Coupling reverse osmosis with electrodialysis to isolate natural organic matter from fresh waters

Reverse osmosis (RO) has proven to be an effective method for the concentration of natural organic matter (NOM) from fresh waters, but an undesirable consequence of this process is the co-concentration of some inorganic solutes. Accordingly, current practice yields solutions of NOM that, upon desalting and freeze-drying, are converted into dry solids containing finely dispersed sulfuric acid and silicic acid (H(4)SiO(4)). These acids will contribute to the apparent carboxylic and phenolic contents of NOM, leading to an overestimation of both. NOM may also be chemically altered by sulfuric acid, which reacts strongly with many classes of organic compounds. The sulfur content and ash content of NOM will be elevated in the presence of sulfuric acid and H(4)SiO(4). The goal of this study is to develop and test a method in which the removal of water by RO is coupled with the removal of salts by electrodialysis (ED). Like RO, ED is a relatively mild treatment that enables the desalting of NOM solutions without subjecting those samples to conditions of extremely high or low pH. The end product of the coupled process is a desalted, concentrated liquid sample from which low-ash NOM can be obtained as a freeze-dried solid material. In this study, the efficacy of ED for desalting NOM is evaluated using concentrated synthetic river waters and actual concentrated (by RO) river waters. Under optimal operating conditions, both sulfate and silica can be largely removed from RO-concentrated solutions of riverine NOM with only an average loss of 3% of total organic carbon.     

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”.     

Systematic features in the study of humic substances in natural surface waters

The article has considered systematic approaches to the investigation of humus substances of natural surface water. It has been shown that the use of chromatographic methods for extraction humic matter from water and investigation of their molecular-weight distribution is accompanied by losses of these compounds owing to phenomena of irreversible adsorption. The comparative assessment of the methods of determining concentration of humus mater, in particular, spectrophotometry and fluorescent spectroscopy as well as methods based on the results of determination of water color and reaction of azo-combination using diazotized 4-nitroaniline has been made.     

Removal of heavy metals from waters by means of natural zeolites

The natural zeolite clinoptilolite can be used as an ion exchanger for the removal of ammonium ions from drinking waters. These waters may also contain small amounts of heavy metals. The retention possibilities of these cations by the zeolite are presented.

The study of the selectivity of the Na-exchanged clinoptilolite in presence of ammonium ions has been achieved by plotting the exchange isotherms relative to the various cations. This leads to the following order of decreasing efficiency of the zeolite.     

Removal of humic substances (HS) from water by electro-microfiltration (EMF)

Humic substances (HS) represent the common agents contributing to flux decline during membrane filtration of natural water. In order to minimize the fouling during microfiltration (MF) of HS, modifying the operation of MF presents a promising alternative. A laboratory-scale electro-microfiltration (EMF) module was used to separate Aldrich HS from water by applying a voltage across the membrane. The presence of an electric field significantly reduced the flux decline. A flux comparable to that of ion-free water was attained when the voltage was near the critical electric field strength (Ecritical), i.e., the electrical field gradient that balances the advective and electrophoretic velocities of solute. At an applied voltage of 100 V (approximately 110 V/cm), it was able to reduce UV absorbance at 254 nm (UV254), total organic carbon (TOC) and trihalomethane formation potential (THMFP) by over 50% in the permeate. Results from 1H nuclear magnetic resonance (1H NMR) analysis suggest that the aromatic and functionalized aliphatic fractions decreased significantly in the permeate. The charged HS have large molecule weight compared with those passing through membrane. Results clearly indicate that a combination of electric force with MF can increase HS rejection and decrease flux decline. Electrophoretic attraction was the major mechanism for the improvement of flux and rejection over time.     

Purification of waters containing fluorine by low pressure reverse osmosis for their complex treatment

The paper has investigated main regularities of the purification process of brackish waters containing fluorine by low-pressure reverse osmosis. It has demonstrated high efficiency of the method with the use of a membrane of a TFC-75 brand. It has been established that chlorides have a negative influence on purification of the specified waters. We have proposed a technological flow chart for their complex processing using reverse osmosis.     

Influence of humic substances on the ultraviolet disinfection of surface waters

The efficiency of ultraviolet (UV) disinfection and the regrowth potential of total coliforms were investigated in humic waters. Experiments were conducted according to an experimental design in which a UV radiation dose range of 68–681 mW s/cm² was applied to waters containing various concentrations (0–10 mg/L) of fulvic acid. Experimental results strongly suggested that the harmful effect of UV radiation on bacteria was diminished in humic waters due to absorption of UV light. Increasing concentrations of fulvic acid appeared to enhance its influence with elevated doses of UV radiation. Measured inactivation responses were on the order of 1–6 log₁₀ units for the UV dose range used. Comparison of k (inactivation coefficient) values for nonhumic water to highly humic water indicated that k decreased proportionally as the fulvic acid concentration of water was increased. The results of dark‐incubation tests indicated significant regrowth of bacteria in fulvic acid‐containing waters.     

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.     

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.     

Removal of humic compounds from aqueous solutions by the complexation-ultrafiltration method

The process of ultrafiltration treatment of aqueous solutions to remove humic acids has been studied under conditions of adding complexation polymers (polydiallyl dimethylammonium chloride, polyacrylamide, polyvinylpyrrolidone, and chitosan). It was shown that the highest values of the retention coefficient for humic acids were observed in case of using polydiallyl dimethylammonium chloride (R = 98.1%) and chitosan (R = 90.5%). The impact of the solution pH on the degree of removing humic acids was also investigated.     

Removal of biological and non-biological viral surrogates by spiral-wound reverse osmosis membrane elements with intact and compromised integrity

The removal of bacteriophage MS2 and fluorescent-dyed polystyrene microspheres with intact and purposely compromised spiral-wound RO membrane elements was investigated. MS2 rejection with intact membrane elements was >99.9995%. A model developed for data evaluation revealed that the advective passage of MS2 through imperfections of intact membrane elements was <2 x 10(-5)% of the overall product water flow produced. The advective passage of MS2 and microspheres through a pinhole induced in one of the elements was 0.05-0.1% of the overall product water flow. Prolonged testing of both intact and compromised elements resulted in increased MS2 rejection corresponding to advective MS2 passage through membrane imperfections of <3 x 10(-7)% of the overall product water flow. The permeate flow rate obtained with an element with a larger pinhole was 5-13% greater than that of the intact element, and the corresponding rejection of MS2 and microspheres was similar to that observed for sodium chloride. The use of a cracked o-ring in the connection of the permeate tube to the element vessel end-cup resulted in advective passage of MS2 through the crack of <0.0001% of the overall permeate flow.     

Water purification of nitrates by low-pressure reverse osmosis method

The paper has investigated possibilities and basic regularities of water purification of nitrates by low pressure reverse osmosis. The negative influence of chlorides and sulfates on the process characteristic have bee shown. A conclusion has bee made on expediency of using the low pressure reverse osmosis method for purifying water of nitrates.     

Removal of dissolved organic matter by granular-activated carbon adsorption as a pretreatment to reverse osmosis of membrane bioreactor effluents

The adsorption of dissolved organic matter (DOM) on granular-activated carbon (GAC) as a pretreatment to reverse osmosis (RO) desalination of membrane bioreactor (MBR) effluents was studied in lab- and pilot-scale columns. The pattern and efficiency of DOM adsorption and fate of the hydrophobic (HPO), transphilic (TPI) and hydrophilic (HPI) fractions were characterized, as well as their impact on organic fouling of the RO membranes. Relatively low DOM adsorption capacity and low intensity of adsorption were observed in batch studies. Continuous adsorption experiments performed within a range of hydraulic velocities of 0.9-12m/h depicted permissible values within the mass transfer zone up to 1.6m/h. The breakthrough curves within this range displayed a non-adsorbable fraction of 24+/-6% and a biodegradable fraction of 49+/-12%. Interestingly, the adsorbable fraction remained almost constant ( approximately 30%) in the entire hydraulic range studied. Comparative analysis by HPO interaction chromatography showed a steady removal (63-66%) of the HPO fraction. SUVA index and Fourier Transform Infrared (FTIR) spectra indicated that DOM changes during the adsorption phase were mainly due to elution of the more HPI components. GAC pretreatment in pilot-scale columns resulted in 80-90% DOM removal from MBR effluents, which in turn stabilized membrane permeability and increased permeate quality. FTIR analysis indicated that the residual DOM present in the RO permeate, regardless of the pretreatment, was mainly of HPI character (e.g., low-molecular-weight humics linked to polysaccharides and proteins). The DOM removed by GAC pretreatment is composed mainly of HPO and biodegradable components, which constitutes the fraction primarily causing organic fouling.     

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.     

Contact flocculation-filtration of humic substances

Contact flocculation-filtration of humic dispersions with alum in medium grain (0.62 mm) sand gave short runs to breakthrough, while in coarse (1.2 mm) media clarification was inefficient. Alum with cationic polymer gave in coarse media acceptable runs, but shorter then for mineral dispersions. Analysis of results by parameters of existing filtration models showed that using the Ives-Iwasaki model and plotting filtration coefficient λ vs specific deposit σ gave different λ_max for different polymer dose, plot of λ_max vs dose goes through maximum at optimum dose.Plotting the cube root of headloss vs square root of specific deposit gave a straight line corresponding to Shektman formula.Analysis by Adin-Rebhun model showed that for contact flocculation-filtration humic dispersions attachment coefficient k₁ was similar to, detachment k₂ was many fold higher then and theoretical maximum mass capacity much lower then in case of mineral dispersions.Calculations showed that the solid content and the density of alumo-organic deposits in bed are much lower than of alumo-mineral deposits. Polymers and minerals increase the solid content, the density of deposits and the mass capacity of filter beds.     

Characterization of aquatic humic substances

Aquatic natural organic matter (NOM) is composed of a large variety of compounds. In this study, NOM from Uzquiza Reservoir in Burgos (Spain) was fractionated using a resin adsorption procedure, and three humic substances – natural fulvic and humic acids (NFA and NHA, respectively) extracted from the mentioned reservoir and a commercially supplied humic acid (CHA) – were characterized by molecular weight (MW) distribution, elemental composition, organic acidity and ultraviolet and infrared spectroscopy. The results indicate that Uzquiza Reservoir NOM is mainly composed of fulvic acids (45%) and low MW hydrophilic acids (27%). MWs obtained are in the following order: CHA (4500 Da)>NHA (2500 Da)>NFA (1000 Da). Both humic acids (CHA and NHA) have the highest specific UV absorbance values (SUVA, an aromaticity indicator) whereas NFA shows a higher total and carboxylic acidity, which is in accordance with its higher solubility in water. Infrared spectra confirm these results.     

Electrosorptive separation of humic substances

A method has been proposed for electrosorptive separation of humic substances, associated organic substances and cations of metals from water solutions of soil extracts. For separation we used beds of activated carbon polarized from an external EMF source. Essential differences were found in compositions of organic and inorganic substances concentrated in beds of the cathode and anode carbons. The proposed method is considered as an option of a comprehensive approach to studying the nature of humic compounds combining electrochemistry and instrumental methods of analysis.     

Characteristics of humic substances in the Kuji River waters as determined by high-performance size exclusion chromatography with fluorescence detection

Direct measurement by high-performance size exclusion chromatography with fluorescence detection was applied to the characterization of humic substances in river waters from the Kuji River system, which runs through forest hills and an agricultural plain in Japan. The monitoring wavelength of excitation 320 nm and emission 430 nm corresponds to the fluorescence maxima for aquatic fulvic acid. Chromatograms of the river waters showed four peaks; each peak position was in good agreement among these samples. Peak height ratios for the samples from the upstream Kuji River and its tributaries were different from those of the midstream and downstream sections of the Kuji River, which may reflect differences in the characteristics of humic substances and other organic materials supplied from soil to river.     

Mineral recovery from inland reverse osmosis concentrate using isothermal evaporation

Mineral recovery from reverse osmosis (RO) concentrate after concentration by a secondary sea water-type RO system with lime–soda pretreatment was the focus of this study. Lime–soda pretreatment removed Ca, Mg and Si allowing for the application of sea water-type RO resulting in a concentrate composed of sodium, potassium, sulfate and chloride. The overall objective was reduction in concentrate volume that will require disposal by evaporation while producing by-products with potential resale value. Thermodynamic phase equilibrium calculations using Pitzer’s correlations for 25 °C, accurately predicted the solubility and evaporation path of the sodium sulfate minerals as potential by-products. Bench-scale evaporation experiments verified the model predictions and indicated that 81–88% of the sodium sulfate by-products were Na₂SO₄.     

The current state of the problem of removing boron from the sea and brackish waters in the process of reverse osmosis desalination

The technical and economic analysis of existing methods for reducing the boron concentration in permeate of the reverse-osmosis plants to the maximum allowable levels for the drinking and irrigation waters has been performed. Relevant tasks were defined that have to be solved for creating practical reverse-osmosis plants of desalting the sea and subterranean waters.