Hydrodynamics And Structural Stability Of Aerobic Granular Sludge

Aerobic granulation technology has been widely investigated for efficient treatment of municipal and industrial wastewater. After nearly 20 years of development, researchers worldwide have cultivated aerobic granule sludge by using different wastewater and operating conditions. The physical, chemical and biological properties of granules have been studied comprehensively. Aerobic granules have many advantages, such as regular shapes, densities, strong microbial structures, and good settling abilities. However, the instability of aerobic granule sludge is still the main barrier to the practical applications. Although many scholars proposed qualitative approach to strengthen the stability of aerobic granules through the study of the factors affecting the stability of granular sludge, but there is no quantitative analysis of the structural damage from the mechanical point of view.The first step in force analysis of aerobic granule sludge is to understand its mechanical parameters. In this work, the elastic modulus of aerobic granule sludge was investigated using experimental approach and numerical simulations. First, the aerobic granular sludge was compressed by elastic compression test, and then the model was established and solved in ANSYS. Finally, the elastic modulus of aerobic granular sludge was 0.018 MPa by spreadsheet. After the elastic deformation of granule sludge, the uniaxial compression experiments were continue to carry out until the granule was completely broken. The stress and strain of granule sludge were recorded by the instrument, and the stress-strain curves of aerobic granular sludge were obtained.In this paper, the hydraulic parameters of aerobic granule sludge were obtained by fractal theory, and the hydraulic characteristics of a single granule sludge with permeability under different porosity and external fluid velocity were simulated by using the method of computational fluid dynamics in FLUENT, such as internal and external flow field distribution, surface stress, etc.. Through these simulation results, it was demonstrated that the convection of the granules increase as the Reynolds number and the primary particle diameter increase. The model also indicated that improving the permeability of a granule can reduce the total pressure on the granule surface, but has less effect on the strain rate. In addition, a new simulation was developed to evaluate the fluid collection efficiency, which proved to be in good agreement with the calculated values determined using formulas and experimentation.On the basis of the gas-liquid flow, the fluid-solid interaction model of aerobic granule sludge was established, and the stress analysis was investigated. The simulation results show that the shear stress increases with the gas holdup, while the granule size had little effect on the surface force. The stress and deformation of the granules increase with the increase of the granule size at low flow velocity. In addition, the porosity of granules plays a key role in reducing the stress.