Lessons Learned Converting an Oxidation Ditch to a BPR Plug Flow Reactor with Fine Bubble Aeration

Authors:

  • Brian Duane - Hazen and Sawyer

Oxidation ditches are promoted as an energy efficient, low maintenance activated sludge treatment process. The Gwinnett County (Georgia) Department of Water Resources operates three oxidation ditches at their Crooked Creek Water Reclamation Facility; the largest of the three is rated at 10 mgd (maximum month) and the two smaller units are rated at 3 mgd each. They found the aeration system on the large ditch difficult to maintain and supplemental aeration was required for summertime operation. The three-ring oxidation ditch operates by introducing screened influent and RAS into the outer ring before flowing to a center island outlet structure. The plant discharges into the Chattahoochee River and has permit limitations on ammonia and phosphorous of 0.7 mg/L and 0.4 mg/L, respectively. Alum is fed into the third ring to meet the total phosphorous discharge limit. This paper documents the design, operational challenges, and the lessons learned while converting the 10 mgd oxidation ditch into a plug flow reactor with fine bubble diffused aeration.

The conversion of the 10 mgd oxidation ditch into a plug flow reactor involved the construction of several baffle walls inside the basin. The plug flow configuration allowed the creation of a new anaerobic selector zone for enhanced biological phosphorus removal. BioWin® modeling indicated the addition of the anaerobic zone will allow the plant to save nearly 400 gallons per day in alum feed costs. In addition, fine bubble membrane diffusers with zone D.O. control will result in significant reductions in process energy requirements. Preliminary engineering indicated that the modifications will pay for themselves in less than 5 years.

Additional BioWin® modeling was performed to evaluate the impact of failing disc aerators, operation during construction, and locating dissolved oxygen probes for optimum DO control.

The challenges encountered were mainly due to the overall physical size of the oxidation ditch; the basin is larger than a football field, covering 1.62 acres. Other challenges included:

- Sequencing construction while maintaining half the basin in service

- Accessibility to instruments and control valves, etc., due to limited walkways

- Noise and the close proximity of a retirement community

- Cleaning the channels of accumulated grit and sludge

- Staff unfamiliarity with a new process and equipment

- Basin shape and depth and the impact on the aeration equipment and mixer installation

- Foam buildup/entrapment

Preliminary operational data indicates that the modeling was correct – power and chemical costs are reduced and the project was completed for less cost than originally estimated. The project has allowed the County to do more with minimal capital investment.

For more information, please contact the author at bduane@hazenandsawyer.com.

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