September 2007, Vol. 19, No.9

Letters

Letters to the Editor

Dear Editor,

Thank you for the attention given to grit removal in your July 2007 issue. This area is historically undervalued in terms of coverage and appreciation across the industry.

As a leading provider of engineered grit removal solutions with thousands of installations in North America since the early 1970s, our organization boasts significant experience and expertise of wastewater grit dynamics. Our technical staff has undertaken extensive grit study and testing, both independently and in concert with many of the nation’s top consulting firms and wastewater treatment plants.

As such, we take issue with the material presented in the Operations Forum article entitled, “The Nitty Gritty”, authored by Eutek equipment supplier president George Wilson and co-authors. Many key statements made within the article are presented or implied as scientific fact, yet they are not substantively supported with actual data or footnoted studies. With no established bases, the reader is essentially subject to theoretical conjecture and/or biased statements.

The authors assert that to “understand how to remove grit from wastewater most effectively, it is important to understand how grit behaves in that medium…” A few sentences later, the author claims that the article “explains why grit settles and why it doesn’t…” However, the authors fail to adequately develop and support these points. Let us demonstrate:

While introducing settling velocity, the authors discuss shear velocity (or “deposit limit”) and its effects on grit settleability. Why is there no mention of the fundamental concepts of laminar and turbulent flow?
Even more, the authors strangely omit the most important factor: specific gravity. The specific gravity of wastewater grit is documented and defined as 2.65 or greater (U.S. EPA, 1987)1. Understanding this value is critical because a grit particle that becomes “maintained in suspension” must have a specific gravity of less than one. Quite simply, grit doesn’t float in water.
Later, the authors state “grit particles that have the physical size to settle might not do so because surface active agents are making them buoyant.” The authors offer neither documented data on grit buoyancy nor validate the degree to which “froth” occurs. Questions arise. How much froth accumulation is required to cause particle suspension (enough to significantly lower its specific gravity)? How long does a particle’s froth remain stout and effective while traveling (for miles) in the sewer or channel without being eroded by the water velocity, turbulence or constant contact with other grit particles?
The authors not only downplay the importance of removing grit continuously, but also suggest that “care must be taken to avoid pulling samples from the bottom of the channel…” Grit removal devices should be designed to remove grit, which includes gravel, at all flow conditions to protect WWTP operations. A majority of the grit entering a plant will be found as a moving bed on the bottom of the channel. An accurate grit test will capture all sizes of grit – large, medium, and fine.
On the discussion of sampling equipment, the use of a vertically integrated sampler in the center of the channel is questionable because grit is not uniformly distributed, either vertically or horizontally. Additionally, sampling devices should remain narrow and slender — not bulky — so as to limit disruption in channel velocity and potential disruption of samples.
Finally, the use of sand equivalent size (SES) — historically promoted by a particular manufacturer — is not necessarily applicable to a variety of grit removal devices applied in WWTPs, including aerated grit chambers and flat-floored vortex units. Not all grit removal systems rely on settling as the primary means of grit removal either, therefore SES is not necessarily relevant to general grit system design.
Respectfully submitted,
Frank Weis, P.E.
Senior Engineering Consultant
Smith & Loveless Inc. (Lenexa, Kan.)

Darby Ritter
Smith & Loveless Inc.