Proper Post-Installation Testing of Gravity Filters


Proper Post-Installation Testing of Gravity Filters

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

  • Our competitors offer an overall less-effective underdrain system, often at a higher price, while falsely promising the same results that Orthos delivers time-after-time.
  • Post-installation testing of gravity filters is extremely important to verify structural strength and validate proper distribution. All too often in the filter specifications, Paragraph 3 has minimal, vague, or insufficient testing to prove that the filters will actually perform as they’re supposed to. I’ve repeatedly seen owners and engineers delegate solely to the manufacturer whether the filters operate properly. With no objective testing criteria other than the manufacturer’s opinion, that’s clearly a conflict, and certainly not in the best interest of the owner.
  • Orthos has created a free CSI-format testing spec section for owners and engineers to use to validate gravity filter performance. This Word doc download details 5 important tests for different types of underdrains, to include nozzle-based monolithic or steel plate floors; dual-parallel plastic block underdrains, and wedgewire screen or folded sheet metal lateral systems. These tests must be done, or how does the owner know what they’re buying?
  • The tests detailed in the free download validate the filter underdrains’ structural integrity; air scour and backwash water distribution; and for each type of media layer, the levelness of the post-backwash media surface.

The Two Most Important Tests

ONE: Air scour portion of the second test in our free download, the Underdrain Flow Distribution Test.

After the underdrain’s structural integrity is verified, the filter is filled with water to 6” above the underdrain. The Underdrain Flow Distribution Test provides a straight-forward visual test for proper backwash distribution. At first, the air scour-only pattern is observed, which often reveals the deficiencies of poor-distribution underdrains and if the underdrain was installed incorrectly. During this test, 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, as well as the relatively poor air scour distribution of folded sheet metal and wedgewire screen lateral systems that are 12” apart. Since a filter is nothing more than a solids storage device, if, during air scour and backwash, its underdrain is designed such that the filter can’t properly clean itself, solids will not be properly removed. Thus, 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

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

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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.