Mathematical Simulation And Optimization Of The Nitritation Granular Sludge Reactor

The study establishes a mathematical model of the nitritation granular sludge reactor based on the computational fluid dynamics method.It explores the phenomenon of uneven mixing of granular sludge in the reactor under low dissolved oxygen condition.The model simulates the flowing of water in the reactor,distribution of dissolved oxygen,and movement of granular sludge under turbulent bubbles,mass transfer,and particle tracking physical fields.According to experimental data of nitritation granular sludge in the reactor,the relevant matching parameters of the model are determined.The purpose of the research is to reduce the aeration amount as much as possible when ensuring partial nitrification and conforming effluent water quality.The study provides theoretical basis for designing and optimizing partial nitrification process.The results show that:(1)Low dissolved oxygen causes uneven mixing of granular sludge.Parameters related to the model were measured through experiments.Dissolved oxygen value in the reactor was measured by using a dissolved oxygen probe.Oxygen consumption was determined through experiment.Then a model was established.The flowing of fluid,gas phase volume,dissolved oxygen and movement of granules in the reactor were actually simulated through calculation.(2)Flowing of fluid was simulated in the reactor by changing aeration amount.Calculated results showed that granular sludge settled at the bottom of the reactor when the aeration volume was less than 0.5L-min-1 and granules were mixed evenly in the reactor when dissolved oxygen was more than 1 mg· L-1.Calculated results were consistent with experimental results.And the mathematics model could be used to predict dynamic characteristics of nitritation granular sludge reactor.(3)It made granules mix evenly by changing the method of aeration.Intermittent aeration maintained dissolved oxygen at 0.6mg· L-1.It realized that computer controls the amount of aeration in the reactor through adapting the original program codes.The program codes made the staggered operation between high and low aeration volume.Intermittent aeration was really feasible and practical in the nitritation granular sludge reactor.(4)CFD provides accurate characterization of flowing pattern.Changing the structural size of nitritation granular sludge reactor improved the operating conditions.Aeration amount was set to 0.5L·min-1 and the inclination angles were set as 30°,45°,50°,55°,60°,and 75°.Then,fluid flowing,dissolved oxygen,and moving of granules were calculated.Changing the angle of reactor couldn't maintain dissolved oxygen at 1.0mg·L-1.However,45°?60° is beneficial to the cyclic movement of granular sludge.(5)In order to seek the best ratio of height to diameter,the paper established different models with H/D of 5.5,7,8.5,10,11.5,and 13,respectively.Results showed that the granules were mixed evenly with an aspect ratio of 8?10.Numbers of particles sinking at the bottom of the reactor were almost equal by comparing of H/D of 5.5 and 7.Pressure and energy will be increased when H/D is too high.Construction cost will be increased when H/D is too low.Therefore,H/D of nitritation granular sludge reactor should be designed 8?10.(6)CFD is helpful to reduce the cost of experimental research.Considering CPU of computer,two factors should be considered.First,it's important to simplify the model and seize the main contradiction.Second,the exact parameter values should be determined and simplify the calculation formula.Numerical simulation provides the theoretical basis for designing and optimizing of nitritation granular sludge reactor.