Theoretical and practical considerations on optimal design & operation conditions for the step-feed anoxic-oxic activated sludge process

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The main objective of this project is to establish the optimal design and operation conditions for the step-feed anoxic-oxic activated sludge process. To achieve this objective, we conducted four kinds of research work, (1) review of experimental reports, (2) stoichiometrical analysis, (3) laboratory experiment and (4) numerical calculation. Then, the following results are obtained :(1) In the work of literature review, more than one hundred experiments were collected from journals and conference proceedings, and the quantitative relationships between nitrogen removal efficiency and many design/operation conditions were obtained.(2) In the stoichiometrical analysis, chemical and biological reactions in reactors were simplified to make all chemical changes the mathematical functions of substrate, DO,NH_4-N,NO_3-N and alkalinity. Then, effluent water quality were formulated with such water quality concentrations in influent and operational conditions such as step ratio and recycle ratio. Consequently, the operational conditions which make the effluent T-N minimum were proposed.(3) To verify the above theory, laboratory tests were conducted more than 20 times with artificial sewage. In a series of experiments, step ratio, recycle ratio and influent intensity were selected as experimental parameters and changed in each run. The experimental results approximately supported the above theory, although some gabs remained a little.(4) Numerical calculation was conducted to explain the gabs between experimental results and theoretical values. The models used for this calculation have 18 variables and express many types of water quality items. The simulation showed the gabs are mainly caused by the reactions related to activated sludge, which were not included in the theoretical analysis.Finally, it was demonstrated that the equations obtained from the stoichiometrical analysis are available to know the optimal conditions for step-feed anoxic-oxic activated sludge process.