Optimisation of municipal wastewater biological nutrient removal using computer simulation.
Creators: name: McHarg, Amy Marie.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2002
Subjects: subject: Engineering, Chemical.

description: Due to more stringent regulations for secondary municipal wastewater treatment, municipalities are beginning to implement tertiary treatment in their wastewater treatment plants. Tertiary treatment would be the removal of either phosphorous or nitrogen or both from the wastewater before it is discarded from the plant. Biological treatment is becoming an increasingly popular process used to accomplish this extra removal. There are several processes available that will provide acceptable levels of biological nutrient and BOD removal from wastewater. Three well-known processes were considered in this study - the Modified Bardenpho Process, the Modified UCT Process and the A2/O Process. For each of these processes, 2 1evel fractional factorial designs along with least squares analysis were performed in order to determine the optimal operating variables (recycle rates and anaerobic, anoxic and aerobic zone retention times), with respect to the final nitrogen concentration, the final phosphorous concentration and a combination of the final nitrogen and phosphorous concentrations. The analyses were performed at 10°C and 20°C with low, medium and high primary effluent concentrations. Due to the complexity of the processes, lab scale experiments were not feasible. Therefore, a widely accepted calibrated biokinetic model (Activated Sludge Model No 2d) was used in a computer simulation program (GPS-X) to gather the necessary data for analysis. Actual plant data were used to test the validity of the simulation model with respect to organic and nitrogen removal. Using the published kinetic and stoichiometric parameters for both temperature levels, the Activated Sludge Model provided a good estimation of outlet concentration levels. It was found that all three biological nutrient removal (BNR) process were capable of achieving an effluent soluble phosphorous concentration below the required limit of 1 mgP/L at 10 and 20°C with low, medium and high primary effluent concentration when the effluent nitrogen concentration was neglected. Neither the Modified Bardenpho, the Modified UCT nor the A 2/O process were capable of producing an effluent with nitrogen concentrations below the required limit of 5 mgN/L at high primary effluent concentrations. The Modified Bardenpho and the Modified UCT processes were both successful in achieving a combined nitrogen and phosphorous removal below their regulatory limits for low primary effluent concentrations at 10 and 20°C. The Modified Bardenpho process, at 20°C with medium primary effluent concentrations, was found to achieve an effluent with nitrogen and phosphorous concentrations below 5 mgN/L and 1 mgP/L, respectively. After analyzing the effects of individual operating variables, it was found that the anoxic recycle for the Modified UCT process had an insignificant effect on total nitrogen (TN) and soluble phosphorous (sP) removals and did not need to be included in future experimental studies. All of the input variables to the MB and A2/O process proved to be somewhat significant and it is recommended that they be kept within future experimental designs. From this study it was found that both the MB and MUCT process are capable of achieving the TN, sP and cBOD5 removals that ROPEC requires. However only the MB process proved to be a robust system when subjected to storm conditions (i.e., peaks in influent flow rate) with respect to sP and cBOD5 removal. Neither the MB nor the MUCT process provided acceptable TN removals when subjected to storm conditions. It is recommended that ROPEC further evaluate the MB process as a possible means to achieve simultaneous cBOD5, TN and sP removal.