| Airlift bioreactor is a type of circulating multiphase flow reactor.Due to its internal guide tube structure,the fluid inside the reactor has a good flow circulation effect,strengthens the mixing and mass transfer between different phases,and forms a significant oxygen concentration gradient in the internal space of the reactor,enhancing the effect of nitrogen and phosphorus removal;This type of reactor has attracted attention from researchers due to its simple structure,high mass transfer efficiency,and good treatment efficiency,and has been applied in many fields such as chemical engineering,biology,and sewage treatment.As is well known,in the aerobic treatment technology of wastewater,the operating and structural parameters not only determine the oxygen mass transfer efficiency and energy consumption of the reactor,but also have a significant impact on the treatment efficiency of the reactor.Optimizing it can not only improve the treatment efficiency,but also be a key link in the carbon reduction and emission reduction of the reactor.Currently,using numerical simulation methods to optimize the structure and operating parameters of the wastewater treatment reactor has become a development trend.This article uses the Higbie solute permeation theory and the activated sludge model(ASM)to establish a dissolved oxygen transport model and a simplified ASM1 model through a user defined program(UDF).The accuracy of the models is verified through experiments,and the interphase mass transfer and biological processes are described.The spatiotemporal distribution patterns of various substance concentrations inside the reactor are observed.The main content includes:(1)Establish a three-dimensional model of an air lift bioreactor,simulate and analyze the characteristics of the internal flow field of the reactor under operating conditions.The comparison error between the simulated and experimental values of the overflow flow rate inside the reactor barrel is within 20%,indicating that the overflow flow rate model can accurately obtain the overflow flow rate value.Based on the Higbie solute permeation theory,a dissolved oxygen transport model was established to simulate and predict the oxygen mass transfer process between two phases.The simulation accuracy was verified by the dissolved oxygen over time curve obtained from the clean water aeration oxygenation experiment.The simulated values showed a consistent trend with the experimental values,with an error controlled within 7%,indicating that the dissolved oxygen transport model can accurately describe the oxygen mass transfer process.The coupling ASM1 simplified model attempted to explore the biochemical reaction process of a single nitrifying bacteria in the reactor.By comparing the hourly measurement results of the nitrification process of a single nitrifying bacteria in the reactor,the reliability of the simulation was verified.The trend of changes in dissolved oxygen,ammonia nitrogen,and nitrate nitrogen concentrations was well fitted,and it was believed that the ASM1 simplified model could be used to explore the spatiotemporal changes in the concentration of various substances in the reactor.(2)Based on the distribution law of flow field characteristics reflected by the above dissolved oxygen transport model,the inner part of the reactor is divided into "air water surface" and "bubble water" to explore the oxygen mass transfer performance.According to the mass transfer performance,the best aeration and structural parameters of the reactor are obtained.With the increase of aeration rate(20 L/min～50 L/min),the oxygen mass transfer coefficient at the outer cylinder side increases in a Exponential type trend,and the growth rate gradually slows down.As the diameter of the draft tube decreases(325 mm～250 mm),the oxygen mass transfer coefficient increases Exponential type,and the increasing trend is gradually accelerated.With the decrease of the height of the inner cylinder protruding from the liquid surface(0.025 m～0.01 m),the oxygen mass transfer coefficient increases in a Exponential type trend,and the growth rate gradually slows down.Based on the concept of "low energy consumption and high efficiency",the optimal operating and structural parameters are proposed: aeration rate of 20 L/min～35 L/min,diameter of guide tube of 260mm～310 mm,and protrusion height of inner tube from liquid level of 0.015 m～0.025 m.(3)Based on the verified ASM1 simplified model,the impact of operating parameters(aeration rate,ammonia nitrogen substrate concentration,nitrifying bacteria concentration)and structural parameters(diameter of draft tube)on the Nitration reaction of the bioreactor was analyzed.With the increase of aeration rate(5 L/min～20 L/min),the dissolved oxygen concentration increases from 0.06 mg/L to 2.57 mg/L,the ammonia nitrogen degradation rate increases by 3.79 times,and the nitrate nitrogen generation rate increases by 3.75 times.Increasing the aeration rate has a significant effect on the improvement of treatment efficiency.With the increase of aeration rate,the concentration of various substances presents a Exponential type upward trend,indicating that the cost of achieving the same aeration effect will also be different in different aeration ranges.As the extension height increases(0.01m～0.04 m),the dissolved oxygen concentration decreases by 72%,the ammonia nitrogen degradation rate decreases by 37%,and the nitrate nitrogen generation rate decreases by 25%.The extension height has little effect on the concentration of various substances in the reactor guide tube,but it affects the anaerobic and aerobic areas of the reactor,thereby affecting the complete biochemical process in actual sewage treatment equipment.As the concentration of ammonia nitrogen substrate increases(40 mg/L～100 mg/L),the dissolved oxygen concentration only decreases by 11%,the ammonia nitrogen degradation rate increases by27%,and the nitrate nitrogen generation rate increases from 0.33 mg/(L·h)to 0.35 mg/(L·h).Increasing the concentration of ammonia nitrogen substrate has no effect on the flow field characteristics.The relationship between substrate concentration and the rate of ammonia nitrogen degradation and nitrate nitrogen generation is basically linear,indicating that under this working condition and microbial state,The concentration of ammonia nitrogen substrate is far from reaching the inhibitory concentration line.As the concentration of nitrifying bacteria increases(from 100 mg/L to 2000 mg/L),the dissolved oxygen concentration decreases by 96%,the degradation rate of ammonia nitrogen increases by 56%,and the generation rate of nitrate nitrogen concentration increases by 1.8 times.The concentration of nitrifying bacteria is exponential with the concentration of various substances in the flow field,and the growth rate gradually slows down.When the dosage exceeds a certain range,the cost of achieving the same effect will be higher due to various life activities such as microbial death and capture,But it can directly improve the treatment effect without changing the area ratio and residence time of each zone in the reactor,and is currently the most important and direct optimization method.The established model can observe the concentration distribution law of various substances in the reactor at a low cost and intuitively,provide guidance for the subsequent reactor design and processing and save costs,and have guiding significance for the subsequent simulation of the actual sewage treatment process. |
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