Technology

As an essential resource for life, sustainable growth and healthy ecosystems, pure water has been high on the global agenda. Increasing population numbers, a changing climate, intensive agricultural practices, economic growth and urbanisation will undoubtedly continue to make the issue of water scarcity a global priority for years to come. As part of the solution, various techniques are available to counter this water scarcity problem. Traditional water treatment methods include physical separation techniques for particle removal; biological and chemical treatments to remove suspended solids, organic matter and dissolved pollutants or toxins; and evaporative techniques and other physical and mechanical methods. Membrane separation replaces or supplements these techniques by the use of selectively permeable barriers, with pores sized to permit the passage of water molecules, but small enough to retain a wide range of particulate and dissolved compounds, depending on their nature.

Membrane separation technique is not a recent invention. This is a part of our daily life will be exist forever because of its large number of practical application. Today this technique is used to produce potable water to clean industrial effluents and recover valuable constituents, to concentrate, purify and fractionate macromolecular mixtures in the food, drug and chemical industries. The separation, concentration and purification of molecular mixtures are major problems in the industries. Efficient separation processes are also needed to obtain high grade products in the food and pharmaceutical industries to supply communities and industry with high quality water, and to remove or recover toxic or valuable components from industrial effluents. For this task a multitude of separation techniques such as distillation, precipitation, crystallization, extraction, adsorption, and ion-exchange are used today. More recently, these conventional separation methods have been supplemented by a family of processes that utilize semipermeable membranes as separation barriers.

Membrane filtration processes are classified according to the membrane pore sizes, which dictate the size of the particles they are able to retain e.g. microfiltration, ultrafiltration, nanofiltration and reverse osmosis.

Reverse osmosis is one of the major membrane separation techniques to produce high quality water. Reverse osmosis (RO) is a filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be "selective," this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the solvent) to pass freely.

Most of the natural waters contain relatively high concentrations of scale causing materials. Membrane scaling and choking is a big problem for membrane separation technique which affects the overall performance of the RO plant. Mineral scale deposits such as calcium carbonate and phosphate, calcium oxalate, barium and strontium sulfate, magnesium silicate and others and colloidal inorganic species such as silica present important challenges for RO water applications. When these are left uncontrolled it forms hard and tenacious deposits that are difficult and hazardous to remove. In membrane separation technique at high recovery ratios, the solubility limits of these materials exceed saturation levels leading to crystallization on membrane surfaces. The surface blockage of the scale results in permeate flux decline, reducing the efficiency of the process and increasing operating costs. Another problem is biofouling due to the development of microorganisms. To avoid scaling difficulties, it is essential to restrict the fractional recovery of purified water below a threshold limit at which there is a risk of scale precipitation. In view of the economic benefit of high water recovery, the effective solubility limits of scaling salts, and hence the allowable water recovery, are usually extended by Antiscalant treatment.