Viability of a low-pressure nanofilter in treating recycled water for water reuse applications: a pilot-scale study

The purpose of this study was to investigate the potential of a low-pressure nanofiltration (NF) membrane for treating recycled water for indirect potable water reuse applications. In particular, the tradeoffs in choosing low-pressure NF over reverse osmosis (RO) were investigated including whether or not significantly lowering operating pressures/costs would result in diminished permeate water quality. A NF membrane (Dow/Filmtec NF-4040) with high permeate productivity was selected for pilot-scale testing over a period of 1200h at a water reuse facility employing conventional RO membranes for treating tertiary treated wastewater effluent prior to aquifer recharge. The novel application of an NF membrane in treating wastewater effluent for water reuse applications permitted a comprehensive screening of NF permeate water quality and allowed for the investigation of trace organic contaminant rejection on pilot scale with environmentally relevant feed water concentrations. Results from pilot-scale testing highlighted the selectivity of NF membranes in removing organic solutes present in wastewater effluents at the parts-per-trillion level. While operating pressures were by a factor of 2-3 lower than conventional RO membranes, and bulk and trace organic rejection generally exceeded 90 percent, not surprisingly, the rejection of monovalent ions such as nitrate was poor. The poor-to-moderate rejection of monovalent ions, however, resulted in lowered brine stream total dissolved solids concentration and sodium adsorption ratio as compared with the brine stream of conventional RO membranes, which may be beneficial for brine disposal strategies.     

Transfer of volatile substances from water to the atmosphere

Mass transfer coefficients and the degree of saturation of the exit gas are important parameters in transferring volatile organic substances from water to the atmosphere. Aeration systems in batch and continuous flow reactors are discussed based on theoretical analysis and experimental data. Three different ranges of the degree of saturation of the exit gas are introduced and their significance for model experiments and technical operations is evaluated. The effect of the aeration system and of surface active agents on the mass transfer coefficient of tetrachloroethylene is studied. Mass transfer coefficients for six nonpolar volatile organic compounds are presented. The stripping efficiencies of different types of gas-liquid contact devices used in water and waste water treatment are assessed.     

A QSAR model for predicting rejection of emerging contaminants (pharmaceuticals, endocrine disruptors) by nanofiltration membranes

A quantitative structure activity relationship (QSAR) model has been produced for predicting rejection of emerging contaminants (pharmaceuticals, endocrine disruptors, pesticides and other organic compounds) by polyamide nanofiltration (NF) membranes. Principal component analysis, partial least square regression and multiple linear regressions were used to find a general QSAR equation that combines interactions between membrane characteristics, filtration operating conditions and compound properties for predicting rejection. Membrane characteristics related to hydrophobicity (contact angle), salt rejection, and surface charge (zeta potential); compound properties describing hydrophobicity (log K_ow, log D), polarity (dipole moment), and size (molar volume, molecular length, molecular depth, equivalent width, molecular weight); and operating conditions namely flux, pressure, cross flow velocity, back diffusion mass transfer coefficient, hydrodynamic ratio (J_o/k), and recovery were identified as candidate variables for rejection prediction. An experimental database produced by the authors that accounts for 106 rejection cases of emerging contaminants by NF membranes as result of eight experiments with clean and fouled membranes (NF-90, NF-200) was used to produce the QSAR model. Subsequently, using the QSAR model, rejection predictions were made for external experimental databases. Actual rejections were compared against predicted rejections and acceptable R² correlation coefficients were found (0.75 and 0.84) for the best models. Additionally, leave-one-out cross-validation of the models achieved a Q² of 0.72 for internal validation. In conclusion, a unified general QSAR equation was able to predict rejections of emerging contaminants during nanofiltration; moreover the present approach is a basis to continue investigation using multivariate analysis techniques for understanding membrane rejection of organic compounds.     

热-水力-力学-传质耦合过程模型及工程土障数值模拟

提出了一个非饱和多孔介质中的热 -水力 -力学 -传质耦合过程模型。利用文献 [1]中提出的热 -水力 -力学本构模型 ,对工程土障系统进行数值模拟。数值模拟结果显示了环境条件对废物场中的热量产生和传输过程对工程土障中水力 -力学行为的影响以及可能在工程土障中产生塑性应变 ,导致土障破坏。污染物浓度分布的数值结果显示了温度与降水过程对污染物随地下水向土障周边自然环境渗透的倾向。数学模型及所发展的有限元求解过程可以为评估土障系统长期工作的有效性和土障系统的合理设计提供有力的工具     

Cost factors and chemical pretreatment effects in the membrane filtration of waters containing natural organic matter

This paper compares the membrane processes available for water treatment. Membranes have the advantage of currently decreasing capital cost, a relatively small footprint compared to conventional treatment, generally a reduction in chemicals usage and comparably low maintenance requirements. Three membrane processes applicable to water treatment, micro- (MF), ultra- (UF), and nanofiltration (NF), are compared in terms of intrinsic rejection, variation of rejection due to membrane fouling and increase in rejection by ferric chloride pretreatment. Twelve different membranes are compared on the basis of their membrane pore size which was calculated from their molecular weight cut-off. A pore size of < 6 nm is required to achieve substantial (> 50%) organics removal. For a fouled membrane this pore size is about 11 nm. UV rejection is higher than DOC rejection. Coagulation pretreatment allows a higher rejection of organics by MF and UF and the cut-off criterion due to initial membrane pore size is no longer valid. A water quality parameter (WQP) is introduced which describes the product water quality achieved as a function of colloid, DOC and cation rejection. The relationship between log (pore size) and WQP is linear. Estimation of membrane costs as a function of WQP suggests that open UF is superior to MF (similar cost at higher WQP) and NF is superior to tight UF. Chemical pretreatment could compensate for the difference between MF and UF. However, when considering chemicals and energy costs, it appears that a process operated at a higher energy is cheaper at a guaranteed product quality (less dependent on organic type). This argument is further supported by environmental issues of chemicals usage, as energy may be provided from renewable sources.     

Subsurface multiphase flow of organic contaminants: Model development and validation

A numerical model has been developed to predict the migration of organic contaminants in the subsurface. The formulations take a multiphase approach in describing the flow of organic contaminants in saturated and unsaturated porous media. In a three-phase fluid system of contaminant, gas, and water, simultaneous flow of the water and the contaminant phases is formulated by applying mass conservation principles to each of the phases under the condition of no interphase mass exchange. For each phase, the formulations incorporate the spatial variability of the relative permeability and its direction dependency. The complex formulations are solved numerically using an implicit finite difference scheme. The accuracy of the numerical model is determined against experimental data from the literature. The predicted migration pattern in both homogeneous and stratified media agrees well with the experimental data. Numerical simulations illustrate the strong effects of the medium permeability and the water distribution profiles on the flow pattern and the pressure distribution of the contaminant.     

Partitioning of dissolved chlorinated ethenes into vegetable oil

Food-grade soybean oil (SoyOil) has been used to enhance in situ anaerobic bioremediation at sites contaminated with chlorinated ethenes (CEs). The abiotic interactions of SoyOil with the CEs may be significant and need to be better understood. The oil: water partition coefficients (Kp) of dissolved CEs into SoyOil were measured in batch tests and ranged from 22 to 1200 with increasing chlorination. CE mixtures significantly reduced the Kp for tetrachloroethene (PCE), but not the other CEs. Simple flow tests were used to quantify the mass transfer coefficients (kL) of dissolved CEs into SoyOil. Higher kL values corresponded to the CEs with higher diffusivity in water. CE mixtures reduced the kL for all of the CEs. The results can be used to predict abiotic interactions and distribution of contaminant mass expected after SoyOil injection, and thus provide a more accurate estimate of the mass of CEs removed due to enhanced biodegradation.     

Performance of nanofiltration for arsenic removal

Performance of rapid sand filtration inter-chlorination system was compared with nanofiltration (NF) to reduce the arsenic health risk of drinking water. It was found that rapid sand filtration with inter-chlorination is not effective in removing arsenic. If total arsenic concentration in raw water is below 50 microg/L regardless of the turbidity of raw water, arsenic can be removed below WHO guideline value of 10 microg/L by conventional coagulation (polyaluminum chloride dosage is about 1.5 mg Al/L). However, if the raw water arsenic concentration exceeds 50 microg/L, more coagulant dosage or enhanced coagulation is needed. To adopt optimum coagulant dosage for arsenic removal, it needs to monitor raw water arsenic concentration, but it is difficult because arsenic measurement is time consuming. In addition, if raw water contains As(III), it is difficult for rapid sand filtration inter-chlorination system to meet an arsenic maximum contaminant level of 2 microg/L, which would achieve reduction of cancer risk below 10(-4). On the other hand, the NF membrane (NaCl rejection 99.6%) could remove over 95% of As(V) under relatively low-applied pressure (< 1.1 MPa). Furthermore, more than 75% of As(III) could be removed using this membrane without any chemical additives, while trivalent arsenic could not be removed by rapid sand filtration system without pre-oxidation of As(III) to As(V). Because both As(V) and As(III) removals by NF membranes were not affected by source water composition, it is suggested that NF membrane can be used in any types of waters.     

Solute transport model for trace organic neutral and charged compounds through nanofiltration and reverse osmosis membranes

Rejection of trace organic compounds, including disinfection by-products (DBPs) and pharmaceutical active compounds (PhACs), by high-pressure membranes has become a focus of public interest internationally in both drinking water treatment and wastewater reclamation/reuse. The ability to simulate, or even predict, the rejection of these compounds by high-pressure membranes, encompassing nanofiltration (NF) and reverse osmosis (RO), will improve process economics and expand membrane applications. The objective of this research is to develop a membrane transport model to account for diffusive and convective contributions to solute transport and rejection. After completion of cross-flow tests and diffusion cell tests with target compounds, modeling efforts were performed in accordance with a non-equilibrium thermodynamic transport equation. Comparing the percentages of convection and diffusion contributions to transport, convection is dominant for most compounds, but diffusion is important for more hydrophobic non-polar compounds. Convection is also more dominant for looser membranes (i.e., NF). In addition, higher initial compound concentrations and greater J(0)/k ratios contribute to solute fluxes more dominated by convection. Given the treatment objective of compound rejection, compound transport and rejection trends are inversely related.     

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).     

The influence of natural organic matter and cations on the rejection of endocrine disrupting and pharmaceutically active compounds by nanofiltration

The impact of natural organic matter (NOM) and cations on the rejection of five endocrine disrupting compounds (EDCs) and pharmaceutically active compounds (PhACs) (acetaminophen, carbamazepine, estrone, gemfibrozil, oxybenzone) by nanofiltration (NF) was examined. The water matrices included membrane bioreactor (MBR) effluent, Lake Ontario water and laboratory-prepared waters modelled to represent the characteristics of the Lake Ontario water. The impact of cations in natural waters on compound rejection was also examined by doubling the natural cation concentration (calcium, magnesium, sodium) in both the Lake Ontario water and the MBR effluent. The presence of Suwannee River NOM spiked into laboratory-grade water was found to cause an increase in compound NF rejection. In addition, the presence of cations alone in laboratory-grade water did not have a significant impact on rejection with the exception of the polar compound gemfibrozil. However, when cation concentration in natural waters was increased, a significant decrease in the rejection of EDCs and PhACs was observed. This suggests that the presence of cations may result in a reduction in the association of EDCs and PhACs with NOM.     

A suite of multi-segment fugacity models describing the fate of organic contaminants in aquatic systems: application to the Rihand Reservoir,India

The fugacity-based quantitative water-air-sediment interaction (QWASI) model is described which can be used to establish a mass balance for an organic or metallic contaminant in a lake ecosystem consisting of water, suspended matter, bottom sediments and the atmosphere. A suite of such models is described and discussed with various degrees of complexity including versions treating equilibrium and non-equilibrium situations, steady-state and dynamic conditions with either single or multiple segments. It is suggested that when seeking to apply a mass balance model to a specific lake and contaminant situation, it is desirable to start with a simple model and increase the complexity as circumstances dictate. This approach is illustrated by application of QWASI models to the Rihand Reservoir in India for lindane and benzo(a)pyrene. The roles are discussed by which such models can contribute to improved management of chemicals that may adversely affect aquatic systems, especially in developing regions.     

Simulating the performance of fixed-bed granular activated carbon adsorbers: removal of synthetic organic chemicals in the presence of background organic matter

Granular activated carbon (GAC) adsorption is an effective treatment technology for the removal of synthetic organic chemicals (SOCs) from drinking water supplies. This treatment process can be expensive if not properly designed. Application of mathematical models is an attractive method to evaluate the impact of process variables on process design and performance. Practical guidelines were developed to select an appropriate model framework and to estimate site-specific model parameters to predict GAC adsorber performance. Pilot plant and field-scale data from 11 different studies were utilized to investigate the effectiveness of this approach in predicting adsorber performance in the presence of background organic batter (BOM). These data represent surface and ground water sources from four different countries. The modeling approach was able to adequately describe fixed-bed adsorber performance for the purpose of determining the carbon usage rate and process design variables. This approach is more accurate at predicting bed life in the presence of BOM than the current methods commonly used by practicing engineers.     

Measurements of dissolved organic nitrogen (DON) in water samples with nanofiltration pretreatment

Dissolved organic nitrogen (DON) measurements for water samples with a high dissolved inorganic nitrogen (DIN, including nitrite, nitrate and ammonia) to total dissolved nitrogen (TDN) ratio using traditional methods are inaccurate due to the cumulative analytical errors of independently measured nitrogen species (TDN and DIN). In this study, we present a nanofiltration (NF) pretreatment to increase the accuracy and precision of DON measurements by selectively concentrating DON while passing through DIN species in water samples to reduce the DIN/TDN ratio. Three commercial NF membranes (NF90, NF270 and HL) were tested. The rejection efficiency of finished water from the Yangshupu drinking water treatment plant (YDWTP) is 12%, 31%, 8% of nitrate, 26%, 28%, 23% of ammonia, 77%, 78%, 82% of DOC (dissolved organic carbon), and 83%, 87% 88% of UV₂₅₄ for HL, NF90 and NF270, respectively. NF270 showed the best performance due to its high DIN permeability and DON retention (∼80%). NF270 can lower the DIN/TDN ratio from around 1 to less than 0.6 mg N/mg N, and satisfactory DOC recoveries as well as DON measurements in synthetic water samples were obtained using optimized operating parameters. Compared to the available dialysis pretreatment method, the NF pretreatment method shows a similar improved performance for DON measurement for aqueous samples and can save at least 20 h of operating time and a large volume of deionized water, which is beneficial for laboratories involved in DON analysis. DON concentration in the effluent of different treatment processes at the YDWTP and the SDWTP (Shijiuyang DWTP) in China were investigated with and without NF pretreatment; the results showed that DON with NF pretreatment and DOC both gradually decreased after each water treatment process at both treatment plants. The advanced water treatment line, including biological pretreatment, clarification, sand filtration, ozone-BAC processes at the SDWTP showed greater efficiency of DON removal from 0.37 to 0.11 mg N L⁻¹ than that at the YDWTP, including pre-ozonation, clarification and sand filtration processes from 0.18 to 0.11 mg N L⁻¹.     

Combined influence of membrane surface properties and feed water qualities on RO/NF mass transfer, a pilot study

The impact of membrane surface characteristics and NOM on membrane performance has been investigated for varying pretreatment and membranes in a field study. Surface charge, hydrophobicity and roughness varied significantly among the four membranes used in the study. The membranes were tested in parallel following two different pretreatment processes, an enhanced Zenon ultrafiltration process (ZN) and a compact CSF process (Superpulsator (SP)) prior to RO membrane treatment for a total of eight integrated membrane systems. All membrane systems were exposed to the similar temperature, recovery and flux as well as chemical dosage. The membrane feed water qualities were statistically equivalent following ZN pretreatment and SP pretreatment except for NOM and SUVA. Membrane surface characteristics, NOM and SUVA measurements were used to describe mass transfer in a low-pressure RO integrated membrane system. Solute and water mass transfer coefficients (MTCs) were investigated for dependence on membrane surface properties and NOM mass loading. Inorganic MTCs were accurately described by a Gaussian distribution curve. Water productivity decreased with NOM loading and increased with contact angle and roughness. The negative effects of NOM loading on productivity were reduced as the negative charge on the membrane surface increased. Inorganic MTCs were also correlated to surface hydrophobicity and surface roughness. The permeability change of identical membranes was related to NOM loading, hydrophobicity and roughness. Organic fouling as measured by water, organic and inorganic mass transfer was less for membranes with higher hydrophilicity and roughness.     

A field studies and modeling approach to develop organochlorine pesticide and PCB total maximum daily load calculations: Case study for Echo Park Lake, Los Angeles, CA

Echo Park Lake is a small lake in Los Angeles, CA listed on the USA Clean Water Act Section 303(d) list of impaired water bodies for elevated levels of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) in fish tissue. A lake water and sediment sampling program was completed to support the development of total maximum daily loads (TMDL) to address the lake impairment. The field data indicated quantifiable levels of OCPs and PCBs in the sediments, but lake water data were all below detection levels. The field sediment data obtained may explain the contaminant levels in fish tissue using appropriate sediment-water partitioning coefficients and bioaccumulation factors. A partition-equilibrium fugacity model of the whole lake system was used to interpret the field data and indicated that half of the total mass of the pollutants in the system are in the sediments and the other half is in soil; therefore, soil erosion could be a significant pollutant transport mode into the lake. Modeling also indicated that developing and quantifying the TMDL depends significantly on the analytical detection level for the pollutants in field samples and on the choice of octanol-water partitioning coefficient and bioaccumulation factors for the model.     

Arsenic removal by zero-valent iron: field,laboratory and modeling studies

Field and laboratory studies were conducted to elucidate the design factors and mechanisms of arsenic removal from contaminated ground water using zero-valent iron. Large scale, field pilot experiments demonstrated for more than 8 months that iron filing filters can efficiently remove arsenite from aqueous solutions to levels less than 10 micro g/L. The maximum arsenic accumulation measured was 4.4 mg As/g of media. The iron filing filters leached significant quantities of iron (73% of the iron was leached). A critical design parameter of the system was found to be the hydraulic detention time of the water in the filter. TCLP analyses of the spent media indicated that the arsenic concentration in the leachate was two orders of magnitude lower than the 5mg/L of TCLP for arsenic. Spectroscopic and laboratory arsenic leaching studies (alkaline extraction and TCLP) suggest that the arsenic surface precipitate is related to sulfur. The aging process (due to the longevity of the removal mechanism) makes the precipitation process virtually irreversible. A mathematical model was developed to simulate the removal process using a partitioning coefficient and a mass transfer process. Calibration of these parameters using the data for three columns revealed that the equilibrium-partitioning coefficient was the same for all three columns while the mass transfer coefficient was a function of the flow rate. The calibrated mass transfer coefficients are similar to those reported in the literature if they are normalized to the surface area of the media.     

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.     

Fenton's oxidation of MTBE with zero-valent iron

Methyl tert-butyl ether (MTBE) has become a contaminant of increasing concern in the U.S. Traditional remediation technologies are successful in removing MTBE from contaminated water, but usually transfer the contaminant from the aqueous to another phase. Fenton's oxidation of MTBE provides a promising alternative to traditional remediation techniques in that it may mineralize the contaminant rather than just phase transfer. This bench-scale study investigated the feasibility of Fenton's oxidation of MTBE using zero-valent iron as the source of catalytic ferrous iron. The oxidation reactions were able to degrade over 99% of the MTBE within 10 min, and showed significant generation, and subsequent degradation, of the MTBE oxidation byproduct acetone. Second-order rate constants for MTBE degradation were 1.9 x 10(8) M(-1) s(-1) at pH 7.0 and 4.4 x 10(8) M(-1) s(-1) at pH 4.0. The total organic carbon was reduced by over 86% when a H2O2:MTBE ratio of 220:1 or greater was used.     

Sorption of dichlorodiphenyltrichloroethane (DDT) and its metabolites by activated carbon in clean water and sediment slurries

Polyethylene-water partitioning coefficients (K_PE) and mass transfer coefficients (k_PE) for the ortho and para isomers of the organochlorine pesticide dichlorodiphenyltrichloroethane (DDT) and its metabolites dichlorodiphenyldichloroethane (DDD), dichlorodiphenyldichloroethylene (DDE) and, dichlorodiphenylmonochloroethylene (DDMU) were measured. These data were used to derive activated carbon (AC) sorption isotherms in clean water in the sub-nanogram per litre free aqueous concentration range for a virgin and a regenerated AC. The sorption strength of AC for DDT and its metabolites was very high and logarithmic values of the AC-water partitioning coefficients, log K_AC, ranged from 8.47 to 9.26. A numerical mass transfer model was calibrated with this data to interpret previously reported reductions in DDT uptake by semipermeable membrane devices after AC amendment of sediment from Lauritzen Channel, California, USA. The activated carbon-water partitioning coefficient values (K_AC) measured in clean water systems appear to overestimate the AC sorption capacity in sediment up to a factor 32 for DDT and its metabolites at long contact time with fine-sized AC. Modelling results show decreased attenuation of the AC sorption capacity with increased sediment-AC contact time. We infer that increased resistance in mass transfer of DDTs to sorption sites in the microporous region likely caused by deposits of dissolved organic matter in the macro- and mesopores of AC appears to be the most relevant fouling mechanism. These results suggest that DDTs may diffuse through possible deposits of dissolved organic matter over time, implying that the effects of sediment on the sorption of DDTs by AC may be more kinetic than competitive.