How to Test Gravity Filters Post-Installation

Gravity Filters: Proper Post-Installation Testing

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 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. Paragraph 3 in the filter specifications often has minimal, vague, or insufficient testing to prove that the filters will actually perform as they’re designed. Owners and engineers delegate solely to the manufacturer whether the filters operate properly. There’s clearly a conflict, and certainly not in the best interest of the owner when there’s no objective testing criteria other than the manufacturer’s opinion.
  • Orthos has created a free CSI-format testing spec section for owners and engineers to use to validate gravity filter performance. This document download details 5 important tests for different types of underdrains. It includes 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 the levelness of the post-backwash media surface for each type of media layer.

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.


Make Sure To See All Our Video & Blog Posts!

Read Our Articles

Resilient – Reliable – Sustainable – Permanent

Get In Touch With Orthos

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 replacedpotentially saving $Millions.