3D格子Boltzmann传质模型模拟生物膜降解有机污水

Simulation of biomembrane degrading organic wastewater by 3D lattice Boltzmann mass transfer model

以膜生物法有机污水处理为研究背景,将3D格子Boltzmann传质模型与多孔介质四参数随机生成法耦合,获得生物膜多孔介质详细的孔隙分布,进而对反应器内生化降解反应过程进行模拟计算。研究分析了生物膜孔隙率及孔隙分布对流动传质及生化反应性能的影响,并与试验结果比较证明了模型的可行性。结果表明:各方向生长概率p1-14=0.005,随着生物膜孔隙率增大,反应器内底物降解效率先增大后减小,且在孔隙率ε=0.5时达到最大,50.97%;孔隙率ε=0.5时,改变各方向生长概率重构获得5种不同结构生物膜,其降解效率随之改变,生物膜为结构1(p_(3-4)=0.01,p_(1,2,5-14)=0.005)时,底物降解效率最高,52.54%。因此,3D格子Boltzmann传质模型可用于膜生物反应器内的流动传质及生化反应过程的模拟,研究结果将对反应器的优化具有一定的指导作用。

Biological treatment has been proven as an efficient wastewater treatment technology and widely used in the processes of city sewage and industrial waste. Biomembrane is aggregates of microorganisms suspended in a matrix of extracellular polymeric substances. Especially, photosynthetic bacteria（PSB） have exhibited significant superiorities to degrade the organic compounds in wastewater through utilizing solar energy, and simultaneously generate hydrogen energy, which is considered as a promising candidate due to its advantages of high energy content, high stability of combustion, cleanness and high efficiency. Biofilm is the foundation of biological membrane processing for wastewater treatment systems. In fact, the structure of biofilm has been proven to be a porous membrane, and thereby the degradation process could be considered as bioreaction in a bioreactor with porous media. Recently, numerous bioreactors and experiments have been proposed and implemented with the aim to improve the stability of reactors and performance of hydrogen production. Except experimental study, many theoretical studies have been carried out, and some numerical models have been established to investigate the bioreaction and two-phase flow transport in the bioreactors. Noteworthily, these numerical models are generally based on macro-scale, and require solving the partial difference equations for complex system. Moreover, they are still quite limited to obtain the detail information of fluid flow and mass transport in the biofilm, and also have difficulties in treating complex geometry of biofilm. Therefore, it is necessary to carry out a further numerical study on the flow and mass transport in bioreactor to overcome the limitations. In present study, a lattice Boltzmann method（LBM） was adopted to simulate the biodegradation in the bioreactor. Unlike the conventional numerical methods based on macroscopic continuum equations, the LBM was a mesoscopic approach that incorporates the essential physics of microscopic or mesoscopic process. Lattice Boltzmann models were based on microscopic kinetic equation for the particle distribution function, and the macroscopic quantities were then obtained through moment integrations of the distribution function. The lattice Boltzmann method has the most distinguished advantages, such as the simplicity of algorithm, the flexibility for complex geometries and parallel computing. Therefore, the flow and mass transfer as well as bioreaction were simulated with 3 D lattice Boltzmann model. Moreover, the detailed porous structure of biofilm was generated by quartet structure generation set（QSGS） method, which was closely combined with lattice Boltzmann model. In the simulation, the lattice Boltzmann model was coupled with a multi-block scheme to improve the computational efficiency and accuracy, and the non-equilibrium extrapolation method was used for velocity and concentration boundary condition treatment. The effect of porosity and pore structure of biofilm on flow and mass transfer was investigated, and the simulation results were compared with the experimental data, validated the LB model. The simulation results indicated that with the increasing biofilm porosity, the substrate consumption efficiency increased and reached the maximum of 50.97% at porosity of 0.5, then decreased under the condition of the same growth probability on every discrete direction; different growth probabilities would lead to the biofilm with various pore structures and specific surface areas, and thereby affect the performance of membrane bioreactor, and the substrate consumption efficiency was highest, 52.54%, under the condition of biofilm with structure 1（p3-4=0.01,p1,2,5-14=0.005） at ε=0.5, indicating that this characteristics of porous biofilm is optimal for bioreaction.