两种正渗透膜的对比试验研究

以不同浓度的葡萄糖、蔗糖溶液作为正渗透汲取液,对比了三醋酸纤维素(CTA)膜和聚酰胺(PA)膜在不同膜朝向下的水通量变化情况。结果表明:当膜的活性层朝向汲取液(ALDS)时,相同汲取液浓度的PA膜水通量高于CTA膜,且随着汲取液浓度的增大,两种FO膜的水通量均增大,但水通量的增幅均随浓度的提高而减小,且蔗糖汲取液对应水通量的增幅高于葡萄糖汲取液;当膜的活性层朝向原料液(AL-FS)时,两种FO膜的水通量均远小于AL-DS模式下的水通量,此时相同汲取液浓度的CTA膜水通量反而高于PA膜,对于CTA膜,水通量变化规律与ALDS模式相似,对于PA膜,随着汲取液浓度的增大,水通量几乎不变,增至高浓度时,水通量反而略有下降。     

正渗透膜处理源分离尿液效能与工艺运行特性

采用正渗透膜处理实际尿液,以Na Cl溶液为汲取液,考察了不同汲取液浓度下膜通量的变化规律、正渗透膜对尿液中污染物的截留效果以及清洗后膜通量的恢复情况。结果表明:提高汲取液浓度可以大幅度地提升膜通量,同时,正渗透膜对尿液中的污染物具有很好的截留作用,对总有机碳、总氮和氨氮的截留率均可达到98.5%以上。运行后仅采用去离子水清洗15 min膜通量就可以恢复90%以上,说明正渗透膜具有很好的抗污染性和可恢复性,可用于源分离尿液的浓缩和处理。     

不同因素对正渗透膜截留污染物效果的影响

针对污水厂二级出水不能直接回用问题,采用正渗透膜对二级出水进行深度处理,分析正渗透膜对污染物质的截留效果及其影响因素。结果表明,正渗透分离过程的影响因素作用大小顺序为汲取液浓度汲取液流量原料液温度曝气量;在最佳试验条件下,正渗透膜对TDS的截留率在98%以上,对PO_4~(3-)-P、TP的截留率在96%左右,对TOC、NO_3~--N、TN的截留率在85%~89%之间;正渗透对NH_4~+-N的截留效果不佳,截留率仅约为63%。     

汲取液EDTA-2Na对正渗透MBR运行性能的影响

采用乙二胺四乙酸二钠(EDTA-2Na)作为汲取液运行正渗透(FO)膜生物反应器(MBR),并与NaCl汲取液进行比较,考察系统的运行效果、污泥性质以及膜污染状况。结果表明,与NaCl相比,采用EDTA-2Na作为汲取液能有效减轻生物反应器的盐分积累,并得到更高的水通量;采用两种汲取液时正渗透MBR对污染物的去除效果相当,与汲取液种类无关,正渗透MBR对TOC、TN、氨氮和TP的去除率分别可达到94.6%、71.7%、96.8%和99.2%以上;水通量的减小会导致MLSS和MLVSS的降低;采用EDTA-2Na作为汲取液时,生物反应器中升高的EDTA浓度导致胞外聚合物(EPS)浓度升高,加重了FO膜污染。FTIR、SEM和EDS分析结果表明,采用两种汲取液时,FO膜活性层表面都会覆盖一层滤饼层;以EDTA-2Na作为汲取液时,污染膜表面有机污染物较多、无机沉积物较少。     

正渗透/膜蒸馏组合工艺回收尿液废水中营养物质

利用正渗透/膜蒸馏组合工艺处理源分离尿液,以1 mol/L的NaCl为汲取液,连续运行48 h,通过比较运行过程中的膜通量和物质截留率等参数来评价组合工艺的处理效果。结果表明:通过调节正渗透侧和膜蒸馏侧的参数可以实现两侧水传递速率的匹配,进而实现系统的平稳运行。正渗透/膜蒸馏装置在处理源分离尿液的过程中,对尿液中氨氮、磷酸盐和TOC的截留率均值都在90%以上。源分离尿液中氨氮、磷酸盐和TOC的浓度分别为(783±16)、(79. 91±3)、(208. 14±33) mg/L,而在产水中未检出磷酸盐,氨氮浓度为46. 2 mg/L,TOC浓度为7. 38 mg/L。物料守恒分析表明,氨氮和TOC会在汲取液和馏出液中积累,提升正渗透膜的性能是得到高品质产水的关键。     

减压膜蒸馏浓缩脱硫液试验研究

采用减压膜蒸馏(VMD)技术浓缩脱硫液,研究了减压膜蒸馏过程中脱硫液浓度、流速、热侧料液温度、冷侧真空度及膜组件装实密度对膜通量和截留率的影响。试验结果表明:当脱硫液浓度较低时,膜通量随脱硫液浓度的升高而下降缓慢,当脱硫液浓度较高时,膜通量随脱硫液浓度的升高呈抛物线式下降;随着热侧料液流速的增加,膜通量略有增加;热侧料液温度升高,膜通量线性增加;随着冷侧真空度的增加,膜通量先略有增加,当冷侧真空度超过一临界值时,膜通量显著增加;膜通量随着组件内膜丝数目的增加而线性减少;试验过程中截留率保持在99.0%以上,表明聚偏氟乙烯(PVDF)中空纤维膜具有良好的疏水性。     

三醋酸纤维素正渗透膜的双向离子传质机制研究

采用三醋酸纤维素正渗透膜处理含盐料液,以NaCl溶液为汲取液,KNO_3或KBr为料液,考察了不同料液浓度下正渗透膜的水通量和离子通量变化,分析了料液离子及汲取液离子的传质规律,以及水通量对离子传质的影响。结果表明:料液离子浓度由1 mmol/L增至100 mmol/L对水通量影响较小,而料液中的离子通量具有较大幅度增加,且阴、阳离子的传输满足唐南平衡。汲取液中的离子通量也随料液浓度的增加而变大,说明除经典的溶解-扩散机理外,汲取液中离子会以其他方式进行传递。对离子通量与水通量比值进行分析得知,料液离子强度增加会降低膜对离子的选择性。     

正渗透膜浓缩生活污水效果及膜过程特性

针对低浓度模拟生活污水,构建了正渗透膜污水浓缩系统,并开展了对污染物和其他离子的截留和浓缩效果试验和膜过程特性研究。在体积浓缩至10倍的过程中,膜对污水中的有机物和营养元素具有良好的截留和浓缩效果。随着驱动液浓度的上升,正渗透膜水通量逐渐增加,膜污染发展速度也变快。在错流流速大于一定程度后,膜水通量受错流流速的影响不明显。正渗透膜具有较轻的膜污染潜势,可通过清洗手段实现超过96%的通量恢复。     

胶团强化超滤法(MEUF)去除废水中苯胺的研究

在应用胶团强化超滤(MEUF)技术处理苯胺废水实验中,考察了表面活性剂浓度、温度、p H和盐度对苯胺截留效果和膜渗透通量的影响,分析结果表明,苯胺截留率随SDS浓度增加而增大,温度升高而减小,p H增大而减小,盐度增大而减小;膜渗透通量随SDS浓度增加而减小,随温度升高而升高,p H变化膜通量基本保持不变,盐度增大膜通量减小。     

TFC-ES正渗透膜的性能研究

以TFC-ES正渗透膜分离系统为基础,考察了膜朝向、错流速度2个影响因素对膜的水通量和驱动溶质逆向渗透现象的影响。结果表明,F0模式下膜的初始水通量远低于PRO模式,且F0模式下水通量随运行时间整体变化趋势较小,驱动溶质逆向渗透现象较轻;在保持其他运行条件不变的情况下,水通量随错流速度的增大而增大,驱动溶质逆向渗透现象也随之变得更加严重。     

Flux patterns and membrane fouling propensity during desalination of seawater by forward osmosis

The membrane fouling propensity of natural seawater during forward osmosis was studied. Seawater from the Red Sea was used as the feed in a forward osmosis process while a 2 M sodium chloride solution was used as the draw solution. The process was conducted in a semi-batch mode under two crossflow velocities, 16.7 cm/s and 4.2 cm/s. For the first time reported, silica scaling was found to be the dominant inorganic fouling (scaling) on the surface of membrane active layer during seawater forward osmosis. Polymerization of dissolved silica was the major mechanism for the formation of silica scaling. After ten batches of seawater forward osmosis, the membrane surface was covered by a fouling layer of assorted polymerized silica clusters and natural organic matter, especially biopolymers. Moreover, the absorbed biopolymers also provided bacterial attachment sites. The accumulated organic fouling could be partially removed by water flushing while the polymerized silica was difficult to remove. The rate of flux decline was about 53% with a crossflow velocity of 16.7 cm/s while reaching more than 70% with a crossflow velocity of 4.2 cm/s. Both concentration polarization and fouling played roles in the decrease of flux. The salt rejection was stable at about 98% during seawater forward osmosis. In addition, an almost complete rejection of natural organic matter was attained. The results from this study are valuable for the design and development of a successful protocol for a pretreatment process before seawater forward osmosis and a cleaning method for fouled membranes.     

Relating reverse and forward solute diffusion to membrane fouling in osmotically driven membrane processes

Osmotically driven membrane processes, such as forward osmosis (FO) and pressure retarded osmosis (PRO), are attracting increasing interest in research and applications in environment and energy related fields. In this study, we systematically investigated the alginate fouling on an osmotic membrane during FO operation using four types of draw solutions (NaCl, MgCl₂, CaCl₂ and Ca(NO₃)₂) to elucidate the relationships between reverse (from draw solution to feed solution) and forward (from feed solution to draw solution) solute diffusion, and membrane fouling. At the same water flux level (achieved by adjusting the draw solution concentration), the greatest reverse solute diffusion rate was observed for NaCl draw solution, followed by Ca(NO₃)₂ draw solution, and then CaCl₂ draw solution and MgCl₂ draw solution, the order of which was consistent with that of their solute permeability coefficients. Moreover, the reverse solute diffusion of draw solute (especially divalent cation) can change the feed solution chemistry and thus enhance membrane fouling by alginate, the extent of which is related to the rate of the reverse draw solute diffusion and its ability to interact with the foulant. The extent of fouling for the four types of draw solution followed an order of Ca(NO₃)₂ > CaCl₂ >> MgCl₂ > NaCl. On the other hand, the rate of forward diffusion of feed solute (e.g., Na⁺) was in turn promoted under severe membrane fouling in active layer facing draw solution orientation, which may be attributed to the fouling enhanced concentration polarization (pore clogging enhanced ICP and cake enhanced concentration polarization). The enhanced concentration polarization can lead to additional water flux reduction and is an important mechanism governing the water flux behavior during FO membrane fouling. Findings have significant implications for the draw solution selection and membrane fouling control in osmotically driven membrane processes.     

Impact of humic acid fouling on membrane performance and transport of pharmaceutically active compounds in forward osmosis

The impact of humic acid fouling on the membrane transport of two pharmaceutically active compounds (PhACs) – namely carbamazepine and sulfamethoxazole – in forward osmosis (FO) was investigated. Deposition of humic acid onto the membrane surface was promoted by the complexation with calcium ions in the feed solution and the increase in ionic strength at the membrane surface due to the reverse transport of NaCl draw solute. The increase in the humic acid deposition on the membrane surface led to a substantial decrease in the membrane salt (NaCl) permeability coefficient but did not result in a significant decrease in the membrane pure water permeability coefficient. As the deposition of humic acid increased, the permeation of carbamazepine and sulfamethoxazole decreased, which correlated well with the decrease in the membrane salt (NaCl) permeability coefficient. It is hypothesized that the hydrated humic acid fouling layer hindered solute diffusion through the membrane pore and enhanced solute rejection by steric hindrance, but not the permeation of water molecules. The membrane water and salt (NaCl) permeability coefficients were fully restored by physical cleaning of the membrane, suggesting that humic acid did not penetrate into the membrane pores.     

Rejection of micropollutants by clean and fouled forward osmosis membrane

As forward osmosis (FO) gains attention as an efficient technology to improve wastewater reclamation processes, it is fundamental to determine the influence of fouling in the rejection of emerging contaminants (micropollutants). This study focuses on the rejection of 13 selected micropollutants, spiked in a secondary wastewater effluent, by a FO membrane, using Red Sea water as draw solution (DS), differentiating the effects on the rejection caused by a clean and fouled membrane. The resulting effluent was then desalinated at low pressure with a reverse osmosis (RO) membrane, to produce a high quality permeate and determine the rejection with a coupled forward osmosis - low pressure reverse osmosis (FO-LPRO) system. When considering only FO with a clean membrane, the rejection of the hydrophilic neutral compounds was between 48.6% and 84.7%, for the hydrophobic neutrals the rejection ranged from 40.0% to 87.5%, and for the ionic compounds the rejections were between 92.9% and 96.5%. With a fouled membrane, the rejections were between 44.6% and 95.2%, 48.7%-91.5% and 96.9%-98.6%, respectively. These results suggest that, except for the hydrophilic neutral compounds, the rejection of the micropollutants is increased by the presence of a fouling layer, possibly due to the higher hydrophilicity of the FO fouled membrane compared to the clean one, the increased adsorption capacity of hydrophilic compounds and reduced mass transport capacity, membrane swelling, and the higher negative charge of the membrane surface, related to the foulants composition, mainly NOM acids (carboxylic radicals) and polysaccharides or polysaccharide-like substances. However, when coupled with RO, the rejections in both cases increased above 96%. The coupled FO-LPRO system was an effective double barrier against the selected micropollutants.     

Comparison of the removal of hydrophobic trace organic contaminants by forward osmosis and reverse osmosis

We compared the rejection behaviours of three hydrophobic trace organic contaminants, bisphenol A, triclosan and diclofenac, in forward osmosis (FO) and reverse osmosis (RO). Using erythritol, xylose and glucose as inert reference organic solutes and the membrane pore transport model, the mean effective pore size of a commercial cellulose-based FO membrane was estimated to be 0.74 nm. When NaCl was used as the draw solute, at the same water permeate flux of 5.4 L/m² h (or 1.5 μm/s), the adsorption of all three compounds to the membrane in the FO mode was consistently lower than that in the RO mode. Rejection of bisphenol A and diclofenac were higher in the FO mode compared to that in the RO mode. Because the molecular width of triclosan was larger than the estimated mean effective membrane pore size, triclosan was completely rejected by the membrane and negligent difference between the FO and RO modes could be observed. The difference in the separation behaviour of these hydrophobic trace organics in the FO (using NaCl the draw solute) and RO modes could be explained by the phenomenon of retarded forward diffusion of solutes. The reverse salt flux of NaCl hinders the pore diffusion and subsequent adsorption of the trace organic compounds within the membrane. The retarded forward diffusion effect was not observed when MgSO₄ and glucose were used as the draw solutes. The reverse flux of both MgSO₄ and glucose was negligible and thus both adsorption and rejection of BPA in the FO mode were identical to those in the RO mode.     

Cyclophosphamide removal from water by nanofiltration and reverse osmosis membrane

The rejection of cyclophosphamide (CP) by nanofiltration (NF) and reverse osmosis (RO) membranes from ultrapure (Milli-Q) water and membrane bioreactor (MBR) effluent was investigated. Lyophilization-extraction and detection methods were first developed for CP analysis in different water matrices. Experimental results showed that the RO membrane provided excellent rejection (>90%) under all operating conditions. Conversely, efficiency of CP rejection by NF membrane was poor: in the range of 20-40% from Milli-Q water and around 60% from MBR effluent. Trans-membrane pressure, initial CP concentration and ionic strength of the feed solution had almost no effect on CP retention by NF. On the other hand, the water matrix proved to have a great influence: CP rejection rate by NF was clearly enhanced when MBR effluent was used as the background solution. Membrane fouling and interactions between the CP and water matrix appeared to contribute to the higher rejection of CP.     

Effects of reverse osmosis isolation on reactivity of naturally occurring dissolved organic matter in physicochemical processes

A field reverse osmosis system was used to isolate dissolved organic matter (DOM) from two lacustrine and two riverine surface water sources. The rejection of DOM was on the order of 99% and did not vary significantly with pressure. A simple mass balance model using a single measured value of rejection predicted the concentration within the closed-loop isolation system. The effect of operating pressure and solution flux on mass recovery of DOM was evaluated in laboratory and field trials. Under controlled laboratory conditions, >99% of a lacustrine DOM could be accounted for. A fraction of the isolated DOM was not recoverable using hydrodynamic cleaning; however, this fraction was recovered by using a pH 10 NaOH wash solution. The mass recovered in the NaOH solution increased from <1% to >6% with increasing transmembrane pressures from 414 kPa (60 psi) to1000 kPa (145 psi), respectively. This is consistent with fouling that results from an increase in solution flux, and a decrease in tangential crossflow velocity. Under field conditions, mass balances were generally >95% and mass recovery was >90% in all cases. The effects of temperature on solution flux were consistent with changes in fluid viscosity; effects of temperature on membrane diffusivity or morphological properties were small. RO isolation under low pressure conditions designed to maximize DOM recovery had little effect on DOM reactivity evaluated in terms of nanofiltration membrane fouling, XAD-8 resin adsorption, activated carbon adsorption, competition with trichloroethylene for adsorption sites on activated carbon, and molecular weight distribution measured using size exclusion chromatography.     

Trace organic solutes in closed-loop forward osmosis applications: Influence of membrane fouling and modeling of solute build-up

In this study, trace organics transport in closed-loop forward osmosis (FO) systems was assessed. The FO systems considered, consisted of an FO unit and a nanofiltration (NF) or reverse osmosis (RO) unit, with the draw solution circulating between both units. The rejection of trace organics by FO, NF and RO was tested. It was found that the rejection rates of FO were generally comparable with NF and lower than RO rejection rates. To assess the influence of fouling in FO on trace organics rejection, FO membranes were fouled with sodium alginate, bovine serum albumin or by biofilm growth, after which trace organics rejection was tested. A negative influence of fouling on FO rejection was found which was limited in most cases, while it was significant for some compounds such as paracetamol and naproxen, indicating specific compound-foulant interactions. The transport mechanism of trace organics in FO was tested, in order to differentiate between diffusive and convective transport. The concentration of trace organics in the final product water and the build-up of trace organics in the draw solution were modeled assuming the draw solution was reconcentrated by NF/RO and taking into account different transport mechanisms for the FO membrane and different rejection rates by NF/RO. Modeling results showed that if the FO rejection rate is lower than the RO rejection rate (as is the case for most compounds tested), the added value of the FO-RO cycle compared to RO only at steady-state was small for diffusively and negative for convectively transported trace organics. Modeling also showed that trace organics accumulate in the draw solution.     

Impact of microfiltration treatment of secondary wastewater effluent on biofouling of reverse osmosis membranes

The effects of microfiltration (MF) as pretreatment for reverse osmosis (RO) on biofouling of RO membranes were analyzed with secondary wastewater effluents. MF pretreatment reduced permeate flux decline two- to three-fold, while increasing salt rejection. Additionally, the oxygen uptake rate (OUR) in the biofouling layer of the RO membrane was higher for an RO system that received pretreated secondary wastewater effluent compared to a control RO system that received untreated secondary effluent, likely due to the removal of inert particulate/colloidal matter during MF. A higher cell viability in the RO biofilm was observed close to the membrane surface irrespective of pretreatment, which is consistent with the biofilm-enhanced concentration polarization effect. Bacterial 16S rRNA gene clone library analysis revealed dominant biofilm communities of Proteobacteria and Bacteroidetes under all conditions. The Cramer-von Mises test statistic showed that MF pretreatment did not significantly change the bacterial community structure of RO membrane biofilms, though it affected bacterial community structure of non-membrane-associated biofilms (collected from the feed tank wall). The finding that the biofilm community developed on the RO membrane was not influenced by MF pretreatment may imply that RO membranes select for a conserved biofilm community.     

Fatty acid fouling of reverse osmosis membranes: implications for wastewater reclamation

Effluent organic matter (EfOM) contributes significantly to organic fouling of reverse osmosis (RO) membranes in advanced wastewater reclamation. In this study, the effect of feed solution chemistry (solution pH and Ca²⁺ concentration) on the fouling of RO membranes by octanoic acid—selected to represent fatty acids in EfOM—is investigated. Crossflow fouling experiments demonstrate that RO membrane fouling is much more significant at solution pH below the pK_a of the octanoic acid (pK_a = 4.9) than at an elevated pH. Octanoic acid permeates across the membranes more readily at solution pH below its pK_a than at elevated pH. At pH below the octanoic acid pK_a, fouling behavior is not affected by calcium ions, whereas at elevated pH, the rate of flux decline decreases with higher calcium ion concentration. The effect of calcium on the fouling behavior was further verified from foulant-foulant adhesion forces, determined by atomic force microscopy (AFM) force measurements under solution chemistries identical to those of the crossflow fouling experiments. To investigate the implications of octanoic acid fouling for wastewater reclamation, the effect of octanoic acid on membrane fouling by a combination of organic foulants in the presence of calcium ions is studied. At a solution chemistry simulating that of typical wastewater effluents, the addition of octanoic acid to a feed solution containing alginate, bovine serum albumin, and Suwannee River natural organic matter, does not enhance membrane fouling behavior. This observation could be attributed to the significant contribution of the alginate-calcium complexes within the fouling layer to the total membrane resistance.