Underdrain Filters: Guide to Selection, Installation, & Maintenance

01｜Underdrain Filters Basics

1.1 Functions & Requirements

A qualified Underdrain Filters must achieve the following:

• Support the filtration media
• Collect filtered water uniformly
• Distribute backwash water and air evenly
• Prevent media loss
• Maintain low head loss and low resistance
• Withstand pressure, seismic, corrosion, and last at least 25–30 years

1.2 Main Technology Types

02｜Working Principles of Underdrain Filters

1. Structure and Components

An underdrain filter typically consists of:

• Filter Media: Sand, gravel, anthracite, or multimedia layers that remove suspended solids.

• Underdrain System: Located at the bottom of the filter bed; it usually consists of:

  • Laterals or nozzles: Small openings that allow water to pass while retaining the media.

  • Support gravel layers: Prevent fine media from escaping into the underdrain.

• Air/Water Distribution System: In some designs, compressed air or water can be used for backwashing.

2. Working Principles

The operation of an underdrain filter can be divided into two main stages: filtration and backwashing.

a) Filtration Stage

1. Raw water flows downward through the filter media.

2. Suspended particles (sediment, organic matter, microorganisms) are trapped in the media bed.

3. Filtered water passes through the underdrain system and is collected for distribution.

4. Uniform flow is ensured by the underdrain laterals, which prevent channeling and allow even water distribution across the filter.

Key points:

• The underdrain supports the filter media and prevents it from being washed out.

• Proper hydraulic design ensures minimal head loss and efficient filtration.

b) Backwashing Stage

1. Over time, the filter media becomes clogged with trapped solids, increasing the head loss.

2. Backwashing involves reversing the flow: water or air lifts and fluidizes the media bed.

3. Trapped particles are carried away with the backwash water.

4. After cleaning, the media settles uniformly on the underdrain support.

Key points:

• The underdrain allows uniform distribution of backwash water or air.

• It prevents the media from escaping while efficiently removing accumulated solids.

3. Design Considerations

• Even flow distribution: Ensures the entire bed is utilized.

• Prevention of media loss: Nozzles and gravel layers act as screens.

• Backwash efficiency: Underdrain design must allow sufficient velocity to fluidize media without damage.

• Maintenance: Access for inspection and cleaning is essential.

4. Advantages of Underdrain Filters

• High efficiency in particle removal.

• Supports a variety of media types.

• Easy to backwash, restoring performance.

• Reduces channeling and uneven flow.

In short, the underdrain filter works by allowing water to flow through a media bed while capturing suspended particles, collecting filtered water at the bottom, and enabling easy cleaning through backwashing without losing the filter media.

03｜Types of Underdrain Filters & Engineering Characteristics

3.1 Stainless Steel Underdrain Blocks

Structure: Dual-layer air-water distribution system: primary chamber + secondary compensation chamber, evenly spaced ports along the module, automatic differential compensation.

Advantages: Uniform backwash, no dead zones, suitable for high-intensity backwash filter beds (deep activated carbon, mixed media), high structural strength, corrosion resistant.

Typical Applications: Municipal water plants, denitrification filters, high-head deep-bed filters.

3.2 Modular HDPE Underdrain Blocks

Features: Primary + secondary chamber, return channels to prevent dry zones, HDPE ensures ≥30-year service life, rapid assembly, lightweight to reduce structural load.

Advantages: High cost-performance for large projects, corrosion-resistant, compatible with sand, anthracite, activated carbon.

Applications: Water reuse, biofiltration (BAF), municipal wastewater treatment.

3.3 Continuous Integral Underdrain Pipe

Structure: HDPE/PVC extruded seamless pipe, reinforced ribs and hollow structures, length 6–12 m.

Advantages: Leak-proof, high hoop strength, smooth inner wall for minimal head loss, fast installation, suitable for long, large filter beds.

Applications: Mega water plants (>200,000 m³/day), deep beds, backwash >18 L/s·m², critical reliability projects.

04｜Selection Guide: How to Select the Right Underdrain Filters

4.1 Selection Principles

Selection must consider:

• Filter bed size & depth
• Backwash intensity (water + air)
• Media type & size
• Allowable head loss
• Lifecycle cost (LCC)
• Structural durability (≥25–30 years)

4.2 Product Applicability Table

By Filter Bed Size:

By Backwash Intensity:

By Media Type:

05｜Installation Guidelines

5.1 Site Preparation

• Ensure the foundation is level and structurally stable.

• Clean the surface; remove debris, sharp objects, or hardened mortar to avoid damaging nozzles or laterals.

5.2 Underdrain System Installation

• Install according to the design drawings (main headers, laterals, nozzle arrangements).

• Nozzle installation:

  • Tighten all nozzles securely and ensure vertical alignment.

  • Verify slot/opening size matches the filter media specification.

• Levelness requirement:

  • Must be level within ±3mm per 5m run to ensure uniform backwash distribution.

• Avoid heavy loading on the installed laterals or blocks before media placement.

5.3 Leak Testing

• Before media placement, fill the filter cell with water and check for leaks.

5.4 Media Placement

• Slowly fill the filter bed to prevent damage to the underdrain nozzles.

• The coarsest media layer should be placed first, followed by the finer layers.

5.5 Post-Installation Check

• Verify all connection bolts are tight and valves are correctly set before starting the system.

06｜Operation & Maintenance