Nanofiltration Reverse Osmosis Research Articles

A systematic approach towards optimization of brackish groundwater treatment using nanofiltration (NF) and reverse osmosis (RO) hybrid membrane filtration system.

This research compares the performance efficiencies of Nanofiltration (NF), Reverse osmosis (RO), and Nanofiltration-Reverse Osmosis (NF-RO) hybrid membrane filtration systems, for treatment of brackish groundwater in Delhi-NCR region. Central composite design (CCD) of response surface methodology (RSM) were applied to formulate predictive models for the optimization and simulation of various responses, viz. Water flux, salt rejection, permeate recovery and specific energy consumption (SEC). Three different input factors (feed concentration, pH and pressure) were evaluated. Significance of RSM model was tested and validated with use of an analysis of variance (ANOVA). The models were also compared graphically for their prediction ability. The optimum conditions were determined by numerical optimization of the NF and RO membrane pilot plants. Finally, hybrid configurations of NF and RO were investigated utilising the optimum conditions to identify the technology suitable for the remediation of brackish groundwater. Despite, RO membrane eliminating over 99% of TDS from groundwater, the lower recovery rate renders it environmentally unfavourable. Result indicates that recovery of the hybrid system (40.35%) was greater than that of RO alone (18.796%) and SEC also decreases (5.090-3.8kWH/m3). Hence, combining a hybrid membrane arrangement with NF-RO to treat brackish groundwater for improved recovery and lower SEC is a viable alternative.

Design and Testing of a Separation and Desalination Device for Farmland Saline-Alkaline Water in Arid Areas.

To solve the problem of soil salination and to desalinate saline–alkaline water in arid areas, this study involved the design and testing of a separation and desalination device for farmland saline–alkaline water that is suitable for arid areas. The results of this study indicate that after the pretreatment of farmland saline–alkaline water, the water yielded by the pretreatment device had a mean turbidity of <1 and a mean silt density index (SDI) of <3, which met the working conditions of nanofiltration (NF) and reverse osmosis (RO) membranes. When used to filter saline–alkaline water, the composite NF–RO membrane system achieved a desalination rate of 97.06%, a total hardness removal rate of 97.83%, and a Cl− removal rate of 99.65%, which satisfied the standard for irrigation water quality. Some indicators of the yielded water reached the hygienic standard for drinking water, thus successfully reutilizing water resources. The circulating solar collector tube of the device was designed with a collection area of 6 m2, which could basically satisfy the heat demand of the flash tank for distillation. The design of the flash tank and the shell-and-tube circulating condenser met the requirements for vapor condensation. The crystals in the solar salt box precipitated under solar action. X-ray diffraction was used to identify the primary compound of the crystals as NaCl, suggesting that the precipitates have potential value as industrial salts. This study offers new technical references and helpful engineering guidance for arid saline–alkaline enrichment areas facing the problem of saline farmland irrigation water.

Life Cycle Assessment of Hybrid Nanofiltration Desalination Plants in the Persian Gulf.

Although emerging desalination technologies such as hybrid technologies are required to tackle water scarcity, the impacts of their application on the environment, resources, and human health, as prominent pillars of sustainability, should be evaluated in parallel. In the present study, the environmental footprint of five desalination plants, including multi-stage flash (MSF), hybrid reverse osmosis (RO)–MSF, hybrid nanofiltration (NF)–MSF, RO, and hybrid NF–RO, in the Persian Gulf region, have been analyzed using life cycle assessment (LCA) as an effective tool for policy making and opting sustainable technologies. The comparison was based on the impacts on climate change, ozone depletion, fossil depletion, human toxicity, and marine eutrophication. The LCA results revealed the superiority of the hybrid NF–RO plant in having the lowest environmental impact, although the RO process produces more desalinated water at the same feed and input flow rates. The hybrid NF–RO system achieves 1.74 kg CO2 equivalent, 1.24 × 10−7 kg CFC-11 equivalent, 1.28 × 10−4 kg nitrogenous compounds, 0.16 kg 1,4-DB equivalent, and 0.56 kg oil equivalent in the mentioned impact indicators, which are 7.9 to 22.2% lower than the single-pass RO case. Furthermore, the sensitivity analysis showed the reliability of the results, which helps to provide an insight into the life cycle impacts of the desalination plants.

Renewable energy powered membrane technology: Impact of osmotic backwash on organic fouling during solar irradiance fluctuation

Organic fouling is a major issue of nanofiltration/reverse osmosis (NF/RO) membranes because it causes severe and usually irreversible flux decline. Consequently, NF/RO requires frequent chemical cleaning and/or replacement. In directly coupled photovoltaic (PV) powered NF/RO systems for brackish water desalination, solar energy fluctuations can induce a spontaneous osmotic backwash (OB) process, which may mitigate organic fouling. In this study, the feasibility of spontaneous OB as a self-cleaning method for organic fouling (humic acid with calcium) control under fluctuating conditions was evaluated with an online total organic carbon (TOC) analyser. The OB cleaning mechanisms with different solar energy fluctuating conditions, relevant feedwater chemistry (salinity, pH, and hardness) and membrane type (NF270 and BW30) were investigated. The visualisation of fouling layers by a helium ion microscope (HIM), a cryo scanning electron microscope equipped with a focused ion beam (FIB-SEM) and energy dispersive X-Ray spectroscopy (EDS) showed that partial foulants were removed by spontaneous OB, and the remaining layer was not uniform. Periodic solar irradiance step-response experiments’ results showed that spontaneous OB cleaning restored 46%–98% flux, depending on solar irradiance fluctuating conditions, feedwater chemistry, and membrane type. Specifically, low-level solar irradiance (i.e., more cloud coverage) and short operating time (i.e., high cleaning frequency) showed high OB cleaning efficiency. High salinity was found to enhance the OB performance but it was still ineffective for the enhanced organic fouling layer. Extremely pH (e.g. pH 2 or pH 12) weakened OB efficiency via a compact fouling layer/mixed fouling layer structures (calcite participation) in presence of calcium. Although spontaneous OB cleaning is promising to mitigate organic fouling, it is ineffective for severe organic fouling (e.g. 12.5 mM humic acid with 2.5 mM CaCl2). This work provides the contribution of process and water quality variables that make spontaneous OB effective for fouling control, which facilitates the further development and application of the OB technique in decentralised PV-NF/RO systems.

Nanofiltration-reverse osmosis hybrid process for hardness removal in brackish water with higher recovery rate and minimization of brine discharges

In this paper, the feasibility of hybrid nanofiltration-reverse osmosis (NF-RO) process to remove hardness in brackish water at higher recovery rate and concentrate minimization is investigated. Two nanofiltration (NF) membranes are tested in the first stage for selecting the appropriate membrane on the basis of lower applied pressure and higher permeate flow. Then the evaluation of the performances of hybrid NF-RO system of each stage is studied in terms of recovery rate and Langelier saturation index (LSI) values to prevent the corrosion potential. The blending strategy is adopted with NF brine and reverse osmosis (RO) permeate in order to bring the RO permeate characteristics in accordance with drinking water standards and to minimize the volume of NF brine. Also, energy consumption performance of conventional nanofiltration (NF-NF) and NF-RO hybrid system in hardness removal is compared. The results show that the NF-RO hybrid system combined to blending strategy provides the simplest process but most energy efficient process compared to a single conventional NF-NF. Moreover, including NF upstream of RO is responsible of the increasing of the overall water recovery rate (95 %) and of the permeate quality with lower total dissolved solids (TDS). However, less water with lower salinity treated by RO increases significantly the recovery rate in RO stage up to 80 %. The portion rate of NF brine blending with RO permeate is equal to 32.45 %, corresponding to 240L/h, which meet the water quality regulations for drinking water. Hybrid NF-RO system has a lower energy cost per m3 than NF-NF, it is around of 0.055 $/m3. At the same time, this procedure minimizes the fraction of brine disposal dumping to sewage, making it environmentally friendly.

Degradations of endocrine-disrupting chemicals and pharmaceutical compounds in wastewater with carbon-based nanomaterials: a critical review

Although water occupies 75% of the earth's surface, only 0.0067% of the total water is available for human activities. These statistics further decline with the population growth and consequent multiplication in the amount of annual waste produced. The demand for clean and safe drinking water has always been a prime concern in the global scenario. Among various types of waste materials, endocrine-disrupting chemicals (EDCs) and pharmaceutical effluents have become a constant threat to the aquatic ecosystem and possess challenges worldwide. Endocrine-disrupting chemicals (EDCs) are a mixed group of emerging concern chemicals with the ability to mimic the mechanisms of biosynthesis, transport, and metabolism of hormones. These chemicals pose various health threats such as early puberty, infertility, obesity, diabetes, reproductive disorders, cancerous tumors, and related disorders (immune cells, hormones' activity, and various organs). On the other hand, pharmaceutical compounds such as antibiotics also harm the natural environment, human health, and soil microbiology. Their low concentration, ranging from a few ng/L to μg/L, gives rise to a micro-pollution phenomenon, which makes it difficult to detect, analyze, and degrade in wastewater treatment plants. Activated carbons (ACs) and other adsorbents, including naturally occurring materials (wood, keratin) are considered as nanomaterials (NMs) reference for the separation of organic pollutants. It is generally acknowledged that mass-transfer phenomena control sorption kinetics at the liquid/solid interface, with retention controlled by the sorbent/sorbate properties. Therefore, the type of interaction (strong or weak van der Waals forces) and the hydrophilic/hydrophobic properties of the adsorbent are two crucial factors. Besides, EDCs and pharmaceutical compound sorption on such kinds of nanoporous solids depend on both the molecule size and charge density. The applications of nanomaterials on non-conservative methods, like advanced oxidation processes or AOPs (e.g., photocatalysis and Fenton reaction), are contemplated as more apt in comparison to conservative technology like reverse osmosis nanofiltration, and adsorption, etc. One of the reasons is that AOPs generate free radicals (hydroxyls), which are strong oxidants for the demineralization of organic compounds and the extreme case that hydroxyl radicals can attack any kinds of pollutants with the generation of only water and carbon dioxide as final products. AOPs may imply the use of NMs as either catalysts or photocatalysts, which improve the selective removal of the target pollutant. Therefore, various literature reviews have revealed that there is a timely need to upgrade the efficiency of the remediation approaches to protect the environment against EDCs and pharmaceuticals adequately. There is currently a lack of definitive risk assessment tools due to their complicated detection and associated insufficiency in the health risk database. Hence, our present review focuses on applying carbon-based nanomaterials to remove EDCs and pharmaceuticals from aqueous systems. The paper covers the effect of these pollutants and photocatalytic methods for treating these compounds in wastewater, along with their limitations and challenges, plausible solutions, and prospects of such techniques.

Removal of Naturally Occurring Strontium by Nanofiltration/Reverse Osmosis from Groundwater.

Removal of naturally occurring strontium (Sr) from groundwater is vital as excessive exposure may lead to bone growth problems in children. Nanofiltration/reverse osmosis (NF/RO) is commonly used in groundwater treatment due to the high effectiveness and simple maintenance of these pressure driven membrane processes. In this research, a pilot-scale NF/RO system was used to desalinate a natural groundwater sample containing high Sr concentration (10.3 mg/L) and “old” groundwater organic matter (70.9 mg/L) from Esilalei in northern Tanzania to understand the removal of strontium by NF/RO. The impact of applied pressure (10–15 bar) and groundwater pH (3–12) on the membrane performance including permeate flux, strontium and total organic carbon (TOC) flux and removal was investigated. Increasing applied pressure was found to enhance the flux by increasing the driving force and enhance Sr and TOC removal by dilution effect (water flux higher than Sr passage). The alkaline pH caused severe flux decline likely due to membrane fouling and scaling, while it slightly enhanced Sr removal of RO membranes, but weakened the TOC removal. In contrast, acidic and neutral pH of groundwater enhanced TOC removal. These findings suggest that appropriately high applied pressure and acidic pH condition of groundwater are recommended to apply to the NF/RO membrane system in groundwater desalination to achieve better membrane performance.

Renewable energy powered membrane technology: System resilience under solar irradiance fluctuations during the treatment of fluoride-rich natural waters by different nanofiltration/reverse osmosis membranes

Directly-powered photovoltaic membrane filtration (PV-membrane) systems possess no energy storage components, hence operate under energy fluctuations that result in variations in the membrane performance. Here, the resilience of a PV-membrane system under such conditions as a function of membrane type and feedwater quality is investigated. Five Tanzanian natural waters characterised by high fluoride (F) concentrations were treated using four nanofiltration/reverse osmosis (NF/RO) membranes. The key performance indicators—including flux, permeate electro-conductivity (EC) and specific energy consumption (SEC)—were instantaneously monitored throughout a solar day that exhibited 1-h of extreme fluctuations. The resilience indicators were resilience factor (RF) and recovery time. The PV-membrane system demonstrated resilience—defined in terms of both permeate quantity and quality—that was more dependent on membrane type. Tighter NF/RO membranes showed high resilience to variations in permeate quality, but poorer resilience to SEC variations, due to lower flux compared to the looser NF membrane. System resilience exhibited a recovery time range of 50–350 s which was determined by the duration of the fluctuation. Monitoring the permeate quality in terms of F concentration by using EC as a substitute for F through a linear correlation, showed that EC is a good indicator for F in these waters.

Modern trends in the mathematical simulation of pressure-driven membrane processes

The presented article is an attempt to evaluate the progress in the development of the mathematical simulation of the pressure-driven membrane processes. It was considered more than 170 articles devoted to the simulation of reverse osmosis, nanofiltration, ultrafiltration, and microfiltration and the others published between 2000 and 2010 years. Besides the conventional approaches, which include the irreversible thermodynamics, diffusion and pore flow (and models which consider the membrane surface charge for nanofiltration process), the application of the methods the computational fluid dynamics, artificial neural networks, optimization, and economic analysis have been considered. The main trends in this field have been pointed out, and the areas of using approaches under consideration have been determined. The technological problems which have been solved using the mentioned approaches have also been considered. Although the question of the concentration polarization has not been considered separately, it was defined that, in many cases, the sufficiently accurate model cannot be designed without considering this phenomenon. The findings allow evaluating more thoroughly the development of the simulation of pressure-driven membrane processes. Moreover, the review allows choosing the strategy of the simulation of the considered processes. Keywords: membrane, simulation, model, reverse osmosis nanofiltration, ultrafiltration, microfiltration.

Renewable energy powered membrane technology: Impact of solar irradiance fluctuation on direct osmotic backwash

Direct osmotic backwash (OB) during solar energy fluctuations is an important observation in directly coupled, battery-less solar energy powered nanofiltration/reverse osmosis (NF/RO) membrane systems. OB occurs spontaneously and has the potential to induce membrane self-cleaning. The impact of controlled solar irradiance fluctuation conditions, feed water salinity (1–10 g/L) and membrane type (NF membrane NF270, RO membrane BW30) on OB were investigated by periodic step-response tests. The objective of this research was to investigate the OB mechanism under solar irradiance fluctuations. The results show that solar energy fluctuations lead to a change of filtration hydrodynamic conditions (feed flow rate, applied pressure), operating time, and permissible backwash time. This results in variations of OB driving force which is determined by the osmotic pressure difference during filtration. High feed water salinity and solar irradiance before fluctuation enhance the OB process, while relative high solar irradiance during fluctuation weakens the OB process. Furthermore, the BW30 membrane shows a higher OB flow rate and less accumulated volume than NF270. This was attributed to the higher salt retention of BW30 resulting in a larger backwash driving force and much lower flux of BW30 which in turn caused a thinner CP layer. These findings indicate that solar irradiance fluctuation conditions may potentially delay fouling and hence enhance the reliability and robustness of battery-less photovoltaic-membrane systems.

Physical and Chemical Cleaning for Nanofiltration/Reverse Osmosis (NF/RO) Membranes in Treatment of Tertiary Palm Oil Mill Effluent (POME) for Water Reclamation

Physical and Chemical Cleaning for Nanofiltration/Reverse Osmosis (NF/RO) Membranes in Treatment of Tertiary Palm Oil Mill Effluent (POME) for Water Reclamation

A Preliminary Study on Application of MBR + NF/RO (Membrane Bio-Reactor + Nanofiltration/Reverse Osmosis) Combination Process for Landfill Leachate Treatment in China

Landfill leachate involves high concentrations of organic compounds, ammonia nitrogen, heavy metals and other complex components. Hence, it is very difficult to treat using conventional biological processes alone. This research is a preliminary literature review on using MBR + NF/RO combination process to achieve purification of landfill leachate in China. MBR + NF/RO process is a combination of biological and physico-chemical technology, including MBR system, nanofiltration and reverse osmosis devices. The study found that, the benefit of a complementary combination of MBR and NF/RO process with respect to removal of ammonia nitrogen, trace organic and heavy metal ions appears quite intuitive. And this promising treatment has developed rapidly in China in last few decades. In this paper, representative project examples in China were selected for review. Mechanism and influencing factors of MBR + NF/RO process in China in the past decades will be reviewed and recommendations will be given for the future development of Chinese MBR + NF/RO process.

Seasonal variation of organic matter characteristics and fluoride concentration in the Maji ya Chai River (Tanzania): Impact on treatability by nanofiltration/reverse osmosis

The Maji ya Chai River in Northern Tanzania, a fluoride-rich tropical area, shows a seasonal variation of natural organic matter (NOM) and fluoride concentration. Water samples collected monthly during one year from two locations of the River were characterized. High levels of precipitation in the rainy seasons increased the total organic carbon (TOC) concentration to as high as 36 mgC L−1 and diluted the fluoride concentration from a dry season high of 24 mg L−1 to <4 mg L−1. A black water swamp in the Maji ya Chai River catchment was confirmed as the main source of NOM, fluoride, salinity, and inorganic carbon entering the River in the rainy season.The water samples were filtered by a number of nanofiltration/reverse osmosis (NF/RO) membranes to identify the retention mechanisms and the impact of varying water quality on treatability. While the denser membranes removed fluoride due to size exclusion, for the membranes with bigger pore radius charge repulsion was the dominant mechanism of fluoride retention. Regardless of the seasonal conditions a TOC concentration <2 mgC L−1 was achieved by all membranes at 50% recovery, as NF/RO membranes remove TOC mainly by size exclusion.Two swamp water samples, containing high TOC (79 and 183 mgC L−1), were filtered to determine the characteristics of NOM which permeate the NF/RO membranes. Liquid chromatography organic carbon detection (LC-OCD) was used to characterize the fractions in the permeates, consisting of about 1% of the original NOM. The average molecular weight of the permeate humic substances (HS) was more than four times larger than the membrane molecular weight cut-off. This suggests that large HS can permeate the NF/RO membranes through diffusion. Moreover, the relatively high aromaticity of the permeate HS (1.7–5.2 L mg−1 m−1) indicated the high content of hydrophobic-aromatic fractions.

Hybrid polystyrene nanoparticle-ultrafiltration system for hormone removal from water

Occurrence of hormones in water resources even at low concentrations of ng/L is a potential risk for both environmental and public health. Hybrid sorbent-ultrafiltration (UF) systems are among the technologies under investigation for their potential as a sustainable and energy-efficient process for the removal of hormones from water. In this study polystyrene (PS) nanoparticles were explored as sorbent in a hybrid system. Estrone adsorption capacity of 52nm PS nanoparticles was found to be 79.6ng/g at equilibrium estrone concentration of 5.9ng/L. The performance of the hybrid PS nanoparticle-UF system was studied in terms of adsorption and membrane permeability under varying solution pH, particle size and particle concentration. The results indicated that neutral pH range is optimal for operation of the system and estrone removal with nanoparticles above 465nm is negligible. The highest estrone removal (40%) was achieved by the hybrid system using a 100kDa UF membrane and 84mg/L PS (52nm) nanoparticle concentration. The capacity of the system to remove estrone was found to be lower than most nanofiltration/reverse osmosis (NF/RO) systems but with a final permeability of 75L/m2hbar, at least five times higher than most of the NF/RO systems.

Molecular sieving effects of disk-shaped molecules on reverse osmosis and nanofiltration separation

Abstract The solute separation in reverse osmosis/nanofiltration (RO/NF) membrane processes is mainly controlled by both the diffusivity in a pore and the steric partition factor, and the latter is defined by the geometrical probability at which a solute can be accessible into a pore. The rejection of alcohols has been estimated semi-empirically by using the steric partition factor derived by the approximation of molecular shape as a rectangular parallelepiped. However, the approach was not suitable for crown ethers, and in this work the shape of crown ethers was approximated as a disk: the disk radius and disk thickness were developed as new shape parameters. The calculated rejections of crown ethers by using the disk-shaped model corresponded well to the observed rejections, where the used pore radius was calculated on the basis of the rejections of alcohols. Boric acid in non-dissociated form is also assumed to be a disk-shaped molecule, but the calculated rejection by the disk-shaped model was smaller than the observed one. The results suggest that boric acid transports through membrane pores with hydrated water molecules.

Renewable energy powered membrane technology: Case study of St. Dorcas borehole in Tanzania demonstrating fluoride removal via nanofiltration/reverse osmosis

Abstract A brackish borehole located at a rural school in northern Tanzania contains excessive salinity (TDS = 3632 mg/L) and fluoride concentration (F − = 47.6 mg/L). This field study evaluated the feasibility of a solar-powered nanofiltration/reverse osmosis (NF/RO) system in treating this brackish water to make it potable. Key performance parameters such as permeate productivity, fluoride retention and specific energy consumption (SEC) were compared among four different NF/RO membrane modules (BW30, BW30-LE, NF90, and NF270). The NF90 membrane exhibited the best overall performance, balancing permeate productivity and quality, with 1582 L of clean drinking water being produced with an average SEC of 1.6 kWh/m 3 over a solar day, while the NF270 could not meet the guideline values for fluoride. Further, the impact of fluctuating solar energy on system performance was studied. Very brief periods of heavy cloud cover in the afternoon affected transmembrane pressure and feed flow, and thus caused the permeate quality to temporarily exceed the guideline values. However, when considering that the permeate was stored in a product tank and was produced over the whole solar day, the system equipped with the NF90 membrane was able to produce high-quality drinking water that, on average, easily meets the guideline values despite the energy fluctuations.

Early non-destructive biofouling detection and spatial distribution: Application of oxygen sensing optodes

Biofouling is a serious problem in reverse osmosis/nanofiltration (RO/NF) applications, reducing membrane performance. Early detection of biofouling plays an essential role in an adequate anti-biofouling strategy. Presently, fouling of membrane filtration systems is mainly determined by measuring changes in pressure drop, which is not exclusively linked to biofouling. Non-destructive imaging of oxygen concentrations (i) is specific for biological activity of biofilms and (ii) may enable earlier detection of biofilm accumulation than pressure drop.The objective of this study was to test whether transparent luminescent planar O2 optodes, in combination with a simple imaging system, can be used for early non-destructive biofouling detection.This biofouling detection is done by mapping the two-dimensional distribution of O2 concentrations and O2 decrease rates inside a membrane fouling simulator (MFS).Results show that at an early stage, biofouling development was detected by the oxygen sensing optodes while no significant increase in pressure drop was yet observed. Additionally, optodes could detect spatial heterogeneities in biofouling distribution at a micro scale. Biofilm development started mainly at the feed spacer crossings.The spatial and quantitative information on biological activity will lead to better understanding of the biofouling processes, contributing to the development of more effective biofouling control strategies.

Pressure-driven membrane and ion exchange hybrid system for the treatment of wastewater containing sulfite and nitrite

Denitrification of flue gas can generate a large amount of wastewater. Depending on the denitrification technology used, the water could contain a fair amount of sulfite and nitrite that need to be removed for water reuse or discharge. An integrated pressure-driven membrane and ion exchange (IX) process is therefore designed and tested for this purpose in this work. The process contains a combination of nanofiltration (NF), reverse osmosis (RO), and a mixed bed IX. NF was performed on a DL1812 membrane module at 2.0 MPa using a simulated wastewater containing 63.0 g/L Na2SO3 and 49.5 g/L NaNO2. Sulfite was separated from nitrite with a retention rate of 92.0%, and its overall recovery efficiency achieved 91.09%. The recovery rate of nitrite in the permeate of NF was 71.06%, which was further elevated by RO using a SG1812 membrane module. A four-pass RO process was shown to be capable of recovering 98.15% of NaNO2 in the final retention. Finally, it was demonstrated that followed by IX treatment, sulfite and nitrite were successfully removed to meet the water surface discharge criteria, proving the feasibility of the hybrid NF–RO–IX system as a viable alternative for treating and reusing denitrification wastewater.

The application of microfiltration-reverse osmosis/nanofiltration to trace organics removal for municipal wastewater reuse

The fate of organic micropollutans (MPs) in a membrane system based on microfiltration (MF) and reverse osmosis/nanofiltration (RO/NF) has been investigated for the case of wastewater reuse. Both an operating full-scale water reuse plant and a pilot plant were employed, with 22 individual organic compounds at their ambient concentrations studied for the former and the latter employing two target compounds over a range of feed concentrations. Results revealed removal efficiencies higher than 75% for most compounds in the full-scale plant, though mass flow studies on all streams revealed a significant imbalance of material for some compounds. Rejection efficiencies measured for candidate commercial NF and RO membranes tested at pilot scale challenged with a pharmaceutically active compound (ibuprofen, IBU) and an endocrine-disrupting chemical (nonylphenol, NP) exceeded 99%. Permeate concentrations were 0.005–0.14 μg/L for IBU and below the limit of detection for NP. A mass balance of the MPs for the full-scale plant across the MF and RO stages revealed a significant imbalance associated with the challenge of accurate determination of low concentrations. Differences in pilot plant and full-scale data were otherwise attributed to the impact of membrane ageing (and specifically hydrolysis) on RO rejection of the MPs examined.

The cost and performance of an MF-RO/NF plant for trace metal removal

The performance of a membrane system based on microfiltration (MF) and reverse osmosis/nanofiltration (RO/NF) has been examined with reference to the removal of trace metals from classically-treated municipal wastewater, and associated costs determined. Metals fate was assessed both at full and pilot scale. Metals rejection was found to be 94.5–99% on average; permeate metal concentrations were between 0.01 and 0.7μg/L for Cu and Ni and 0.7–5.7μg/L for Zn, and largely unaffected both by feed concentration and membrane type. The operational expenditure (OPEX) was calculated from information from the full-scale plant, primarily comprised energy demand (47%) and membrane replacement (37%) and was largely independent of plant size for flow rates from 1 to 100megalitres per day (MLD). Results confirmed the membrane-based process to reliably remove metals down to levels below 6μg/L, but only at a cost (~18p/m3 OPEX) which makes the process untenable for this duty exclusively. Moreover, a further cost would be incurred from the management of the concentrate stream. However, the reliably high levels of removal permit possibility of employing RO or NF technology for treating only a fraction of the wastewater and blending with the untreated stream, depending on the required discharged wastewater quality.