Addressing Flow Reduction in Filtration Systems: A Guide to Underdrain Cleaning & Maintenance

Filtration systems are the backbone of many water treatment operations. When these systems begin to show signs of reduced flow, the issue can often be traced back to the underdrain—an essential but often overlooked component. For example, flow loss persistent after the filtration media has been removed reveals that the restriction was occurring within the underdrain itself. This type of buildup signals that it's time to take a closer look at the cleaning process and maintenance practices.

Understanding the Problem: Internal Underdrain Fouling

In a typical downflow filtration cycle, water passes through the media bed and enters the underdrain from the outside. During a backwash cycle, the direction reverses: clean water flows up through the center of the underdrain and out through the media, flushing it of accumulated particles.

When the water used during backwash isn’t clean, or if flow and pressure are insufficient, debris can accumulate inside the underdrain screen instead of being flushed out. Over time, this internal fouling can severely restrict flow, even when the media bed is clean.

Key questions to consider in troubleshooting:

• Is the system being backwashed on a regular schedule?

  
  

• Does the unit experience long periods of stagnation?

  
  

• Is backwash pressure and flow adequate to effectively scour the system?

  
  

• Is the backwash water itself free of solids and contaminants?

If the answer to any of these points is “no,” internal fouling of the underdrain is more likely.

Restoring Performance: Acid Cleaning Procedure

When normal flushing or mechanical cleaning isn't effective, chemical cleaning can help restore performance. Our maintenance guidelines recommend using a diluted muriatic acid solution to break down internal buildup and restore flow.

Acid Mixture Ratio:

1. Prepare a 10% muriatic acid solution

   
   

2. Mix 1 part muriatic acid (commonly 31–33% hydrochloric acid) with 9 parts clean water

   
   

3. Always add acid to water slowly, never the other way around, to avoid dangerous reactions

Recommended Cleaning Steps:

1. Remove the media from the vessel to fully expose the underdrain.

   
   

2. Wear appropriate PPE and ensure good ventilation—acid fumes can be harmful.

   
   

3. Fill the vessel until the underdrain is submerged in the acid solution.

   
   

4. Allow the solution to dwell for 60 to 120 minutes, depending on severity of fouling.

   
   

5. Drain and flush the vessel thoroughly with fresh water to remove any remaining acid.

   
   

6. Test underdrain flow. If restored, proceed to reload the filtration media and resume normal operations.

Long-Term Prevention Strategies

To reduce the likelihood of future flow issues:

• Backwash consistently to avoid sediment buildup and stagnation.

  
  

• Monitor and maintain proper backwash flow rates and pressures.

  
  

• Use clean, filtered water during backwash whenever possible.

  
  

• Inspect system components regularly, especially in high-demand or high-sediment applications.

Proper care of the underdrain system is essential to the health of the entire filtration process. When fouling is allowed to accumulate inside the screen, performance and efficiency suffer. With routine inspections, effective backwash practices, and periodic chemical cleaning when necessary, your system can continue operating at peak performance for years to come.

If you need a detailed maintenance guide or would like technical assistance with your specific setup, our support team is here to help.