Design and Construction of BNR Upgrades at an Operating Secondary Treatment Facility
- Patrick Moore, P.E., Petersen Benjamin, P.E., and Fred Holmes, P.E. - Hazen and Sawyer
- Sondra Lee, P.E. - City of Tallahassee
The Thomas P. Smith Water Reclamation Facility (TPSWRF) is a 26.5-mgd (annual average) secondary treatment facility located in Tallahassee, Florida. Effluent from TPSWRF is disposed of by land application on crop sprayfields. The USGS identified the sprayfield as one suspected source of increased nitrogen levels in nearby Wakulla Springs, one of the largest and deepest natural springs in the world. As a result, the Florida Department of Environmental Protection (FDEP) implemented phased nutrient limits over a six year period. In January, 2014, the TPSWRF will be required to meet Advanced Water Treatment (AWT) standards with effluent limits of 3 mg/L total nitrogen (TN) and 2.5 mg/L total phosphorus (TP). Hazen and Sawyer designed upgrades and oversaw construction to meet the final limits as well as interim improvements to allow the plant to meet the phased limits while construction was underway.
Design and Construction Challenges
Incorporating AWT upgrades into an existing treatment facility presented a number of challenges and constraints. This presentation will cover
• Site Layout and Hydraulic Profile – the space available for new facilities was limited and in some cases was not ideal. The locations available for the preliminary treatment, primary treatment and disinfection facilities had a significant impact on the hydraulic profile.
• Hydraulic Profile – due to the location of the new preliminary and primary treatment facilities, a pump station was required upstream of the BNR Basins. The new pump station eliminated additional pumping of wastewater downstream of BNR basins.
• Existing Subsurface Infrastructure – the existing facilities were constructed in at least six different phases. The quality of available As-Built drawings varied and sometimes conflicted. Subsurface utility location utilized ground penetrating radar, and over 200 “soft-dig” locates to verify pipes at key points.
• Airport Proximity – the TPSWRF is next to the Tallahassee Regional airport and lies directly in the landing path of one of its two runways. This had to be considered when locating and designing several of the tallest structures. This was also a significant challenge during construction.
• Soil Conditions – the existing subsurface conditions required three of the new proposed facilities be supported on auger-cast minipiles (over 500 total).
• Electrical Supply – the existing electrical system did not meet Class I reliability. In addition, the proposed electrical load represented a 55 percent increase over the existing.
• Maintaining Plant Operations – disruptions to the existing treatment facility were minimized by maintaining access for operations and maintenance staff, scheduling shutdowns for tie-in work for off-peak hours and providing temporary bypass piping.
• Testing and Start-up – testing of new facilities often required temporary provisions so that existing operations were not affected. When ready, new facilities had to be integrated into the existing process which sometimes required interim operation and control protocols.
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