Membrane rejection from water

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

In ultrafiltration, the effluent is passed across a semiper-meable membrane (see Chapter 10). Water passes through the membrane, while submicron particles and large molecules are rejected from the membrane and concentrated. The membrane is supported on a porous medium for strength, as discussed in Chapter 10. Ultrafiltration is used to separate very fine particles (typically in the range 0.001 to 0.02 xm), microorganisms and organic components with molar mass down to 1000 kg kmol. Pressure drops are usually in the range 1.5 to 10 bar. 

Membrane Concentration Test. Process potential was demonstrated by concentrating 500 L of synthetic tap water spiked with trace levels of the model compounds. A 50X volumetric concentration was achieved by reducing the sample volume from 500 to 10 L. The recovery of model compounds and membrane rejection of compounds were evaluated, and the location of system losses was approximated. 

The overwhelming conclusion supported by data is the superiority of the FT-30 composite membrane for the majority of organic compounds tested. From arguments presented earlier, improved recovery of organic compounds on the basis of these higher rejection properties would be expected. Data from selected literature sources (6, 10-20) on membrane rejections of organics in water at parts-per-million levels were reviewed. Results are presented by chemical class in Table VI. Data are compiled for cellulose acetate and a cross-linked NS-1-type composite membrane. Differences in the rejection of various compound classes by the two membrane types determined at higher solute levels are similar to those observed and reported here at parts-per-billion levels. 

The membrane cast from chloroform-formic acid mixtures had an anisotropic structure with a 0.9-1.2 p active layer and a 40 p porous support layer. At a water flux of 139 1/m2 day (kg/cm2 at 20 °C), the membrane showed 99.4 % rejection of cytochrome C and 72.7% of Vitamine B12. At 3980 1/m2 day water flux level, the rejection for bovine serum hemoglobin (MW, 66000 68000), cytochrome C, and Vitamine Bl2 were, 95.6, 79.4, and 39.8%, respectively. 

Reverse osmosis membranes were also prepared from polyamides with pendant carboxamide groups 90). For example, 4,4 -diaminodiphenylmethane-3,3 -dicarbox-amide-isophthaloyl chloride copolymer 33 was dissolved in DMF containing LiCl, cast to 250 p thickness, dried at 100 °C for 15 min, and gelled in ice water to give a membrane with the water flux permeability of 900 1/m2 day and salt rejection of 80% (0.5% NaCl aqueous solution, 30 kg/cm2). After heating the membrane in... 

Heat-resistant polyamide membranes containing pendant sulfonamide groups were also fabricated 91 93). Thus the membrane prepared from 2,2 -disulfonamide-4,4 -diaminodiphenyl ether-isophthaloyl chloride copolymer 34 gave the water permeation rate of 1700 1/m2 day and salt rejection of 65 %. The film with 50 p... 

Sulfo group-containing poly(amide imide) semipermeable membrane 35 from trimellitic anhydride sodium 3,5-dicarboxybenzenesulfonate, and 4,4 -diphenyl-methane diisocyanate have been prepared 90>. A 110 p-thick membrane gave water permeation 5601/m2 day and salt rejection 99.0% (0.5% aqueous NaCl solution, 25 °C, 42 kg/cm2), while a trimellitic anhydride-4,4 -diphenylmethanediisocyanate copolymer membrane gave 30 1/m2 day and 98.2%, respectively. 

The origin of this absorption may be water molecules with one OH group bonded to the membrane and the second OH group non-H-bonded or water molecules with H-bonds to weak acceptors of the membranes. In both cases the water transport through these membranes may be related to weak H-bonds water-membrane. In addition the spectroscopic observations show in celluloseacetate — or polyimide — membranes are less water molecules of the type of liquid water. In agreement with this observation we have found in model glass membranes with low salt rejection (about 70%) at high relative humidity water spectra not far from the spectra of liquid water. As result of these first experiments we may discuss two possible mechanism of membranes for desalination processes ... 

Figure 5.5 Water permeability as a function of sodium chloride permeability for membranes made from cellulose acetate of various degrees of acetylation. The expected rejection coefficients for these membranes, calculated for dilute salt solutions using Equation (5.6),...

As Figure 5.12 shows, Toray s PEC-1000 crosslinked furfuryl alcohol membrane has by far the best sodium chloride rejection combined with good fluxes. This explains the sustained interest in this membrane despite its extreme sensitivity to dissolved chlorine and oxygen in the feed water. Hollow fine fiber membranes made from cellulose triacetate by Toyobo or aromatic polyamides by Permasep (Du Pont) are also comfortably in the one-stage seawater desalination performance range, but the water fluxes of these membranes are low. However, because large-surface-area, hollow fine fiber reverse osmosis modules can be... 

A further long-term area of research is likely to be the development of reverse osmosis membranes to recover organic solutes from water. This chapter has focused almost entirely on the separation of ionic solutes from water, but some membranes (such as the PEC-1000 membrane) have excellent organic solute rejections also. The PEC-1000 membrane was chemically unstable, but it demonstrated what is achievable with membranes. A stable membrane with similar properties could be used in many wastewater applications. 

The main problem in membrane usage for water purification is the fouling layer that adheres to the membrane. The source of the fouling layer is the different species existing in the feed water and their increased concentration next to the membrane wall. When water permeates through the membrane, all rejected species accumulate next to the membrane wall, their concentration increases in comparison to the bulk concentration, and the motion away from the membrane is controlled by diffusion to the bulk of flow against the flux of the water flowing to the membrane. 

Because of the carbon dioxide present in most waters, the pH of RO product water is generally lower than the pH of feed water, unless the carbon dioxide is completely removed from the feed water. If carbon dioxide is present in feed water, it will be present in permeate, as gases are not rejected by RO membranes (see Chapter 3.2). However, the membrane rejects carbonate and bicarbonate. Passage of carbon dioxide upsets the equilibrium among these compounds in the permeate. A new equilibrium occurs in the permeate, hence lowering its pH ... 

Membranes used for NF are made of cellulose acetate and aromatic polyamide with characteristics such as salt rejections from 95% for divalent salts to 40% for monovalent salts and an approximate MWCO of 300 for organics. An advantage of NF over RO is that NF can typically operate at higher recoveries, thereby conserving total water usage due to a lower concentrate stream flow rate. NF is not effective on small-molecular-weight organics, such as methanol. 

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