MUD opts to bid-out tube settlers

9:41 p.m. June 10, 2025

DUANE CROSS
MCO Publisher•Editor

The Metro Utility Department board took a key step toward upgrading the county’s water treatment plant on Tuesday, voting unanimously to publish a bid for the installation of new tube settlers – a solution aimed at keeping the plant operational while long-term options are explored.

Tube settlers are used in water and wastewater treatment plants to remove suspended solids (like dirt, sand, and debris) from water. They consist of a series of inclined tubes or channels, often made of PVC or other lightweight materials, arranged in parallel. These channels are typically inclined at a 60-degree angle to maximize settling efficiency.

As water flows through the tubes, suspended solids settle onto the tube surfaces. The sludge then slides down the inclined surfaces and is collected at the bottom of the basin.

Engineer Bryant Griffin presented the board with two options for improving the plant’s water clarification process: replacing the existing tube settlers for roughly $150,000 or upgrading to stainless steel plate settlers, which would increase the plant’s capacity from 800 gallons per minute to 1,200 but carry a price tag closer to $1 million.

“The material cost for tube settlers is around $50,000,” Griffin said, explaining that the full project cost, including installation, would likely run closer to $100,000-$150,000. “That gets us back to the 800-gallon-per-minute capacity the plant was originally designed for.”

By contrast, plate settlers would increase capacity to 1,200 gallons per minute and last significantly longer, but would also require major infrastructure upgrades – including enlarging the clearwell, adding two more filters, and expanding the sludge lagoon. Such an overhaul could take five years or more and require land that the utility district currently does not own.

“We probably ought to go with the tube settlers now because that buys us time," Griffin said.

Board members acknowledged the appeal of the longer-term plate settler option. Still, they agreed with Griffin and Utility Manager Ronnie Cunningham that it was too early to commit to such a large investment.

Tube Settlers vs. Plate Settlers

When it comes to water treatment clarifiers, both tube settlers and plate settlers offer distinct advantages and disadvantages, depending on the needs and resources of the utility.

Tube Settlers Advantages

• Lightweight and Portable: Made from PVC, tube settlers are easy to transport and install.
• Flexible Design: Their adaptability allows them to be fitted into various tank shapes and sizes, making them ideal for retrofits or constrained spaces.
• Cost-Effective: Tube settlers are generally less expensive, making them a good option for budget-conscious utilities.

Disadvantages

• Durability Issues: The PVC material is more prone to damage over time. Broken pieces can clog expensive membranes, leading to costly maintenance.
• Design Limitations: Poor design can impede water flow, thereby reducing treatment efficiency.

Plate Settlers Advantages

• Durable and Long-Lasting: Constructed from stainless steel, plate settlers can last decades – often with minimal wear.
• Low Maintenance: Their sturdy design requires less frequent repair and upkeep.
• Straightforward Installation: Despite their higher cost, the installation process is generally uncomplicated.

Disadvantages

• Higher Initial Cost: The materials and fabrication needed are significantly more expensive.
• Installation Constraints: Plate settlers must meet certain height requirements during installation, which can limit their placement or require structural adjustments.

Key Differences

• Cost: Tube settlers are cheaper upfront, while plate settlers require a higher investment but offer long-term savings in maintenance.
• Material & Durability: Tube settlers use PVC (less durable), while plate settlers use stainless steel (highly durable).
• Maintenance: Tube settlers may create more operational issues over time, whereas plate settlers are known for reliability.
• Flexibility: Tube settlers are more versatile for various tank shapes; plate settlers are more rigid in installation requirements.

Bottom Line

Tube settlers are a flexible, cost-effective solution ideal for short- to mid-term needs, especially where budgets are tight. Plate settlers, while costly, provide durability, lower maintenance, and long-term operational reliability – making them a better fit for long-range planning and higher-capacity systems.

Next Steps

As discussed in the April meeting, expansion of the water treatment plant will require additional land for clearwell and lagoon expansion. If a deal cannot be worked out with the property owner, alternative property will be needed – possibly requiring a more complex layout.

“You’re talking five years,” Board Chairman Barry Posluszny said, referencing the timeline for a plate settlers project. “That’s the only thing [acquiring additional land] that we see that’s going to be the fly in the ointment, if you will. If we can’t negotiate with somebody, that means what is our alternative?”

Griffin confirmed that the Tennessee Department of Environment and Conservation (TDEC) has already approved the tube settler upgrade and stated that the project could proceed to bid immediately, with bids expected to be received by the end of July. The board would then vote on the selected bid at its August meeting.

Glen Thomas made the motion to publish the bid with a second by Greg Guinn. Darrel Richards, Charles "Boo" Johnston, and Posluszny voted "yes" for unanimous consent. The project will be paid for by money already accounted for within MUD.

Griffin said he would begin exploring land availability and survey options and would also evaluate other technologies that could reduce the plant’s reliance on a larger sludge lagoon, which complicates future expansion.

“The county may not be ready in five years to expand,” Posluszny said, “but let’s say toward the end of the life of these tube settlers, somebody better start looking at another option. If [land] is because we can’t expand – that’s the biggest problem, besides money – then I want to take that hurdle away from whoever is sitting here.”