Brackish water membranes high-productivity membrane

Dynamically formed membranes were pursued for many years for reverse osmosis because of their high water fluxes and relatively good salt rejection, especially with brackish water feeds. However, the membranes proved to be unstable and difficult to reproduce reliably and consistently. For these reasons, and because high-performance interfacial composite membranes were developed in the meantime, dynamically formed reverse osmosis membranes fell out of favor. A small application niche in high-temperature nanofiltration and ultrafiltration remains, and Rhone Poulenc continues their production. The principal application is poly(vinyl alcohol) recovery from hot wash water produced in textile dyeing operations. 

Production of high-purity water, concentration of food and heavy metal recovery from well water, river water, lake water, or industrial waste water Lower pressure brackish water membrane Ideal for small drinking water systems due to low energy consumption... 

RO membrane separation has been traditionally used for seawater and brackish water desalination, and production of high-purity water for food, pharmaceutical processing and industrial waste treatment, as discussed in Chapter 1. The development of nanofiltration (NF) membranes has opened up many areas of apphcation including water softening, removal of disinfection by-product precurson (trihalomethanes), removal of total organic carbon (TOC), food processing and industrial water treatment [5]. 

The pressure to be used for reverse osmosis depends on the salinity of the feedwater, the type of membrane, and the desired product purity. It ranges from about 1.5 MPa for low feed concentrations or high flux membranes, through 2.5—4 MPa for brackish waters, and to 6—8.4 MPa for seawater desalination. In desalination of brackish or sea water, typical product water fluxes through spiral-wound membranes are about 600—800 kg/m /d at a recovery ratio RR of 15% and an average salt rejection of 99.5%, where... 

Leading Examples Electrodialysis has its greatest use in removing salts from brackish water, where feed salinity is around 0.05-0.5 percent. For producing high-purity water, ED can economically reduce solute levels to extremely low levels as a hybrid process in combination with an ion-exchange bed. ED is not economical for the produc tion of potable water from seawater. Paradoxically, it is also used for the concentration of seawater from 3.5 to 20 percent salt. The concentration of monovalent ions and selective removal of divalent ions from seawater uses special membranes. This process is unique to Japan, where by law it is used to produce essentially all of its domestic table salt. ED is very widely used for deashing whey, where the desalted product is a useful food additive, especially for baby food. 

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. 

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. 

NF is used when high molecular weight solutes have to be separated from a solvent. It is effective in the production of drinking water, especially in the case of water softening. Compared to RO, a lower retention is found for monovalent ions. But very recently [9], it has been found that NF separates the ions of the same valency for a selective defluorination of brackish water. RO and UF have shown, respectively, solution-diffusion and convection mass transfers. In NF, a synergism between both can be observed but strongly depends on the operational conditions (pH, ionic strength, flow rate, transmembrane pressure) and on the membrane material used. 

A detailed study of SPSF desalination membranes was carried out by Brousse and coworkers43. Sulfonation was effected by chlorosulfonic acid on a commercial material (Polysulfone P 1700, Union Carbide), and the products as well as their sodium salts were cast from highly polar solvents. Their performance was compared to that of noncharged cellulose-acetate membranes, largely being used for desalination of brackish water. 

During RO/NF operation water is forced into the membrane module pressure vessel by a high-pressure pump at pressures in the range of 10—30 bar g for brackish water and from 55 to 80 bar g for seawater. The desalted product (permeate) is removed from the opposite side of the membrane at low pressure. A flow-regulating valve on the reject side is used to create back-pressure and increase recovery, as shown in Figure 2.20. The total pressure drop from the feed inlet to reject outlet is minimal (<2 bar g), which allows the high-pressure reject to be fed to successive RO stages to increase recovery or productivity. 

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]. 

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