Recent developments on catalytic membrane for gas cleaning

Catalytic membrane, a novel membrane separation technology that combines catalysis and separation, exhibits significant potential in gas purification such as formaldehyde, toluene and nitrogen oxides(NO_x). The catalytic membrane can remove solid particles through membrane separation and degrade gaseous pollutants to clean gas via a catalytic reaction to achieve green emissions. In this review, we discussed the recent developments of catalytic membranes from two aspects: preparation of catalytic membrane and its application in gas cleaning.Catalytic membranes are divided into organic catalytic membranes and inorganic catalytic membranes depending on the substrate materials. The organic catalytic membranes which are used for low temperature operation(less than 300 °C) are prepared by modifying the polymers or doping catalytic components into the polymers through coating, grafting, or in situ growth of catalysts on polymeric membrane. Inorganic catalytic membranes are used at higher temperature(higher than 500 °C). The catalyst and inorganic membrane can be integrated through conventional deposition methods, such as chemical(physical) vapor deposition and wet chemical deposition. The application progress of catalytic membrane is focused on purifying indoor air and industrial exhaust to remove formaldehyde, toluene, NO_x and PM2.5, which are also summarized. Perspectives on the future developments of the catalytic membranes are provided in terms of material manufacturing and process optimization.

Effect of the Ultrasound Generated by Flat Plate Transducer Cleaning on Polluted Polyvinylidenefluoride Hollow Fiber Ultrafiltration Membrane

Polyvinylidenefluoride （PVDF） hollow fiber ultrafiltration membrane is frequently employed in water treatment. However, the fouling of ultrafiltration membranes affects the economic effectiveness of such process significantly. The ultrasound generated by flat plate transducer （UFPT） was used to clean the polluted PVDF ultrafiltration membrane with 2 g·L^-1 of citric acid aqueous solution in our study. The effects of UFPT intensity on the membrane surface were studied. The new membrane was easy to be polluted by the saturated CaCl2 solution. A synergistic effect of UFPT and 2 g·L^-1 citric acid aqueous solution could remove the foul of the membrane, and its flux could be recovered about 81%. The flux recovery of old membrane polluted was increased to 73.2% after 7 h soaking in citric acid aqueous solution, but its flux recovery without soaking was only increased to 56.2%.

Fouling and cleaning of membrane——a literature review

Membrane fouling curtails severely the economical and practical implementation of membrane process. The fundamental principles and mechanisms of membrane fouling as well as factors affecting fouling have been summarized in this paper. It also has covered three fouling resistance models and four kinds of approaches to improve membrane performance. Membrane cleaning methods are also discussed including physical, chemical, physico\|chemical and biological methods. In the four groups of basic cleaning methods, biological cleaning has considerable advantages and potentials. Extensive research work should be carried out further to explore and develop new ideas and techniques in the field of membrane cleaning and restoration.

Fouling behavior of poly(vinylidene fluoride)(PVDF)ultrafiltration membrane by polyvinyl alcohol(PVA)and chemical cleaning method

Severe fouling to poly(vinylidene fluoride)(PVDF)membrane is usually caused as filtrating the papermaking wastewater in the ultrafiltration(UF)process.In the paper,fouling behavior and mechanism were investigated,and the low-concentration polyvinyl alcohol(PVA)contained in the sedimentation tank wastewater was found as the main foulant.Consequently,the corresponding cleaning approach was proposed.The experiment and modeling results elaborated that the fouling mode developed from pore blockage to cake layer along with filtration time.Chemical cleaning conditions including the composition and concentration of reagents,cleaning duration and trans-membrane pressure were investigated for their effect on cleaning efficiency.Pure water flux was recovered by over 95% after cleaning the PVDF membrane using the optimal conditions 0.5 wt% NaClO(as oxidant)and 0.1 wt% sodiumdodecyl benzene sulfonate(SDBS,as surfactant)at 0.04MPa for 100 min.In the chemical cleaning method,hypochlorite(ClO−)could first chain-scissor PVA macromolecules to small molecules and SDBS could wrap the fragments in micelles,so that the foulants were removed from the pores and surface of membrane.After eight cycling tests,pure water flux recovery maintained above 95% and the reused membrane was found intact without defects.

Characterization of polysulfone hollow-fiber ultrafiltration membrane and its cleaning efficiency by streaming potential and flux method

Polysulfone(PS)hollow-fiber ultrafiltration membrane was characterized combined with flux and streaming potential in single electrolyte solutions.The effects of trans-membrane pressure,electrolyte concentration,ion valence and pH value of electrolyte solution on the streaming potential(SP)of the membrane were investigated.The zeta potential and surface charge density of the membrane were calculated on the basis of Helmholtz-Smoluchowski equation and Gouy-Chapmann theory.The results indicate that the valence and concentration of cation have a greater influence on the SP and surface charge density of PS membrane than those of anion,and the pH value of electrolyte solution has great effects on the SP and zeta potential of the membrane surface. Both the absolute value of the streaming potential and water flux of the adsorbed membrane decrease,compared with those of the clean membrane.The streaming potential and flux of the cleaned membrane can be completely recovered by cleaning with the mass fraction of 0.8%EDTA at pH=10.

Application of heat-activated peroxydisulfate process for the chemical cleaning of fouled ultrafiltration membranes

Na Cl O has been widely used to restore membrane flux in practical membrane cleaning processes,which would induce the formation of toxic halogenated byproducts.In this study,we proposed a novel heatactivated peroxydisulfate(heat/PDS)process to clean the membrane fouling derived from humic acid(HA).The results show that the combination of heat and PDS can achieve almost 100%recovery of permeate flux after soaking the HA-fouled membrane in 1 mmol/L PDS solution at 50℃ for 2 h,which is attributed to the changes of HA structure and enhanced detachment of foulants from membranes.The properties of different treated membranes are characterized by scanning electron microscopy(SEM),atomic force microscope(AFM),attenuated total reflection Fourier transform infrared spectroscopy(ATRFTIR),and X-ray photoelectron spectroscopy(XPS),demonstrating that the reversible and irreversible foulants could be effectively removed by heat/PDS cleaning.The filtration process and fouling mechanism of the cleaned membrane were close to that of the virgin membrane,illustrating the good reusability of the cleaned membrane.Additionally,heat/PDS which can avoid the generation of halogenated byproducts shows comparable performance to Na Cl O on membrane cleaning and high performance for the removal of fouling caused by sodium alginate(SA),HA-bovine serum albumin(BSA)-SA mixture and algae,further suggesting that heat/PDS would be a potential alternative for membrane cleaning in practical application.

Pilot-plant Study of Anoxic-aerobic Membrane Bioreactor （AO-MBR） on the Changes of Membrane Flux under Constant Pressure

Under a constant pressure, a pilot-plant test was conducted through the use of anoxic-aerobic membrane bioreactor （AO-MBR）, and this test operated steadily for 251 days. During the experiment, there were a total of four membrane cleaning processes, for the 90th day, the 150th day, the 210th day and the 240th day, respectively （The cleaning cycle was 90 days, 60 days, 60 days and 30 days, respectively）. From the initial 33.26 L/m^2.b dropped to 20.03 L/m^2.h after the fourth membrane cleaning, membrane flux reduced to 60.22% of the initial flux. During the 180 thd-210 thd of the experiment, the powdered activated carbon （PAC）, the segment size of which is 80-100, was put into anoxic reactor. Membrane flux decreased from 16.02 L/m^2·h to 15.29 L/m^2·h, and then rose to 15.65L/m^2·h. The dosing of PAC had a significant effect on the maintenance of a high membrane flux and extending running time. Before several membrane cleanings, a wire of membrane was intercepted from membrane module. It was found that the membrane surface sediments seemed to the inorganic colloid formed by Fe^2＋, Ca^2＋ and biofilm formed by some micro-organisms after the membrane surface pollutants were analyzed preliminarily with scanning electron microscopy （SEM）.

Characteristics of Membrane Fouling in an Anaerobic-(Anoxic/Oxic)~n-MBR Process

The characteristics of membrane fouling and cleaning, in a hybrid MBR process, was investigated. Under the condition of sub-critical flux operation, a characteristic three-stage trans-membrane pressure （TMP） profile is observed as time passes. The initially extended period of slow pressure rise, followed by a somewhat faster rise, is then sup- planted by a sudden transition to rapid pressure rise. Membrane cleaning experiments and SEM examination make it apparent that the rapid TMP rise is mainly caused by the accumulation of a surface cake layer, which is a reversible fouling that can be removed by tap water washing. Fouling caused by a gel layer, which is an irreversible fouling, can be removed efficiently by chemical cleaning. NaC10 can oxidize the gel layer, which is formed mainly of macromo-lecular organic substances. The HC1 can remove inorganic particles formed by Ca^2＋, Mg^2＋ ions etc. The sequence of chemicals used in membrane cleaning has an influence on the cleaning result. The effect of the NaC1O＋HC1 cleaning procedure is superior to that of the HCI＋NaC1O one. Particle size distribution measurements （PSD） reveal that fine particles are inclined to deposit or attach on the membrane surface, or in the membrane pores, and caused rapid fouling.

Chemical cleaning of fouled PVC membrane during ultrafiltration of algal-rich water

Cleaning of hollow-fibre polyvinyl chloride （PVC） membrane with different chemical reagents after ultrafiltration of algal-rich water was investigated. Among the tested cleaning reagents （NaOH, HCl, EDTA, and NaClO）, 100 mg/L NaClO exhibited the best performance （88.4% ± 1.1%） in removing the irreversible fouling resistance. This might be attributed to the fact that NaClO could eliminate almost all the major foulants such as carbohydrate-like and protein-like materials on the membrane surface, as confirmed by Fourier transform infrared spectroscopy analysis. However, negligible irreversible resistance （1.5% ± 1.0%） was obtained when the membrane was cleaning by 500 mg/L NaOH for 1.0 hr, although the NaOH solution could also desorb a portion of the major foulants from the fouled PVC membrane. Scanning electronic microscopy and atomic force microscopy analyses demonstrated that 500 mg/L NaOH could change the structure of the residual foulants on the membrane, making them more tightly attached to the membrane surface. This phenomenon might be responsible for the negligible membrane permeability restoration after NaOH cleaning. On the other hand, the microscopic analyses reflected that NaClO could effectively remove the foulants accumulated on the membrane surface.

Fenton cleaning strategy for ceramic membrane fouling in wastewater treatment

Membrane fouling is an obstacle impeding the wide applications of ceramic membranes and organics are responsible for most of the membrane fouling issues in wastewater treatment.In this study,Fenton cleaning strategy was firstly proposed to clean ceramic membrane fouling in wastewater treatment.Fe2+ efficiently catalyzed fouling cleaning with H2O2 (1.5%) to recover the filterability of ceramic membrane.The maximum ΔTMP recovery (over 99%) was achieved at an optimal Fe2+ dosage of 124 mg/L after 6 hr of immersion cleaning.The total residual membrane fouling resistance decreased gradually from this optimum value as the Fe2+ dosage increased above 124 mg/L.The residual hydraulically reversible fouling resistance accounted for most of the membrane fouling and was basically removed (≤3.0 × 109 m-1) when Fe2+ dosages higher than 124 mg/L were used.The foulants responsible for the formation of a residual hydraulically reversible fouling layer (DOC (dissolved organic carbon),proteins,polysaccharides,EEM (fluorescence excitationemission matrix spectra),SS (suspended solids),and VSS (volatile suspended solids)) were gradually removed as the Fe2+ dosage increased.These residual organic foulants were degraded from biopolymers (10-200 kDa) to low molecular weight substances (0.1-1 kDa),and the particle size of these residual foulants decreased significantly as a result.The strong oxidation power of hydrogen peroxide/hydroxy radicals towards organic foulants was enhanced by Fe2+.Fe2+ played a significant role in the removal of hydraulically reversible fouling and irreversible fouling from the ceramic membrane.However,Fe2+(≥124 mg/L) increased the likelihood of forming secondary iron-organics aggregates.

Towards a better hydraulic cleaning strategy for ultrafiltration membrane fouling by humic acid: Effect of backwash water composition

As a routine measurement to alleviate membrane fouling, hydraulic cleaning is of great significance for the steady operation of ultrafiltration（UF） systems in water treatment processes. In this work, a comparative study was performed to investigate the effects of the composition of backwash water on the hydraulic cleaning performance of UF membranes fouled by humic acid（HA）. Various types of backwash water, including UF permeate, Milli-Q water, Na Cl solution, CaCl_2 solution and HA solution, were compared in terms of hydraulically irreversible fouling index, total surface tension and residual HA. The results indicated that Milli-Q water backwash was superior to UF permeate backwash in cleaning HA-fouled membranes, and the backwash water containing Na＋or HA outperformed Milli-Q water in alleviating HA fouling. On the contrary, the presence of Ca^（2＋） in backwash water significantly decreased the backwash efficiency. Moreover, Ca^（2＋） played an important role in foulant removal, and the residual HA content closely related to the residual Ca^（2＋） content.Mechanism analysis suggested that the backwash process may involve fouling layer swelling, ion exchange, electric double layer release and competitive complexation. Ion exchange and competitive complexation played significant roles in the efficient hydraulic cleaning associated with Na＋and HA, respectively.

Pilot study on treatment of low turbidity and low temperature water by coagulation-immersed membrane process

The objective of this paper was to investigate the practicability of coagulation-immersed membrane process during low-temperature period through the study of steady operation,chemical cleaning methods,water quality and agent consumption.Experimental results showed that:membrane performance decreases with the reduction of temperature,but low temperature has little effect on stable operation of immersed membrane when coagulation as pretreatment.EFM with 1200 mg/L sodium hypochlorite after every 48 filtration cycles was made for reducing membrane fouling efficiently,and the method,with 1.5% sodium hydroxide and 3500 mg/L sodium hypochlorite for 10 h and then 2% hydrochloric acid for 4 h,is an appropriate cleaning method under low temperature.Compared with convention treatment process,immersed membrane process not only has same agent consumption,but also permeated water quality is more superior such as fine removal effect on turbidity with average 0.10 NTU.Therefore,coagulation-immersed membrane process is more appropriate for increasing water quality demand and the treatment of low turbidity and low temperature water.

Fabrication of cost effective iron ore slime ceramic membrane for the recovery of organic solvent used in coke production

Improvement of coking properties of sub-bituminous coal （A） and bituminous coal （B） was done using blended organic solvents, namely, n-methyl-2-pyrrolidinone （NMP） and ethylenediamine （EDA）. Various solvent blends were employed for the coal extraction under the total reflux condition. A low-cost ceramic membrane was fabricated using industrial waste iron ore slime of M/s TATA steel R＆D, Jamshedpur （India） to separate out the dissolved coking fraction from the solvent-coal mixture. Membrane separations were carried out in a batch cell, and around 75 % recovered NMP was reused. The fractionated coal properties were determined using proximate and ultimate analyses. In the case of bituminous coal, the ash and sulfur contents were decreased by 99.3 % and 79.2 %, respectively, whereas, the carbon content was increased by 23.9 % in the separated coal fraction. Three different cleaning agents, namely deionized water, sodium dodecyl sulphate and NMP were used to regain the original membrane permeability for the reusing.

Eco-friendly regeneration of end-of-life PVDF membrane with triethyl phosphate:Efficiency and mechanism

Membrane will inevitably reach the end of its lifespan due to the irrecoverable fouling accumulation in membrane bioreactors(MBRs)during long-term operation.Herein,we developed an eco-friendly membrane regeneration strategy with triethyl phosphate(TEP),which successfully prolonged the lifespan of end-of-life(EOL)polyvinylidene fluoride(PVDF)membranes in a large-scale MBR.The regenerated(Rg)membrane exhibited a water permeance of 534.8±45.7 L m^(-2)h^(-1)bar-1,along with stable rejection rate,which was comparable with that of the new membrane.Furthermore,compared to the membrane subjected solely to preliminary cleaning,the Rg membrane presented a more hydrophilic surface due to the combination of preliminary cleaning and solvent-based processing.Besides,the Rg membrane presented less fouling propensity with the critical flux of 15.2 L m^(-2)h^(-1),significantly higher than that of the EOL membrane(4.0 L m^(-2)h^(-1)).Importantly,the membrane regeneration strategy was capable of guaranteeing the effluent quality in MBR systems for treating real municipal wastewater.This study provides an eco-friendly membrane regeneration strategy for effectively removing the irrecoverable foulants,thereby promoting the advancement of sustainable membrane-based wastewater treatment technology.

Ceramic Ultra Filtration Membrane Bioreactor for Domestic Wastewater Treatment

A long term domestic wastewater treatment experiment was conducted using a recirculating ceramic ultra filtration membrane bioreactor (CUFMB) system. Three experiments were run with a hydraulic retention time of 5h, sludge retention times of 5d, 15d, and 30d and a membrane surface flow rate of 4m/s. The experiment studied the membrane fouling mechanism and cleaning techniques. The results show that a CUFMB system can provide continuous good quality effluent which is completely acceptable for reuse. The system is also not affected by fluctuations of the inlet flow. The CUFMB sludge loading rate is similar to that of conventional biological treatment units. However, the volumetric loading rate of the CUFMB is 24 times that of conventional biological treatment units. Membrane fouling occurs due to channel clogging, which could be easily removed, and surface fouling, which can be effectively removed using the method described in this work which includes water rinsing, base cleaning, and acid washing.

Using loose nanofiltration membrane for lake water treatment:A pilot study

Nanofiltration(NF)using loose membranes has a high application potential for advanced treatment of drinking water by selectively removing contaminants from the water,while membrane fouling remains one of the biggest problems of the process.This paper reported a seven-month pilot study of using a loose NF membrane to treat a sand filtration effluent which had a relatively high turbidity(∼0.4 NTU)and high concentrations of organic matter(up to 5 mg/L as TOC),hardness and sulfate.Results showed that the membrane demonstrated a high rejection of TOC(by<90%)and a moderately high rejection of two pesticides(54%–82%)while a moderate rejection of both calcium and magnesium(∼45%)and a low rejection of total dissolved solids(∼27%).The membrane elements suffered from severe membrane fouling,with the membrane permeance decreased by 70%after 85 days operation.The membrane fouling was dominated by organic fouling,while biological fouling was moderate.Inorganic fouling was mainly caused by deposition of aluminum-bearing substances.Though inorganic foulants were minor contents on membrane,their contribution to overall membrane fouling was substantial.Membrane fouling was not uniform on membrane.While contents of organic and inorganic foulants were the highest at the inlet and outlet region,respectively,the severity of membrane fouling increased from the inlet to the outlet region of membrane element with a difference higher than 30%.While alkaline cleaning was not effective in removing the membrane foulants,the use of ethylenediamine tetraacetate(EDTA)at alkaline conditions could effectively restore the membrane permeance.

Green production of sugar by membrane technology:How far is it from industrialization?

Application of membrane filtration in sugar production is attractive because it can reduce the usage of chemicals,produce high-quality clarified juice,and obtain various high value-added products.However,some technical problems,such as insufficient membrane performance,high sucrose loss in membrane retentate,severe membrane fouling,and incomplete cleaning protocols,limit its industrial applications.In order to facilitate the development of membrane technology for sugar production,this review summarizes recent progress on the applications of membrane filtration in different stages of sugar production as well as the integrated membrane processes for various purposes.Moreover,some important issues including membrane fouling,membrane cleaning,economic feasibility and engineering problems of the membrane-based sugar production process are discussed.Finally,the existing challenges and future research prospects for the industrialization of this green technique are pointed out.

废水处理中纳滤膜化学清洗技术研究现状、挑战与展望

纳滤技术由于其操作压力较低、对二价离子截留率高等优势,在市政污水、纺织废水、制药废水等废水处理领域有广阔的应用前景。然而膜污染问题的存在限制了纳滤技术的发展,膜污染不仅会导致膜通量衰减、分离性能下降,还会大大缩短膜的使用寿命。化学清洗可有效缓解膜污染,但常用的清洗剂均会对膜结构和分离性能产生影响,还可能引发生物耐药性及膜清洗废水二次污染等问题。本文对不同类型废水处理中纳滤膜的污染特征及化学清洗方法进行综述,并且重点阐释了酸、碱、次氯酸钠等化学清洗剂与聚酰胺纳滤膜的作用机制,以及频繁的化学清洗对膜面微生物的影响和所引发的生物风险,最后对有关问题的解决措施提出展望,以期为膜污染控制方法的优化提供参考。

浸没式超滤膜在大型给水厂的应用与思考

浙江省某地两个大型水厂(J水厂和T水厂)的生产规模分别为2.0×10^(5)m^(3)/d和5.0×10^(5)m^(3)/d,其运行时间分别为2年和7年,均采用浸没式超滤膜为核心净水工艺。文章结合两个水厂的实际运行情况,研究和总结了两个水厂的运行管理经验,得到如下结论:膜运行方面,两水厂膜进水浑浊度日常在0.15~4.25 NTU,台风季偶有短时13.6~35.2 NTU的情况下,膜出水浑浊度一直稳定在0.1 NTU以下,2μm以上的颗粒数偶有检出;膜污染控制方面,物理清洗时两水厂均在产水中间阶段进行一次气冲清洗,可有效缓解膜污染;化学清洗时发现J水厂仅采用柠檬酸清洗效果好,T水厂采用先次氯酸钠、后柠檬酸清洗效果好;水厂设计方面,J水厂泵吸式产水整体能耗高达27 kW·h/km^(3),T水厂虹吸式产水最低能耗仅为13 kW·h/km^(3),有效降低了能耗;运行管理方面,扩大清水池容积可有效解决因清水池调蓄能力不足导致的超滤膜长时间超负荷运行的问题;同时在膜池的进水管上增设膜前加氯点,保证出水余氯质量浓度稳定在0.2 mg/L左右可有效解决藻类滋生问题。