Performance and mechanistic study on electrocoagulation process for municipal wastewater treatment based on horizontal bipolar electrodes

• EC modified with BPEs enhances pollutant removal and reduce energy consumption. • Increasing BPE number cannot increase flocculants yield exponentially. • Simulations help to predict the distribution of electrochemical reactions on BPEs. The design of electrodes is crucial to electrocoagulation process (EC), specifically, with respect to pollutant removal and energy consumption. During EC, the mechanisms for interaction between different electrode arrangement and electrode reactions remain unclear. This work presents an integrated EC process based on horizontal bipolar electrodes (BPEs). In the electrochemical cell, the graphite plates are used as driving cathode while either Fe or Al plates serves as driving anode and BPEs. The BPEs are placed horizontally between the driving electrodes. For municipal wastewater treatment, the pollutant removal efficiency and energy consumption in different configurations of two-dimension electrocoagulation (2D-EC) system with horizontal BPEs were investigated. The removal efficiency of turbidity, total phosphorus and total organic carbon increased significantly with the number of BPEs. Noted that the energy consumption for TP removal decreased by 75.2% with Fe driving anode and 81.5% with Al driving anode than those of 2D-EC, respectively. In addition, the physical field simulation suggested the distributions of potential and current in electrolyte and that of induced charge density on BPE surface. This work provides a visual theoretical guidance to predict the distribution of reactions on BPEs for enhanced pollutant removal and energy saving based on electrocoagulation process for municipal wastewater treatment.     

Electrocoagulation process for the treatment of metal-plating wastewater: Kinetic modeling and energy consumption

The wastewater from industrial area was treated by EC via Fe and Al electrodes. Cu, Ni, Cr and Zn were highly removed at the first minutes, simultaneously. Pseudo-2nd-order was found to be more suitable for kinetics. Adsorption capacities based on kinetic modeling were observed as Cr>Cu>Ni>Zn. The chemical cost in the case of pH adjustment after EC was less as 3.83 $/m³. It is known that wastewater produced by the metal-plating industry contains several heavy metals, which are acidic in nature and therefore toxic for the environment and for living creatures. In particular, heavy metals enter the food chain and accumulate in vital organs and cause serious illness. The precipitation of these metals is mostly achieved by pH adjustment, but as an alternative to this method, the electrocoagulation process has investigated in this study using iron and aluminum electrodes. The effects of the pH adjustment on removal before and after the electrocoagulation process were investigated, and cost analyses were also compared. It was observed that a high proportion of removal was obtained during the first minutes of the electrocoagulation process; thus, the current density did not have a great effect. In addition, the pH adjustment after the electrocoagulation process using iron electrodes, which are 10% more effective than aluminum electrodes, was found to be much more efficient than before the electrocoagulation process. In the process where kinetic modeling was applied, it was observed that the heavy metal removal mechanism was not solely due to the collapse of heavy metals at high pH values, and with this modeling, it was seen that this mechanism involved adsorption by iron and aluminum hydroxides formed during the electrocoagulation process. When comparing the ability of heavy metals to be adsorbed, the sequence was observed to be Cr>Cu>Ni>Zn, respectively.     

Pesticide wastewater treatment using the combination of the microbial electrolysis desalination and chemical-production cell and Fenton process

• MEDCC combined with Fenton process was developed to treat real pesticide wastewater. • Pesticide removal was attributable to desalination in the MEDCC. • High COD removal was attributable to organic distributions in different chambers. The combination of the microbial electrolysis desalination and chemical-production cell (MEDCC) and Fenton process for the pesticide wastewater treatment was investigate in this study. Real wastewater with several toxic pesticides, 1633 mg/L COD, and 200 in chromaticity was used for the investigation. Results showed that desalination in the desalination chamber of MEDCC reached 78%. Organics with low molecular weights in the desalination chamber could be removed from the desalination chamber, resulting in 28% and 23% of the total COD in the acid-production and cathode chambers, respectively. The desalination in the desalination chamber and organic transfer contributed to removal of pesticides (e.g., triadimefon), which could not be removed with other methods, and of the organics with low molecular weights. The COD in the effluent of the MEDCC combined the Fenton process was much lower than that in the perixo-coagulaiton process (<150 vs. 555 mg/L). The combined method consumed much less energy and acid for the pH adjustment than that the Fenton.     

Biological removal of selenate in saline wastewater by activated sludge under alternating anoxic/oxic conditions

Removal of selenate in saline wastewater by activated sludge was examined. Sequencing batch reactor was operated under alternating anoxic/oxic conditions. Above 97% removal of soluble selenium (Se) was achieved continuously. Major Se removal mechanism varied depending on the length of aeration period. Various Se-reducing bacteria likely contributed to coordinately to Se removal. Selenium (Se)-containing industrial wastewater is often coupled with notable salinity. However, limited studies have examined biological treatment of Se-containing wastewater under high salinity conditions. In this study, a sequencing batch reactor (SBR) inoculated with activated sludge was applied to treat selenate in synthetic saline wastewater (3% w/v NaCl) supplemented with lactate as the carbon source. Start-up of the SBR was performed with addition of 1–5 mM of selenate under oxygen-limiting conditions, which succeeded in removing more than 99% of the soluble Se. Then, the treatment of 1 mM Se with cycle duration of 3 days was carried out under alternating anoxic/oxic conditions by adding aeration period after oxygen-limiting period. Although the SBR maintained soluble Se removal of above 97%, considerable amount of solid Se remained in the effluent as suspended solids and total Se removal fluctuated between about 40 and 80%. Surprisingly, the mass balance calculation found a considerable decrease of Se accumulated in the SBR when the aeration period was prolonged to 7 h, indicating very efficient Se biovolatilization. Furthermore, microbial community analysis suggested that various Se-reducing bacteria coordinately contributed to the removal of Se in the SBR and main contributors varied depending on the operational conditions. This study will offer implications for practical biological treatment of selenium in saline wastewater.     

One-step ball milling-prepared nano Fe2O3 and nitrogen-doped graphene with high oxygen reduction activity and its application in microbial fuel cells

• Nano Fe₂O₃ and N-doped graphene was prepared via a one-step ball milling method. • The maximum power density of Fe-N-G in MFC was 390% of that of pristine graphite. • Active sites like nano Fe₂O₃, pyridinic N and Fe-N groups were formed in Fe-N-G. • The improvement of Fe-N-G was due to full exposure of active sites on graphene. Developing high activity, low-cost and long durability catalysts for oxygen reduction reaction is of great significance for the practical application of microbial fuel cells. The full exposure of active sites in catalysts can enhance catalytic activity dramatically. Here, novel Fe-N-doped graphene is successfully synthesized via a one-step in situ ball milling method. Pristine graphite, ball milling graphene, N-doped graphene and Fe-N-doped graphene are applied in air cathodes, and enhanced performance is observed in microbial fuel cells with graphene-based catalysts. Particularly, Fe-N-doped graphene achieves the highest oxygen reduction reaction activity, with a maximum power density of 1380±20 mW/m² in microbial fuel cells and a current density of 23.8 A/m² at –0.16 V in electrochemical tests, which are comparable to commercial Pt and 390% and 640% of those of pristine graphite. An investigation of the material characteristics reveals that the superior performance of Fe-N-doped graphene results from the full exposure of Fe₂O₃ nanoparticles, pyrrolic N, pyridinic N and excellent Fe-N-G active sites on the graphene matrix. This work not only suggests the strategy of maximally exposing active sites to optimize the potential of catalysts but also provides promising catalysts for the use of microbial fuel cells in sustainable energy generation.     

Simultaneous removal of hydrogen sulfide and volatile organic sulfur compounds in off-gas mixture from a wastewater treatment plant using a two-stage bio-trickling filter system

A two-stage BTF system was established treating odorous off-gas mixture from a WWTP. The two-stage BTF system showed resistance for the lifting load of H₂S and VOSC. Miseq Illumina sequencing showed separated functional microbial community in BTFs. Avoiding H₂S inhibition and enhancement of VOSC degradation was achieved. Key control point was discussed to help industrial application of the system. Simultaneous removal of hydrogen sulfide (H₂S) and volatile organic sulfur compounds (VOSCs) in off-gas mixture from a wastewater treatment plant (WWTP) is difficult due to the occasional inhibitory effects of H₂S on VOSC degradation. In this study, a two-stage bio-trickling filter (BTF) system was developed to treat off-gas mixture from a real WWTP facility. At an empty bed retention time of 40 s, removal efficiencies of H₂S, methanethiol, dimethyl sulfide, and dimethyl disulfide were 90.1, 88.4, 85.8, and 61.8%, respectively. Furthermore, the effect of lifting load shock on system performance was investigated and results indicated that removal of both H₂S and VOSCs was slightly affected. Illumina Miseq sequencing revealed that the microbial community of first-stage BTF contained high abundance of H₂S-affinity genera including Acidithiobacillus (51.43%), Metallibacterium (25.35%), and Thionomas (8.08%). Analysis of mechanism demonstrated that first stage of BTF removed 86.1% of H₂S, mitigating the suppression on VOSC degradation in second stage of BTF. Overall, the two-stage BTF system, an innovative bioprocess, can simultaneously remove H₂S and VOSC.     

Metallic wastewater treatment by sulfate reduction using anaerobic rotating biological contactor reactor under high metal loading conditions

An-RBC reactor is highly suited to treat metallic wastewater. Metal removal is due to sulfide precipitation via sulfate reduction by SRB. Cu(II) removal was the best among the different heavy metals. Maximum metal removal is achieved at low metal loading condition. Metal removal matched well with the solubility product values of respective metal sulfide salts. This study was aimed at investigating the performance of anaerobic rotating biological contactor reactor treating synthetic wastewater containing a mixture of heavy metals under sulfate reducing condition. Statistically valid factorial design of experiments was carried out to understand the dynamics of metal removal using this bioreactor system. Copper removal was maximum (>98%), followed by other heavy metals at their respective low inlet concentrations. Metal loading rates less than 3.7 mg/L?h in case of Cu(II); less than 1.69 mg/L?h for Ni(II), Pb(II), Zn(II), Fe(III) and Cd(II) are favorable to the performance of the An-RBC reactor. Removal efficiency of the heavy metals from mixture depended on the metal species and their inlet loading concentrations. Analysis of metal precipitates formed in the sulfidogenic bioreactor by field emission scanning electron microscopy along with energy dispersive X-ray spectroscopy (FESEM-EDX) confirmed metal sulfide precipitation by SRB. All these results clearly revealed that the attached growth biofilm bioreactor is well suited for heavy metal removal from complex mixture.     

Municipal wastewater treatment in China: Development history and future perspectives

The history of China’s municipal wastewater management is revisited. The remaining challenges in wastewater sector in China are identified. New concept municipal wastewater treatment plants are highlighted. An integrated plant of energy, water and fertilizer recovery is envisaged. China has the world’s largest and still growing wastewater sector and water market, thus its future development will have profound influence on the world. The high-speed development of China’s wastewater sector over the past 40 years has forged its global leading treatment capacity and innovation ability. However, many problems were left behind, including underdeveloped sewers and sludge disposal facilities, low sustainability of the treatment processes, questionable wastewater treatment plant (WWTP) effluent discharge standards, and lacking global thinking on harmonious development between wastewater management, human society and the nature. Addressing these challenges calls for fundamental changes in target design, policy and technologies. In this mini-review, we revisit the development history of China’s municipal wastewater management and identify the remaining challenges. Also, we highlight the future needs of sustainable development and exploring China’s own wastewater management path, and outlook the future from several aspects including targets of wastewater management, policies and technologies, especially the new concept WWTP. Furthermore, we envisage the establishment of new-generation WWTPs with the vision of turning WWTP from a site of pollutant removal into a plant of energy, water and fertilizer recovery and an integrated part urban ecology in China.     

A review on anammox process for the treatment of antibiotic-containing wastewater: Linking effects with corresponding mechanisms

• Anammox is promising for nitrogen removal from antibiotic-containing wastewater. • Most antibiotics could inhibit the anammox performance and activity. • Antibiotic pressure promoted the increase in antibiotic resistance genes (ARGs). • Antibiotic-resistance mechanisms of anammox bacteria are speculated. Antibiotic is widely present in the effluent from livestock husbandry and the pharmaceutical industry. Antibiotics in wastewater usually have high biological toxicity and even promote the occurrence and transmission of antibiotic resistant bacteria and antibiotic resistance genes. Moreover, most antibiotic-containing wastewater contains high concentration of ammonia nitrogen. Improper treatment will lead to high risk to the surrounding environment and even human health. The anaerobic ammonium oxidation (anammox) with great economic benefit and good treatment effect is a promising process to remove nitrogen from antibiotic-containing wastewater. However, antibiotic inhibition has been observed in anammox applications. Therefore, a comprehensive overview of the single and combined effects of various antibiotics on the anammox system is conducted in this review with a focus on nitrogen removal performance, sludge properties, microbial community, antibiotic resistance genes and anammox-involved functional genes. Additionally, the influencing mechanism of antibiotics on anammox consortia is summarized. Remaining problems and future research needs are also proposed based on the presented summary. This review provides a better understanding of the influences of antibiotics on anammox and offers a direction to remove nitrogen from antibiotic-containing wastewater by the anammox process.     

Sludge fermentation liquid addition attained advanced nitrogen removal in low C/N ratio municipal wastewater through short-cut nitrification-denitrification and partial anammox

• Sludge fermentation liquid addition resulted in a high NAR of 97.4%. • Extra NH₄⁺-N from SFL was removed by anammox in anoxic phase. • Nitrogen removal efficiency of 92.51% was achieved in municipal wastewater. • The novel system could efficiently treat low COD/N municipal wastewater. Biological nitrogen removal of wastewater with low COD/N ratio could be enhanced by the addition of wasted sludge fermentation liquid (SFL), but the performance is usually limited by the introducing ammonium. In this study, the process of using SFL was successfully improved by involving anammox process. Real municipal wastewater with a low C/N ratio of 2.8–3.4 was treated in a sequencing batch reactor (SBR). The SBR was operated under anaerobic-aerobic-anoxic (AOA) mode and excess SFL was added into the anoxic phase. Stable short-cut nitrification was achieved after 46d and then anammox sludge was inoculated. In the stable period, effluent total inorganic nitrogen (TIN) was less than 4.3 mg/L with removal efficiency of 92.3%. Further analysis suggests that anammox bacteria, mainly affiliated with Candidatus_Kuenenia, successfully reduced the external ammonia from the SFL and contributed approximately 28%–43% to TIN removal. Overall, this study suggests anammox could be combined with SFL addition, resulting in a stable enhanced nitrogen biological removal.     

Wastewater treatment meets artificial photosynthesis: Solar to green fuel production,water remediation and carbon emission reduction

• Mitigating energy utilization and carbon emission is urgent for wastewater treatment. • MPEC integrates both solar energy storage and wastewater organics removal. • Energy self-sustaining MPEC allows to mitigate the fossil carbon emission. • MPEC is able to convert CO₂ into storable carbon fuel using renewable energy. • MPEC would inspire photoelectrochemistry by employing a novel oxidation reaction. Current wastewater treatment (WWT) is energy-intensive and leads to vast CO₂ emissions. Chinese pledge of “double carbon” target encourages a paradigm shift from fossil fuels use to renewable energy harvesting during WWT. In this context, hybrid microbial photoelectrochemical (MPEC) system integrating microbial electrochemical WWT with artificial photosynthesis (APS) emerges as a promising approach to tackle water-energy-carbon challenges simultaneously. Herein, we emphasized the significance to implement energy recovery during WWT for achieving the carbon neutrality goal. Then, we elucidated the working principle of MPEC and its advantages compared with conventional APS, and discussed its potential in fulfilling energy self-sustaining WWT, carbon capture and solar fuel production. Finally, we provided a strategy to judge the carbon profit by analysis of energy and carbon fluxes in a MPEC using several common organics in wastewater. Overall, MPEC provides an alternative of WWT approach to assist carbon-neutral goal, and simultaneously achieves solar harvesting, conversion and storage.     

Degradation of metronidazole by dielectric barrier discharge in an aqueous solution

The discharge characteristics during the degradation of MNZ by DBD were investigated. Increasing the discharge frequency can promote the degradation of MNZ. MNZ removal reaches 99.1% at the initial concentration of 40 ppm within 120 min. Coexisting organic matter inhibits the degradation of MNZ. The energy efficiency of DBD for MNZ removal is higher than other technologies. Degradation of metronidazole (MNZ) which is a representative and stable antibiotic by dielectric barrier discharge (DBD) in an aqueous solution has been studied. Effects of initial MNZ concentration, solution pH and coexisting organics on the degradation were investigated. The results illustrated that increasing the input power and the discharge frequency can improve the removal of MNZ. At low initial concentration, the removal of MNZ can reach up to 99.1%. Acidic and neutral conditions are more favorable for MNZ removal than alkaline condition in the early stage of degradation. However, the difference in MNZ removal between those in acidic or neutral media and that in alkaline one could be neglected with prolonging of the treatment time. Therefore, this method can be applied to MNZ degradation with a wide pH range. Coexisting organic matter in water can attenuate the removal to some extent. This study could provide valuable references for the degradation of nitroimidazole antibiotics by DBD.     

Distribution and removal of antibiotic resistance genes during anaerobic sludge digestion with alkaline,thermal hydrolysis and ultrasonic pretreatments

Sludge digestion is critical to control the spread of ARGs from wastewater to soil. Fate of ARGs in three pretreatment-AD processes was investigated. UP was more efficient for ARGs removal than AP and THP in pretreatment-AD process. The total ARGs concentration showed significant correlation with 16S rRNA gene. The bacteria carrying ARGs could be mainly affiliated with Proteobacteria. Sewage sludge in the wastewater treatment plants contains considerable amount of antibiotic resistance genes (ARGs). A few studies have reported that anaerobic digestion (AD) could successfully remove some ARGs from sewage sludge, but information on the fate of ARGs in sludge pretreatment-AD process is still very limited. In this study, three sludge pretreatment methods, including alkaline, thermal hydrolysis and ultrasonic pretreatments, were compared to investigate the distribution and removal of ARGs in the sludge pretreatment-AD process. Results showed that the ARGs removal efficiency of AD itself was approximately 50.77%, and if these three sludge pretreatments were applied, the total ARGs removal efficiency of the whole pretreatment-AD process could be improved up to 52.50%–75.07%. The ultrasonic pretreatment was more efficient than alkaline and thermal hydrolysis pretreatments. Although thermal hydrolysis reduced ARGs obviously, the total ARGs rebounded considerably after inoculation and were only removed slightly in the subsequent AD process. Furthermore, it was found that the total ARGs concentration significantly correlated with the amount of 16S rRNA gene during the pretreatment and AD processes, and the bacteria carrying ARGs could be mainly affiliated with Proteobacteria.     

Combined Fenton process and sulfide precipitation for removal of heavy metals from industrial wastewater: Bench and pilot scale studies focusing on in-depth thallium removal

Addition of alkali to pH 10 is effective for precipitation of precipitable metals. Fenton treatment is effective for substantial removal of Tl, Cd, Cu, Pb, and Zn. Sulfide precipitation is a final step for removal of trace Tl, Cd, Cu, Pb, and Zn. Bench and pilot studies demonstrated the effectiveness of this combined technique. Thallium (Tl) in industrial wastewater is a public health concern due to its extremely high toxicity. However, there has been limited research regarding Tl removal techniques and engineering practices to date. In this investigation, bench and pilot studies on advanced treatment of industrial wastewater to remove Tl to a trace level were conducted. The treatment process involved a combination of hydroxide precipitation, Fenton oxidation, and sulfide precipitation. While hydroxide precipitation was ineffective for Tl+ removal, it enabled the recovery of approximately 70%–80% of Zn as Zn hydroxide in alkaline conditions. The Fenton process provided good Tl removal (>95%) through oxidation and precipitation. Tl was then removed to trace levels (<1.0 µg/L) via sulfide precipitation. Effective removal of other heavy metals was also achieved, with Cd<13.4 µg/L, Cu<39.6 µg/L, Pb<5.32 µg/L, and Zn<357 µg/L detected in the effluent. X-ray photoelectron spectroscopy indicated that Tl2S precipitate formed due to sulfide precipitation. Other heavy metals were removed via the formation of metal hydroxides during hydroxide precipitation and Fenton treatment, as well as via the formation of metal sulfides during sulfide precipitation. This combined process provides a scalable approach for the in-depth removal of Tl and other heavy metals from industrial wastewater.     

Treating wastewater under zero waste principle using wetland mesocosms

• Smart wetland was designed to treat wastewater according to zero waste principle. • The system included a dynamic roughing filter, Cyperus papyrus (L.) and zeolite. • It removed 98.8 and 99.8% of chemical and bacterial pollutants in 3 days. • The effluent reused to irrigate a landscape and the sludge recycled as fertilizer. • The plant biomass is a profitable resource for antibacterial and antioxidants. The present investigation demonstrates the synergistic action of using a sedimentation unit together with Cyperus papyrus (L.) wetland enriched with zeolite mineral in one-year round experiment for treating wastewater. The system was designed to support a horizontal surface flow pattern and showed satisfactory removal efficiencies for both physicochemical and bacteriological contaminants within 3 days of residence time. The removal efficiencies ranged between 76.3% and 98.8% for total suspended solids, turbidity, iron, biological oxygen demand, and ammonia. The bacterial indicators (total and fecal coliforms, as well as fecal streptococci) and the potential pathogens (Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa) showed removal efficiencies ranged between 96.9% and 99.8%. We expect the system to offer a smart management for every component according to zero waste principle. The treated effluent was reused to irrigate the landscape of pilot area, and the excess sludge was recycled as fertilizer and soil conditioner. The zeolite mineral did not require regeneration for almost 36 weeks of operation, and enhanced the density of shoots (14.11%) and the height of shoots (15.88%). The harvested plant biomass could be a profitable resource for potent antibacterial and antioxidant bioactive compounds. This could certainly offset part of the operation and maintenance costs and optimize the system implementation feasibility. Although the experiment was designed under local conditions, its results could provide insights to upgrade and optimize the performance of other analogous large-scale constructed wetlands.     

Removal of arsenic by pilot-scale vertical flow constructed wetland

• VFCWs are effective for the treatment of arsenic-containing wastewater. • Arsenic removal did not affect the removal of nutrients, except for TP in CW500. • Arsenic removal was highest when the temperature peaked and the reed was in bloom. • Substrate accumulation contributed more to arsenic removal than plant absorption. Four pilot-scale Vertical Flow Constructed Wetlands (VFCWs) filled with gravel and planted with Phragmites australis were operated for seven months in the field to study the efficiency of arsenic removal in contaminated wastewater. The average arsenic removal efficiency by the VFCWs was 52.0%±20.2%, 52.9%±21.3%, and 40.3%±19.4% at the theoretical concentrations of 50 μg/L (CW50), 100 μg/L (CW100), and 500 μg/L (CW500) arsenic in the wastewater, respectively. The results also showed no significant differences in the removal efficiency for conventional contaminants (nitrogen, phosphorus, or chemical oxygen demand) between wastewater treatments that did or did not contain arsenic (P>0.05), except for phosphorus in CW500. The highest average monthly removal rate of arsenic occurred in August (55.9%–74.5%) and the lowest in November (7.8%–15.5%). The arsenic removal efficiency of each VFCW was positively correlated with temperature (P<0.05). Arsenic accumulated in both substrates and plants, with greater accumulation associated with increased arsenic concentrations in the influent. The maximum accumulated arsenic concentrations in the substrates and plants at the end of the experiment were 4.47 mg/kg and 281.9 mg/kg, respectively, both present in CW500. The translocation factor (TF) of arsenic in the reeds was less than 1, with most of the arsenic accumulating in the roots. The arsenic mass balance indicated that substrate accumulation contributed most to arsenic removal (19.9%–30.4%), with lower levels in plants (3.8%–9.5%). In summary, VFCWs are effective for the treatment of arsenic-containing wastewater.     

Aggravation of membrane fouling and methane leakage by a three-phase separator in an external anaerobic ceramic membrane bioreactor

The existence of three-phase separator did not affect COD removal in the EAnCMBR. The existence of three-phase separator aggravated methane leakage of EAnCMBR. The existence of three-phase separator aggravated membrane fouling rate of EAnCMBR. Start-up of EAnCMBR equipped three-phase separator was slightly delayed. The three-phase separator is a critical component of high-rate anaerobic bioreactors due to its significant contribution in separation of biomass, wastewater, and biogas. However, its role in an anaerobic membrane bioreactor is still not clear. In this study, the distinction between an external anaerobic ceramic membrane bioreactor (EAnCMBR) unequipped (R1) and equipped (R2) with a three-phase separator was investigated in terms of treatment performance, membrane fouling, extracellular polymers of sludge, and microbial community structure. The results indicate that the COD removal efficiencies of R1 and R2 were 98.2%±0.4% and 98.1%±0.4%, respectively, but the start-up period of R2 was slightly delayed. Moreover, the membrane fouling rate of R2 (0.4 kPa/d) was higher than that of R1 (0.2 kPa/d). Interestingly, the methane leakage from R2 (0.1 L/d) was 20 times higher than that from R1 (0.005 L/d). The results demonstrate that the three-phase separator aggravated the membrane fouling rate and methane leakage in the EAnCMBR. Therefore, this study provides a novel perspective on the effects of a three-phase separator in an EAnCMBR.     

Effective removal of Cd2+ and Pb2+ pollutants from wastewater by dielectrophoresis-assisted adsorption

Dielectrophoresis (DEP) process could enhance the removal the Cd²⁺ and Pb²⁺ with less absorbent. The removal rates of both Cd²⁺ and Pb²⁺ increased with the increase of voltage. The overall removal rate of Cd²⁺ and Pb²⁺ in the binary system is higher than that of Cd²⁺ or Pb²⁺ in the single system. DEP could cause considerable changes of the bentonite particles in both surface morphology and microstructure. Dielectrophoresis (DEP) was combined with adsorption (ADS) to simultaneously and effectively remove Cd²⁺ and Pb²⁺ species from aqueous solution. To implement the process, bentonite particles of submicro-meter size were used to first adsorb the heavy metal ions. These particles were subsequently trapped and removed by DEP. The effects of the adsorbent dosage, DEP cell voltage and the capture pool numbers on the removal rate were investigated in batch processes, which allowed us to determine the optimal experimental conditions. The high removal efficiency, 97.3% and 99.9% for Cd²⁺ and Pb²⁺, respectively, were achieved when the ions are coexisting in the system. The microstructure of bentonite particles before and after ADS/DEP was examined by scanning electron microscopy. Our results suggest that the dielectrophoresis-assisted adsorption method has a high capability to remove the heavy metals from wastewater.     

Membrane-based treatment of shale oil and gas wastewater: The current state of knowledge

• Shale oil and gas production generates wastewater with complex composition. • Membrane technologies emerged for the treatment of shale oil and gas wastewater. • Membrane technologies should tolerate high TDS and consume low primary energy. • Pretreatment is a key component of integrated wastewater treatment systems. • Full-scale implementation of membrane technologies is highly desirable. Shale oil and gas exploitation not only consumes substantial amounts of freshwater but also generates large quantities of hazardous wastewater. Tremendous research efforts have been invested in developing membrane-based technologies for the treatment of shale oil and gas wastewater. Despite their success at the laboratory scale, membrane processes have not been implemented at full scale in the oil and gas fields. In this article, we analyze the growing demands of wastewater treatment in shale oil and gas production, and then critically review the current stage of membrane technologies applied to the treatment of shale oil and gas wastewater. We focus on the unique niche of those technologies due to their advantages and limitations, and use mechanical vapor compression as the benchmark for comparison. We also highlight the importance of pretreatment as a key component of integrated treatment trains, in order to improve the performance of downstream membrane processes and water product quality. We emphasize the lack of sufficient efforts to scale up existing membrane technologies, and suggest that a stronger collaboration between academia and industry is of paramount importance to translate membrane technologies developed in the laboratory to the practical applications by the shale oil and gas industry.     

Comparison of CNT-PVA membrane and commercial polymeric membranes in treatment of emulsified oily wastewater

CNT-PVA membrane was fabricated and compared with polymeric membranes. The separation performance was evaluated by homemade and cutting fluid emulsions. The three membranes show similar oil retention rates. CNT-PVA membranes have higher permeation fluxes compared with polymeric membranes. CNT-PVA membrane shows higher fouling resistance. Membrane separation is an attractive technique for removal of emulsified oily wastewater. However, polymeric membranes which dominate the current market usually suffer from severe membrane fouling. Therefore, membranes with high fouling resistance are imperative to treat emulsified oily wastewater. In this study, carbon nanotube-polyvinyl alcohol (CNT-PVA) membrane was fabricated. And its separation performance for emulsified oily wastewater was compared with two commercial polymeric membranes (PVDF membrane and PES membrane) by filtration of two homemade emulsions and one cutting fluid emulsion. The results show that these membranes have similar oil retention efficiencies for the three emulsions. Whereas, the permeation flux of CNT-PVA membrane is 1.60 to 3.09 times of PVDF membrane and 1.41 to 11.4 times of PES membrane, respectively. Moreover, after five consecutive operation circles of filtration process and back flush, CNT-PVA membrane can recover 62.3% to 72.9% of its initial pure water flux. However, the pure water flux recovery rates are only 24.1% to 35.3% for PVDF membrane and 6.0% to 26.3% for PES membrane, respectively. Therefore, CNT-PVA membrane are more resistant to oil fouling compared with the two polymeric membranes, showing superior potential in treatment of emulsified oily wastewater.