Field Testing And Cost – Benefit Analysis of Anoxic/Anaerobic Selectors


  • Griborio, Alonso; Castilla, Edwin; Vinci, Paul; Pitt, Paul - Hazen and Sawyer, PC
  • Ralph Aliseo - Broward County Water and Wastewater Services

It has been well documented that anoxic and anaerobic selector zones in wastewater treatment facilities are very effective in improving the activated sludge settleability. Selectors are very effective in reducing both the average SVI and its variance (Mangrum, 1998) and are a good strategy for bulking control in full-scale activated sludge plants (Parker et al., 2003). Griborio et al. (2008) indicated that sludge settleability is the single most important factor affecting clarifier performance and capacity, and therefore the design SVI in a wastewater treatment plant has an enormous economic impact.
This paper will present the evaluation of a full scale selector zone installed in a facility that requires increasing the MLSS to enhance nitrification. The selector zone was installed to evaluate its potential benefits versus building additional clarifiers.

Implementing a Selector Zone
The study was conducted at the Broward County North Regional Wastewater Treatment Plant (BCNRWWTP) which is a facility composed of five individual activated sludge modules and rated at 100 mgd Annual Average Daily Flow (AADF). Modules A, B and D have mechanical surface aerators and Modules C and E utilize fine bubble aeration. The BCNRWWTP conducted a pilot study in one module (Module C) to evaluate the reduction of the total nitrogen (TN) in the secondary effluent. As a part of this study an anoxic selector at the front of Module C was implemented by shutting down the air diffusers. Module E is provided with an anaerobic selector. A schematic layout of the BCNRWWTP is presented in Figure 1.

Effect of the Anoxic Selector on the SVI
The Sludge Volume Index or SVI is normally used in wastewater treatment plants to indicate the settleability of the activated sludge, with lower values representing better settling conditions. The SVI determination is empirical, depends on sludge concentration and is prompt to significant errors, but it is easy to apply and therefore widely used. An SVI value of 100 mL/g is normally considered good settling sludge and values over 150 mL/g are normally considered poor settling. Table 1 shows the SVI values measured for all modules at the BCNRWWTP during the course of the pilot study and Table 2 compares the SVI values for Module C before and after the implementation of the anoxic selector. The data presented in Table 1 indicate considerably better settling for Modules C and E compared to Modules A, B and D. The average SVIs for Module C and E are 75 and 76 mL/g, respectively, while SVIs for the rest of the Modules are over 200 mL/g. Module C (during the pilot program) and Module E are provided with anoxic and anaerobic zones at the front of the biological reactors. These unaerated zones act like biological selectors that promote the growth of floc-foarming bacteria over filamentous bacteria resulting in an activated sludge with better settling and thickening characteristics. While the difference between Modules C and E compared to Modules A, B and D could be also associated with the use of mechanical surface aerator versus the use of fine bubble diffused aeration, the SVI values presented in Table 2 confirm that the anoxic selector zone considerably improved the settleability of the activated sludge in Module C.

Clarifier Evaluation and Cost Analysis
There are four secondary clarifiers currently in operation in Module C of the BCNRWWTP. The basic dimensions of the secondary clarifiers are presented in Table 3. A State Point Analysis (SPA) was conducted to determine the number of clarifiers needed for a peak flow of 40 mgd and a design MLSS of 2,000 mg/L (traditionally Module C has been operated at MLSS close to 1,000 mg/L). Using the SVI presented in Table 2, the analyses were conducted for two options: with and without the installation of an anoxic selector. The 95th percentile SVIs were selected for the evaluations. Figure 2 shows the SPA results for 4 and 5 clarifiers in operations. The results indicate that four clarifies are needed for an SVI of 120 mL/g (design SVI with a selector zone) and five clarifiers are needed for an SVI of 270 mL/g (design SVI without a selector zone). Table 4 summarizes the results for both options and compares the estimated costs. This study concludes that having an anoxic selector in the first part of the flow-through activated sludge basin in Module C resulted in low and stable SVI values that require 20% less clarifier area than without selector and results in savings of 1.0 million dollars for Module C alone.

Griborio, A., Rohrbacher, J., Taylor, R., Pitt, P. and Latimer, R. (2008). Evaluation of Wet Weather Strategies and Clarifier Optimization Using State-of-the-Art Tools. Proceedings Water Environment Federation 81st Annual Conference & Exposition, Chicago, Illinois, pp. 4957 – 4968.
Parker, D., Appleton, R., Bratby, J., and Melcer, H. (2003). Anoxic or Anaerobic Slectors: Which is Better?. Proceedings Water Environment Federation 76th Annual Conference & Exposition, Los Angeles, California, pp. 582 – 598.
Mangrum, Robert (1998). The effect of Anoxic Selectors on the Control of Activated Sludge Bulking and Foaming. Master thesis submitted to the Faculty of the Virginian Polytechnic Institute and State University. October 1998, Falls Church, Virginia, 70 pages.

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