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Some Guidelines for the Selection of Filters to Reduce Particles

When one of the goals of treating water is to reduce the concentration of particles (called suspended solids), several different types of filters can be effective…but they will not all be equally effective under all circumstances.  Selecting the most appropriate filter requires some laboratory testing of a sample of the water.  The two most helpful tests are:

  • TSS (Total Suspended Solids) and
  • Particle Size Distribution

When TSS levels are very high and particle sizes are very small, the addition of polymers (coagulants and/or flocculants) will make removal of the particles much easier and more efficient.  (It’s easier to remove a larger particle than a smaller one.)  Particles formed through the addition of polymers are called “flocs”.  The first step in filtration will often be a clarifier.  It removes the larger flocs and reduces TSS to a level that can be handled by the next stage of filtration.

When TSS levels are very high and particles sizes are larger, dewatering filters and filter presses should be considered.  They can be used either as the final step in treatment, or the first step to reduce TSS to a manageable level for further filtration.

When TSS levels are very high and the particles are inorganic and mostly larger than 40 micron, consideration should be given to a sand separator (sometimes called a hydrocyclone).  Through a spinning action, the particles are spun out of the water and drop to the bottom of the unit where they are periodically flushed out.

When TSS levels are < 70 mg/l and automatic operation is desired, several different types of media filters will be effective.  The specific configuration of filter media and the rate at which water passes through the filter determines how small a particle can be removed.

Sand filters remove particles as small as 40 micron.  Dual-media filters remove particles as small as 10 micron.  Multi-media filters remove particles as small as 5 micron.  A particle size distribution can be very helpful in selecting the most appropriate removal goal for your particular situation.  It can be used to estimate the reduction in TSS using the different media filters.

When TSS levels are < 50 mg/l and manual operation is acceptable, consideration should be given to bag and cartridge filters.  They are available in a wide range of micron ratings.  Some cartridges can now be backwashed, which extends their useful life and reduces the frequency of replacement.  When automatic operation is required and disposable filter elements are not preferred, screen filters are a suitable alternative.

Selecting the right type of filter for your situation is both science and some judgment.  At Everfilt our Applications Engineers are happy to discuss your particular requirements, review test results, and recommend the most cost-effective solution.  Call us at: (800) 360-8380 or send an e-mail to:  everfilt@everfilt.com .

Written by Alfred Martinez

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