Posted by William Vander Plaats on Thu, Nov 05, 2009 @ 11:00 AM
Sand media filters may look the same, but they don't all operate the same. Some claim to use a "magical media" sand of one sort or another that has some mystical power that lends itself to finer filtration. But under a microscope, those particles appear no different than many others.
Independent testing by 3rd party agencies, however, has determined there is ONE measurable trait of a sand media filter which actually DOES make one filter more efficient than the next. That trait is "available surface area" of the underdrain. That is, the total area in square inches and mm that are available to take the flush of the filtered liquid as it passes through the sand/media/gravel pack. 
It makes sense when you stop and think about it. Water passes through the media tank, carrying debris which are caught in the media/sand as the water migrates towards the holes in the underdrain and out of the tank. What if there are only a FEW holes, spaced closely together? The debris "bumps" into other debris, as does the water, and the debris starts to pile up in certain places. The media sand does too. This leads to the "channeling effect" as the once level and flat bed of sand/media is turned into one that is uneven due to uneven flow through the underdrain. Watch this animation showing exactly how channeling takes place and how water flows through different styles of underdrains.
Now think of the opposite situation. Water passing through a sand filter carrying debris and leaving through a LARGE number of holes in an expansive underdrain, where the holes are further apart. The water is more evenly dispersed as it passes through the sand/media and underdrain, and no channeling takes place. The bed of sand remains relatively level and uniform because the water has more holes on the greater surface area of the underdrain to pass through the entire media filter.
Why Should You Care?
The reasons you should care about the efficiency of the sand media filter are:
- Greater efficiency and no "channeling" means fewer backwash cycles are required, which reduces wear and tear on ALL components of the system. This translates to longer equipment life.
- Water Savings.
- Lower pressure loss through the sand filter and reduced energy usage.
Don't forget to read the Independent 3rd Party testing results comparing the underdrains of some of the largest media tank filters manufacturers. See link below:
Comparison Performance Testing Results of Various Sand Media Filters