TETRA® Modular Underdrain Cleanwater Filters

As a complete filtration specialist, Severn Trent Services offers the TETRA® “LP” and “U” modular underdrain filters to meet a variety of needs.

• New Filter Design
• Retrofit Existing Filters
• Underdrain
• Filter Media
• Wash Troughs, Surface Sweeps
• Instrumentation and Controls
• Filter Problem Solutions and Service

The proven TETRA “U” Block will provide superior distribution for both water and air, separately and concurrently. 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 bed.

Total media cleaning is the key requirement for a filter to function effectively and efficiently under all loading conditions. Backwashing the gravity filters is necessary to remove the solids collected by the media during the filtration run. An upward flow of water or a combination of air and water to fluidize and scour the media is required. The effectiveness of this backwash operation is determined by the resulting cleanliness of the media.

The superior design of the TETRA “U” Block allows for the following technical advantages:

• Lightweight, easy-to-handle - reduces installation costs
• Snap fit, single gasket, bell and spigot joint assembly ensures a tight fit with no leaking
• Reduced installation time and costs due to simple assembly
• Long maintenance-free life
• No moving or wearing parts
• No corrosive materials used
• Easy-to-adapt to new and old filter designs for either hydraulic only or air/water backwash applications
• Even air and water distribution across the entire filter bed during backwash guarantees high efficiency backwash and a clean filter bed. No dead spaces, no mud balling and no filter failures
• Low headloss and non-clogging underdrain - no filter blow-ups

The TETRA “LP” underdrain block has the same superior design as the “U” Block and share most of the same features and benefits. The LP Block
differs by only:

• Lower Profile: allows for the retrofit of existing shallow filters and air/water backwash and greater media depth
• Wider Profile: allows for fewer blocks to cover the filter floor which limits joints and grout
• Maldistribution: plus or minus 5% for all lateral lengths up to 30 feet long
• Flexibility: Can be cut in half, lengthwise, to avoid filling the remaining space with grout that will not accommodate a full width block

The TETRA media retention plate is designed and installed to ensure long-term stability and durability. It was designed as a replacement for the gravel supporting filter media beds equipped with the TETRA “U” Block and “LP” Block underdrains.

The media retention plate is manufactured of ultraviolet resistant HDPE which is fitted to the top of the underdrain block. The plate is fastened to the block with three rows of stainless steel self-tapping screws (18 minimum per block) and the edges are sealed in place by NSF certified, gasket forming, polyurethane sealant. The exposed edges of the plate will also be covered with the sealant to prevent side leakage. The media retention plate will provide uniform distribution at headloss equivalent to, or less than gravel headloss.

The dual parallel lateral underdrain block provides flow distribution during backwash with a variation usually of no more than plus or minus five percent across the filter. Mounting the plate on the top of each individual underdrain block assures uniform flow distribution. The flow distribution of the plate, for both air and water, is equal to that of the standard reverse gradation support gravel. The headloss is the same as it is for the gravel it replaces. The headloss for the 12 inch depth of reverse gradation support gravel is 6 inches of water at 22.5 gpm/sf.

Most existing filters can be rehabilitated using the “U” Block or the “LP” Block with the integrally mounted plate. Since the need for support gravel is eliminated, a deeper media load can be installed in any shallow filter. Filters with the TETRA underdrain block can be retrofitted with any one of the TETRA media retention plates.

The TETRA Media Retention Plate shall be a porous, tri-molded, bi-molded or single-molded plate. The tri-molded plate will have a top and bottom layer of 500 microns average and a middle layer of 300 micros average. The bi-molded plate will have a top layer of 300 microns average and a bottom layer of 500 microns average. The single layer plate is 500 microns average.

• Lightweight, easy-to-install and removable for inspection
• Provide uniform flow distribution at a headloss equivalent to or less than gravel
• Compatible with water treatment chemicals
• Factory sealed and mounted