Developments in membrane technology for water treatment

Membrane technology is widely accepted as a means of producing various qualities of water from surface water, well water, brackish water and seawater. Membrane technology is also used in industrial processes and in industrial wastewater treatment, and lately membrane technology has moved into the area of treating secondary and tertiary municipal wastewater and oil field produced water. In many cases one membrane process is followed by another with the purpose of producing water of increasing purity and quality for various purposes. One type of membrane may thus enhance the function of another to meet goals ranging from disposal of wastewater to production of drinking water from unexpected sources. In this way membrane technology offers the possibility of managing the total water resources in a region, which is of special interest in geographical areas where the natural water resources are scarce. The spiral wound membrane element configuration is the most widely used due to its high packing density and relatively low price. This paper will describe some technological advance in the area of innovative new membranes and application concepts for spiral wound membrane elements. Spiral wound elements span the four commonly defined membrane technologies, which are microfiltration (0.01–0 microns), ultrafiltration (500– 100,000 Da), nanofiltration (100–500 Da), and reverse osmosis (up to 100 Da). A sandwich consisting of two membrane sheets with an inserted permeate carrier is glued together and to complete the membrane package a feed spacer is inserted between the opposing membrane surfaces. The membrane package is wound around a perforated central tube through which the permeate exits the element. The physical shape of a membrane element is secured by applying a suitable outer wrap. The physical and chemical properties of the various materials, including the membrane, are chosen according to the operating parameters. The typical reverse osmosis elements have limitations with respect to temperature (45°C), pH value (2–10), silt density index (less than 3 SDI), chlorine (dechlorination mandatory) and several other parameters. This is acceptable for conventional pure water applications, but for more complex membrane applications these limitations must be diminished or removed. Advanced materials and material science have been applied to the membranes, materials and construction of spiral wound elements. This effort has resulted

Membrane technology has undergone a rapid development as it pertains to most applications and in particular as it pertains to water purification. The last twenty years have witnessed new membranes working at ever lower pressures and with increasing salt rejection from the original cellulose acetate membrane requiring 400 psi (28 bar) to modern polyamide thin-film membranes requiring only 100 psi (7 bar) net driving pressure. Salt rejection of the reverse osmosis membranes has increased from 97.0 to 99.5%, with some special membrane types exhibiting even higher salt rejections. Are we nearing the end of the possible and will we have to accept today’s standards in the future? The answer is that membrane technology will continue to develop with huge benefits for the user of membrane filtration equipment for water purification. Material science and molecular modeling are some of the tools, which are used in the advancement of membrane technology. Net driving pressures will continue to decrease. Salt rejection will continue to increase, although they are already close to 100%. Resistance of membrane materials to oxidizing agents will increase to the point that cleaning with chlorine is possible. The tolerance for solvents and other aggressive chemicals will be improved. In short, the end is not in sight and membranes will claim an increasing role in water purification to the advantage of our thirsty humanity.