A bench scale activated sludge study of nitrification and carbon treatment uning municiple wastewater

Author

Shawn Allen Hawkins

Date of Award

12-2000

Degree Type

Thesis

Degree Name

Master of Science

Major

Environmental Engineering

Major Professor

Kevin G. Robinson

Committee Members

Gary S. Sayler, Chris D. Cox

Abstract

A new bench scale biological treatment system was constructed to simulate the conventional activated sludge wastewater treatment process. This system included a novel bench scale external secondary clarifier design in place of the traditional internal bench scale clarifier. Special emphasis was placed on this design feature since external clarification and solids recycle place strong selective pressure on the bacterial population within the bench scale reactor, namely selecting for flocculent species of bacteria that are predominant in full-scale reactors. EPA’s effluent suspended solids (SS_eff) standard of 30 mg/L was adopted to benchmark the success of this new bench scale clarifier design. During subsequent operation of the system under solids quasi steady state conditions, SS_eff averaged 11, 16, 17, and 20 mg/L for bench scale reactors with 20-, 10-, 5-, and 2- day sludge ages. These performance data clearly demonstrate that the external clarifier design was successful. The newly designed bench scale treatment system (BSTS) was used to study carbon treatment and nitrification of settled municipal wastewater collected from the Kuwahee wastewater treatment plant in Knoxville, TN. For this study, the BSTS reactors were operated with a dissolved oxygen (DO) concentration ≥ 2.0 mg/L, a temperature of 20 °C, and a pH between 7.0 and 8.0. During operation, special emphasis was placed on tight sludge age control. An evaluation of sludge age control techniques revealed that current day measures of SS_eff and MLSS were required to strictly maintain reactor sludge age. A primary objective of the treatability study was an evaluation of the activated sludge kinetic coefficients for both carbon treatment and nitrification. The yield coefficient, half-saturation constant, decay rate, and the maximum specific rates of substrate utilization and biomass growth were evaluated for both the heterotrophic and autotrophic populations within the BSTS’s activated sludge. To more accurately define these kinetic coefficients, estimates of only the active autotrophic and heterotrophic biomass were utilized in quasi steady state substrate utilization calculations. The total active biomass fraction (ABF_t) was modeled as a function of reactor sludge age, while the heterotrophic and autotrophic populations therein were defined using a ratio characteristic of municipal wastewater treatment sludges. This method of modeling the active fractions was based on conclusions gathered during a thorough literature review of the subject. Heterotrophic kinetic coefficients were proposed for Knoxville’s settled municipal wastewater; the yield coefficient, half-saturation constant, decay rate, and the maximum specific rates of substrate utilization and biomass growth were found to be 0.47 mg VSS/mg COD_T, 20 mg/L COD_T, 0.12 day^-1, 2.0 mg COD_T/mg VSS-day, and 0.9 day^-1, respectively. These carbon treatment kinetic parameters are within a published range of values identified during the Literature Review, suggesting the proposed coefficients are reliable. Practical conclusions were also drawn from the carbon treatment study of Knoxville’s settled municipal wastewater. Namely, the total chemical oxygen demand (COD_T) mean removal efficiency should exceed 65% with sludge ages ≥ 2 days for this waste stream. Further, an evaluation of the carbon treatment residual revealed that approximately 18% of the influent COD_T appears refractory. Non-refractory COD_T removal efficiency was excellent, averaging between 81% (2-day sludge age reactor) and 97% (20-day sludge age reactor) during quasi steady state carbon treatment. While good carbon treatment was possible at very low sludge ages (2-5 days), greater operational stability and sludge settleability were maintained with longer sludge ages (10-20 days). Overall nitrification (NH₄⁺ to NO₃^- conversion) kinetic coefficients were evaluated during a period of quasi steady state nitrification; the yield coefficient, half-saturation constant, decay rate, and the maximum specific rates of substrate utilization and biomass growth were found to be 0.26 mg VSS/mg-N, 0.2 mg/L NH₄⁺, 0.17 day^-1, 2.3 mg-N/mg VSS-day, and 0.6 day^-1, respectively. These kinetic coefficients are also within a published range of values identified during the Literature Review, suggesting that the proposed coefficients are reliable. Practical operational conclusions were also drawn for Knoxville’s settled municipal wastewater based on nitrification and alkalinity data produced during the study. While nitrification was possible at sludge ages as low as 2 days, a 10-day sludge age was required to avoid effluent nitrite residuals. The study also supported a proposed effluent alkalinity safety factor of 60 mg/L as CaCO₃. Effluent alkalinity below this concentration may result in a reactor pH that drops unexpectedly and perhaps precipitously in the presence of significant nitrifier activity.

Recommended Citation

Hawkins, Shawn Allen, "A bench scale activated sludge study of nitrification and carbon treatment uning municiple wastewater. " Master's Thesis, University of Tennessee, 2000.
https://trace.tennessee.edu/utk_gradthes/9400