Gravity Filters: Proper Post-Installation Testing

Why does Orthos want the toughest post-installation testing of gravity filters in the water/wastewater treatment industry?

The Two Most Important Tests

ONE:  Air Scour Portion of The Underdrain Flow Distribution Test

 The filter is filled with water to 6” above the underdrain after the underdrain’s structural integrity is verified. The Underdrain Flow Distribution Test provides a straight-forward visual test for proper backwash distribution. At first, the air scour-only pattern is observed. This often reveals the deficiencies of poor-distribution underdrains and if the underdrain was installed incorrectly. The water surface must present a very uniform turbulent appearance without dead zones or boils. 

One may easily see the distribution difference between nozzles on 6″-centers each way versus 8”‑centers.  They may also see relatively poor air scour distribution of folded sheet metal and wedgewire screen lateral systems that are 12” apart. If the underdrain is designed such that the filter can’t properly clean itself, solids will not be properly removed during air scour and backwash. The next filtration cycle will be compromised with shorter filter runs, lower UFRV, and other operating issues as a result.

TWO: Flow Distribution Tests for Sand, Anthracite, and GAC

Media may be placed into the filter per AWWA guidelines after the structural and air-water distribution tests are passed. Drain the water to below the media surface after each type of media is installed and at least three backwash cycles are performed on that media layer. We want to assess the levelness of the media surface, which should be extremely level and smooth as glass. Measure down to the media in 7 different places using a laser level or string line datum and a ruled measuring stick. 
Variations more than plus or minus ½” maximum results in failure. Some specification call for plus or minus 1”, which is far too much—that’s an allowable 2” total variation! Get ready for flow short-circuiting, low relative UFRV, spouts, and other operational problems with that much allowable variation. 

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Remember Orthos’ HUGE life cycle cost benefits– At the end of the filter’s life cycle (~25 years), in contrast to a complete plastic block or folded sheet metal underdrain overhaul, nozzles are simply replaced
potentially saving $Millions.

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