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.     

Combined coagulation-disk filtration process as a pretreatment of ultrafiltration and reverse osmosis membrane for wastewater reclamation: an autopsy study of a pilot plant

The effects of the combined coagulation-disk filtration (CC-DF) process on the fouling characteristics and behavior caused by interactions between effluent organic matter (EfOM) and the membrane surfaces of the ultrafiltration (UF) and reverse osmosis (RO) membranes in a pilot plant for municipal wastewater reclamation (MWR) were investigated. The feed water from secondary effluents was treated by the CC-DF process used as a pretreatment for the UF membrane to mitigate fouling formation and the permeate from the CC-DF process was further filtered by two UF membrane units in parallel arrangement and fed into four RO modules in a series connection. The CC-DF process was not sufficient to mitigate biofouling but the UF membrane was effective in mitigating biofouling on the RO membrane surfaces. Fouling of the UF and RO membranes was dominated by hydrophilic fractions of EfOM (e.g., polysaccharide-like and protein-like substances) and inorganic scaling (e.g., aluminum, calcium and silica). The desorbed UF membrane foulants included more aluminum species and hydrophobic fractions than the desorbed RO membrane foulants, which was presumably due to the residual coagulants and aluminum-humic substance complexes. The significant change in the surface chemistry of the RO membrane (a decrease in surface charge and an increase in contact angle of the fouled RO membranes) induced by the accumulation of hydrophilic EfOM onto the negatively charged RO membrane surface intensified the fouling formation of the fouled RO membrane by hydrophobic interaction between the humic substances of EfOM with relatively high hydrophobicity and the fouled RO membranes with decreased surface charge and increased contract angle.     

Rejection of pharmaceutically active compounds and endocrine disrupting compounds by clean and fouled nanofiltration membranes

Rejections of 9 pharmaceuticals and 5 endocrine disruptors by clean and fouled nanofiltration membranes were investigated in this study. Waters containing a cocktail of compounds were filtered by clean and pre-fouled membranes. The rejection of hydrophilic neutral compounds by the clean NF-200 membrane varied from 35 to 70% under steady state conditions while that of NF-90 membrane was in the range of 62-96%. The clean NF-90 membrane rejected nearly all of the hydrophobic neutral compounds (95-98%) predominantly due to size exclusion. Nevertheless, electrostatic repulsion was the main mechanism of rejection of ionic compounds by both membranes (71-94% by NF-200 and 99% by NF-90). Fouling with sodium alginate deteriorated the performance of the NF-200 membrane in rejecting hydrophilic neutral compounds as well as hydrophilic and hydrophobic ionic compounds. In contrast, rejections of hydrophobic neutral compounds by the fouled NF-200 membrane increased by 5-38%. This may be attributed to the incipient interaction of the solutes with the membrane foulant layer resulting in less partitioning and diffusion across the membrane surface. On the other hand, rejections of hydrophobic compounds by NF-90 were not observed to be affected by fouling; however, hydrophilic neutral compounds showed increased rejections by 7-30%.     

Forward osmosis for concentration of anaerobic digester centrate

The nutrient-rich liquid stream produced during the dewatering of digested biomass (i.e., the centrate) is commonly mixed with the influent raw wastewater at wastewater treatment facilities. This increases the nitrogen and phosphorus loading on biological processes, increases operating costs, and in some cases, results in increased nutrient concentrations in the final effluent. Forward osmosis (FO) is a membrane treatment process that was investigated at bench scale to determine its feasibility to concentrate centrate under both batch and continuous operating conditions. The continuous bench-scale system used FO as pretreatment for reverse osmosis (RO). Results demonstrated that high water flux and high nutrient rejection could be achieved. The combined FO/RO process exhibited sustainable flux over an extended time period. A mathematical model was developed in order to determine the specific energy, power, and membrane area requirements for a larger-scale centrate treatment process. Modeling results indicated that to optimize power and membrane area requirements, the system should be operated at approximately 70% water recovery.     

Treatability of organic fractions derived from secondary effluent by reverse osmosis membrane

Dissolved organic matters (DOMs) from two batches of secondary effluent collected from a local water reclamation plant were fractionated using column chromatographic method with non-ionic resins XAD-8, AG MP-50 and IRA-96. Seven isolated fractions were obtained from the fractionation study and these fractions were quantified using DOC, UV(254) and SUVA values. The fractionation study revealed that the secondary effluent samples comprised about 47.3-60.6% of hydrophobic and 39.4-52.7% of hydrophilic solutes. The treatability of each isolated fraction was investigated by subjecting each fraction to reverse osmosis (RO) treatment individually. It was noted that RO process could achieve high DOC rejections for acid and neutral fractions (ranging from 80% to 98% removal) probably due to the negative charge of RO membrane. The results obtained also indicated that hydrophobicity of DOMs is significant in determining treatability of organic species by RO process. The performance of RO in terms of DOC rejection of un-fractionated secondary effluent was also investigated to assess possible effects of interactions among organic fractions on their treatability by RO process. It was noted that DOC rejection associated with the un-fractionated secondary effluent was generally higher (ranging from 2% to 45%) than the corresponding rejection obtained from each individual fraction isolated from the secondary effluent. This finding suggested there is a beneficial interaction among the fractions that in turn has contributed towards a better overall DOC rejection performance by RO treatment.     

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.     

微滤/反渗透净化污水厂二级处理出水

以微絮凝和微滤作为反渗透的预处理工艺,采用连续流微滤-反渗透集成膜工艺深度处理城市污水厂二级生物处理出水,比较了投加聚合氯化铝和氯化铁进行微絮凝控制微滤膜污染的效果.结果表明,投加氯化铁对微滤膜污染具有更好的控制效果;微滤出水浊度＜0.5 NTU,对TOC的去除率为50%左右,对UV260的去除率＜30%.     

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.     

Distillery wastewater treatment by the membrane-based nanofiltration and reverse osmosis processes

A hybrid nanofiltration (NF) and reverse osmosis (RO) pilot plant was used to remove the color and contaminants of the distillery spent wash. The feasibility of the membranes for treating wastewater from the distillery industry by varying the feed pressure (0-70 bar) and feed concentration was tested on the separation performance of thin-film composite NF and RO membranes. Color removal by NF and a high rejection of 99.80% total dissolved solids (TDS), 99.90% of chemical oxygen demand (COD) and 99.99% of potassium was achieved from the RO runs, by retaining a significant flux as compared to pure water flux, which shows that membranes were not affected by fouling during wastewater run. The pollutant level in permeates were below the maximum contaminant level as per the guidelines of the World Health Organization and the Central Pollution Control Board specifications for effluent discharge (less than 1,000 ppm of TDS and 500 ppm of COD).     

Impacts of operating conditions on reverse osmosis performance of pretreated olive mill wastewater

Management of the effluent from the olive oil industry is of capital importance nowadays, especially in the Mediterranean countries. Most of the scarce existing studies concerning olive mill wastewater (OMW) treatment by means of membrane processes not only do fix their aims simply on achieving irrigation standards, but lack suitable pretreatments against deleterious fouling issues. With the target of achieving the parametric requirements for public waterways discharge or even for reuse in the production process, a bench-scale study was undertaken to evaluate the feasibility of a thin-film composite reverse osmosis (RO) membrane (polyamide/polysulfone) for the purification of OMW. Previously, OMW was pretreated by means of chemical oxidation based on Fenton’s reagent, flocculation-sedimentation and biosorption through olive stones. Impacts of the main operating parameters on permeate flux and pollutants rejection of the RO process, as well as fouling on the membrane surface, were examined for removing the significant ionic concentration and remaining organic matter load of the pretreated OMW. Combining operating parameters adequately in a semibatch operating regime ensured high and sustainable permeate flux, yielding over 99.4% and 98.5% removal efficiencies for the chemical oxygen demand and ionic content respectively, as well as complete rejection of phenols, iron and suspended solids.     

Removal of natural hormone estrone from secondary effluents using nanofiltration and reverse osmosis

The rejection of steroid hormone estrone by nanofiltration (NF) and reverse osmosis (RO) membranes in treated sewage effluent was investigated. Four NF/RO membranes with different materials and interfacial characteristics were utilized. To better understand hormone removal mechanisms in treated effluent, effluent organic matters (EfOM) were fractionated using column chromatographic method with resins XAD-8, AG MP-50 and IRA-96. The results indicate that the presence of EfOM in feed solution could enhance estrone rejection significantly. Hydrophobic acid (HpoA) organic fraction made a crucial contribution to this “enhancement effect”. Hydrophobic base (HpoB) could also improve estrone rejection while hydrophobic neutral (HpoN) and hydrophilic acid (HpiA) with low aromaticity had little effects. The increment in estrone rejection was predominantly attributed to the binding of estrone by EfOM in feed solutions, which led to an increase in molecular weight and appearance of negative charge (for the HpoA case) and thus an increased level of estrone rejection. However, the improvement of estrone rejection by HpoA decreased with increasing calcium ion concentration. The important conclusion of this study is, first, hydrophobic acid macromolecules are recommended to be added into feed water to improve the rejection of trace hormone during NF/RO membrane process, and, second, removal of calcium ions via pretreatment and application of membrane with more negative charge at its interface can greatly intensify this “enhancement effect”.     

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.     

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.     

Reverse osmosis membrane rejection for ersatz space mission wastewaters

Adequate rejection of a variety of inorganic and organic compounds is necessary if reverse osmosis (RO) and nanofiltration (NF) membranes are to be used for space mission wastewater reuse. Three ersatz space mission wastewaters defined by NASA having different pH (2.6-8.9), conductivities (3980-12,640 microS/cm), and amounts of organic compounds (50-2400 mg/L as carbon) were tested to determine the membrane flux and the solute rejection for five RO and two NF membranes that are commercially available. The results show that the rejection of ions depends upon the solution pH which influences electrostatic repulsion. However, the rejection of dissolved organic carbon (DOC) depends upon the composition of the wastewater. The DOC rejection (80-95%) was the highest for the wastewater containing dextran (molecular weight 15-20 k) compared with the other ersatz wastewaters having detergent and urea as the major carbon sources (31-83%). The wastewater having the greatest conductivity (12,640 microS/cm) and DOC (2400 mg/L) showed a greater flux decline (71-96%) than the other ersatz wastewaters (37-82%) having lower conductivities (3980-6980 microS/cm) and DOC (50-660 mg/L) for the RO and NF membranes. The ratio of solute radius (r(i,s)) to effective membrane pore radius (r(p)) was employed to compare ion rejection. For ionic compounds, the rejection is higher than 70% when the r(i,s)/r(p) ratio is greater than 0.5 for both the RO and NF membranes with all wastewaters.     

Ozone and biofiltration as an alternative to reverse osmosis for removing PPCPs and micropollutants from treated wastewater

This pilot-scale research project investigated and compared the removal of pharmaceuticals and personal care products (PPCPs) and other micropollutants from treated wastewater by ozone/biofiltration and reverse osmosis (RO). The reduction in UV₂₅₄ absorbance as a function of ozone dose correlated well with the reduction in nonbiodegradable dissolved organic carbon and simultaneous production of biodegradable dissolved organic carbon (BDOC). BDOC analyses demonstrated that ozone does not mineralize organics in treated wastewater and that biofiltration can remove the organic oxidation products of ozonation. Biofiltration is recommended for treatment of ozone contactor effluent to minimize the presence of unknown micropollutant oxidation products in the treated water. Ozone/biofiltration and RO were compared on the basis of micropollutant removal efficiency, energy consumption, and waste production. Ozone doses of 4-8 mg/L were nearly as effective as RO for removing micropollutants. When wider environmental impacts such as energy consumption, water recovery, and waste production are considered, ozone/biofiltration may be a more desirable process than RO for removing PPCPs and other trace organics from treated wastewater.     

An autopsy study of a fouled reverse osmosis membrane element used in a brackish water treatment plant

The fouling of a spiral wound reverse osmosis (RO) membrane after nearly 1 year of service in a brackish water treatment plant was investigated using optical and electron microscopic methods, Fourier transform infrared spectroscopy (FTIR) and inductively coupled plasma atomic emission spectrometry (ICP-AES). Both the top surface and the cross-section of the fouled membrane were analysed to monitor the development of the fouling layer. It has been found that the extent of fouling was uneven across the membrane surface with regions underneath or in the vicinity of the strands of the feed spacer being more severely affected. Fouling appeared to have developed through different stages. In particular, it consisted of an initial thin fouling layer of an amorphous matrix with embedded particulate matter. The amorphous matrix comprised organic-Al-P complexes and the particulate matter was mostly aluminium silicates. Subsequently, as the fouling layer reached a thickness of about 5-7microm, further amorphous material, which is suggested to include extracellular polymeric substances such as polysaccharides, started to deposit on top of the existing fouling layer. This secondary amorphous material did not seem to contain any particulate matter nor any inorganic elements within it, but acted as a substrate upon which aluminium silicate crystals grew exclusively in the absence of other foulants, including natural organic matter (NOM).     

N-nitrosamine rejection by reverse osmosis membranes: A full-scale study

This study aims to provide longitudinal and spatial insights to the rejection of N-nitrosamines by reverse osmosis (RO) membranes during sampling campaigns at three full-scale water recycling plants. Samples were collected at all individual filtration stages as well as at a cool and a warm weather period to elucidate the impact of recovery and feed temperature on the rejection of N-nitrosamines. N-nitrosodimethylamine (NDMA) was detected in all RO feed samples varying between 7 and 32 ng/L. Concentrations of most other N-nitrosamines in the feed solutions were determined to be lower than their detection limits (3–5 ng/L) but higher concentrations were detected in the feed after each filtration stage. As a notable exception, in one plant, N-nitrosomorpholine (NMOR) was observed at high concentrations in RO feed (177–475 ng/L) and permeate (34–76 ng/L). Overall rejection of NDMA among the three RO systems varied widely from 4 to 47%. Data presented here suggest that the feed temperature can influence rejection of NDMA. A considerable variation in NDMA rejection across the three RO stages (14–78%) was also observed. Overall NMOR rejections were consistently high ranging from 81 to 84%. On the other hand, overall rejection of N-nitrosodiethylamine (NDEA) varied from negligible to 53%, which was considerably lower than values reported in previous laboratory-scale studies. A comparison between results reported here and the literature indicates that there can be some discrepancy in N-nitrosamine rejection data between laboratory- and full-scale studies probably due to differences in water recoveries and operating conditions (e.g. temperature, membrane fouling, and hydraulic conditions).     

Full-scale simulation of seawater reverse osmosis desalination processes for boron removal: Effect of membrane fouling

The objective of this study is to further develop previously reported mechanistic predictive model that simulates boron removal in full-scale seawater reverse osmosis (RO) desalination processes to take into account the effect of membrane fouling. Decrease of boron removal and reduction in water production rate by membrane fouling due to enhanced concentration polarization were simulated as a decrease in solute mass transfer coefficient in boundary layer on membrane surface. Various design and operating options under fouling condition were examined including single- versus double-pass configurations, different number of RO elements per vessel, use of RO membranes with enhanced boron rejection, and pH adjustment. These options were quantitatively compared by normalizing the performance of the system in terms of E_min, the minimum energy costs per product water. Simulation results suggested that most viable options to enhance boron rejection among those tested in this study include: i) minimizing fouling, ii) exchanging the existing SWRO elements to boron-specific ones, and iii) increasing pH in the second pass. The model developed in this study is expected to help design and optimization of the RO processes to achieve the target boron removal at target water recovery under realistic conditions where membrane fouling occurs during operation.     

A comparative life cycle assessment of hybrid osmotic dilution desalination and established seawater desalination and wastewater reclamation processes

The purpose of this study was to determine the comparative environmental impacts of coupled seawater desalination and water reclamation using a novel hybrid system that consist of an osmotically driven membrane process and established membrane desalination technologies. A comparative life cycle assessment methodology was used to differentiate between a novel hybrid process consisting of forward osmosis (FO) operated in osmotic dilution (ODN) mode and seawater reverse osmosis (SWRO), and two other processes: a stand alone conventional SWRO desalination system, and a combined SWRO and dual barrier impaired water purification system consisting of nanofiltration followed by reverse osmosis. Each process was evaluated using ten baseline impact categories. It was demonstrated that from a life cycle perspective two hurdles exist to further development of the ODN-SWRO process: module design of FO membranes and cleaning intensity of the FO membranes. System optimization analysis revealed that doubling FO membrane packing density, tripling FO membrane permeability, and optimizing system operation, all of which are technically feasible at the time of this publication, could reduce the environmental impact of the hybrid ODN-SWRO process compared to SWRO by more than 25%; yet, novel hybrid nanofiltration-RO treatment of seawater and wastewater can achieve almost similar levels of environmental impact.     

Development of antifouling reverse osmosis membranes for water treatment: A review

With the rapidly increasing demands on water resources, fresh water shortage has become an important issue affecting the economic and social development in many countries. As one of the main technologies for producing fresh water from saline water and other wastewater sources, reverse osmosis (RO) has been widely used so far. However, a major challenge facing widespread application of RO technology is membrane fouling, which results in reduced production capacity and increased operation costs. Therefore, many researches have been focused on enhancing the RO membrane resistance to fouling. This paper presents a review of developing antifouling RO membranes in recent years, including the selection of new starting monomers, improvement of interfacial polymerization process, surface modification of conventional RO membrane by physical and chemical methods as well as the hybrid organic/inorganic RO membrane. The review of research progress in this article may provide an insight for the development of antifouling RO membranes and extend the applications of RO technology in water treatment in the future.