City Of Tallahassee’s T.P. Smith WRF Upgrade from Secondary Limits To ENR Nitrogen Limits

Authors:

  • Ron J. Latimer, P.E., Paul A. Pitt, Ph.D., P.E, Alonso Griborio, Ph.D., P.E. - Hazen and Sawyer
  • Joe Cheatham, Sondra Lee, Darby Dressel - City of Tallahassee, Florida

Introduction
The T.P. Smith Water Reclamation Facility (TPSWRF) provides wastewater treatment and water reclamation for the majority of the City of Tallahassee, Florida. The existing 26.5 mgd facility has not historically had nitrogen or phosphorus permit limits. The City is now under a Settlement Agreement to meet interim nitrogen limits which progressively become more stringent over the next 6 years, and at the end of the 6 year period, requires AWT standards including an ENR total nitrogen limit of 3 mg/L to be met.

Background
Hazen and Sawyer is currently proceeding with the design of improvements required to meet the interim and final permit limits defined by the Settlement Agreement. The existing facility consists of three different and independent activated sludge treatment trains: one train has mechanical surface aerators (Train 2), one train has jet aeration (Train 3), and the third train is an MLE configuration with fine bubble diffusers (Train 4).

Special Sampling And Model Calibration
Special sampling was conducted to better understand the performance of the existing facilities and to develop a calibrated BioWin model of the existing facility. This sampling showed that all trains in the existing facility were close to meeting the first anticipated interim nitrogen level of 12 mg/L TN. Unexpectedly, the mechanical aeration train and jet aeration train were achieving lower effluent TN concentrations than the MLE train through simultaneous nitrification/denitrification. Another unexpected result from the special sampling was that all three trains were achieving enhanced biological phosphorus removal without formal anaerobic zones.

Microscopic examination was conducted to further understand the filamentous population of each train and to further verify the presence of EBPR microorganisms. The results verified the EBPR populations as significant and showed major differences between the independent treatment trains in terms of filamentous levels (major impacts on settling).

The special sampling data and historical data were used to calibrate a BioWin model of the existing facility. Reasonable calibration was achieved using a whole plant calibration approach.

Interim Nitrogen Limits
The calibrated BioWin model was used to evaluate interim modifications required to meet the interim nitrogen limits of 12 mg/L by July 2008 and 9 mg/l by 2011. Using the calibrated model, supported with the special sampling data, it was determined that with minor operational modifications, the existing facility could meet the initial interim TN limit of 12 mg/L. The model was used to optimize the flow split between each train, the cycling of the aerators in Train 2 and Train 3, and to show that increased RAS flow rates on all trains and increased nitrified recycle flow on Train 4 would significantly lower the effluent TN. These modifications were implemented the 1st of April 2008.

Secondary Clarifier Stress Testing And CFD Modeling
Stress testing and CFD calibration and analysis were used to evaluate the existing secondary clarifier capacity. The results indicated severe limitations in one set of secondary clarifiers. The limitation was found to be due to missing target baffles in the peripheral feed/peripheral outlet secondary clarifiers. The outcome of the CFD modeling indicated significantly different capacities between the four older, shallower clarifiers and the newer, deeper clarifiers (even with target baffles in place). Therefore, the proposed flow split to the secondary clarifiers will have differential loading to optimize performance and maximize capacity. The paper will further discuss the use of the CFD modeling tool linked with BioWin for integrated design will.

Achieving ENR Nitrogen Limits
To achieve the final permit limits, including TN = 3 mg/L, the existing reactors will all be converted to 4-stage BNR reactors with fine bubble diffusers with optional supplemental carbon addition to the post anoxic zone. The 1st anoxic zone and internal recycle will be configured to allow optional 5-stage operation to minimize metal salt requirements to meet the TP limit. In addition, new deep bed filters will be included and will allow operation in a denitrification mode for nitrate trim.

Optimization Of Design
The calibrated BioWin model was used to evaluate options to minimize chemical use and operating costs. This analysis considered alum cost, methanol cost, additional aeration cost and cost to purchase additional gas for the sludge dryer because of primary sludge fermentation reducing methane production in the digesters. This analysis showed that primary sludge fermentation was cost effective, even considering lost digester gas and the capital cost for the fermenter. The optimum 4-stage and 5-stage operating conditions resulted in approximately the same yearly cost at the initial 2.5 mg/L TP limit with the preferred location for supplemental carbon at the filters. However, if the TP limit was reduced to 0.5 mg/L, the 5-stage process with supplemental carbon addition to the BNR basins was most cost effective.

This presentation will summarize the results of this study and optimization efforts to cost effectively and quickly achieve the interim nitrogen limits and the final design for the ENR nitrogen limits. The unique findings, lessons learned and benefits achieved through the use of BioWin and CFD modeling with stress testing and special sampling will be further discussed.

For a copy of the full paper, please contact the author at rlatimer@hazenandsawyer.com

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