and forward osmosis (FO) draw solution, were used to clean UF membranes fouled by shale gas FPW. The UF fouling behaviors were characterized by total and non-backwashable fouling rates. Membrane surface characteristics were analyzed by scanning electron microscopy (SEM), total tension surface and FTIR spectra.

RESULTS

Protein-like substances in terms of fluorescence intensity in the backwash water decreased with the order of UF permeate, RO concentrate, NF permeate, RO permeate and FO draw solution. Compared with UF permeate backwashing, alleviated UF fouling was observed by using demineralized backwash water including ultrapure water and RO permeate, irrespective of hollow fiber and flat-sheet membranes. NF permeate and RO concentrate after NF used as backwash water resulted in low and comparable membrane fouling with that in integrated coagulation-UF process under optimal dosage. Among the backwash water tested, FO draw solution backwashing corresponded to the lowest UF fouling rates, which were even lower than that in the presence of coagulant under optimal dosage. The superiority of these backwash water sources to UF permeate was further confirmed by SEM images and FTIR spectra. The residual foulant mass on membrane surface and the total surface tension correlated well with non-backwashable and total fouling rates, respectively.

CONCLUSIONS

FTIR microscopy was a powerful surface mapping technique to characterize UF membrane fouling caused by shale gas FPW. Backwash water sources significantly influenced the fouling of UF membranes. In the integrated UF-NF-RO or UF-FO process, RO concentrate or FO draw solution were proposed as backwash water to enhance UF fouling control and decrease waste discharge simultaneously.

Pretreatment Techniques for Produced Water with Subsequent Forward Osmosis Remediation

Unconventional oil and gas extraction is on the rise across the United States and comprises an integral component in meeting the nation’s energy needs. The primary by-product of this industrious process is produced water, which is a challenging matrix to remediate because of its complex physical and chemical composition. Forward osmosis is a viable option to treat high-salinity produced water; however, fouling has been an issue. This study aimed to treat produced water before using forward osmosis as a remediation option. Trials consisted of a series of five experiments in order to evaluate the performance of the membrane. Samples were treated by centrifugation, activated carbon, filtration, ferric chloride, as well as coagulants and a polymer. It can be concluded that forward osmosis can be used to extract water from high-salinity oil field brines and produced water, and that pretreating the produced water decreased the tendency for fouling. The pretreatment with the overall best performance was activated carbon, which also yielded the lowest total organic carbon concentrations of 1.9 mg/L. During remediation trials using produced water pretreated with activated carbon as the feed solution, there was a 14% decrease in flux over the course of the 7 h trials. The membrane performance was restored after washing.

High total dissolved solids in shale gas wastewater inhibit biodegradation of alkyl and nonylphenol ethoxylate surfactants

Hydraulic fracturing fluids are injected into unconventional oil and gas systems to stimulate hydrocarbon production, returning to the surface in flowback and produced waters containing a complex mixture of xenobiotic additives and geogenic compounds. Nonionic polyethoxylates are commonly added surfactants that act as weatherizers, emulsifiers, wetting agents, and corrosion inhibitors in hydraulic fracturing fluid formulations. Understanding the biodegradability of these ubiquitous additives is critical for produced water pre-treatment prior to reuse and for improving treatment trains for external beneficial reuse. The objective of this study was to determine the effect of produced water total dissolved solids (TDS) from an unconventional natural gas well on the aerobic biodegradation of alkyl ethoxylate and nonylphenol ethoxylate surfactants. Changes in surfactant concentrations, speciation and metabolites, as well as microbial community composition and activity were quantified over a 75-day aerobic incubation period. Alkyl ethoxylates (AEOs) were degraded faster than nonylphenol ethoxylates (NPEOs), and both compound classes and bulk organic carbon biodegraded slower in TDS treatments (10 g L^-1, 40 g L^-1) as compared to controls. Short-chain ethoxylates were more rapidly biodegraded than longer-chain ethoxylates, and changes in the relative abundance of metabolites including acetone, alcohols, and carboxylate and aldehyde intermediates of alkyl units indicated metabolic pathways may shift in the presence of higher produced water TDS. Our key finding that polyethoxylated alcohol surfactant additives are less labile at high TDS has important implications for produced water management, as these fluids are increasingly recycled for beneficial reuse in hydraulic fracturing fluids and other purposes.

Enhanced organic removal for shale gas fracturing flowback water by electrocoagulation and simultaneous electro-peroxone process

Colloids and organics in shale gas fracturing flowback water (SGFFW) during shale gas extraction are of primary concerns. Coagulation combined with oxidation might be a promising process for SGFFW treatment. In this study, a novel electrocoagulation-peroxone (ECP) process was developed for SGFFW treatment by simultaneous coagulation and oxidation process with a Al plate as the anode and a carbon-PTFE gas diffusion electrode as the cathode, realizing the simultaneous processes of coagulation, H₂O₂ generation and activation by O₃ at the cathode. Compared with electrocoagulation (EC) and peroxi-electrocoagulation (PEC), COD removal efficiency mainly followed the declining order of ECP, PEC and EC under the optimal current density of 50 mA cm^-2. The appearance of medium MW fraction (1919 Da) during ozonation and PEC but disappearance in ECP indicated that these intermediate products couldn't be degraded by ozonation and PEC but could be further oxidized and mineralized by the hydroxyl radical produced by the cathode in ECP, demonstrating the hydroxyl radical might be responsible for the significant enhancement of COD removal. The pseudo-first order kinetic model can well fit ozonation and EC process but not the PEC and ECP process due to the synthetic effect of coagulation and oxidation. However, the proposed mechanism based model can generally fit ECP satisfactorily. The average current efficiency for PEC was 35.4% and 12% higher than that of ozonation and EC, respectively. This study demonstrated the feasibility of establishing a high efficiency and space-saving electrochemical system with integrated anodic coagulation and cathodic electro-peroxone for SGFFW treatment.

Characterization and implications of solids associated with hydraulic fracturing flowback and produced water from the Duvernay Formation, Alberta, Canada

Public concern is heightened around flowback and produced water (FPW) generated by the hydraulic fracturing process. FPW is a complex mix of organic and inorganic solutes derived from both the injected hydraulic fracturing fluid and interactions with the subsurface lithology. Few studies to date have systematically investigated the composition of FPW or its individual components. Here, we provide the first systematic characterization of the composition of the solids associated with FPW by analyzing samples from three wells drilled into the Duvernay Formation in Alberta, Canada. The FPW initially returned to the surface with high total dissolved solids (greater than 170 000 mg L-1) and enriched with Fe(ii), silica, sulfate, barium, and strontium. The solids form two distinct phases once the FPW reached the surface: (1) silica-enriched Fe(iii) oxyhydroxides, and (2) a barite-celestine solid solution. We hypothesize that the precipitation of the amorphous silica-enriched Fe(iii) oxyhydroxide is a two-step process, where first the silica precipitates as a function of the cooling of the FPW from elevated subsurface temperatures to ambient surface temperatures. Next, the silica acts as a template for the precipitation of Fe(iii) oxyhydroxide as the diffusion of oxygen into the subsurface causes oxidation of aqueous Fe(ii). The barite-celestine solid solution precipitates solely as a function of cooling. Elevated dissolved Fe concentrations in FPW and modeled saturation indices from five North American shale plays (Marcellus, Fayetteville, Barnett, Bakken, and Denver-Julesburg) indicate that solids similar to those found in Duvernay FPW, specifically Fe(iii) oxyhydroxides, barite and quartz, are likely to occur. With the solids known to carry a significant portion of FPW's toxicity and organic contaminant load, the development of new treatment technologies, such as the oxidation of the Fe(ii) in FPW, may increase FPW reuse and reduce the environmental risk posed by FPW.

The treatment of flowback water in a sequencing batch reactor with aerobic granular sludge: Performance and microbial community structure

The extensive application of hydraulic fracturing technology has significantly promoted the large-scale development of shale gas. However, it is a great challenge for shale gas extraction to effectively manage large-volume flowback water (FW) with high salinity and complex organic substances. Here, we report an aerobic granular sludge (AGS) tolerable to high salinity, and suited to the treatment of FW. The performance of a sequencing batch reactor (SBR) with the AGS for the treatment of the synthetic FW and the microbial community structure at different salinity levels were investigated. The AGS fed with synthetic FW possessed a larger average particle size and a higher settling rate (50 m h^-1). When NaCl concentration increased to 50.0 g L^-1, the removal efficiency of total organic carbon (TOC) increased to 79 ± 1%, and the removal rate of polyacrylamide (PAM) raised up to 42.7 ± 0.7 g m^-3 d^-1. Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Sphingobacteriia dominated in the microbial community of AGS. Cellvibrionaceae, Rhodocyclaceae, Enterobacteriaceae, Moraxellaceae, Pseudomonadaceae, and Halomonadaceae belonging to Betaproteobacteria and Gammaproteobacteria played important role in degrading PAM, polycyclic aromatic hydrocarbons (PAH), and some other organics in FW at high salinity. These results suggest that an AGS-based SBR is a promising technology for the treatment of FW.

Detection of contaminants in water supply: A review on state-of-the-art monitoring technologies and their applications

Water monitoring technologies are widely used for contaminants detection in wide variety of water ecology applications such as water treatment plant and water distribution system. A tremendous amount of research has been conducted over the past decades to develop robust and efficient techniques of contaminants detection with minimum operating cost and energy. Recent developments in spectroscopic techniques and biosensor approach have improved the detection sensitivities, quantitatively and qualitatively. The availability of in-situ measurements and multiple detection analyses has expanded the water monitoring applications in various advanced techniques including successful establishment in hand-held sensing devices which improves portability in real-time basis for the detection of contaminant, such as microorganisms, pesticides, heavy metal ions, inorganic and organic components. This paper intends to review the developments in water quality monitoring technologies for the detection of biological and chemical contaminants in accordance with instrumental limitations. Particularly, this review focuses on the most recently developed techniques for water contaminant detection applications. Several recommendations and prospective views on the developments in water quality assessments will also be included.

Chemical Degradation of Polyacrylamide during Hydraulic Fracturing

Polyacrylamide (PAM) based friction reducers are a primary ingredient of slickwater hydraulic fracturing fluids. Little is known regarding the fate of these polymers under downhole conditions, which could have important environmental impacts including decisions on strategies for reuse or treatment of flowback water. The objective of this study was to evaluate the chemical degradation of high molecular weight PAM, including the effects of shale, oxygen, temperature, pressure, and salinity. Data were obtained with a slickwater fracturing fluid exposed to both a shale sample collected from a Marcellus outcrop and to Marcellus core samples at high pressures/temperatures (HPT) simulating downhole conditions. Based on size exclusion chromatography analyses, the peak molecular weight of the PAM was reduced by 2 orders of magnitude, from roughly 10 MDa to 200 kDa under typical HPT fracturing conditions. The rate of degradation was independent of pressure and salinity but increased significantly at high temperatures and in the presence of oxygen dissolved in fracturing fluids. Results were consistent with a free radical chain scission mechanism, supported by measurements of sub-μM hydroxyl radical concentrations. The shale sample adsorbed some PAM (∼30%), but importantly it catalyzed the chemical degradation of PAM, likely due to dissolution of Fe²⁺ at low pH. These results provide the first evidence of radical-induced degradation of PAM under HPT hydraulic fracturing conditions without additional oxidative breaker.

Superhydrophilic Functionalization of Microfiltration Ceramic Membranes Enables Separation of Hydrocarbons from Frac and Produced Water

The environmental impact of shale oil and gas production by hydraulic fracturing (fracking) is of increasing concern. The biggest potential source of environmental contamination is flowback and produced water, which is highly contaminated with hydrocarbons, bacteria and particulates, meaning that traditional membranes are readily fouled. We show the chemical functionalisation of alumina ceramic microfiltration membranes (0.22 μm pore size) with cysteic acid creates a superhydrophilic surface, allowing for separation of hydrocarbons from frac and produced waters without fouling. The single pass rejection coefficients was >90% for all samples. The separation of hydrocarbons from water when the former have hydrodynamic diameters smaller than the pore size of the membrane is due to the zwitter ionically charged superhydrophilic pore surface. Membrane fouling is essentially eliminated, while a specific flux is obtained at a lower pressure (<2 bar) than that required achieving the same flux for the untreated membrane (4-8 bar).

An overview on exploration and environmental impact of unconventional gas sources and treatment options for produced water

Rising global energy demands associated to unbalanced allocation of water resources highlight the importance of water management solutions for the gas industry. Advanced drilling, completion and stimulation techniques for gas extraction, allow more economical access to unconventional gas reserves. This stimulated a shale gas revolution, besides tight gas and coalbed methane, also causing escalating water handling challenges in order to avoid a major impact on the environment. Hydraulic fracturing allied to horizontal drilling is gaining higher relevance in the exploration of unconventional gas reserves, but a large amount of wastewater (known as "produced water") is generated. Its variable chemical composition and flow rates, together with more severe regulations and public concern, have promoted the development of solutions for the treatment and reuse of such produced water. This work intends to provide an overview on the exploration and subsequent environmental implications of unconventional gas sources, as well as the technologies for treatment of produced water, describing the main results and drawbacks, together with some cost estimates. In particular, the growing volumes of produced water from shale gas plays are creating an interesting market opportunity for water technology and service providers. Membrane-based technologies (membrane distillation, forward osmosis, membrane bioreactors and pervaporation) and advanced oxidation processes (ozonation, Fenton, photocatalysis) are claimed to be adequate treatment solutions.