Tuesday, September 10, 2024
2:30 PM - 3:00 PM
Location Name
Gaining Clarity – Secondary Clarifier Upgrades at Nashville CWRF
Clean Water Technology

As part of the Central Water Reclamation Facility (WRF) Capacity Improvements and CSO Reduction Project, the eight north secondary clarifiers needed a major overhaul in order to achieve the process and hydraulic capacity required for secondary treatment. The existing clarifiers had documented issues with out-of-plumb center columns and torn seals on the suction manifolds, resulting in high sludge blankets and reduced capacity. Several years ago, Metro Water Services operations and maintenance staff implemented some short-term fixes to continue operations and provide temporary improvements. However, it became clear as the Central WRF Project was underway that long-term solutions for these clarifiers were needed to achieve consistent and reliable performance. A change order was issued to include new clarifier mechanisms with key design improvements, structural refurbishment, and electrical & controls upgrades. Two design improvements to the clarifiers will be particularly highlighted in this presentation: the larger energy dissipating inlets (EDI) and rectangular-notch weirs: • Larger EDIs with strategically angled baffles forced the clarifier’s influent flow to slow down, reducing eddies and waves that inhibit the clarification and settling process. In conjunction with the aeration system improvements that lowered the overall Sludge Volume Index (SVI), the larger EDIs created conditions for improved sludge settling. • Replacing the existing V-notch weirs with rectangular-notch weirs had many benefits. Previously, algae growth on the clarifiers consistently plugged the V-notch weirs, resulting in reduced capacity. The new rectangular-notch weirs are inherently self-cleaning, can operate under both submerged and unsubmerged conditions, and are less impacted by minor inconsistencies in leveling and fabrication. Now the clarifiers can achieve their design capacity with higher effluent quality and UV transmittance. Because the Central WRF was operating continuously during this overall construction project, Maintenance of Plant Operations (MOPO) directed the schedule and logistics for upgrading these clarifiers. Coordination between Metro Water Services staff, the construction contractor, and the rest of the project team was paramount to ensuring the appropriate number of tanks were in service and operationally sound for expected and unexpected weather conditions. A maximum of two north secondary clarifiers were taken out of service at a time to complete the demolition, upgrades, and start-up and testing periods. Crane placement was another key consideration the team had to account for. Thanks to this coordination, the improvements to all eight north secondary clarifiers were integrated into the Project without delays to the overall schedule. This presentation will cover the design development, construction, punch-walk and startup, and performance improvements of the upgraded north secondary clarifiers. The design was submitted as part of a change order in November 2021, and the overall project construction is anticipated to be complete in the first quarter of 2024.