PollutionEngineering August 2012 : Page 22
MEMBRANES By ROY BIGHAM We all have heard or even used the word. Do we really know what it is? Above: Two RO trains from Koch Membrane Systems provide 2.4 MGD of high-quality water. M embranes used in environmental appli-cations are special filters. According to most definitions, a membrane is a thin pliable animal or vegetable material that covers or connects tissue. In cell biology, a membrane is a thin covering that holds everything together. It turns out that there are many types of what we call membranes that perform a variety of functions. Not all of them are necessarily thin or pliable. In the environmental industry, mem-brane technology can be applied to air or water applications. There is also an applica-tion that separates dissolved gases from liq-uids (see the article, Membrane Application to Split Gas and Liquids , by Rakesh Govind, Ph. D. on page 30 in the July issue). Materials range from special plastics to ceramics. Some are flexible sheets, some are flexible like reeds and some are similar to solid sheets. Membranes can be built in many shapes including cylindrical. In nearly any application, the important parameter is the size of the pores in the materials. One would not use a filter with holes one inch in diameter to filter sand. Today’s technology allows manufacturing companies to control pore size to very 22 Pollution Engineering AUGUST 2012 exacting specifications. Membranes can be made with pore holes small enough to allow gas that is dissolved in a liquid to pass through while holding back the liquid materials. They can also be sized to remove certain dissolved chemical components. For example, membranes are often used in desalination plants to remove the salt from the seawater. They can also be sized to remove certain organic materi-als such as germs. Membrane technology is commonly used to treat surface water, brackish water, ground water, chemical wastewater treatment systems, municipal wastewater and water for reuse projects. Membranes can be highly selective in design to remove specific elements. For more details, refer to an article in the Pollution Engineering archives from the September 2003 issue titled, Selective Membranes. A lesser-known application for mem-brane technology is gas separation or purification. For example, biogas is easily formed as organic materials ferment or rot. Such gases can be collected and used as fuel but often it is necessary to clean up the gas before combustion. Biogas is typically composed of CH 4 , CO 2 and sometimes H 2 S. The CH 4 material is what is needed to burn for energy recovery. A membrane can separate the gases and isolate the CH 4 to provide the purity needed. The flow-through rate of membrane filters is not really high. In order to get the necessary volume, the membranes are used in numerous collections. See Figure 1 showing a reverse osmosis sys-tem used to clean brackish water for drinking purposes. Some people have expressed concern that these special types of filters would have a problem with clogging. In most instances, that turns out to be of no con-cern. The system pictured above had been operating for eight years with no cleaning required at that time. Some systems are designed so that the flow is across the sur-face to constantly flush it. Some systems are designed with flexible reed like tubes. The system automatically fires air bubbles at regular intervals to force the reeds to flex and dislodge any materials sticking to the surfaces. There are certain environ-ments such as hard water that could create problems for membrane filtration and the suppliers will certainly ask the right ques-tions to avoid problems. Users can expect many years of good performance when the systems are properly designed. PE
We all have heard or even used the word.Do we really know what it is?
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