Treatment plant selects innovative underdrain for high performance

The Lake County Public Water District, in northern Illinois, needed to upgrade its filtration and backwash system to ensure reliable, long-term performance. Selecting Xylem’s Leopold Type 360 Underdrain wasn’t a hard choice. Here’s why.
 
The District processes 6.5 million gallons (24,600 cubic meters) of water per day to provide drinking water to several neighboring communities in northern Illinois. There’s simply no cutting corners on quality, safety or reliability, which is why the District has worked with Leopold, a Xylem brand, for more than half a century.
 
Based in Zion, Illinois, the District has long enjoyed dependable performance from Leopold underdrain systems. Their previous two systems each performed for 25 years before being upgraded. Now, the District has installed Leopold’s latest innovation in underdrains, the Type 360 Underdrain, featuring a cutting-edge design for optimal filtration and backwash.

“Xylem was instrumental in getting us on the right path and in the right direction in engineering the new system,” said Don White, the District’s General Manager.

How gravity media filtration systems work

Filtration, which removes dissolved particles from water, is one of the last steps in treating water for drinking. In gravity media filtration systems, water flows down through media, such as sand and anthracite, to filter out the particles. During a filtration run, solids collect in the media and periodically must be removed to maintain performance. The run typically lasts 24 to 48 hours, but can be longer depending on the influent water and media arrangement.
 
Solids are removed through backwashing, a process where underdrains at the bottom of the filtration tank send water up through the media to dislodge the particles. A combination of water and air (called air scouring) can be used to optimize the process.
 
Monitoring head loss is essential to ensuring performance in filtration. Head loss is measured as the distance between the bottom of the undrain system and the surface of the water. When a filtration run begins, the head loss is less, since the water can move more freely through the filter. As the run cycle continues, the head loss grows, since the build-up of particles blocks some of the flow. The typical maximum head loss for a filter run is 5 feet (1.5) meters, since above this limit the system does not perform optimally.

Designing the optimal filtration system

Xylem’s Leopold team worked closely with local engineers and contractors to design a solution that would perform reliably over the long term. One of the main considerations was ensuring the system had the necessary piping for air scouring in the future, which will reduce backwash water use.

“They were proactive and wanted to move on to a new underdrain system that would require less maintenance or be maintenance-free,” said Bruce Wolfe, Territory Manager for Leopold.

The Leopold Type 360 Underdrain

The Type 360 is a groutless underdrain system that is bolted in place, allowing service to individual laterals. The underdrain delivers best-in-class fluidization during backwash. By allowing for 100% backwash of the media above and between the laterals, the Type 360 Underdrain eliminates dead zones in the filter media to increase filter run times, reduce backwash water consumption, extend media life, and lower energy costs.
 
It also features an efficient self-cleaning system for maximum reliability and uptime. These were among the many qualities that appealed to the District, which was looking decades ahead when they made the decision to upgrade.
 
Leopold also provided the engineered filter media, consisting of 15 inches of silica sand and 20 inches of anthracite. The installation was completed in March 2022, and the District is already seeing promising results.

“The system is functioning above expectations,” White said. “The filter run hours are above 150 hours. The filter effluent turbidities are tremendous. Loss of heads are below five feet when we’re doing a backwash. The filter is performing outstandingly.”

And when the health of local residents is on the line, that’s precisely the kind of performance one should expect.
 
“An underdrain system is one of the final steps in a water treatment plant,” Wolfe said. “And that’s probably the most crucial piece of equipment. So, you want to have a reliable system that will provide safe drinking water to the community.”
 

 
Learn more about Xylem’s Leopold gravity media filtration systems.
 
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