Membrane scaling brackish waters

Polyphosphinocarboxylic acid. Products based on this chemical tend to be suitable for brackish waters up to say 10,000 to 15,000 ppm TDS and where high sulfates are present (200 to 300 ppm as S04). A feature of this type of chemical is not only its ability to deal effectively with carbonate and sulfate scaling in higher TDS waters but also the fact that it has dispersant properties of benefit in physically moving potential foulants away from the membrane surface. 

As discussed by Pletcher 24, electrodialysis is an electrically driven membrane separation process. The main use of electrodialysis is in the production of drinking water by the desalination of sea-water or brackish water. Another large-scale application is in the production of sodium chloride for table salt, the principal method in Japan, with production exceeding 106 tonne per annum. 

Research effort at Albany International Research Co. has developed unit processes necessary for pilot scale production of several species of reverse osmosis hollow fiber composite membranes. These processes include spin-dope preparation, a proprietary apparatus for dry-jet wet-spinning of microporous polysul-fone hollow fibers, coating of these fibers with a variety of permselective materials, bundle winding using multifilament yarns and module assembly. Modules of the membrane identified as Quantro II are in field trial against brackish and seawater feeds. Brackish water rejections of 94+% at a flux of 5-7 gfd at 400 psi have been measured. Seawater rejections of 99+% at 1-2 gfd at 1000 psi have been measured. Membrane use requires sealing of some portion of the fiber bundle for installation in a pressure shell. Much effort has been devoted to identification of potting materials which exhibit satisfactory adhesion to the fiber while... 

EDR is typically used for demineralization of brackish water, which often contains poorly soluble minerals such as calcium bicarbonate and calcium sulfate, as well as colloids such as humic and fulvic acids and iron hydroxides The periodic reversal of the direction of the electric current avoids scaling and fouling of the membranes by such substances. 

To enable an impression of the large number of possible applications of ion-exchange membranes, a brief survey is here given of the pertinent literature. The major application, which has already been implemented on a technical scale, is the electrodialytical desalting of brackish water in order to obtain drinkingwater (19, 64, 65, 66, 177, 178, 179, 180). 

In the case of clarified feeds and low current densities, membrane lifetime can be as long as seven or 10 years for brackish water desalination or drinking water nitrate removal, respectively. However, if the feed solution is fouling or scaling or the ED separation plant has not been well designed or is not properly conducted, membrane lifetime is no longer than a year. 

The relationship between brine solution concentration factor and water recovery rate is shown in Figure 5.20. With plants that operate below a concentration factor of 2, that is, 50 % recovery rate, scaling is not normally a problem. However, many brackish water reverse osmosis plants operate at recovery rates of 80 or 90 %. Salt concentrations on the brine side of the membrane may then be far above the solubility limit. In order of importance, the salts that most commonly form scale are ... 

Electromembrane processes such as electrolysis and electrodialysis have experienced a steady growth since they made their first appearance in industrial-scale applications about 50 years ago [1-3], Currently desalination of brackish water and chlorine-alkaline electrolysis are still the dominant applications of these processes. But a number of new applications in the chemical and biochemical industry, in the production of high-quality industrial process water and in the treatment of industrial effluents, have been identified more recently [4]. The development of processes such as continuous electrodeionization and the use of bipolar membranes have further extended the range of application of electromembrane processes far beyond their traditional use in water desalination and chlorine-alkaline production. 

A variety of reverse osmosis membrane systems based on cellulose acetate, aromatic polyamides, and other polymers have been tested for their potential applications. Reverse osmosis membrane equipment is available for large-scale operation since the process is widely used for the production of potable water from sea or brackish waters and upstream of ion exchange in the preparation of ultrapure water for steam-generating boilers. In these applications, the feed concentrations may vary from 500 to 40,000 mg/L of dissolved solids. The RO technique can be used at pH values between 3 and 12 and up to 45°C. 

Membrane processes of the reverse osmosis (hyperfiltration) or electrodialysis types are used, but usually for smaller scale facilities (Fig. 5.4). Reverse osmosis units use high pressures of brackish water or seawater charging on one side of a semipermeable membrane, sufficient to exceed the osmotic... 

Many large-scale applications of clectrodialysis use ion-selective membranes and a potential gradient to speed migration of ions through the membranes. Brackish water can be made potable by passage through an array with alternate cation- and anion-permeable membranes, as shown in Fig. 26.13. In half of the spaces, cations move out to one side and anions move out to the other side, leaving... 

Electrodialysis was developed first for the desalination of saline solutions, particularly brackish water. The production of potable water is still currently the most important industrial application of electrodialysis. But other applications, such as the treatment of industrial effluents [45], the production of boiler feed water, demineralization of whey [46], de-acidification of fruit juices [47], etc. are gaining increasing importance with large-scale industrial installations. An application of electrodialysis which is limited regionally to Japan has gained considerable commercial importance. This is the production of table salt from sea water. Diffusion dialysis and the use of bipolar membranes have significantly expanded the application of electrodialysis in recent years [48]. 

Electrodialysis (ED) is used to remove ionized substance from hquids through selective ion-permeable membranes. ED is the most widely commercialized electromembrane technology. Desalination of brackish water is the area of electrodialysis application with the largest number of installations. This chemical-free technology competes with reverse osmosis. Electrodialysis shows better resistance to fouling and scaling. It also has an economical advantage in desalination of low-salinity solutions [13]. Also, it should be kept in mind that because of small material consumption ED is the most environmental friendly process for solution desalination [14]. 

Brackish water desaHnation was the first successful appHcation of RO with the first large-scale plant built in the late 1960s using cellirlose acetate membranes. The first seawater RO (SWRO) was built in 1973 with the advent of high permeabifity polyamide membranes. By 1993, the SWRO total capacity had reached 56,800 tn /d. In 2008, membrane desaHnation constituted 50% of total desaHnation capacity of which 45% was RO and 5% was EDR, and the rest 50% was thermal. However, 80% of aU desaHnation plants were membrane — 90% RO and 10% EDR. Desalination dominates the RO market and breaks down to 51 % desaHnation, 35% industrial and 14% residential/commercial and non-desal water [44]. In 2012, the global desaHnation capacity exceeded 60 M tn /d with more than 60% produced by RO membranes. The global water production by desaHnation in 2016 is projected to be 100 M m /d, twice the rate ofglobal water production by desalination in 2008 [45,46]. 

Membranes and membrane processes were first introduced as an anafytical tool in chemical and biomedical laboratories they developed veiy rapidly in to industrial products and methods with significant technical and commercial impact. Today, membranes are used on a large scale to produce potable water from sea and brackish water, to clean industrial effluents and recover valuable constituents, purity and to separate gases, and vapor in petrochenucal process (Baker, 2004 Bhattacharya and Misra, 2003). 

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