Ozone Cleaning of Fouled Poly(Vinylidene) Fluoride/Carbon Nanotube Membranes

This research demonstrates for the first time that ozone is an effective cleaning agent for polyvinylidene fluoride (PVDF) membranes fouled by natural organic matter (NOM). Bare PVDF membranes as well as PVDF impregnated with CNTs (pristine (CNTs–P) and oxidized (CNTs–O)) at 0.3% mass membranes were used. Three different methods were investigated for cleaning the fouled membranes including; A: 10-min cleaning by pure water, B: 5-min water followed by 5-min ozonated water, and C: 10-min fully ozonated water. It was found that the application of fully ozonated water for 10 min was very effective to reinstate the flux to almost its original value of unfouled membrane. The CNTs–P/PVDF membrane exhibited the highest fouling with a total fouling ratio of 81%, while for the bare PVDF and the CNTs–O/PVDF membranes, the fouling ratios were 76% and 74%, respectively. The full ozonated water cleaning method gave the highest removal of fouling leaving the lowest irreversible fouling on the membrane as compared to the other cleaning methods. On the other hand, the highest removal of NOM fouling was obtained for CNTs–O/PVDF membranes indicating that fouling on CNTs–O/PVDF membrane was less bound than the other membranes. Contact angle measurements of the fouled membranes showed that all membranes exhibited increased contact angles due to the NOM deposition but after cleaning, particularly with ozonated water, the membrane contact angles returned to almost their original values. FTIR analysis of the membranes corroborated the results obtained.     

BPAC-UF组合工艺对微污染原水处理效果及膜污染控制

采用生物粉末活性炭-超滤(BPAC-UF)分体式工艺的中试装置,探究该工艺对西氿微污染原水的处理效果,在此基础上对比研究聚偏氟乙烯(PVDF)和聚氯乙烯(PVC)两种材质超滤膜的膜污染情况。PVDF膜和PVC膜两种不同材质的膜组合工艺对浊度去除率均超过95%,超滤膜可保证出水浊度要求。PVDF膜对尺寸不超过2μm的颗粒物去除效果要略优于PVC膜。初期PAC未转化为BPAC时,工艺对氨氮和CODMn的去除主要靠活性炭吸附。当PAC转化为BPAC,BPAC-UF对氨氮的去除主要是通过生物作用,后期去除率稳定在80%以上,BPAC-UF对CODMn去除率不高,稳定在25%左右。在膜污染控制方面,PVDF膜和PVC膜的跨膜压差增长趋势相似,但PVC膜的跨膜压差增速要略高于PVDF膜。经过强化物理反冲洗和化学反冲洗后,PVDF膜和PVC膜的产水性能均能恢复至较优状态,且PVDF膜恢复程度略优于PVC膜。     

Characteristics of soluble microbial products in membrane bioreactor and its effect on membrane fouling

In this study, the accumulation and characteristics of soluble microbial products (SMP) in the mixed liquor and the effluent of the membrane bioreactor (MBR) were measured and compared. It was found that the concentration of SMP decreased when the SRT was increased from 10 days to 30 days, and then stabilized as SRT was increased to 60 days. The molecular weight (MW) distributions of SMP indicated that the SMP of larger MW (> 30 kDa) was the most abundant fraction in the MBR. The similar MW distributions of SMP in the mixed liquor and effluent implied that membrane fouling due to SMP in the initial slow fouling stage was not due to size sieving. After the MBR was operated for a period of time, only the SMP of relatively large MW (> 30 kDa) was detected in the mixed liquor. The result indicated that size sieving of SMP occurred only after a cake layer was formed on the membrane surface although the effect was not significant and only worked on larger molecules. The accumulation of hydrophilic components of SMP in the mixed liquor of the bioreactor suggested that the hydrophilic fraction (in carbohydrates) could be the major cause for membrane fouling.     

Facile surface modification of PVDF microfiltration membrane by strong physical adsorption of amphiphilic copolymers

To endow the surface of poly(vinylidene fluoride) (PVDF) microfiltration (MF) membranes with hydrophilicity and antifouling property, physical adsorption of amphiphilic random copolymers of poly(ethylene glycol) methacrylate (PEGMA) and poly(methyl methacrylate) (PMMA) (P(PEGMA‐r‐MMA)) onto the PVDF membrane was performed. Scanning electron microscopy (SEM) images showed that the adsorption process had no influence on the membrane structure. Operation parameters including adsorption time, polymer concentration, and composition were explored in detail through X‐ray photoelectron spectroscopy (XPS), static water contact angle (CA), and water flux measurements. The results demonstrated that P(PEGMA‐r‐MMA) copolymers adsorbed successfully onto the membrane surface, and hydrophilicity of the PVDF MF membrane was greatly enhanced. The antifouling performance and adsorption stability were also characterized, respectively. It was notable that PVDF MF membranes modified by facile physical adsorption of P(PEGMA₅₈‐r‐MMA₃₃) even showed higher water flux and better antifouling property than the commercial hydrophilic PVDF MF membranes. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3112–3121, 2013     

Effects of pre-oxidant (KMnO4) on structure and performance of PVDF and PES membranes

The structure and performance of membrane materials are very important to the efficient and stable operation in membrane drinking water purification technology. Potassium permanganate (KMnO₄), which can change the characteristics of organic matters and control membrane surface fouling, has been widely used as pre-oxidant in the front of membrane drinking water process. This study investigates the evolution of membrane surface structure and performance when polyvinylidene fluoride (PVDF) and polyethersulfone (PES) were exposed to 10, 100 and 1000 mg·L^-1 KMnO₄ solution for 6 and 12 d, respectively. The aged membrane physicochemical characteristics such as membrane surface morphology, chemical composition, hydrophilicity, porosity and zeta potential were evaluated by modern analytical and testing instruments. The anti-fouling property of membrane surface was also investigated by the filtration-backwash experiment. The results indicated that the different concentrations and exposure time of KMnO₄ led to a different variation on PVDF and PES membrane surface structure and performance, which could further affect the membrane separation performance and the membrane fouling behaviors. The membrane surface pore size and porosity increased due to the dislodgment and degradation of membrane additive (PVP), which improved membrane permeability and enhanced the adsorption and deposition of pollutants in the membrane pores. With the increase of exposure time, the membrane surface pore size and porosity reduced for the reactions of chain scission and crosslinking on membrane materials, and the backwashing efficiency declined, leading to a more serious irreversible fouling. Compared with PVDF membranes, the formation of sulfonic group for PES membranes increased the negative charge on membrane surface due to the oxidation of KMnO₄. The present study provides some new insights for the regulation of the pre-oxidant dose and the selection of the membrane materials in KMnO₄ pre-oxidation combined with membrane filtration system.     

Antifouling enhancement of PVDF membrane tethered with polyampholyte hydrogel layers

The preparation and property of antifouling poly(vinylidene fluoride) (PVDF) membrane tethered with polyampholyte hydrogel layers were described in this work. In fabricating these membranes, the [2‐(methacryloyloxy)ethyl] trimethylammonium chloride and 2‐acrylamide‐2‐methyl propane sulfonic acid monomers were grafted onto the alkali‐treated PVDF membrane to yield polyampholyte hydrogel layers via radical copolymerization with N,N′‐methylenebisacrylamide as crosslinking agent. The analyses of fourier transform infrared attenuated total reflection spectroscopy and X‐ray photoelectron spectroscopy confirm the covalent immobilization of polyampholyte hydrogel layer on PVDF membrane surface. The grafting density of polyampholyte hydrogel layer increases with the crosslinking agent growing. Especially for the membrane with a high grafting density, a hydrogel layer can be observed obviously, which results in the complete coverage of membrane pores. Because of the hydrophilic characteristic of grafted layer, the modified membranes show much lower protein adsorption than pristine PVDF membrane. Cycle filtration tests indicate that both the reversible and irreversible membrane fouling is alleviated after the incorporation of polyampholyte hydrogel layer into the PVDF membrane. This work provides an effective pathway of covalently tethering hydrogel onto the hydrophobic membrane surface to achieve fouling resistance. POLYM. ENG. SCI., 55:1367–1373, 2015. © 2015 Society of Plastics Engineers     

纳米氧化石墨烯改性PVDF微滤膜在MBR中的抗污染性能

利用强亲水性氧化石墨烯（GO）作为添加剂,自制了PVDF/GO复合微滤膜,并将其应用于MBR系统中,进行长期应用研究。研究发现,共混膜的表面性质得到改善,其表面含氧基团达到29.33%（GO=3%）;共混膜与PVDF膜对污染物的去除效果相近,但共混膜（GO=3%）仅进行了一次清洗,清洗周期是PVDF的4倍;PVDF/GO共混膜的膜阻力要低于PVDF膜,并且共混膜的不可逆污染阻力显著下降,其数值仅仅是PVDF膜的9.0%（GO=3%）;EPS分析发现,共混膜表面的EPS浓度明显减少,其降幅分别达到59.98%（GO=1%）和69.57%（GO=3%）,抗污染性能显著提高;通过SEM与CLSM分析发现,随着氧化石墨烯加入量的提高,共混膜表面形成了滤饼层变得疏松,厚度也变薄,甚至露出了部分膜表面（GO=3%）,保证了共混膜稳定的渗透率。     

A comparative study of tertiary membrane filtration of industrial wastewater treated in a granular and a flocculent sludge SBR

Membrane fouling has been identified by many researchers as an inconvenience for the industrial application of membranes in wastewater treatment plants. Membrane fouling decreases permeability and therefore permeates flow, increasing costs. Although fouling is the result of complex phenomena not completely known, it can be said that fouling takes place by the presence of three different kinds of compounds in the water: suspended solids, colloids and solutes. In this sense, the characteristics of the suspended solid aggregates might be an important aspect in order to diminish the impact of suspended solids on membrane fouling. The main objective of this study was to compare the operation of two similar tertiary membrane filtration units treating the effluent of two different SBRs, respectively: A granular sludge SBR (GSBR) and a membrane flocculent sludge SBR system, at laboratory scale. Two PVDF microfiltration membrane modules were used for tertiary filtration of the effluent treated in the SBRs. Both reactors were used for treating the wastewater generated in a factory of the fish freezing sector. COD of the wastewater was between 700 and 1100 mg/L, total nitrogen concentration was between 110 and 180 mg N/L and total phosphorus ranged around 110 mg P/L. The chemical characteristics of both permeates were similar. Moreover, the presence of either granules or flocs in the tertiary membrane filtration systems did not have an appreciable impact on the membrane filtration. Nevertheless, it was observed that the operation of the membrane on the flocculent system tends to be more instable, showing a major tendency to achieve critical flux.     

Sub-critical flux fouling in membrane bioreactors — a review of recent literature

Membrane fouling is one of the main factors affecting the diffusion of membrane bioreactors to wastewater treatments. Fouling is generally held to persist above a so-called “critical flux”, below which a steady-state membrane permeability is assumed to be attainable. However, a number of recent studies have demonstrated that low-level fouling can take place at very low fluxes which are substantially below the critical value. This short survey is focused on reported evidences of membrane fouling under sub-critical conditions and examines possible links between operational paramenters, mixed liquor characteristics and fouling propensity. The reported experimental evidences suggest no unequivocal connection between parameters such as permeability decline (dK/dt) and fouling rate (dP/dt) and sustainable long term sub-critical operation. In this respect, test size and plant configuration also play a relevant role. Step-flux experiments may provide information on fouling propensity under sub-critical flux, but the fouling rates determined with this method are generally not applicable to long term operation. Mixed liquor quality determinants such as EPS and SMP and their relative protein/carbohydrate ratios are likely to contribute to sub-critical fouling in MBRs. Possible relationships between these parameters and plant operation (sludge age, biomass concentration, hydraulic retention time, etc.) are briefly discussed.     

Structural and performance properties of UV-assisted TiO2 deposited nano-composite PVDF/SPES membranes

In this research, the surface of poly (vinylidene fluoride) (PVDF)/sulfonated polyethersulfone (SPES) blend membrane prepared via immersion precipitation was modified by depositing of TiO₂ nano-particles followed by UV irradiation to activate their photocatalytic property. The membranes were characterized by FTIR, SEM, AFM, contact angle, dead end filtration (pure water flux and BSA solution flux), antifouling analysis and antibacterial activity. The FTIR spectrum confirmed the presence of OH functional groups on the PVDF/SPES membrane structure, which was the key factor for deposition, and self-assembly of TiO₂ nanoparticles on the membrane surface. The SEM and AFM images indicated that the TiO₂ nanoparticles were deposited on the PVDF/SPES membrane. The contact angle measurements showed that the hydrophilicity of PVDF/SPES membrane was strongly improved by TiO₂ deposition and UV irradiation. The filtration results indicated that the initial flux of TiO₂ deposited PVDF/SPES membranes was lower than the initial flux of neat PVDF/SPES membrane. However, the former membranes showed lower flux decline compared to the neat PVDF/SPES membrane. The BSA rejection of modified membranes was improved. The fouling analysis demonstrated that the TiO₂ deposited PVDF/SPES membranes showed the fewer tendencies to fouling. The results of antibacterial study showed that the UV irradiated TiO₂ deposited PVDF/SPES membranes possess high antibacterial property.     

投加助滤剂对高盐度冲击下MBR运行及膜污染影响

以膜生物反应器（membrane bioreactor,MBR）受高盐度废水冲击前后的活性污泥为研究对象,考察了助滤剂（硅藻土）投加对于MBR污染物去除及膜污染减缓的影响。实验结果表明投加硅藻土对高盐废水冲击后的MBR性能恢复效果显著。与对照反应器相比,投加60 mg·L^-1硅藻土可提高系统对传统污染物的去除效率,COD、NH₄⁺-N及TP去除效率分别提高了4.9%、3.2%及74.5%;高盐度废水冲击显著增加了对照反应器的膜污染速率,其膜污染速率是投加硅藻土MBR的4倍。硅藻土具有的吸附性能及絮凝能力显著降低了MBR本体溶液中的溶解性微生物代谢产物含量,减小了膜污染速率。进一步研究发现,投加硅藻土增加了平均絮体粒径（mean particles sizes, d_p）及相对疏水性（related hydrophobic, RH）,有利于减缓膜污染。     

Electrospun fiber membranes from blends of poly(vinylidene fluoride) with fouling‐resistant zwitterionic copolymers

Nonwoven super‐hydrophobic fiber membranes have potential applications in oil–water separation and membrane distillation, but fouling negatively impacts both applications. Membranes were prepared from blends comprising poly(vinylidene fluoride) (PVDF) and random zwitterionic copolymers of poly(methyl methacrylate) (PMMA) with sulfobetaine methacrylate (SBMA) or with sulfobetaine‐2‐vinylpyridine (SB2VP). PVDF imparts mechanical strength to the membrane, while the copolymers enhance fouling resistance. Blend composition was varied by controlling the PVDF‐to‐copolymer ratio. Nonwoven fiber membranes were obtained by electrospinning solutions of PVDF and the copolymers in a mixed solvent of N,N‐dimethylacetamide and acetone. The PVDF crystal phases and crystallinities of the blends were studied using wide‐angle X‐ray diffraction and differential scanning calorimetry (DSC). PVDF crystallized preferentially into its polar β‐phase, though its degree of crystallinity was reduced with increased addition of the random copolymers. Thermogravimetry (TG) showed that the degradation temperatures varied systematically with blend composition. PVDF blends with either copolymer showed significant increase of fouling resistance. Membranes prepared from blends containing 10% P(MMA‐ran‐SB2VP) had the highest fouling resistance, with a fivefold decrease in protein adsorption on the surface, compared to homopolymer PVDF. They also exhibited higher pure water flux, and better oil removal in oil–water separation experiments. © 2018 Society of Chemical Industry     

Interaction mechanisms associated with organic colloid fouling of ultrafiltration membrane in a drinking water treatment system

The Extended Derjaguin Landau Verwey Overbeek (XDLVO) approach was introduced to predict organic colloid fouling of membranes in an initial ultrafiltration (UF) phase. Two polymeric UF membranes, made of polyvinylchloride (PVC) and polyvinylidene fluoride (PVDF) respectively, were selected to investigate membrane fouling by filtering water samples with different organic colloid compositions. The experiment was performed to determine the fouling contributions of van der Waals (LW) interactions, electrostatic (EL) interactions, as well as double layer and short-range acid–base (AB) interactions, to the total interaction energy caused by organic colloids attaching to UF membrane surfaces. The results showed that LW interaction energy predominated when the distance between the membrane surface and organic colloid was > 5 nm, while AB accounted for a key contribution to total interaction energy over short distances (< 2.5 nm). The influence of EL interaction energy was ignored in the total interaction energy composition. The surface energy, among all characteristics of membrane material, was a dominant factor affecting membrane fouling. The experimental results of initial ultrafiltration of raw water from the actual water source were in accordance with the predictions based on XDLVO theory, indicating that it was a feasible option for predicting membrane fouling during the initial ultrafiltration phase.     

基于小孔径PVDF内衬膜A/O-MBR膜污染分析

为研究膜生物反应器膜污染问题,在缺氧-好氧一体式膜生物反应器中,对模拟生活污水的处理效果进行考察,考察了4种不同孔径聚偏氟乙烯(PVDF)内衬膜对膜污染的影响规律。结果表明,基于小孔径PVDF膜的MBR工艺对模拟生活污水中COD、NH3-N的去除率分别达到96%、90%,出水浊度在0.35 NTU以下;在操作条件固定的情况下,随着膜孔径增加,临界通量逐渐降低;孔径越小,跨膜压差上升越缓慢,膜污染较轻。膜污染模型分析结果显示:无论何种孔径的膜,在MBR系统中,滤饼层和凝胶层污染所占的比例都最大(>80%),因此,在使用过程中减缓滤饼层及凝胶层的形成至关重要。     

臭氧-活性炭技术对膜生物反应器膜污染减缓研究

本研究目的是探讨臭氧-活性炭技术对膜生物反应器（membrane bioreactor,MBR）膜污染减缓的影响。通过短期批式实验表明,粉末活性炭（power activated carbon,PAC）可强化臭氧的氧化效果,臭氧投加量超过0.25mg/(gSS)将恶化污泥混合液可滤性;对滤出液残余臭氧浓度检测表明,PAC的加入有利于维持本体溶液臭氧浓度。臭氧-活性炭技术引入MBR系统有助于膜污染的减缓,反应器内微生物活性受到一定的抑制作用,但对MBR出水水质影响较小;臭氧-活性炭减小了反应器内溶解性微生物产物（soluble microbial products,SMP）中的蛋白质及多聚糖含量,显著降低了污泥絮体中松散的胞外聚合物（loosely bound EPS,LB）及胞外聚合物（extracellular polymeric substances,EPS）中蛋白质浓度,以上结果表明应用臭氧-活性炭技术来延缓MBR膜污染是可行的。     

膜材料对膜曝气生物膜反应器性能影响的比较

选择聚偏氟乙烯(PVDF)和聚丙烯(PP)中空纤维曝气膜作为膜曝气生物膜反应器(MABR)的膜组件材料,比较两种膜材料由于亲疏水性能、表面形态、生物相容性等性质的差别,对MABR挂膜启动速度、生物附着量、脱氮除碳及膜污染等性能的影响。研究显示,运行末期PVDF和PP膜纤维生物附着量分别为35.62 g·m^-2和30.63 g·m^-2。通过场发射扫描电子显微镜观察两种膜表面生物污染情况,PVDF膜纤维表面呈鱼鳞状结构,有效保护了膜孔不被微生物完全堵塞。在90 d的运行周期内以PVDF为曝气膜材料的MABR获得了90%以上的COD去除率和78%的TN去除率;而以PP为曝气膜的MABR由于运行后期曝气膜纤维污染严重仅得到了76.5%的COD去除率和49.1%的TN去除率。因此,PVDF曝气纤维更适于作膜曝气生物膜反应器的曝气膜。     

Surface immobilization of polymer brushes onto porous poly(vinylidene fluoride) membrane by electron beam to improve the hydrophilicity and fouling resistance

Poly(vinylidene fluoride) (PVDF) membrane was pre-irradiated by electron beam, and then poly(ethylene glycol) methyl ether methacrylate (PEGMA) was grafted onto the membrane surface in the aqueous solution. The degree of grafting was significantly influenced by the pH value of the reaction solution. The surface chemical changes were characterized by the Fourier transform infrared attenuated total reflection spectroscopy (FTIR-ATR) and X-ray photoelectron spectroscopy (XPS). Combining with the analysis of the nuclear magnetic resonance proton and carbon spectra (¹H NMR and ¹³C NMR), PEGMA was mainly grafted onto the membrane surface. Morphological changes were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The porosity and bulk mean pore size changes were determined by a mercury porosimeter. The surface and bulk hydrophilicity were evaluated on the basis of static water contact angle, dynamic water contact angle and the dynamic adsorption process. Furthermore, relative high permeation fluxes of pure water and protein solution were obtained. All these results demonstrate that both hydrophilicity and fouling resistance of the PVDF membrane can be improved by the immobilization of hydrophilic comb-like polymer brushes on the membrane surface.     

Characteristics of soluble microbial products and extracellular polymeric substances in the membrane bioreactor for water reuse

In biological treatment processes, the effluent contains some portion of organic material originating from soluble microbial products (SMP) that is closely related with extracellular polymeric substances (EPS), although the biological activity is maximized. These SMP should be considered important target materials if additional processes are necessary to satisfy the required water quality for the intended reuse. In this study, the characteristic of SMP and EPS were investigated using a submerged membrane bioreactor, which is one of the most promising water reuse process. Most protein SMP in the reactor existed at a molecular weight (MW) above 10 kDa. Over 86% of the carbohydrate SMP contained in the permeate had a MW below 1 kDa. The protein and carbohydrate SMP in the permeate account for 83–91% of the COD. The MW of the protein and carbohydrate SMP in the reactor, when converted to COD showed a bimodal pattern (less than 1 kDa or greater than 10 kDa), but over 81% of those in the permeate existed below 1 kDa due to the membrane rejection. The relative hydrophobicity of the protein and carbohydrate SMP was affected by the influent characteristics, and the hydrophobicity was decreased after the membrane filtration. The total EPS concentrations increased with increases in the food to microorganism ratio in this study. The modified fouling index (MFI) was used to investigate the biofouling characteristics, in terms of soluble and suspended solid parts closely related with the SMP and EPS, respectively.     

Membrane filtration characteristics in membrane-coupled activated sludge system - the effect of physiological states of activated sludge on membrane fouling

The effect of sludge physiology on membrane fouling was investigated in a membrane-coupled activated sludge (MCAS) system. A series of ultrafiltrations were performed to assess the flux behaviors according to foaming potential, solids retention time (SRT), growth phase and nutrient condition of the activated sludge. The foaming sludge showed greater flux decline than the non-foaming sludge. The extraordinary increase, that is, more than 100 times in membrane fouling for the foaming sludge, was attributed to the hydrophobic and waxy nature of the foaming sludge surface, which was confirmed by a comparison with relative hydrophobicity. Membrane fouling tendency was increased as SRT decreased. A greater flux decline was observed at the endogenous phase than at the log growth phase. The activated sludge acclimated to the nitrogen deficient substrates produced less extracellular polymeric substances (EPS) and exhibited higher flux than the control activated sludge. The quantitative measurements of EPS content in order to estimate the extent of membrane fouling in various activated sludges showed that, in any physiological states of activated sludge, the higher the content of EPS the activated sludge had, the greater the membrane fouling proceeded. The EPS content of activated sludge is suggested as a probable index for the membrane fouling in a MCAS system.     

相转化法制备PVDF超滤膜及其亲水化改性研究进展

综述了聚偏氟乙烯(PVDF)超滤(UF)膜在相转化制备方法和亲水化改性两方面的最新研究进展。在相转化制备方法中,介绍了聚合物用量、溶剂的选择、添加剂的种类和用量及凝固浴组成和温度对膜结构和性能的影响;在PVDF超滤膜亲水化改性方面,介绍了共混改性、共聚改性、辐照接枝改性、等子体改性等方法的机理、特点和近年来的研究进展,指出了共混改性是今后亲水改性的主要方向。