The MULTIBLOCK® gravity filter underdrain system utilizes compensating lateral technology through a design of continuous, interlocking laterals formed from structural high-density polyethylene (HDPE). This configuration provides even hydraulic distribution for consistent performance in both service and backwash operations in water treatment filters.
For any water treatment facility relying on gravity filters, the underdrains are the backbone of the system, responsible for ensuring reliable water collection and efficient backwashing. Whether undergoing new construction, expansion, or rehabilitation, the performance of these components is paramount.
What is the MULTIBLOCK® Gravity Filter Underdrain System?
The MULTIBLOCK® system is a modular underdrain composed of high-strength, interlocking blocks. These blocks are joined on-site using a simple snap-together construction to form robust, continuous laterals. Each block is manufactured from lightweight, corrosion-proof structural foam HDPE (High-Density Polyethylene) and is certified to NSF® Standard 61 for use in potable water applications.
The system’s core strength lies in its compensating orifice design. This hydraulic engineering ensures that filtered water is collected evenly across the entire filter bed and that backwash air and water are distributed with exceptional uniformity, keeping filter media clean and performing at its peak.
Key Features and Core Benefits of Multiblock Gravity Filter Underdrains
- Superior Hydraulic Performance: A dual-lateral design with compensating flow orifices guarantees uniform distribution during both filtering and backwashing cycles, eliminating dead zones and maximizing filter efficiency.
- Simplified, Robust Installation: The lightweight blocks feature a unique gasket seal that eliminates the common issues of o-ring rolling and the need for lubrication during assembly. This provides a more positive, reliable seal and simplifies the installation process.
- Durable, Corrosion-Proof Construction: Made from structural foam HDPE, MULTIBLOCK® underdrains are immune to corrosion and designed to withstand a design loading of 850 psf and a downward force of 1,400 psf, ensuring a long, low-maintenance service life.
- Proven and Tested: The MULTIBLOCK® design has been rigorously validated through extensive hydraulic and structural testing in dedicated facilities and test flumes, verifying the strength and performance of every component.
- Versatile Applications: The system is adaptable to conventional concrete gravity filter cells.
Advanced Media Support: Tailored for Your Application
The MULTIBLOCK® system offers a complete range of media support options, allowing engineers to select the perfect solution for any project requirement, from traditional gravel beds to advanced direct retention plates.
Laser Shield™: The Premium Direct Media Retention Solution
For applications demanding direct media retention without compromising performance, the Laser Shield™ is the state-of-the-art choice.
- Ultra-Low Profile: At less than a tenth of an inch thick (<0.1 inches), it saves valuable filter depth compared to porous plate designs.
- Minimal Fouling Potential: The stainless-steel, open-slot design has up to 200 times less open surface area than porous bead designs, drastically reducing the potential for biological or chemical fouling.
- Superior Cleanability: Should any fouling occur, the open slots are easily and effectively cleaned, a significant advantage over porous structures.
- Application: Ideal for media with an effective size of 0.20 mm and larger, with precise 0.008” slot openings.
MULTIBLOCK® Shields: Reliable HDPE Media Retention
When project specifications call for a porous bead design, we provide factory-installed HDPE media retaining shields. The MS-500 Shield is designed for media larger than 0.45 mm, while the MS-250 Shield is suited for finer, mixed-media applications.
Traditional Gravel Support
For systems utilizing a gravel support bed, MULTIBLOCK® underdrains are designed to match the application perfectly. When air scour is required, the system can incorporate a reverse-graded bed or an in-bed air wash grid located at the media-gravel interface to achieve the widest range of air and water rates.
Unmatched Backwash Performance with MULTIWASH® Integration
When coupled with the proven MULTIWASH® baffled trough system, MULTIBLOCK® underdrains provide virtually unlimited and sustained simultaneous air and water backwashing capabilities. This powerful combination, which has benefitted hundreds of installations, ensures the most aggressive and effective media cleaning, leading to longer filter runs and superior performance.
Technical Specifications at a Glance
| Parameter | Value |
|---|---|
| Filtering Operation | 2−10 gpm/ft2 |
| Water Only Backwashing | 5−25 gpm/ft2 |
| Air Scour Only | 2−5 scfm/ft2 |
| Combined Air/Water Backwash | 5 gpm/ft2 water and 3 scfm/ft2 air |
| Nominal Block Dimensions | 11″ W×12″ H×36″ L |
| Installation Tolerance | ±1/8″ |
Frequently Asked Questions (FAQ)
Q: What are the dimensions of a MULTIBLOCK® lateral? A: A single block is nominally 11” wide x 12” high x 36” long. Blocks are joined to form laterals that can extend up to 50 feet from the central flume.
Q: Why is a gasket used between blocks instead of an O-ring? A: The flat gasket is simpler to install, held in place by an adhesive backing, and compresses to create a highly effective seal. This design eliminates the need for lubricant and prevents the common problem of O-rings rolling out of their groove during assembly.
Q: What makes the Laser Shield™ so effective against fouling? A: Its effectiveness comes from three key features:
- Low Profile: The plate is only 0.06” thick, preventing depth fouling.
- Minimal Surface Area: It has only 16 in2 of open surface area per square foot, compared to as much as 3,000 in2 for porous bead designs, leaving far less area for fouling to occur.
- Hydraulic Control: The total slot area is greater than the underdrain’s final distribution orifices, meaning flow is controlled by the underdrain itself, not the shield.
Q: What components do we provide versus the installer? A: We provide the underdrain blocks, sealing gaskets, anchors, end caps, and, if required, the Laser Shield™ retainer, flume bridges, and air headers. The installing contractor is responsible for providing the grout and all labor for assembly and installation.
Q: Is the MULTIBLOCK® system considered a “Base Bid” standard? A: Yes, due to its proven track record and reliability, the MULTIBLOCK® system is often specified as the Base Bid equipment on major water treatment projects. Alternate suppliers must undergo a rigorous prequalification process to demonstrate their equipment provides comparable or superior performance, quality, and features