Reverse osmosis for the separation of organics from aqueous solutions

The properties of modern polymer membranes with respect to organic aqueous solutions are discussed to some extent. The potential of reverse osmosis for the treatment of organics-contaminated waste water is demonstrated by a discussion of two examples—landfill-drainage water and ‘desorbate’. It is shown that in the case of partial immiscibility, a combination of reverse osmosis and phase separation allows operation at very high recovery rates without exceeding tolerable concentrations of organics within the reverse osmosis modules. Furthermore, it is shown that a purification of the organic phase by pervaporation is economically feasible.     

Reverse osmosis of ionic aqueous solutions on aMFI zeolite membrane

separation of ions from aqueous solutions was performed by reverse osmosis (RO) on an -alumina-supported MFI:-type zeolite membrane synthesized by in-situ crystallization. For the 0.1 M chloride single-salt solutions, the separation efficiency in terms of ion rejection was found to increase with the ion valence in the order r_Al³⁺ > r_Mg²⁺ > r_Na⁺, while the ion and water fluxes changed in the reverse order. The charge density, size, and apparent dynamic hydration number of the ion as well as the mobility of the hydrated ion were found to have critical influences on ion diffusion and water permeation through the polycrystalline zeolite membrane.     

Reverse osmosis applied to metal finishing wastewater

The electroplating industry is a great water consumer and, as a consequence, one of the biggest producers of liquid effluent. The metal finishing industry presents one of the most critical industrial waste problems. There is therefore growing interest in developing methods for reclaiming metals from plating waste stream and recovery of water using membrane technology [1,2]. The application of reverse osmosis (RO) to the global effluent from the electroplating industry has been studied in this paper. The results obtained show that there is 75-95% recovery of water and nearly total removal of metals in the permeate.     

Reverse osmosis separation of hydrocarbons in aqueous solutions using porous cellulose acetate membranes

A physicochemical parameter, represented by the symbol Σs^*, based on molar solubility in water and molar attraction constants of Small, has been developed to express quantitatively the relative hydrophobicity, or nonpolar character, of the hydrocarbon molecule. The value of Σs^* can be calculated for a hydrocarbon from its chemical structure. The scale of Σs^* is consistent within each group of aromatic, cyclic, and noncyclic hydrocarbons. Reverse osmosis data have been obtained at 250 psig for single-solute aqueous feed solution systems involving low concentrations of 39 different hydrocarbons (including 13 aromatics, 10 cyclic, and 16 noncyclic compounds) and several samples of cellulose acetate membranes of different surface porosities. The effect of operating pressure on membrane performance has also been studied for two aromatic hydrocarbon solutes. The values of Σs^* for the solutes used were in the range of 425 to 924 for aromatic hydrocarbons, 521 to 931 for cyclic hydrocarbons, and 369 to 960 for noncyclic hydrocarbons. The reverse osmosis data have been correlated with Σs^* for each group of hydrocarbons studied. In all cases, positive solute separations were obtained, and the ratio [PR]/[PWP] was less than 1. With respect to each film, solute separation increased with increase in Σs^*, and decreased with increase in operating pressure. Also, solute separation decreased in the order aromatic hydrocarbon > cyclic hydrocarbon > noncyclic hydrocarbon at any given value of Σs^*. At a given operating pressure, for low values of Σs^* (～500 or less) solute separation increased with progressive decrease in average pore size on the membrane surface. For high values of Σs^* (～800 or more), solute separation initially increased with decrease in average pore size, then passed through a maximum and minimum with further decrease in average pore size, and again increased with still further decrease in average pore size. The results are discussed on the basis of preferential sorption of solute at the membrane–solution interface under the experimental conditions studied.     

Reverse osmosis separation of phenols in aqueous solutions using porous cellulose acetate membranes

Reverse osmosis separations of phenol (9.4 to 108 ppm), p-cresol (108 ppm), and p-chlorophenol (129 ppm) were studied using Loeb-Sourirajan-type porous cellulose acetate membranes, and single-solute aqueous feed solutions at 500 psig and the indicated solute concentrations. It was found that, by dissociating the solute by changing the pH of the feed solution, all the above phenols could be separated by reverse osmosis. Solute separation increased with increase in the degree of dissociation of the solute in the feed solution; and, by the appropriate choice of pore size on the membrane surface, separations of phenol approaching the degree of dissociation of phenol in the feed solution could be obtained under the operating conditions used. Similar experiments using aniline (93 ppm) as the solute showed that dissociation of solute molecules in the feed solution could be a technique generally applicable for the reverse osmosis separation of nonionic solutes in aqueous solution. The effects of operating pressure in the range 250 to 1500 psig and pore size on the membrane surface on the separation of un-ionized phenol and p-chlorophenol showed that, with respect to single-solute aqueous feed solutions of phenols, the component whose relative acidity was greater was preferentially sorbed at the cellulose acetate membrane—aqueous solution interface, and the solute concentration in the membrane-permeated product solution was a function of the extent and mobility of each of the sorbed species.     

Reverse osmosis separation of inorganic solutes in aqueous solutions using porous cellulose acetate membranes

Using a modified form of the Born expression for the free energy of ion-solvent interaction, to both the bulk solution phase and the membrane–solution interface, a parameter is obtained to express the repulsion of the ion at the interface. This parameter, called the free energy parameter for ions, is then related to solute transport parameter obtained from reverse osmosis experiments. Numerical values of this free energy parameter have been obtained for six monovalent and four divalent cations and for 12 monovalent anions. Using the experimental data for the reverse osmosis separation of sodium chloride as reference, the utility of the above parameter for predicting solute separation in reverse osmosis is illustrated for 32 other inorganic salts.     

Reverse osmosis to achieve water control and recycle

A 648,000 GPD reverse osmosis (RO) facility at ERDA's Rocky Flats Plant near Golden, Colorado will convert tertiary sewage plant effluent for recycle as cooling tower makeup to reduce external water demand and achieve “zero discharge” off-site of tertiary sewage effluent.

Design parameters for the facility, determined by three years of pilot plant testing, include 98% feedwater recovery, 100 ppm T.D.S. product water, and minimum brine production for evaporation to dryness.

Pretreatment consists of RO feed attenuation in a large pond, chlorination, sand filtration, softening, diatomaceous earth filtration, feed-water heating and pH adjustment. The RO plant will have three 150 GPM trains, each with a combination of HFF modules producing about 90% of the permeate, followed by SW modules producing the final 10%. Permeate from the SW modules can be combined with permeate from the HFF modules or returned to the RO feed stream.

Unique design considerations include heating the 40–70°F fee to 77°F by means of heat recovery from the permeate and supplemental steam heating, recycling of pretreatment backwash streams wherever possible to reduce the volume of brine, and precautions to avoid silica scaling of the modules.     

Reverse osmosis separation of organic acids in aqueous solutions using porous cellulose acetate membranes

Reverse osmosis data obtained using porous cellulose acetate membranes and aqueous feed solutions involving one of 22 monocarboxylic acids, seven dicarboxylic acids, and four hydroxycarboxylic acids have been analyzed. The operating pressure used was 250 psig in all cases, and the solute concentration used was ～100 ppm in most cases. The results yield the following physicochemical criteria for preferential sorption at the membrane–solution interface with respect to the un-ionized acid. At pK_a = 4.2 (for monocarboxylic acids), or Taft number (σ^*) = 0.6 or Hammett number (σ) = 0, neither the un-ionized acid nor water is preferentially sorbed at the membrane–solution interface; at pK_a < 4.2 (for monocarboxylic acids) or σ^* > 0.6 or σ > 0, the un-ionized acid is preferentially sorbed at the membrane–solution interface. For practical purposes, preferential sorption of water at the membrane–solution interface may be considered negligible in the σ^* region of 0 to 0.6. The results also show that the criterion of acidity of the molecule governing the extent of its repulsion or attraction at the membrane–solution interface is valid for both ionized and un-ionized acid. Further, when the acid molecule contains three or more straight-chain carbon atoms not associated with a ? COOH group, the nonpolar character of the molecule also affects its separation in reverse osmosis.     

Reverse osmosis separation of amino acids in aqueous solutions using porous cellulose acetate membranes

Polar and steric effects together govern the reverse osmosis separation of amino acids in single-solute aqueous solution systems. The solute transport parameter for the completely ionized aliphatic amino acids (with no additional polar groups other than one ? NH₂ and one ? COOH) in the pK₁ range of 4.03 to 1.71 can be represented as a function of pK₁ and the steric parameter ΣE_s. The latter parameter has a relatively greater influence with respect to the separation of zwitter ions. The effect of the polar parameter pK₁ on solute separation increases with increase in the concentration of the ionic species R⁺ (or decrease in the concentration of the ionic species R^?) in the feed solution. The effect of the presence of additional polar groups in the amino acid molecule is to increase its basicity. Experiments with p-aminobenzoic acid solutions indicate that the undissociated acid is preferentially sorbed at the membrane–solution interface. With respect to both aliphatic and aromatic amino acid ions, solute separation is in the order R^? > R^± > R⁺ for the cellulose acetate membrane material studied.     

Reverse osmosis separations of polyethylene glycols in dilute aqueous solutions using porous cellulose acetate membranes

Reverse osmosis separations of eight polyethylene glycol (PEG) solutes in the average molecular weight range of 200 to 6750 in single-solute dilute aqueous solutions have been studied using porous cellulose acetate membranes at the operating pressures of 50, 75, and 100 psig. Diffusivity data for the above PEG solutes have also been obtained from experimental data on intrinsic viscosities. From an analysis of all experimental data, numerical values for the parameters representing the polar (?ΔΔG/RT), steric (δ^*ΣE_s), and nonpolar (ω^*Σs^*) forces governing reverse osmosis separations of PEG solutes have been generated. These numerical values are useful for precise characterization of cellulose acetate membranes for whose specifications sodium chloride is not the appropriate reference solute because of its low or practically negligible separation under reverse osmosis operating conditions. This work also illustrates that solute separation in reverse osmosis can predictably increase or decrease with increase in operating pressure depending on experimental conditions.     

Reverse osmosis separations for some alcohols and phenols in aqueous solutions using aromatic polyamide membranes

The performance of aromatic polyamide membranes for reverse osmosis separations of eight alcohol and four phenol solutes in dilute aqueous solutions has been studied. The Taft polar parameter σ^* for the solutes studied were in the range of ?0.3 to 1.388. Positive solute separations were obtained for each one of the solutes. In the σ^* value range of ?0.3 to 0, data on PR/PWP ratio scattered close to 1, and solute separation decreased with increase in σ^*. For the phenol solutes, PR/PWP ratio decreased and solute separation increased with increase in σ^*. The results are interpreted on the following basis. The aromatic polyamides are more nonpolar than cellulose acetates. In the σ^* range of ?0.115 to ?0.3, solute separation is governed primarily by polar interactions; in this range, solute transport parameter D_AM/Kδ is well correlated by the expression D_AM/Kδ = C^* exp (ρ^*σ^*). The solute separation for ethyl and methyl alcohol solutes (σ^* = ?0.1 and 0, respectively) is reduced by the nonpolar character of the membrane material. Positive solute separation for each of the phenolic solutes is due to preferential sorption of solute at the membrane-solution interface caused by both the nonpolar character of the membrane material and acidity of the solutes.     

Reverse osmosis separations of some organic and inorganic solutes in aqueous solutions using aromatic polyamide membranes

The reverse osmosis separations of some ethers, ketones, aldehydes, monocarboxylic acids, and inorganic salts in single-solute (55–454 ppm) aqueous solution systems using aromatic polyamide membranes have been studied at 250 psig. It was found that reverse osmosis separation was essentially a function of steric parameter for ethers, and of both polar and steric parameters for ketones, aldehydes, and alcohols. Solute separations for monocarboxylic acids passed through a minimum at a pK_a value of ～4.8. The values for the free-energy parameter for Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and F^?, Cl^?, Br^?, I^?, and IO₃^? ions have been calculated for the polyamide membranes used. The above values for cations are negative and those for anions are positive, suggesting that the polyamide membrane surface behaves as if it is positively charged. Further, the data show that the polyamide membrane material is only about 40% as polar as that of the cellulose acetate membrane material studied earlier.     

Reverse osmosis separation of single and mixed alcohols in aqueous solutions using porous cellulose acetate membranes

Reverse osmosis transport for alcohol-water systems in the Taft number (σ^*) region of 0 to ?0.3 is explored in detail. The numerical value of the polar functional constant for alcohols is 15.5 for the above σ^* region in the operating pressure range of 50 to 500 psig for the cellulose acetate membrane material used. An analysis of the combined effect of operating pressure and mass transfer coefficient on the high-pressure side of the membrane shows that, under certain conditions, solute separation could pass through a maximum with increase in operating pressure. A general experession for solute separation is derived as a function of pore structure on membrane surface, polarity of solute, and operating conditions of the experiment. Alcohols behave independently in mixed solute systems. A method is described and illustrated for predicting alcohol separation in alcohol–sucrose–water feed solutions from data on single solute systems.     

Reverse osmosis in the desalination of brackish water and sea water

To make RO an economical process, plant and operating costs have been reduced by developing compact modules, led by polyamide hollow fibre modules which recover energy from the reject when desalinating highly saline brackish water and seawater. Such plants only start to pay their way at product rates of around 400m³ /d +. Energy costs for desalination systems with lower capacities can also be reduced considerably by operating the RO plants at conversion rates of more than 30%. To what extent the required higher conversion rates can be obtained for a particular application depends on concentration of scales-forming content matter (calcium sulphate, strontium sulphate, barium sulphate and calcium carbonate), salt content of the raw water, concentration of fouling substances such as iron, manganese, anorganic and organic matter, risk of biological growth due to algae, shells and bacteria. The scope and type of conditioning and pre-treatment are based on these factors.     

Reverse osmosis brine staging treatment of subsurface water

An earlier Kuwait Institute for Scientific Research (KISR) study on subsurface water rise problems in residential areas of Kuwait recommended drainage by deep wells to lower levels and maintain the subsurface water at a desired level. A large quantity of water needs to be permanently pumped for this purpose. The quality of the pumped water is likely to prohibit its direct use. Disposing of the water as waste is a loss that Kuwait cannot afford in view of its meager, over-stressed and practically nonrenewable natural groundwater resources. Therefore, experiments were conducted by KISR at the Kifan site to desalinate the rising subsurface water using reverse osmosis (RO) brine staging. Operation of these experiments lasted for about 5 months. The aim of these experiments was to test the technical feasibility of increasing the productivity of the tested RO systems using brine staging. This paper highlights the brine-staging concept and presents the data collected and an evaluation of the data. The data collected indicate that the average recovery obtained was 48%, availability of the brine staging RO unit was 97% and the increase in recovery using brine staging resulted in a significant reduction in the specific power consumption by about 37%.     

Removal of organic substances from aqueous solutions by reagent enhanced reverse osmosis

The reagent enhanced reverse osmosis treatment of aqueous solutions containing phenol and its derivatives has been studied. A combination of the reverse osmosis with the oxidation of organic substances by hydrogen peroxide in the presence of FeCl₂ salt as a catalyst was shown to lead to the effective removal of phenol and its derivatives from aqueous solutions. The phenol catalytic decomposition was believed to occur not only in the bulk solution but also on the membrane surface. This phenomenon is of particular interest, but additional studies are necessary. In the case of solutions containing lignosulphonates (LS), a combination of the reverse osmosis treatment with the LS oxidation by hydrogen peroxide was found to provide the high degree of solution purification from both organic substances and inorganic salts. Furthermore, it shows a possibility for the recirculation of water.     

高盐废水的反渗透膜污染与膜清洗实验研究

研究模拟高盐废水中碳酸钙、有机胶体以及它们的混合溶液对反渗透膜XR和SE的污染与清洗。在无机、有机以及混合污染试验中,XR和SE膜最大通量降分别为0. 32和0. 39,0. 20和0. 32以及0. 30和0. 33;脱盐率最大分别降低了8. 3%和9. 6%,6. 3%和9. 6%以及6. 5%和8. 6%。结果表明,XR膜要比SE膜更抗污染一些。对3种污染膜元件分别进行酸洗、碱洗以及先碱洗后酸洗的处理。测量其通量及脱盐率的恢复效果,结合清洗前后的SEM电镜图片,表明3种膜污染中的可逆污染占90. 0%以上。     

高盐废水的反渗透膜污染与膜清洗实验研究

研究模拟高盐废水中碳酸钙、有机胶体以及它们的混合溶液对反渗透膜XR和SE的污染与清洗。在无机、有机以及混合污染试验中,XR和SE膜最大通量降分别为0. 32和0. 39,0. 20和0. 32以及0. 30和0. 33;脱盐率最大分别降低了8. 3%和9. 6%,6. 3%和9. 6%以及6. 5%和8. 6%。结果表明,XR膜要比SE膜更抗污染一些。对3种污染膜元件分别进行酸洗、碱洗以及先碱洗后酸洗的处理。测量其通量及脱盐率的恢复效果,结合清洗前后的SEM电镜图片,表明3种膜污染中的可逆污染占90. 0%以上。     

Reverse osmosis membranes from aromatic polymers

A new reverse osmosis membrane composed of the following aromatic polymer containing benzimidazolone ring was studied.

Lonsdale's permeability parameters of PBIL (e.g., P₁/P₂ = 21.3 × 10³ g/cm³) show its excellent potential as permselective membrane material. Asymmetric membranes were successfully prepared from PBIL by the conventional phase-inversion method. A typical example of the basic membrane performances is as follows; water flux, 35 l/m² h and salt rejection, 99.5% determined under the pressure of 80 kg/cm² using 1% NaCl solution at 36°C. The PBIL membrane was characterized by its excellent chemical and thermal resistances, especially against acid, alkali and the oxidative atmospheres. These features suggest that the PBIL membrane is a promising candidate for one-stage sea water desalination.