Tetra – Filter LP Block Underdrain

DeNora Tetra – TETRA LP Blocks™ dual parallel lateral underdrain and the TETRA U Blocks™

In gravity filters, the underdrain is one of the most important components contributing to overall system performance and operation. TETRA LP blocks are popular with plant owners for their low headloss, lightweight construction and easy installation either as a retrofit or for new filter underdrain applications. The easy-to-assemble TETRA underdrain filters ensure exceptional installation strength, integrity and maintenance-free life with no moving or wearing parts.

The underdrain filters feature the GroutGrip™ design, which helps to alleviate the buoyancy inherent in any plastic underdrain installation. Additionally, the wide, low-profile design requires that fewer blocks be installed to cover a filter floor. Therefore, fewer joints and less grout are needed. The system’s proven design provides superior distribution for both water and air, separately or concurrently. The blocks are formed covering the floor of the gravity filters, supporting the filter media. Air is distributed evenly across the entire filter bottom area to scour the media and to provide an air lift which, with the water, removes the released solids from the filter. This dual backwash action provides intense washing energy throughout the filter. TETRA LP Block filter floor system to negate the need for a separate air header, which significantly reduces the capital cost of rapid gravity filters.

TETRA™ LP and U block drinking water filter underdrain advantages

  • Low headloss
  • Easy installation
  • No moving parts
  • Even backwash distribution
  • Reduced backwash water
  • Lower operating cost
  • High resistance to uplift from the filter floor
  • Fewer blocks and grout required
  • Lower profile allows for the retrofit of existing filters with air/water backwash and greater media depth
  • Wider profile allows for fewer blocks to cover the filter floor, therefore, fewer joints and less grout
  • Eliminates “Maldistribution” for all lateral lengths up to 30 feet long
  • The LP Underdrain Block™ can be cut in half lengthwise to avoid filling the remaining space with grout that will not accommodate a full width block
  • Select “LP” or “U” block underdrain, with or without a media retaining plate (MRP), for placement in the base of the filter basin
    • Underdrain may be designed for support of nearly all media configurations (mono-media, dual-media, GAC).
    • The media retaining plate (MRP) eliminates the need for support gravel,common with other underdrains.

Project Profile: Kansas City, Missouri (KCMO) Water Services

DeNora furnished TETRA® LP Block™ Filter Underdrain System for the City of Kansas City, Mo., Water Services Department’s primary water treatment plant – a 240-mgd plant located along the Missouri River. The equipment was specified by the project’s engineer, Black & Veatch.

The contract called for the installation of 24 14’ X 50’ filter cells covering a surface area of 16,800 square feet. In addition, DeNora provided new stainless steel backwash air headers for the filter to simultaneously backwash with air and water. The 10”-diameter air headers were uniquely designed to fit under the floor of the filters with J-Risers to bring air through holes in the concrete floors to the underdrain.

The TETRA LP Block is a dual parallel lateral underdrain system constructed from HDPE molded plastic blocks that provide effective distribution of both air and water for either concurrent or sequential backwashing. The system’s lightweight construction and easy assembly and installation make it ideal for both new and retrofit filter underdrain applications. Its nozzle-less design ensures there are no blockages that could potentially affect filter performance.

TETRA underdrain filters feature the patented Grout Grip™ design, which increases resistance to uplift from the filter floor and offers twice the pullout strength of a conventional underdrain filter block. The wide, low-profile design requires fewer blocks to be installed to cover a filter floor, so fewer joints and less grout are needed.

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