Transition to Excellence in Wet Weather Treatment

Aug. 10, 2022

The Client

Knoxville Utilities Board (KUB) provides electric, natural gas, water, and wastewater services to more than 400,000 customers in and around Knoxville, TN. Fourth Creek WWTP is one of four wastewater treatment plants owned by KUB.

The Client’s Needs

The Fourth Creek WWTP was originally built in 1966 as an activated sludge facility with blending to accommodate peak flows during rain events. KUB desired to eliminate blending activities and provide full biological treatment for all flows entering the plant. In addition, KUB wanted to increase treatment flexibility in a relatively short process time while maintaining as small of a footprint on plant grounds as possible.

The Solution

This upgrade utilized plant grounds where unused tankage once existed. The phased approach converted a dual train 9.5 MGD ACTIFLO® system, originally installed in 2014, to the BIOACTIFLO™ system. BIOACTIFLO™ is a high rate clarifier that includes a biological solids contact tank, which can be incorporated into the treatment flow path to improve soluble biological oxygen demand (BOD) removal throughout the system. A small stream of return activated sludge (RAS) is borrowed from the existing secondary system and combined with the peak excess flows within the solids contact tank. A targeted mixed liquor suspended solids (MLSS) concentration is maintained in the contact tank to facilitate rapid uptake of soluble BOD via contact stabilization. Clarification with ACTIFLO® follows, producing exceptional TSS and total BOD removal rates. The plant upgrades were completed in 2018 with an ease of operation and exceptional performance.

 The Benefits

  • Operational Flexibility
  • Proven Performance
  • Compact Footprint
  • Resilient Process
  • Unlimited Treatment Capacity

Process Description

BIOACTIFLO™ for wet weather SSO/CSO treatment, combines the benefits of ACTIFLO® high rate ballasted clarification with contact stabilization. The contact tank blends the diverted peak excess flows with borrowed RAS from the main plant which facilitates sBOD uptake onto the MLSS. Followed by exceptional TSS removal through the clarification step, the pairing of technologies allows for BIOACTIFLO™ to also meet total BOD removals and therefore providing 100% biological treatment of all wet weather flows meeting secondary standards. The BIOACTIFLO™ process was established, and built upon, over 20 years of wet weather treatment experience. This versatile treatment system is ideal for handling the variability associated with peak wet weather flows due to its short start-up times and ability to rapidly respond to varying influent conditions. 


The BIOACTIFLO™ system provided at the Fourth Creek WWTP has been a success and has been proven to be a reliable solution for handling peak excess flows. The clarification process and the ability to remove both soluble and insoluble BOD in a very short detention time has proven to meet regulatory standards while being easy to operate. This was put to the test in 2018 and 2019, when the area received record rainfall resulting in widespread flooding that lasted for days. KUB treated over 4 billion gallons of water during this time period and BIOACTIFLO™ helped Fourth Creek WWTP handle these historic flows and remain in compliance.

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