Micro-mechanical Analysis Of Aerobic Granular Sludge Based On ANSYS

Aerobic granular sludge has many advantages, such as the compact structure, the excellent settling property and microbial diversity. It has excellent performance in biological denitrification, nitrogen and phosphorus removal and treating municipal sewage, et al. However, aerobic granular sludge has poor stability, so it can not be widely used in the practical wastewater treatment process. The instability of the granular sludge may cause filamentous bulking or disintegration, the deterioration of settling performance, the decrease of the removal rate and even the paralysis of treatment process. Although the effects of operating conditions and granule characteristics on stability have been investigated by experiments, the other important factor, i.e., force analysis has rarely been studied.In this study, the hydrodynamic characteristics of an aerobic granule of different primary particle diameter at different outflow Reynolds numbers was investigated using FLUENT software. It is suggested that the enhancement of the surrounding flow could increase the granule internal convection, thus, could enhance the mass-transfer efficiency of the granules. Besides, the maximum strain rate of the aerobic granules also increased.Elastic modulus is one of the most important indicators of mechanical property of aerobic granular sludge. An equipment was provided including dial gauge and electronic balance to measure the vertical displacement of aerobic granular sludge under certain force. Meanwhile, using the ideas of the finite element method, the finite element model was created by using ANSYS software. Combined with the measured data, the elastic modulus of aerobic granular sludge was predicted as 0.2 Mpa through the trail method.The hollow, cracked and pore models of a 2-mm granule were created by using ANSYS software. The results of numerical simulation reveal that under the condition of certain shear force, the stress distribution was not uniform, and appeared significant stress concentration which might cause the disintegration of the granules. Besides, it is found that:(1) The larger the radius of the hollow is, the more easily broken the granule is. The crack area of the aerobic granular sludge was mainly on the upper and lower ends of the sphere; (2) Compared to the crack which was parallel to the flow direction, the granule was more likely to break up when the crack was perpendicular to the flow direction. On the other hand, the area of other cracks appeared was consistent, appearing at the flank guard of the granule. The main way of disintegration of cracked aerobic granular sludge was likely to be outside-in collapsing. (3) As the porosity increased gradually, the max Von Mises stress and max deformation became large, and the granules became more easily broken. It is hoped that the results can provide foundation for stable operation and application of aerobic granules.