Stainless Steel Filter Underdrain

MAXIMUS™ “free-flow” filter underdrain is constructed of robust, 10-gauge stainless steel (SST).  Chemically-anchored to the filter floor using SST hardware for quick installation, the patent-pending MAXIMUS™ features a low, 6” profile that provides increased filter media depth or operational freeboard in retrofit applications. 

Equalizing orifices between nozzle plate supports provide free-flow of water and air across the entire underdrain plenum footprint, eliminating the flow distribution and pressure differential problems associated with plastic block and triangular lateral style systems.

Maximus Free Flow Orifices
Type D Nozzle for Stainless Steel Filter Underdrain

ORTHOS Type D nozzles with built-in pressure differential are installed into the MAXIMUS plates on 6” centers each way to further ensure excellent distribution and to cast aside problematic gravel or other support media.  Long-lived gaskets and SST hardware seal adjoining nozzle plates, the plates to the perimeter frame, and the frame to the filter floor to fully encapsulate MAXIMUS.



A: Retrofits of concrete-tankage, granular media filters are a perfect application, though because of its relatively-quick installation, new, “greenfield” applications certainly deserve consideration of MAXIMUS installation.

A: Front- and Center-Flume as well as H-Flume (or Upper/Lower Gullet) arrangements may all be adapted into by MAXIMUS.

A:  The free-flow orifices of MAXIMUS allow for excellent distribution characteristics down nozzle plate lengths in excess of 20 feet away from a flume.  Plastic block and triangular header-lateral systems with 6-inch height are not recommended for use past this dimension. 

A: Yes! Any remaining odd or fractional remaining space between the perimeter frame and the filter wall is to be filled with grout and filleted.  The dimensions of MAXIMUS are custom-designed for each specific application.

A: Yes!  ORTHOS Type D nozzles with tailpipes are installed into the MAXIMUS underdrain plates.  As scour air is introduced into the underdrain, an air blanket collects immediately under the plates.  Once the air blanket reaches the air metering holes on the side of the nozzle tailpipes, scour air is delivered up through the nozzles and into the media bed.

A: If at all practicable, scour air should be introduced under the nozzle plates using a manifold in the flume.  Manifold riser pipes are connected each nozzle plate and span plate volume and terminate slightly below the plates.  A less desirable method is to connect drop pipes to MAXIMUS from an above manifold, requiring piping to extend through the media, which is not allowed in many US states.

A: When the air scour process is terminated, air will continue to exit the underdrain through the air metering holes in the nozzle tailpipe. Once the blanket rises to the top of the metering holes, air will escape through a small-diameter air bleed hole at the top of each nozzle tailpipe and located immediately under the screen section.

A: Once all components are installed, MAXIMUS may be tested and placed into service immediately.  As part of installation, the filter floor leveling coarse and flume end plug (as necessary) must cure for 24 hours minimum before MAXIMUS may be installed.  The HILTI chemical anchors for the perimeter frame and nozzle plates require 12-hour minimum bond time.  Afterward, the remaining components of MAXIMUS may be installed, and the underdrain promptly tested and placed into service.

A: The thick, 10-gauge stainless steel perimeter frame, nozzle plates with its robust stiffeners, and anchors will withstand a minimum of 1600 lbs/ft2 uplift force.  Safety factors are built into MAXIMUS to ensure minimal deflection.

A: Yes! ORTHOS has teamed with independent, nationally-recognized FEA and CFD firms to model MAXIMUS and all of its potential configurations.

Stainless Steel Underdrain Finite Element Analysis

A:  The perimeter frame of MAXIMUS is secured using anchor bolts with nut and washer on 6” centers using NSF-61 certified HILTI HIT-HY 500 V3 epoxy.  A gasket is placed between the perimeter frame, floor, flume support angle, and flume end plug (as necessary) to seal the bottom perimeter of MAXIMUS.

A: Nozzle plates are secured to the filter floor using square anchor tubes, each with a nut, washer, and bolt that is chemically-anchored into the floor using NSF-61 certified HILTI HIT-HY 500 V3 epoxy.  Span plates with gaskets are bolted through the nozzle plates and into nuts that are factory-welded to the underside of the nozzle plate.  Nozzle plates and span plates extend over the perimeter frame and are connected with bolts and gaskets to further seal the top of MAXIMUS.

A: Triangular-shaped underdrain laterals present three concerns: (1) Spacing on 12” centers that creates large dead zones between lateral lengths; (2) the inherent problem of maldistribution down each lateral in a header-lateral configuration; and (3) structural integrity of 20 gauge (0.0375 inch) sheet metal.  MAXIMUS solves all of these problems by effectively creating a 10 gauge (0.1406 inch—4x as thick) plate underdrain with free-flow orifices (creating an open plenum) and with nozzles on 6-inch centers each way (with excellent flow distribution).

A: No.  The free-flow orifices of MAXIMUS enable backwash water and scour air to flow freely between nozzle plates, effectively creating a wide-open plenum that ensures excellent distribution through ORTHOS’ long-lived nozzles.

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