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

Abstract: 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 3D 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 ?=0.5, indicating that this characteristics of porous biofilm is optimal for bioreaction.