两种膜的前端处理技术减缓膜污染的试验研究

通过超滤膜过滤粉末活性炭预处理长江水和混凝沉淀预处理太湖水的上清液试验,比较各工艺中有机物极性含量的变化,结合膜的扫描电镜照片,探讨粉末活性炭和混凝沉淀预处理技术对膜污染的减缓作用。结果表明,疏水性有机物是造成膜污染的主要因素,亲水性有机物对膜污染的影响较小。超滤膜分别过滤经20 mg/L投量的粉末活性炭预处理的长江水样和25 mg/L投量的聚合氯化铝预处理的太湖水样时,相比直接过滤2种原水,超滤膜反洗后膜通量可有效恢复。粉末活性炭和混凝沉淀预处理后,膜表面形成的滤饼层较松散,易被周期性反洗排出膜组件,滤饼层可阻止疏水性有机物直接沉积在膜表面,减缓超滤膜污染,同时也可提高超滤膜出水的化学安全性。     

紫外/次氯酸钠预处理对超滤膜污染的影响

针对污水再生超滤处理过程中的膜污染问题,以某污水处理厂深度处理出水为原水,通过对比紫外/次氯酸钠处理前后有机指标、膜通量及膜污染的变化,对该工艺作为超滤预处理措施的可行性进行研究。结果表明,紫外/次氯酸钠预处理对大分子(>100 kDa)有机物去除率为96.02%,优于其他分子量有机物;对疏水性有机物的去除率为21.32%,优于亲水性有机物及过渡亲水性酸。原水未经预处理直接采用超滤膜过滤时,膜污染阻力快速升高至2.52×10¹¹m^-1,可逆污染阻力占总阻力的69.84%;经过紫外/次氯酸钠预处理后超滤膜的总污染、可逆污染和不可逆污染膜阻力分别降低了50.40%,69.31%,6.58%,膜比通量提高了10.17%。紫外/次氯酸钠预处理措施能够有效去除原水中有机物,降低其对膜单元的污染负荷,从而缓解超滤膜污染。     

基于XDLVO理论的超滤膜污染机理研究

应用XDLVO理论定量解析腐殖酸中不同分子质量组分在PVC和PVDF两种超滤膜污染中的界面作用,评价范德华力(LW)、极性力(AB)、静电荷作用力(EL)在腐殖酸引起的超滤膜污染中的相对贡献,确定膜污染的主要作用力。另外,取长江南京段沉后水进行臭氧氧化预处理,进一步探究XDLVO理论在实际原水膜污染中的适用性。结果表明,试验采用的PVC和PVDF膜总表面能相近,同一分子质量有机物导致两种膜污染相近;腐殖酸膜污染中AB相互作用能起主导作用,LW相互作用能次之。腐殖酸中小分子质量有机物以及臭氧氧化预处理后的长江原水,具有更强的亲水性和极性,AB、LW相互作用能降低,膜污染减轻。     

粉末活性炭吸附对超滤膜污染的影响研究

进行了粉末活性炭(PAC)吸附缓解浸没式和内压式超滤膜污染试验研究,并探讨了PAC吸附缓解膜污染的机理。浸没式超滤膜试验结果表明:PAC通过吸附溶解小分子有机物,减少了膜孔堵塞和膜孔内吸附污染,缓解了运行初期通量的快速下降;但PAC在膜表面的累积会降低浸没式超滤膜的起始通量。内压式超滤膜试验结果表明:膜前PAC吸附预处理能提高对有机物的去除效能,降低膜表面的污染负荷,从而降低TMP及其增长速度。     

粉末活性炭/污泥回流工艺强化膜前预处理的研究

采用粉末活性炭(PAC)吸附/混凝沉淀/浸没式超滤膜组合工艺处理苏州市某河水,考察了PAC/污泥回流工艺对膜前预处理的强化效果及对膜污染的影响,并与常规混凝沉淀、污泥回流强化混凝沉淀、PAC吸附/混凝沉淀等3种预处理工艺进行了对比。结果表明,PAC/污泥回流强化预处理工艺对浊度、DOC、UV254和THMFP的去除率分别为80.2%、47.5%、42.3%和52.3%,均比其他预处理工艺的高,对MW30 ku和MW1 ku有机物的去除效果明显。PAC/污泥回流强化预处理和超滤膜组合工艺对浊度、DOC、UV254和THMFP的去除率分别可达到99.2%、54.1%、47.2%和60.2%;经过15 d的运行,超滤膜的跨膜压差基本保持稳定,而其他预处理工艺虽能在一定程度上减轻膜污染,但无法避免不可逆膜污染的发生。     

混凝/粉末炭组合预处理改善超滤膜污染可逆性的效能

通过投加聚合氯化铝(PACl)和粉末活性炭(PAC)进行预处理,研究二者组合减缓超滤过程中膜通量下降的效果,以及预处理效果与膜的不可逆污染之间的关系,并分析了膜污染机理。结果表明,组合预处理相比于单独预处理能够有效减轻膜污染,使膜污染的不可逆性大大降低。同时,相较于混凝/吸附(先混凝后吸附)预处理,吸附/混凝(先吸附后混凝)组合方式在缓解膜污染和改善可逆性方面都更具优势。对经预处理的水样进行膜过滤,不可逆污染与相关指标的拟合结果表明,不可逆污染与预处理过程对UV_(254)和DOC的去除率呈负相关,与SUVA值呈正相关,不可逆污染与UV_(254)的相关性较DOC好,原水中的UV_(254)类污染物是造成超滤膜不可逆污染的主要物质。     

活性炭表面性质对其控制超滤膜不可逆污染的影响

不可逆污染是影响超滤技术在水处理领域推广的重要因素。选取三种粉末活性炭A、B、C作为吸附剂进行膜前预处理后,通过进行浸没式超滤膜污染试验,考察活性炭吸附对超滤膜不可逆污染的控制作用,并分析活性炭的物理化学性质及其对天然有机物的吸附效果与控制不可逆膜污染之间的关系,探讨活性炭吸附前处理对不可逆膜污染的影响机理。试验表明,活性炭吸附对超滤膜不可逆污染有一定的控制作用,三种活性炭对不可逆膜污染的控制效果为A≈BC;而三种活性炭对原水中有机物的吸附效果顺序为ABC,说明活性炭对水中天然有机物的整体吸附性能并不能完全反映其控制不可逆膜污染的效果。进一步分析活性炭理化性质后发现,针对东江原水,活性炭的比表面积、中孔结构以及表面酸性和碱性基团的数量,均影响活性炭吸附前处理控制不可逆膜污染的效果。     

前表面三维荧光表征超滤膜污染的方法优化研究

超滤膜污染的表征是膜污染控制的关键基础,前表面三维荧光技术能够直接针对膜表面的污染物进行定量表征。对前表面三维荧光表征的方法进行了优化,对比了湿润样品直接检测、烘干样品检测和浸没样品检测3种样品预处理及检测方法。实验结果表明:湿润样品直接检测,荧光会受到样品湿度的影响;烘干样品检测,荧光会受到膜老化的影响;对浸没样品检测,前表面三维荧光技术能够稳定、准确地定量膜表面的污染物。将该方法进一步应用于超滤BSA污染试验的结果证明:优化后的前表面三维荧光表征方法能够准确定量膜污染物质含量并有效指示膜污染情况,从而为膜污染机理解析和膜污染控制提供参考。     

预处理对东江原水超滤过程中膜污染的控制作用

以珠江流域东江水作为原水,研究不同预处理(混凝、吸附、氧化)及其组合对水体中有机污染物的去除效果及对超滤膜污染的控制作用。试验结果表明,针对东江原水中天然有机物的去除,聚合氯化铝(PACl)、粉末活性炭(PAC)和高锰酸钾(KMnO4)的最佳投加量分别为20、30、0. 1 mg/L;三种单一预处理方法能够在一定程度上缓解膜通量衰减,而两两组合预处理则能够进一步提高膜运行通量;对于聚偏氟乙烯膜,PACl+PAC组合预处理对膜污染的控制作用最好。对于UV254和蛋白质,PACl和KMn O4对其去除效果优于PAC;对于多糖,三种预处理方法对其去除效果均不佳(<40%),其中PAC略好于PACl和KMn O4。此外,三种单一预处理方法对腐殖酸类荧光物质的去除效果高于蛋白质类荧光物质,而组合预处理能够更加显著地降低这两类荧光物质的响应强度,其中PACl+PAC组合预处理对有机物各荧光组分的去除效果最佳。通过对膜污染物成分的识别分析可知,东江原水中造成超滤膜污染的物质有腐殖酸类、多糖类和蛋白质类物质,而化学不可逆污染物主要为多糖类物质及少量的腐殖酸类物质,化学可逆污染物主要为蛋白质类物质及部分腐殖酸类物质。     

MIEX/超滤一体化工艺净化长江原水的研究

水体中的天然有机物是造成超滤膜污染的重要因素。考察了磁性离子交换树脂(MIEX)/超滤膜一体化净水工艺处理长江原水的效果,并通过与超滤膜直接过滤进行比较,探讨了MIEX预处理对去除有机物的影响及控制膜污染的效果。结果表明,与原水直接进行超滤处理相比,组合工艺对CODMn、DOC、UV254的去除率分别提高了40.70%、38.20%和43.90%。MIEX/超滤工艺控制消毒副产物的优势更为明显,对THMFP和HAAFP的去除率分别达62.54%和55.83%。由于MIEX预处理去除了原水中56.72%的疏水性有机物,降低了超滤膜的负荷,延缓了膜表面致密凝胶层的形成,因而减少了膜孔堵塞,可有效控制运行压力的增长速度,延长过滤时间。     

UF膜组件的有机污染及清洗

近年来,超滤（UF）膜组件的污染问题主要集中在有机物污染的范围。利用几种化学药剂对聚砜超滤膜有机污染造成的堵塞的清洗方式进行了试验研究,研究了UF件的有机污染行为及膜组件污染物的去除。实验证明NaClO,H2O2和NaOH对于UF膜的有机污染去除都有一定的效果,其中H2O2的去除效果最好。控制原水的PH值有利于减轻膜污染。同时,以NaOH／NaClO及NaOH／H2O2水溶液相结合的方式对于膜的蛋白质污染的去除也有很好的效果,能够使超滤膜的渗透水通量恢复到初始值。同时还对UF膜的有机污染形成基理进行了分析。     

Identification and quantification of major organic foulants in treated domestic wastewater affecting filterability in dead-end ultrafiltration

Ultrafiltration (UF) membranes can be used after conventional wastewater treatment to produce particle free and hygienically safe water for reuse. However, membrane fouling affects the performance of UF to a large extent. Stirred cell tests with UF membrane show high flux decline filtering treated domestic wastewater. Investigation on the impact of size fractioned substances indicates that dissolved substances are major foulants affecting water filterability. Dissolved organic substances in feed and permeate samples of the stirred cell tests are analyzed by liquid chromatography with online organic carbon detection (LC-OCD). The resulting chromatograms displayed a significant difference of feed and permeate samples in the range of large molecules identified as biopolymer peak. The substances detected in this peak (mostly macro polysaccharide-like and protein-like molecules) are almost completely retained by UF membranes. Quantified investigation shows that biopolymer concentration influences filterability of corresponding water sample proportionally. The apparent magnitude of delivered biopolymer to membrane has a striking correlation with fouling resistance. The relationship was verified to be reproducible using different water samples. Mechanism analysis demonstrates that based on the delivered biopolymer load to membrane pore blocking or cake/gel fouling is the main fouling mechanism in the present experiment conditions.     

Adsorption combined with ultrafiltration to remove organic matter from seawater

Organic fouling and biofouling are the major severe types of fouling of reverse osmosis (RO) membranes in seawater (SW) desalination. Low pressure membrane filtration such as ultrafiltration (UF) has been developed as a pre-treatment before reverse osmosis. However, UF alone may not be an effective enough pre-treatment because of the existence of low-molecular weight dissolved organic matter in seawater. Therefore, the objective of the present work is to study a hybrid process, powdered activated carbon (PAC) adsorption/UF, with real seawater and to evaluate its performance in terms of organic matter removal and membrane fouling. The effect of different PAC types and concentrations is evaluated. Stream-activated wood-based PAC addition increased marine organic matter removal by up to 70% in some conditions. Moreover, coupling PAC adsorption with UF decreased UF membrane fouling and the fouling occurring during short-term UF was totally reversible. It can be concluded that the hybrid PAC adsorption/UF process performed in crossflow filtration mode is a relevant pre-treatment process before RO desalination, allowing organic matter removal of 75% and showing no flux decline for short-term experiments.     

Biopolymer fouling in dead-end ultrafiltration of treated domestic wastewater

Ultrafiltration (UF) is considered as a suitable treatment process after conventional wastewater treatment to produce reuse water. Nevertheless, fouling affects the performance of UF to a large extent. As biopolymers (mostly macro polysaccharide-like and protein-like molecules) have been identified as major foulants affecting the filterability of water in dead-end UF, the present study focuses on investigating the reversibility of biopolymer fouling occurring at different biopolymer mass loads to the membrane and under different compression conditions. UF-membrane stirred cell tests using five cycles show that filtering treated domestic wastewater leads to a significant permeability reduction due to the accumulation of biopolymers on the membrane surface and/or in the membrane pores. Although they can be removed by hydraulic backwashing, an increased mass load of biopolymers reduces the removal efficiency. This correlation was verified using a UF pilot plant filtering treated wastewater (secondary effluent or slow sand filtrate). The effect of biopolymer fouling layer deformation on its reversibility was studied using multi-cycle membrane filtration tests under different filtration pressures. The results showed that higher filtration pressures result in more compact biopolymer fouling which is more difficult to be hydraulically backwashed. This phenomenon was also confirmed by pilot-scale UF experiments.     

Effects of magnetic ion exchange pretreatment on low pressure membrane filtration of natural surface water

Magnetic ion exchange (MIEX) pretreatment has been increasingly employed by water treatment plants for removal of dissolved organic carbon (DOC). In this study, the effects of MIEX pretreatment on low pressure membrane filtration of natural surface water were investigated under different feedwater qualities, membrane properties, and MIEX dosing conditions. Regardless of feedwater DOC, moderate decrease in the total and hydraulically irreversible fouling was observed for a polyvinylidene fluoride (PVDF) microfiltration membrane and a polyethersulfone ultrafiltration (UF) membrane after MIEX pretreatment, which was coincident with moderate removals of high molecular weight DOC in the feedwaters. Comparatively, the fouling of a PVDF UF membrane did not decrease after MIEX pretreatment, revealing the impact of membrane properties on membrane fouling in the presence of MIEX pretreatment. Reuse of virgin or regenerated MIEX resulted in similar membrane fouling as observed with single use of the virgin MIEX. The level of DOC removal by MIEX was similar to the removal of MS2 bacteriophage spiked in the feedwater, suggesting a potential similarity in the removal of organic and microbial particles. In conclusion, MIEX pretreatment was effective for DOC removal, but less effective in controlling short-term membrane fouling or removing viruses.     

Reversible and irreversible low-pressure membrane foulants in drinking water treatment: Identification by principal component analysis of fluorescence EEM and mitigation by biofiltration pretreatment

With the increased use of membranes in drinking water treatment, fouling - particularly the hydraulically irreversible type - remains the main operating issue that hinders performance and increases operational costs. The main challenge in assessing fouling potential of feed water is to accurately detect and quantify feed water constituents responsible for membrane fouling. Utilizing fluorescence excitation-emission matrices (EEM), protein-like substances, humic and fulvic acids, and particulate/colloidal matter can be detected with high sensitivity in surface waters. The application of principal component analysis to fluorescence EEMs allowed estimation of the impact of surface water constituents on reversible and irreversible membrane fouling. This technique was applied to experimental data from a two year bench-scale study that included thirteen experiments investigating the fouling potential of Grand River water (Ontario, Canada) and the effect of biofiltration pre-treatment on the level of foulants during ultrafiltration (UF). Results showed that, although the content of protein-like substances in this membrane feed water (= biofiltered natural water) was much lower than commonly found in wastewater applications, the content of protein-like substances was still highly correlated with irreversible fouling of the UF membrane. In addition, there is evidence that protein-like substances and particulate/colloidal matter formed a combined fouling layer, which contributed to both reversible and irreversible fouling. It is suggested that fouling transitions from a reversible to an irreversible regime depending on feed composition and operating time. Direct biofiltration without prior coagulant addition reduced the protein-like content of the membrane feed water which in turn reduced the irreversible fouling potential for UF membranes. Biofilters also decreased reversible fouling, and for both types of fouling higher biofilter contact times were beneficial.     

High-performance thin-layer hydrogel composite membranes for ultrafiltration of natural organic matter

Thin-layer hydrogel composite (TLHC) ultrafiltration (UF) membranes were synthesized by photo-grafting of either poly(ethylene glycol) methacrylate (PEGMA) or N,N-dimethyl-N-(2-methacryloyloxyethyl-N-(3-sulfopropyl) ammonium betaine (SPE) onto commercial polyethersulfone (PES) UF membranes. The performance of TLHC UF membranes was evaluated for natural organic matter (NOM) filtration and compared to commercial PES UF membranes. The fouling evaluation was done by investigation of membrane-solute interactions (adsorptive fouling) and membrane-solute-solute interactions (UF). The results suggest that the TLHC membranes convincingly displayed a higher adsorptive fouling resistance than unmodified PES UF membranes. In long-term stirred dead-end UF, a much lower fouling was observed for TLHC membranes than for commercial membranes with the same flux and rejection. Further, water flux recovery was also much higher. An analysis using an existing blocking model was performed in order to elucidate the effect of a polymer hydrogel layer on fouling mechanism as well as cake layer characteristics. The TLHC membranes synthesized by photo-grafting of PEGMA (40 g/L) and PEGMA with a low concentration of cross-linker monomer in the reaction mixture (ratio: 40/0.4 (g/L)/(g/L)) showed a much better performance than the other composite membranes. Those membranes could reduce the cake resistance on the membrane surface. This work has relevance for the design of high-performance UF membranes for applications in water treatment.     

中空纤维超滤膜处理滦河水中试研究

采用混凝-超滤工艺对滦河原水进行了中试研究,重点考察了PVC中空纤维超滤膜对不同水质期的滦河水中污染物的去除情况。结果表明：水温降低使膜性能下降。膜污染较严重;在高藻期进行预氯化有助于减缓膜污染;针对不同的水质期要采用不同的预处理方法。在低温期适当增加铁盐投量,高藻期进行预氯化都有助于系统稳定运行;膜出水在3个水质期（低温、常温、高藻期）均很稳定,与常规工艺相比,膜工艺具有一定的优越性,在去除浊度和颗粒物方面尤为突出。     

Identifying fouling events in a membrane-based drinking water treatment process using principal component analysis of fluorescence excitation-emission matrices

The identification of key foulants and the provision of early warning of high fouling events for drinking water treatment membrane processes is crucial for the development of effective countermeasures to membrane fouling, such as pretreatment. Principal foulants include organic, colloidal and particulate matter present in the membrane feed water. In this research, principal component analysis (PCA) of fluorescence excitation-emission matrices (EEMs) was identified as a viable tool for monitoring the performance of pre-treatment stages (in this case biological filtration), as well as ultrafiltration (UF) and nanofiltration (NF) membrane systems. In addition, fluorescence EEM-based principal component (PC) score plots, generated using the fluorescence EEMs obtained after just 1 hour of UF or NF operation, could be related to high fouling events likely caused by elevated levels of particulate/colloid-like material in the biofilter effluents. The fluorescence EEM-based PCA approach presented here is sensitive enough to be used at low organic carbon levels and has potential as an early detection method to identify high fouling events, allowing appropriate operational countermeasures to be taken.     

Degradation of natural organic matter by TiO2 photocatalytic oxidation and its effect on fouling of low-pressure membranes

Natural organic matter (NOM) fouling continues to be the major barrier to efficient application of microfiltration (MF) and ultrafiltration (UF) in drinking water treatment. In this study, the potential of TiO2/UV photocatalytic oxidation to control fouling of membranes by NOM was evaluated. Decomposition kinetics of NOM was investigated using a commercial TiO2 catalyst, and the effect of various experimental parameters including TiO2 dosage and initial total organic carbon (TOC) concentration were also determined. The reaction kinetics was found to increase with increasing TiO2 dosage, but decrease with increasing initial TOC concentration. Even though the rate of TOC removal was relatively low, the TiO2/UV process was very effective in controlling membrane fouling by NOM. At a TiO2 concentration of 0.5 g/L, fouling of both an MF and a UF membrane was completely eliminated after 20 min of treatment. Careful analyses of specific UV absorbance (SUVA) and molecular weight (MW) distribution of NOM revealed that the effectiveness in membrane fouling control is the result of the changes in NOM molecular characteristics, namely MW and SUVA due to the preferential removal and transformation of large, hydrophobic NOM compounds. Results from this study show that TiO2/UV photocatalytic oxidation is a promising pretreatment method for MF and UF systems.