Study On Mass Transfer Capacity And Stability Of Granular Sludge Based On CFD Technology

Anaerobic ammonium oxidation(Anammox)granular sludge is favored by the environmental engineering community due to its compact structure,high biological retention rate,sedimentation performance and impact resistance,which has been applied to various practical wastewater treatment.in process.However,granular sludge is prone to problems such as granule disruption and floating,which greatly affects the stable operation of the process.Mass transfer capacity of the granular sludge is closely related to the operating efficiency of the process.The mass transfer process and stability of granules are mainly affected by the internal pore distribution of granular sludge.In addition,the hydraulic shear in the expanded granular sludge bed(EGSB)reactor has a significant impact on the physicochemical properties and stability of granular sludge.The research on the three-dimensional pore structure,permeation properties and stability of granular sludge is of great significance for guiding the stable and efficient operation of granular sludge and anammox processes.In this paper,the mass transfer capacity and stability of granular sludge were studied by X-ray micro tomography(CT)imaging technology and computational fluid dynamics(CFD)method,which provided the basis for stable and efficient operation of granular sludge process.The main research and findings are as follows:Anammox granular sludge was scanned by X-ray micro-CT imaging technology of synchrotron radiation technology,and the spatial distribution characteristics that could accurately describe the internal structure and pores of granular sludge were obtained.The results show that the internal pore diameter of granular sludge is mainly less than 100μm,and this part of the pores accounts for about 85%of the total pore volume.The spatial distribution of pores basically follows a normal distribution,and the porosity reaches the maximum in the interval of about 1125-1250μm from the center.The porosity of granular sludge distributed between 44.71%-66.73%,the fractal dimension distributed between2.52-2.68,and the curved fractal dimension distributed between 0.65-0.96,and the porosity was positively correlated.The independent pores in the granular sludge are mostly distributed on the surface of the granular sludge,the pore size is mostly less than40μm,and the volume ratio is generally not more than 10%,which is similar to the spatial distribution of the overall pores.The three-dimensional structure of granular sludge was used to calculate the permeability,and the fractal dimension,porosity and permeability of granular sludge in the whole scale and small-scale area were analyzed.The results show that the overall permeability of the particles is isotropic,and the average permeability distribution is between 260.50-380.48μm~2.The independent pore volume ratio and permeability in granular sludge have a negative correlation trend.The permeability of granular sludge is most significantly affected by the curved fractal dimension.In the granular sludge,the fractal dimension is distributed in the small-scale interval of 2.27-2.72,the permeability was distributed between 15.42-182.67μm~2,and the porosity distributed between 45.24%-71.11%.There is a negative correlation between fractal dimension and permeability and porosity.In the small-scale interval,the Kozeny-Carman equation can describe the relationship between fractal dimension,porosity and permeability.A simplified CFD model of spatial distribution characteristics of reactive granular sludge porosity was established.Taking four EGSB reactors with water chambers at different angles as the object,the physical and chemical properties of the granular sludge,such as particle size,convexity,viscoelasticity and EPS content,were statistically analyzed.The results showed that with the decrease of the water distribution cavity angle from 60 ~o to 22.5~o,the stability of granular sludge was significantly enhanced,and the average particle size increased from1089.56±27.70μm to 2391.55±24.90μm.The average convexity value increased from0.906±0.041 to 0.950±0.052.The content of protein(PN)in extracellular polymeric substances(EPS)increased from 76.1±1.1 mg/g VSS to 80.0±1.5 mg/g VSS,and the content of polysaccharide(PS)increased from 7.4±0.5 mg/g VSS to 9.0±0.3 mg/g VSS,the elasticity of granular sludge is also enhanced.Combined with the analysis of the flow field and concentration field of the reactor by CFD,it can be seen that reducing the angle of the water distribution cavity is conducive to building a good hydraulic shear environment in the reactor,producing particles with larger particle size,smoother surface and better mechanical properties.It is beneficial to the growth of Anammox bacteria and the rapid start-up and stable operation of the EGSB system.