Tango Uniform - Triangular Underdrains

Triangular Underdrains, or using the military phonetic alphabet, “Tango Uniformare a type of underdrain made of thin stainless steel sheet metal with laterals connected directly to a center flume or to an end flume box with gaskets.

Tango Uniform laterals are short with a small cross-sectional area compared to plastic block or the open plenum of Orthos Centurion™ monolithic.

Tango Uniform Performance - Water Backwash

Tango Uniform has an upper and lower conduit for air and water. Water flow rates are commonly 4 to 8 gpm/ft2 during combined air-water backwash.  Flow rates during the fluidizing rinse are 3-5 times as high, 15 to 22 gpm/ft2, depending on the media, water temperature, and other factors.


If you have a really long tube with holes, will flow be the same from holes all the way down the tube at 4 gpm/ft2 and at 22 gpm/ft2? It’s impossible and the reason why specifications for Tango Uniform only list one backwash flow rate because that’s the rate that is used to size the slots. 


If you want to use 15 gpm/ft2 because the water is really cold – get ready for maldistribution! Or if you try to use 4 gpm/ft2 during combined air-water wash – better have the air on or what’s happening with the water will be a disappointment.


Triangular Underdrains Flow Length
maldistribution tests performed by Orthos

The three-dimensional plenum of an Orthos Centurion™ buffers the pressures with its nozzles equalizing the distribution no matter the flow rate

To our knowledge, Tango Uniform manufacturers have yet to agree to perform any of these stringent, multiple flow-rate tests.  Instead, they go to great lengths in attempt to explain why AWWA’s methods of peer-reviewed filter testing are incorrect and demand test boxes instead.  

Tango Uniform Performance - Test Boxes

AWI using test boxes for underdrain testing

These vertical towers are connected to a lateral and the rise of water level measured during backwash.  The problem is if test boxes are installed near each other on the same lateral, they are hydraulically connected, and with the same slot openings, they will of course have the same water level.  It’s magic—they’re perfectly equal with zero maldistribution! 

Location of 7 different underdrain testing locations

A more proper test would be at the same time every second measure the level in seven boxes on seven different laterals in seven different places along the lateral lengths at different flowrates.  

Other Related Blogs:

Why does Orthos want the toughest post-installation testing of gravity filters in the W/WW treatment industry? Our competitors offer an overall less-effective underdrain system while falsely promising the same results that Orthos delivers time-after-time.

Orthos has demonstrated repeated success in BAFs and BACs with our monolithic design. Monolithic filter plenums allow backwash water and air to move in two directions, this advantage is extremely important in BAFs/BACs.  

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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.