Doing More With Less: BNR Upgrades During the Great Recession

Authors:

  • K. Bilyk, B. Duane, T. Bruton - Hazen and Sawyer
  • J. Mahagan - Town of Hillsborough, NC

This paper will present creative solutions at two biological nutrient removal (BNR) facilities that resulted in the ability to defer a major capital investment with a small investment in a short-term improvement that freed up available funds to address other critical needs.

In 2008, the Town of Hillsborough NC initiated a preliminary engineering study to upgrade their 3 mgd treatment plant to meet nutrient standards of 3 mg/L TN and 0.3 mg/L TP. The study resulted in a phased approach to meeting these standards, which included construction of new BNR tanks in the second phase at a cost of approximately $6.9 million. The plant had never had a liquid train upgrade since its construction in the 1970s. The total available funding for Phase 1 improvements, provided through the State Revolving Loan Fund, was $16.4 million. Approximately $1.3 million was available to improve the existing BNR tanks. It is expected that this investment will defer the need for the new BNR tanks by at least 5 to 8 years.

The 16 mgd secondary treatment system at the Crooked Creek WRF in Georgia consists of three Orbal oxidation ditches and six clarifiers. Oxidation Ditches 1 and 2 are each rated for 3 mgd, and Oxidation Ditch 3 is rated for 10 mgd. The Gwinnett County Department of Water Resources (GCDWR) planned to replace and upgrade the secondary treatment system at the plant. The phasing for the upgrade project required that Oxidation Ditch 3 remain in reliable operation for at least 6 to 7 years while the final design and construction of the upgrade was completed. Oxidation Ditches 1 and 2 would be demolished to make way for the new treatment facilities.

In 2012, after experiencing several aerator shaft and gearbox failures, the County realized that the 20-year old aeration equipment in Oxidation Ditch 3 would not provide reliable operation for the duration of the planned upgrade project. The County initiated a project to replace the aeration equipment in Oxidation Ditch 3 with in-kind equipment. However, the preliminary design showed a diffused aeration system with an anaerobic selector zone would pay for itself in less than 5 years. The diffused air conversion of Oxidation Ditch 3 involves construction of several baffle walls to create a plug flow reactor. This configuration allows for the creation of an anaerobic selector for enhanced biological phosphorus removal. Significant reductions in process energy and chemical requirements are anticipated due to the proposed modifications. The bid price for the aeration improvements to Oxidation Ditch 3 was $5.1 million.

In 2013, GCDWR began re-evaluating the proposed upgrade project because estimated construction costs were greater than $200 million, revenues were down due to the economy, and the County had to allocate funding for other critical projects. Therefore, the interim solution of replacing the aeration equipment in Oxidation Ditch 3 is now potentially a longer-term solution that allows the County to meet treatment and capital project funding requirements while realizing reduced operating costs.

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

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