Does Height Matter? You Bet!

February 9, 2017 -- Not being the tallest person, I recognize that height comes with some advantages, particularly in business--see the following interesting texts: Gladwell and Adams et alWhat is also clear is that for filter underdrains, height is important for effective water and air distribution.

Results from a long-term, full-scale pilot by Greater Cincinnati Water Works were recently presented by Ying Hong at the Ohio AWWA Conference in September 2016.  Four types of "header-lateral" type underdrains were evaluated: (1) Clay block [±10" height], (2) Type S block [±12"], (3) Type SL block [±8"], and (4) Triangular folded sheet metal [Δ-Metal, ±5.5"].  Unfortunately, monolithic suspended floor underdrains were not evaluated; however, it is straight-forward that a wide-open, 18"-tall underdrain plenum provides better air/water distribution than 12" or shorter header-lateral systems.
Examination of the pilot data provides the following summary graph and table:

What is apparent is that underdrain height (or lateral cross-sectional area) is largely proportional to filter run time and unit filter run volume (UFRV).  Taller underdrains performed better due to improved media cleaning, which occurs through enhanced scour air and backwash water distribution.  

Type S Block and Δ-Metal Laterals
Open-Plenum Monolithic Underdrain

As compared to header-lateral underdrains, velocities under a monolithic underdrain are relatively low, which reliably leads to good backwash and air scour distribution.  The plenum volume buffers changes and provides uniform water and air flowrates across the filter floor.  Underdrains with laterals produce higher velocities, friction loss, and the Bernoulli effect, resulting in limits of effective lateral length, additional filter design concerns, and increased maldistribution.  Why risk non-performance for a few inches of additional monolithic floor height?

Orthos offers plenum designs as short as 14 to 18 inches--these can be inspected with a sewer camera and cleaned using an inserted hose, unlike header/lateral systems.  Remember, Orthos has never had an underdrain failure.  Needing to get into the plenum after commissioning is a very unlikely event.  Please contact me to discuss this or any of the above!
Orthos never underdrain orange

Say "NO" to floor failures! 
Providing filtration solutions since 1978, Orthos Liquid Systems, Inc., is the leading manufacturer of nozzle-based filter systems and nozzle products in water/wastewater treatment. With thousands of North American installations in the municipal and industrial markets and millions of nozzles sold, Orthos delivers unparalleled quality and system longevity. Orthos is also the North American master licensee for Schlick® atomizing nozzles for pharmaceutical and other precision application industries. For more information, visit or call 843-987-7200.

Stuart Humphries
Orthos Liquid Systems, Inc.
Orthos Liquid Systems, Inc.
P.O. Box 1970
Bluffton, SC 29910
© 2017 Orthos Liquid Systems, Inc. All Rights Reserved.

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