Membrane softening

Bergman, R.A.(Nov. 1995) Florida - A Cost Comparison Update, Membrane Softening vs. Lime Softening. International Desalination and Water Reuse. 

Nanofiltration spans the gap in particle size between reverse osmosis (hyperfiltration) and ultrafiltration. It can separate high molecular weight compounds (100-1(X)0) from solvents, and can also separate monovalent from multivalent ions. The driving force is a pressure difference of about 0.3-3 MPa (even greater than ultrafiltration). The nanofiltration process can reject selected (typically polyvalent) salts and may be used for selective removal of hardness ions in a process known as membrane softening [10]. 

The catalysts can also be bonded onto each face of the membrane under pressure and at a temperature (22) usually between the glass transition temperature and the thermal degradation temperature of the membrane (17,23,24). At such temperatures the membrane softens and can flow under pressure, such that the adhesion force of the membrane is at a maximum, and an intimate contact between the catalyst and the membrane can be achieved (17). The heating process is rather short, so that the membrane is not over-dehydrated. A dehydrated membrane gives poor bonding (17). 

Suratt W.B., Pinto T.G., O Keefe B. (1993), Low cost membrane softening-two years of operation with a hybrid membrane system, Proc. of AWWA Membrane Technology Conf., Baltimore, Aug 93, 491-512. 

W.J. CoNLON, S.A. McClellan, Membrane softening a treatment process comes to age. Journal of American Water Works Association SI (1989) 47. 

Membrane softening by NF is a relatively new appHcation as discussed in Chapter 1. NF membranes ( loose RO ) operate at a lower feed pressure than RO membranes, and have a high rejection (99%) of divalent hardness ions. It is a more attractive alternate to hme softening and IX softening because not only is it a rehable process, no regeneration is required and, thus, there is no chemical wastewater. NF separation Hke RO is a continuous process and is independent of the plant capacity (flow rate) and feed water... 

The anode GDM (with or without MPL), the anode CL bound to either the GDM or the PEM, the PEM, the cathode CE bound to either the GDM or the PEM, and the cathode GDM (with or without MPE) are often hot-bonded together to form a multilayered structure. Such a structure is called a membrane electrode assembly (MEA). When two catalyst layers are separately applied to either side of a PEM, the resulting PEM is a CCM. Hot-bonding is carried out under a suitable pressure and a suitable temperature for a few minutes. The temperature is chosen such that the membrane softens. For Nation, it is often around 130°C. The pressure is often around 100 bars. 

Bergman, R.A. (1995) Membrane softening versus lime softening in Florida a cost comparison update. Desalination, 102,11-24. 

O M) costs were lower than for membrane softening, but the relative difference in costs decreased with larger facihties from a factor 2 for a production of 4000 m /d to 15% for a production capacity of about 50,000 m /d. If additional treatment processes are added to hme softening to match the better membrane softening permeate quality, or if some water can be bypassed around the membranes and blended to produce water comparable to the finished water in the lime softening plant, the cost of NF is much lower than for hme softening. 

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. 

Home desalinators are possible only for industrialized countries with a central service organization. They will eventually become available on a rental/service contract basis, as is standard practice for water softeners in many communities. Although rental of water softeners is common in the United States, home membrane-system rental is not estabUshed. 

While the ambient-temperature operation of membrane processes reduces scaling, membranes are much more susceptible not only to minute amounts of scaling or even dirt, but also to the presence of certain salts and other compounds that reduce their ability to separate salt from water. To reduce corrosion, scaling, and other problems, the water to be desalted is pretreated. The pretreatment consists of filtration, and may include removal of air (deaeration), removal of CO2 (decarbonation), and selective removal of scale-forming salts (softening). It also includes the addition of chemicals that allow operation without scale deposition, or which retard scale deposition or cause the precipitation of scale which does not adhere to soHd surfaces, and that prevent foam formation during the desalination process. 

Whey concentration, both of whole whey and ultrafiltration permeate, is practiced successfully, but the solubility of lactose hmits the practical concentration of whey to about 20 percent total sohds, about a 4x concentration fac tor. (Membranes do not tolerate sohds forming on their surface.) Nanofiltration is used to soften water and clean up streams where complete removal of monovalent ions is either unnecessary or undesirable. Because of the ionic character of most NF membranes, they reject polyvalent ions much more readily than monovalent ions. NF is used to treat salt whey, the whey expressed after NaCl is added to curd. Nanofiltration permits the NaCl to permeate while retaining the other whey components, which may then be blended with ordinaiy whey. NF is also used to deacidify whey produced by the addition of HCl to milk in the production of casein. 

The degree of concentration that can be achieved by RO may be limited by the precipitation of soluble salts and the resultant scaling of membranes. The most troublesome precipitate is calcium sulfate. The addition of polyphosphates to the influent will inhibit calcium sulfate scale formation, however, and precipitation of many of the other salts, such as calcium carbonate, can be prevented by pretreating the feed either with acid or zeolite softeners, depending on the membrane material. 

A second consideration is that RO tends to be sensitive to incoming suspended matter. Comprehensive and sometimes expensive pre-treatment technologies are generally needed with RO, whereas ion exchange is less sensitive to the suspended matter. Further, RO systems are sensitive to hardness, so that softening is usually required as a pre-treatment. As a rule, RO membranes cannot handle high silica waters. 

Where the feed contains a large proportion of treated water, softening is a minimum requirement and the raw water quality dictates whether a more sophisticated form of external treatment would be preferable. If the water has a high alkalinity it calls for de-alkalization and base exchange. De-ionization is the ideal water treatment, but is usually avoided if possible because of its cost and use of corrosive chemicals. Membrane processes giving partial de-ionization are not normally installed at present, but are certain to become important in the future. 

Recently developed softening membranes reject most of the hardness in water while passing sodium salts, and operate at pressures of about 5 bar. This can be used to provide substantially softened make-up water for shell... 

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