Mathematical Modeling Of Nitrogen/phosphorus Removal And N₂O Production In Biological Wastewater Treatment

The activated sludge method is the most commonly used technology in the municipal wastewater treatment,and the continuous flow A²/O?anaerobic-anoxic/oxic?process is the preferred one in China due to the simultaneous biological removal of nitrogen and phosphorus.Recently,the modified processes,such as the simply reversed A²/O process and the division point inflow reversed A²/O process,have been recognized as the better technologies than the conventional A²/O process.It is necessary to comprehensively analyze the performances of nitrogen and phosphors removal for these three A²/O processes.Moreover,as N₂O is an important greenhouse gas,it also contributes to the destruction of the ozone layer and causes the acid rain pollution,N₂O production and accumulation during biological nitrogen removal cannot be ignored.Most of studies on N₂O emission from the biological nitrification-denitrification reactors were carried out through lab-scale experiments,and some activated sludge mathematical models which developed to explore the mechanisms of N₂O production and accumulation during biological nitrogen removal processes were incomplete and imperfect,the necessary improvements and supplement works are still to be done.The biological processes of nitrogen/phosphorus removal and N₂O production in wastewater treatment were studied in this thesis and the major conclusions were summarized as below:?1?Based on ASM2 D,the removal performances of nitrogen and phosphorus for the continuous flow A²/O process,the simply reversed A²/O process and the division point inflow reversed A²/O process were modeled and predicted using GPS-X software.Results indicated that the satisfied pollutant removal performances of these three A²/O processes were achieved at different influent C/N ratios.As the carbon source is adequate for the heterotrophic denitrification in the prepositive anoxic tank,the TN removal efficiencies in the two reversed A²/O processes were higher than the conventional A²/O process.Because of that the amounts of the anaerobic phosphorus-release and the aerobic phosphorus-uptake in the conventional A²/O process were far larger than the two reversed A²/O processes,and the denitrifying phosphorus removal reaction occurred in the anoxic tank,the TP removal efficiency in the conventional A²/O process was higher than the two reversed A²/O processes.?2?A mathematic model including the AOB?ammonium oxidation bacteria?denitrification pathway and the incomplete NH2 OH oxidation pathway for describing N₂O production in the oxygen-limited nitritation process was newly proposed in this work.By omitting the intermediates of NO and NOH,this two-pathway model was more easily to use as the model parameters were less and the model structure was more concise than the published models.A series of experimental data from nitritaion-nitrification processes were used to calibrate and validate the model.Modeling results showed that the prediction results were in good agreement with the measured data from experiments.N₂O production by AOB denitrification pathway occupied the dominant part in oxygen-limited nitritation processes and the incomplete NH2 OH oxidation pathway acted as the primary part in nitrification processes at high DO levels.?3?By adding the heterotrophic denitrification on intracellular polymers?XSTO?into the autotrophic two-pathway model,a new mathematical model that integrates the AOB denitrification pathway,the incomplete NH2 OH oxidation pathway and the heterotrophic denitrification pathway on XSTO for N₂O simulation was firstly proposed in this work.This three-pathway model was used to describe the storage and consumption of intracellular polymers,the oxidation of ammonium,the denitrification on intracellular polymers and the growth and decay of organisms for the full-course sequent AOA?anaerobic-oxic-aerobic?processes and A/O?anaerobic/oxic?processes.Modeling results displayed good agreement with the measured data,suggesting that the model was capable to predict the carbon/nitrogen conversion dynamics and the N₂O production variations for the AOA and A/O systems.Results indicated that N₂O was primarily generated in the aerobic stage by AOB denitrification?67.84⁸1.64%?in the AOA system.Smaller amounts of N₂O were produced via incomplete NH2 OH oxidation?15.61³2.17%?and heterotrophic denitrification on intracellular polymers?0¹2.47%?.The high FNA?free nitrite acid?inhibition on N₂O reductase led to the increased N₂O accumulation in heterotrophic denitrification process.?4?The three-pathway model was further improved by introducing the competition for intracellular polymers among four denitrification steps to represent the electron competitions among Nar?nitrate reductase?,Nir?nitrite reductase?,Nor?nitric oxide reductase?and Nos?nitrous oxide reductase?under electron-limited conditions.The newly improved model was applied to simulate and analyze the role of electron competitions among four denitrification reductases on N₂O production and accumulation in three independent sequent A²/O systems.Results demonstrated that the modeling results fitted the measured data well in terms of SS?readily biodegradable substrate?,NO3-,NO2-,NH4+ and N₂O variations,and the improved model predicted the NO2-and N₂O accumulations in A²/O processes more accurately than the original three-pathway model.Since Nar and Nor have great advantages in capturing electrons,most of intracellular polymers were taken by the NO3-and NO reduction,little intracellular polymers were consumed during NO2-and N₂O denitrification.Due to the weak electron affinity of Nos,N₂O accumulation resulted from the more rapid decline of N₂O reduction rate than NO2-reduction rate under conditions of insufficient XSTO in these A²/O systems.Mathematical model is an effect tool for predicting pollutant removal dynamics and exploring N₂O production mechanisms in biological wastewater treatment.