Membrane bioreactor technology for wastewater treatment and reuse

Treatment technology for water recycling encompasses a vast number of options. Membrane processes are regarded as key elements of advanced wastewater reclamation and reuse schemes and are included in a number of prominent schemes world-wide, e.g. for artificial groundwater recharge, indirect potable reuse as well as for industrial process water production. Membrane bioreactors (MBRs) are a promising process combination of activated sludge treatment and membrane filtration for biomass retention. This paper will provide an overview of the status of membrane bioreactor applications in municipal wastewater reclamation and reuse in Europe and will depict their potential role in promoting more sustainable water use patterns. Particular attention will be paid to the impact of MBR technology on emerging pollutants. A case study will be presented on a full-scale MBR plant for municipal wastewater which is operated by Aquafin in Belgium.     

Removal of nitrogen from wastewater for reusing to boiler feed-water by an anaerobic/aerobic/membrane bioreactor

The innovative process anaerobic/aerobic/membrane bioreactor (A/O/MBR) was developed to enhance pre-denitrification without the energy consumption of the recirculation pump for reusing wastewater to boiler feed-water. The performance of this bioreactor was investigated. Firstly, the septic tank wastewater with low ratio of COD/TN was disposed by a dynamic membrane bioreactor (DMBR). It was found that, although the high concentration of NO₂⁻–N in the effluent implied the potential ability of DMBR to realize shortcut nitrification and denitrification, the effluent of single DMBR was difficult to reach the criteria of reusing to boiler feed-water. Then, the process A/O/DMBR in disposing the septic tank wastewater was studied. The results indicated that this process not only accomplished the removal of 91.5% COD, 90.3% NH₄⁺–N and 60.2% TN, but also successfully realized pre-denitrification without additional recirculation pump. At last, based on the A/O/DMBR, a pilot plant A/O/MBR was built to dispose the municipal raw sewage. In the stable operation period, the average removal efficiencies for COD, NH₄⁺–N, TP and turbidity reached 90%, 95%, 70% and 99%, respectively. During the tested HRT run of 9.0 h, the effluent of COD, NH₄⁺–N, TP and turbidity was about 10 mg/L, 3 mg/L, below 1 mg/L and 1.2 NTU, respectively, which reached the criteria of the boiler feed-water in China.     

Biological COD reduction and inorganic suspended solids accumulation in a pilot-scale membrane bioreactor for traditional Chinese medicine wastewater treatment

A pilot-scale test was conducted in a membrane bioreactor (MBR) for 452 days to treat high-strength traditional Chinese medicine (TCM) wastewater from two-phase anaerobic digest effluent. This study focuses on the chemical oxygen demand (COD) reduction and inorganic suspended solid (ISS) accumulation. The wastewater was high in COD, varying daily between 259 and 12,776 mg L⁻¹. Almost all the COD was removed by the MBR system, leaving a COD of <50 mg L⁻¹ in the MBR effluent. This indicated a great potential of the MBR in TCM wastewater reuse. ISS produced in the bioreactor by metabolism of microorganism increased from 265 to 4912 g h⁻¹, which showed that there were large numbers of ISS accumulation in the bioreactor. Two models, built on the material balances of COD and ISS, were developed for the simulation of MBR system performance in the biodegradation of TCM wastewater. Consequently, the kinetic constants including the maximum substrate specific biodegradation rate (V_max), the half-saturation coefficient (K_s) and the inorganic suspended solids growth rate (k) were calculated as V_max, 3.64, 3.82, 4.39 d⁻¹, K_s, 56.4, 225, 394 mg L⁻¹ and k, 265, 888, 4912 mg L⁻¹ d⁻¹ using the operational data at different hydraulic retention times (HRTs). The models well fitted the pilot-scale experimental data, and were able to simulate the COD reduction and ISS accumulation.     

Hohlfaser-Membrankontaktoren zur Stickstoffrückgewinnung aus Prozesswasser einer kommunalen Kläranlage

Process water deriving from digested sludge dewatering at municipal wastewater treatment plants (WWTP) represents an additional load for the plant in terms of nitrogen. Münster WWTP is the first German plant to operate an innovative full-scale membrane contactor facility for nitrogen removal and recovery from process water. Within the scope of this study, aspects influencing the membrane performance and operation as well as utilization of the produced fertilizer are investigated. First results reveal nitrogen removal efficiencies around 85 % and a regional usability of the produced fertilizer.     

Survey of MBR market: Trends and perspectives in China

Membrane bioreactor (MBR) has gained considerable attention for wastewater treatment and reuse in China in the last two decades. Comparing with the global MBR market, which has an average annual growth rate of 10.9%, the average annual growth rate in China is nearly 100% in recent years. In the past 10 years, publications on MBR researches and applications for wastewater treatment have increased sharply. Over three hundred MBR plants have been successfully applied into practice for different wastewater treatments, such as municipal wastewater, bathing wastewater, restaurant wastewater, landfill leachate, hospital wastewater, petrochemical wastewater and high-concentration industrial wastewater. These plants have capacities ranging from 10 to 100,000 m³/d, among which over 12 MBR plants have capacities exceeding 10,000 m³/d. The largest MBR plant, i.e. Beijing Kunyu River WWPT, which has a capacity of 100,000 m³/d for municipal wastewater treatment and reuse, was constructed in Beijing by Origin Water Technology Co., Ltd. The largest MBR plant for industrial wastewater treatment was located in Tianjin and installed by Motimo Membrane Inc., which has a capacity of 30,000 m³/d. The largest MBR application for industrial sectors was petrochemical wastewater treatment, and over ten MBR plants each exceed a capacity of 5000 m³/d. In South-east China, the constructed MBRs are mostly involved in the high-strength industrial wastewater treatment while in North China MBRs mainly focused on municipal wastewater treatment and reuse.For an MBR commercial application in China, MBR plants were constructed by a lot of home-grown companies such as Tianjin Motimo Membrane Technology Co., Ltd., Beijing Origin Water Technology Co., Ltd. and Omexell Environmental Engineering Co., Ltd. and overseas-funded companies like Zenon-GE and CNC-Simens. Origin Water occupies the majority of the MBR market in China, whereas CNC-Simens and Zenon-GE have a larger number of installations in other parts of China. MBR unit key suppliers in China are Zenon (Canada), Mitsubishi-Rayon (Japan), Toray (Japan), Kubota (Japan), Norit (Netherlands), Motimo (China) etc.Due to more stringent regulations and wastewater reuse strategies, it is expected that a significant increase in MBR plant capacity and a widening of application areas will occur in the future.     

An integrated wastewater treatment and reuse concept for the Olympic Park 2008, Beijing

In a Sino-German research project, a joined developed sustainable water reclamation concept was developed for different applications of municipal water reuse at the Olympic Green 2008. The concept combines advanced technological processes like membrane bioreactors, specific phosphorus (P) adsorption columns and ultrafiltration (UF) with nature-based treatment processes like wetland and bank filtration mechanisms. The project’s approach is not only to comply with the reclamation requests of the Olympic Green 2008, but also to give an example for better, adapted and energy efficient reuse applications throughout China. The study shows that fixed-bed granular ferric hydroxide (GFH) adsorbers after a membrane bioreactor (MBR) can maintain a total phosphorus (TP) concentration of <0.03 mg L⁻¹. A low P concentration will be necessary to control eutrophication in the artificial Olympic Lake filled with treated wastewater. With an adsorption capacity of approx. 20 mg g⁻¹ d.m. at a corresponding equilibrium concentration of 1 mg L⁻¹ P, GFH reaches long operation times and can be repeatedly regenerated by caustic solutions with an efficiency of 50%. Apart from scenic impoundments, treated wastewater will be used for irrigation and toilet flushing. The latter requires a superior quality that will be delivered by low pressure UF treatment after lake (bank) filtration. A crucial reduction of fouling potential for dead-end UF is expected.     

Effect of COD/NO3-N ratio on the performance of a hybrid UASB reactor treating phenolic wastewater

The effect of COD/O₃⁻-N ratio on the biodegradation of complex phenolic mixture was studied in bench scale hybrid upflow anaerobic sludge blanket (HUASB) reactors. HUASB reactor is a combination of a UASB unit at the lower part and an anaerobic fixed film at the upper end. The aim of this study was to evaluate the biodegradability of phenolic mixture (from synthetic coal wastewater) as the only carbon and energy source in continuous experiments using nitrate as the final electron acceptor. Synthetic coal wastewater contained phenol (490 mg/L); m-,o-,p-cresols (123.0 mg/L, 58.6 mg/L, 42 mg/L); 2,4-, 2,5-, 3,4- and 3,5 dimethyl phenols (6.3 mg/L, 6.3 mg/L, 4.4 mg/L and 21.3 mg/L) as major phenolic compounds representing the complex phenolic mixture. Nitrate nitrogen loading was increased from 0.11 g/m³/d to 0.5 g/m³/d in order to keep COD/NO₃⁻-N ratio as 20.1, 14.85, 9.9, 6.36 and 4.45. An input phenolics concentration of 752 mg/L and hydraulic retention time (HRT) of 24 h was maintained through out the study. Removal of phenolic mixture was found to increase with the lowering of COD/NO₃⁻-N ratio. Maximum phenolics removal of 98% was achieved at a COD/NO₃⁻-N ratio of 6.36. However, phenolics removal got adversely affected when COD/NO₃⁻-N ratio was reduced below 6.36. A nitrogen production efficiency of 78% was obtained according to nitrate consumption. Simultaneous denitrification and methanogenesis was observed in all the reactors throughout the study, demonstrating that denitrification is a feasible alternative for the treatment of coal wastewater. Granules degrading complex phenolic mixture were of diameter 1.6–2.25 mm.     

Winery wastewater management and treatment in the Niagara Region of Ontario,Canada: A review and analysis of current regional practices and treatment performance

The winemaking process produces a large volume of wastewater with highly variable characteristics. Methods in place to manage winery wastewater (WWW) vary, with some wineries treating their WWW on-site, while others direct the WWW to municipal wastewater treatment plants (WWTPs). Despite the significant presence and growth of the wine industry in the Niagara Region of Ontario, Canada, few data are available regarding the characteristics and treatment of WWW in this geographic region. This work presents a comprehensive review of full-scale treatment methods currently in place to manage and treat WWW at 53 wineries in the Niagara Region. The most common type of on-site treatment is the constructed wetland process, providing overall removal rates of >98% for chemical oxygen demand (COD) and >98% for total suspended solids (TSS) for the combined treatment of WWW and site-generated sanitary sewage. Subsurface effluent discharge is the most widely used disposal strategy, while larger systems treating WWW alone also reuse the effluent for vineyard irrigation. The construction and operation of on-site treatment systems at wineries does not address all WWW treatment needs, requiring a portion of the WWW to be co-treated at WWTPs. While full-scale operating data indicate that anaerobic co-digestion with municipal sludges is effective (89% COD removal), the co-digestion capacity is limited. Co-treatment in the municipal WWTPs' liquid treatment train leads to negative operational and performance impacts. No efficient WWW co-treatment options, viable in the long-term, are currently available at municipal WWTPs. Improved co-treatment strategies are required to address WWW treatment needs in the Niagara Region.     

Monitoring and analysis of the energy cost of an MBR

Certain regions in the south of Europe are suffering an acute lack of water. One way to solve this hydric deficit is to use membrane bioreactors (MBR) to reuse the treated wastewater in tasks where drinking water is not required, e.g. irrigation. The aim of this research was to monitor physical-chemical (COD, BOD, and TN) and microbiological (EF, TC, and FC) parameters, as well as the energy costs in a Kubota MBR pilot plant located in Southeast Spain for one year, as well as to check the feasibility of the MBR process. The system showed a high robustness providing a fairly constant effluent with a large reduction of the entry pollutants and thus providing a highly reliable operation. High TSS, COD, BOD, NH⁺₄ and TN removal efficiencies up to 97, 94, 95, 98 and 81% respectively where achieved. The removal of the different microbials was quite high, with values in the permeate below 10 colony forming units (CFUs) most of the time. Some problems like blackouts, high temperatures and cleaning processes altered the normal operation lowering the retention of the different compounds and microorganisms. Regarding the energy consumption, two fluxes, 19 and 25 LMH were tested. Assuming a price for the electric supply of 0.0806 €/kW, the prices of the treated water for the two fluxes were 0.49 and 0.39 €/m3 and the total consumptions were 6.06 and 4.88 kWh/m³ for 19 and 25 LMH respectively. Aeration comprised almost 50% of the total energy requirements.     

Degradation of dye wastewater in a thin-film photoelectrocatalytic (PEC) reactor with slant-placed TiO2/Ti anode

A thin-film photoelectrocatalytic (PEC) reactor with slant-placed TiO₂/Ti anode was developed and successfully applied to degrade Rhodamine B (RhB) and textile effluent. Using a 5–150 mg L⁻¹ RhB solution as the model system, thin-film PEC removed total color and TOC by 99–28% and 78–15%, respectively, in 1 h, which is much higher than 82–7% and 60% to zero by conventional PEC. The enhanced treatment efficiency achieved by thin-film PEC process was attributed to the significantly reduced path length of irradiation light source. The wastewater was kept circulating during the experiments to timely refresh the aqueous film on the TiO₂/Ti anode and promote the mass transfer of the target pollutants and the degradation products in the bulk solution. The thin-film PEC reactor can degrade both simulated and real dye wastewater efficiently under UV light irradiation. Results suggested that thin-film PEC was particularly superior for treating a high concentration solution. The thin-film PEC reactor was also applied to treat RhB solution efficiently under solar light irradiation. The recycle experiments demonstrated excellent stability and reliability of the slant-placed TiO₂/Ti anode. This study proposed a simple and effective method to design PEC reactor applicable for industrial dye wastewater treatment.     

Effect of the sulfide concentration on zinc bio-precipitation in a single stage sulfidogenic bioreactor at pH 5.5

Dissolved zinc is present in natural waters and process streams generated by the mining and metallurgical industry. These streams usually have a low pH. By using sulfate reducing bacteria, sulfide can be produced that precipitates with zinc as zinc sulfide (sphalerite), which can be easily separated from the wastewater and even reused as zinc concentrate. In this study, a sulfate reducing gas-lift bioreactor was operated at pH 5.5 using hydrogen as electron donor for sulfate reduction. We demonstrate effective zinc removal (7.2 mmol L⁻¹ d⁻¹) with low zinc effluent concentrations (0.65–8.8 μM) in a system combining sulfide generation by sulfate reducing bacteria (7.2–10.6 mmol SO₄²⁻ L⁻¹ d⁻¹) at low pH (5.5) with the bio-precipitation of crystalline sphalerite. To investigate the effect of the sulfide excess on the settling properties of the sphalerite precipitates, the sulfide excess concentration was varied about two orders of magnitude (0.008–2.2 mM). The results show that crystalline sphalerite was formed in all cases, but larger particles with better settling properties were formed at lower sulfide concentrations.     

Enhanced mesoporosity and capacitance property of spherical carbon aerogel prepared by associating Mg(OH)2 with non-ionic surfactant

Non-ionic surfactant F127 (PEO₁₀₆–PPO₇₀–PEO₁₀₆) has been employed to enhance the mesoporosity of carbon aerogel (CA) for electric double layer capacitor application. Field emission scanning electron microscopy images exhibit that CA spheres prepared with F127 possess much coarser surface and smaller diameter. Furthermore, nitrogen sorption measurements show that the total pore volume of CA prepared with 0.6 wt.% F127 is 0.90 cm³ g⁻¹ and the ratio of its mesopore volume to the total pore volume (mesoporosity) could reach 86%. The specific capacitance of this CA electrode is approximately 130.8 F g⁻¹ in 4 M KOH, which is 45% higher than that of CA solely catalyzed by Mg(OH)₂. The high specific capacitance of the CA is believed to be associated with its enhanced mesoporosity as well as the high pore volume. It also performs well in the galvanostatic charge/discharge measurement for supercapacitor.     

Catalytic wet air oxidation of dye pollutants by polyoxomolybdate nanotubes under room condition

In order to develop a catalyst with high activity and stability for catalytic wet air oxidation of pollutant dyes at room condition, a new polyoxometalate Zn_1.5PMo₁₂O₄₀ with nanotube structure was prepared using biological template. The structure and morphology were characterized using infrared (IR) spectra, UV–vis diffuse reflectance spectra (DR-UV–vis), elemental analyses, X-ray powder diffraction (XRD), and transmission electron microscopy (TEM). And the degradation of Safranin-T (ST), a hazardous textile dye, under air at room temperature and atmospheric pressure was studied as a model experiment to evaluate the catalytic activity of this polyoxomolybdate catalyst. The results show that the catalyst has an excellent catalytic activity in treatment of wastewater containing 10 mg/L ST, and 98% of color and 95% of chemical oxygen demand (COD) can be removed within 40 min. And the organic pollutant of ST was totally mineralized to simple inorganic species such as HCO₃⁻, Cl⁻ and NO₃⁻ during this time (total organic carbon (TOC) decreased 92%). The structure and morphology of the catalyst under different cycling runs show that the catalyst are stable under such operating conditions and the leaching tests show negligible leaching effect owning to the lesser dissolution. So this polyoxomolybdate nanotube is proved to be a heterogeneous catalyst in catalytic wet air oxidation of organic dye.     

Dehydrochlorination behavior of a chloride ion-intercalated hydrotalcite-like compound during thermal decomposition

We prepared a chloride ion-intercalated hydrotalcite-like compound (Cl⁻-HT) in a co-precipitation reaction, and investigated its thermal properties using thermogravimetry-differential thermal analysis (TG–DTA) and simultaneous thermogravimetry-mass spectrometry (TG-MS). In addition, we examined the effects of temperature and atmosphere on the dehydrochlorination of Cl⁻-HT during thermal treatment. The thermal decomposition of the Cl⁻-HT was divided into three stages: the evaporation of surface adsorbed water and interlayer water in the Cl⁻-HT, the dehydroxylation of the brucite-like octahedral layers in the Cl⁻-HT, and the elimination of HCl from the Cl⁻-HT. H₂O was produced at four steps: the evaporation of surface adsorbed water and interlayer water in the first stage, two steps of H₂O production caused by the dehydroxylation of the parts of Cl⁻-HT with properties similar to Mg(OH)₂ and Al(OH)₃, respectively, in the second stage, and H₂O production due to the dehydroxylation of the part of Cl⁻-HT with properties similar to Al(OH)₃ in the third stage. HCl was produced at two steps: the dehydrochlorination of the parts of Cl⁻-HT with properties similar to AlOCl and Mg(OH)Cl, respectively, in the third stage. Hydrochloric acid was obtained from the Cl⁻-HT on thermal treatment. The degree of dehydrochlorination of the Cl⁻-HT increased with temperature. The degree was greater under water vapor than under nitrogen at all temperatures, indicating that water vapor promoted the dehydrochlorination of Cl⁻-HT. However, the concentration of hydrochloric acid obtained under water vapor was lower than that under nitrogen.     

Uptake of heavy metal ions from aqueous solution using Mg–Al layered double hydroxides intercalated with citrate, malate, and tartrate

Mg–Al layered double hydroxide (Mg–Al LDH) was modified with organic acid anions using a coprecipitation technique, and the uptake of heavy metal ions from aqueous solution by the Mg–Al LDH was studied. Citrate·Mg–Al LDH, malate·Mg–Al LDH, or tartrate·Mg–Al LDH, which had citrate³⁻ (C₆H₅O₇³⁻), malate²⁻ (C₄H₄O₅²⁻), or tartrate²⁻ (C₄H₄O₆²⁻) anions intercalated in the interlayer, was prepared by dropwise addition of a mixed aqueous solution of Mg(NO₃)₂ and Al(NO₃)₃ to a citrate, malate, or tartrate solution at a constant pH of 10.5. These Mg–Al LDHs were found to take up Cu²⁺ and Cd²⁺ rapidly from an aqueous solution at a constant pH of 5.0. This capacity was mainly attributable to the formation of the citrate–metal, malate–metal, and tartrate–metal complexes in the interlayers of the Mg–Al LDHs. The uptake of Cu²⁺ increased in the order malate·Mg–Al LDH < tartrate·Mg–Al LDH < citrate·Mg–Al LDH. The uptake of Cd²⁺ increased in the order malate·Mg–Al LDH < tartrate·Mg–Al LDH = citrate·Mg–Al LDH. These differences in Cu²⁺ and Cd²⁺ uptake were attributable to differences in the stabilities of the citrate–metal, malate–metal, and tartrate–metal complexes. These results indicate that citrate³⁻, malate²⁻, and tartrate²⁻ were adequately active as chelating agents in the interlayers of Mg–Al LDHs.     

钙钛矿型混合导体透氧膜SrFe0.6Cu0.3Ti0.1O3－δ的制备与表征

Dense membrane with the composition of SrFe0.6Cu0.3Ti0.1O3-δ （SFCTO） was prepared by solid state reaction method. Oxygen permeation flux through this membrane was investigated at operating temperature ranging from 750℃ to 950℃ and different oxygen partial pressure. XRD measurements indicated that the compound was able to form single-phased perovskite structure in which part of Fe was replaced by Cu and Ti. The oxygen desorption and the reducibility of SFCTO powder were characterized by thermogravimetric analysis and temperature programmed reduction analysis, respectively. It was found that SFCTO had good structure stability under low oxygen pressure at high temperature. The addition of Ti increased the reduction temperature of Cu and Fe. Performance tests showed that the oxygen permeation flux through a 1.5 mm thick SFCTO membrane was 0.35-0.96 ml·min ^-1·cm^-2 under air/helium oxygen partial pressure gradient with activation energy of 53.2 kJ·mol^-1. The methane conversion of 85%, CO selectivity of 90% and comparatively higher oxygen permeation flux of 5 ml·min^-1·cm^- 2 were achieved at 850℃, when a SFCTO membrane reactor loaded with Ni-Ce/Al2O3 catalyst was applied for the partial oxidation of methane to syngas.     

Factors influencing the hydration of layered double hydroxides (LDHs) and the appearance of an intermediate second staging phase

The hydration of layered double hydroxides (LDHs) was investigated by changing the interlayer anion species, the Mg/Al ratio of the LDH hosts and the relative humidity (RH). The anions were CO₃²⁻, Cl⁻, Br⁻, NO₃⁻, I⁻, SO₄²⁻, and ClO₄⁻ (listed in the order of ion size, small to large) and LDHs with Mg/Al = 1.90 (LDH2) and 2.91 (LDH3) were used. Their XRD profiles were measured by an XRD diffractometer while controlling the RH in the range 0–95% at 25 °C. Only I⁻, SO₄²⁻, and ClO₄⁻ LDH2s and SO₄²⁻ LDH3 showed a large step-wise basal-spacing expansion, 0.24–0.28 nm, under high RH conditions (> ca. 60%) probably due to the insertion of one water layer into the interlayer space. Such hydration occurred more favorably for the LDHs with larger anions and those with a higher layer charge (LDH2). Among them, I⁻ and ClO₄⁻ LDH2s exhibited the second staging – alternate stacking of hydrated (H) and non-hydrated (NH) interlayers – in the intermediate RH region.     

The effect of recycle ratio on nitrogen removal in the combined pre‐denitrification/nitrification biofilter system

Previous research has shown that nitrogen in municipal wastewater could be eliminated by a biofilter system. This study investigates the effect of the recycle ratio on total nitrogen removal and the nitrogen component in the effluent. In this study, a system combining pre‐denitrification/nitrification biofilters was set up. The experiments which varied the recycle ratio of the nitrifying solution to the influent were carried out in order to determine the amount of the total nitrogen removal and the form of nitrogen in the effluent. These experimental results show that total nitrogen removal might be dependent on nitrification but not on denitrification. Although the operation at a low recycle ratio would result in a lower value of total nitrogen removal, the NH₃‐N content of the effluent would be lower and the flow condition would be more stable. In comparison, operation at a higher recycle ratio would result in a lower NO₃^?‐N content in the effluent, while the NH₃‐N content in the effluent would be higher. At a recycle ratio of 2.5, the total nitrogen removal is at its maximum. The relationship between specific nitrification rate and recycle ratio is also discussed. © 2001 Society of Chemical Industry     

The Electro-Reduction of Carbon Dioxide in a Continuous Reactor

This paper reports an investigation into the electro-reduction of CO₂ in a laboratory bench-scale continuous reactor with co-current flow of reactant gas and catholyte liquid through a flow-by 3D cathode of 30^# mesh tinned-copper. Factorial and parametric experiments were carried out in this apparatus with the variables: current (1–8 A), gas phase CO₂ concentration (16–100 vol%) and operating time (10–180 min), using a cathode feed of [CO₂ + N₂] gas and 0.45 m KHCO₃(aq) with an anolyte feed of 1 m KOH(aq), in operation near ambient conditions (ca. 115 kPa(abs), 300 K). The primary and secondary reactions here were respectively the reduction of CO₂ to formate (HCOO⁻) and of water to hydrogen, while up to ca. 5% of the current went to production of CO, CH₄ and C₂H₄. The current efficiency for formate depended on the current density and CO₂ pressure, coupled with the hydrogen over-potential plus mass transfer capacity of the cathode, and decreased with operating time, as tin was lost from the cathode surface. For superficial current densities ranging from 0.22 to 1.78 kA m⁻², the measured values of the performance indicators are: current efficiency for HCOO⁻ = 86–13%, reactor voltage = 3–6 Volt, specific energy for HCOO⁻ = 300–1300 kWh kmol⁻¹, space-time yield of HCOO⁻ = 2 × 10⁻⁴–6 × 10⁻⁴ kmol m⁻³ s⁻¹, conversion of CO₂ = 20–80% and yield of organic products from CO₂ = 6–17%.     

Synthesis of proton conductive membranes based on inorganic–organic hybrid structure bound with phosphonic acid

Proton conductive inorganic–organic hybrid membranes were synthesized from styrene derivatives of alkoxysilane and ethyl 2-[3-(dihydroxyphosphonyl)-2-oxopropyl] acrylate (EPA) through copolymerization followed by sol–gel reaction. Self-standing, homogeneous and transparent hybrid membranes with chemically bound phosphonic acid groups were synthesized. FT-IR analysis exhibited the hybrid membranes included phosphonic acid groups. ¹³C and ²⁹Si NMR studies indicated that alkoxysilyl functionalized styrene derivatives were not only copolymerized with EPA but also condensed yielding Si–O–Si linkages by sol–gel reaction. TG–DTA analysis revealed that these membranes were thermally stable up to 200 °C in dry O₂. The proton conductivities of the hybrid membranes increased with phosphonic acid content and temperature. The P/Si ratio of the membrane was dependent on the number of alkoxy group in the starting alkoxysilane. The hybrid membrane from (dimethylmethoxysilylmethyl)styrene (DMMSMS(M))/EPA = 1/6 revealed proton conductivities of 6.3 × 10⁻³ and 2.4 × 10⁻⁴ S cm⁻¹ at 68.0% relative humidity and 18.8% relative humidity, respectively, at 140 °C.