Self-Cleaning Pre-Filtration for RO and Other Membrane Systems

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Courtesy of Amiad Filtration Systems Ltd.

Abstract

A few years ago physical processes could only remove suspended solids from a water stream. Recent advances in technology now allow the removal of selected molecules and ions from a water stream by physical means using membranes. The membrane process has recently become very efficient and economically feasible for many industrial, commercial, municipal and even residential customers. Because of the infinitesimally small entities dealt with in this technology, blinding of the membrane surface becomes a chronic problem. To maintain the efficiency of this technology, pre-treatment must remove the greater bulk of the suspended solids to allow the membranes to efficiently remove selected dissolved solids. Five-micron bag and cartridge pre-filters have traditionally been used to treat the influent to membrane systems. The inherent nature of the elements in these pre-filters has restricted them to one-time use then being discarded and replaced by new elements. Under many conditions these pre-filter elements may be useful only for a few hours before replacement. This replacement activity can be very labor-intensive and quite expensive. A very cost saving alternative is the use of automatic self-cleaning filters to remove the bulk of the suspended solids. These filters remove all suspended solids above ten microns achieving a nominal five-micron removal. This type of filter utilizes a weavewire screen mesh woven from 316-L stainless steel. Such filters are PLC controlled and automatically clean themselves based on a preset differential pressure across the screen. Units with a footprint of four square feet are capable of filtering hundreds of gallons per minute to below ten microns. Because there is only one moving part during the short cleaning cycle and no mechanism comes in contact with the screen element, maintenance and repair are minimal. Installation requires only an electrician to bring power to the filter unit and a plumber to bolt the filter's inlet and outlet flanges into the piping system. No pneumatic or hydraulic lines are required. Another great advantage of this technology is the fact that filtered water is continuously supplied to the membrane system even during the twenty-second cleaning cycle without duplicity. Other models using 'thread' technology can boast three-micron removal. Self-cleaning pre-filtration drastically reduces the operational costs of membrane filtration systems and paves the way for economically increasing the trend in America, and elsewhere around the world, for water reuse. This trend is being propagated by the exponentially growing demands of industrial, municipal and commercial establishments for limited water supplies. Many areas of the U.S. and world are tapping fixed deposits of freshwater or depleting freshwater at a rate greater than it can be renewed constituting the mining of this precious natural resource.

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