Hydrodynamics Of Microbial Granule-Based Reactors

Microbial granule is a special microbial aggregate in the biological wastewater treatment, including aerobic and anaerobic forms. Up to the present, with the formation and development of the granular sludge, research work of granular sludge has been focused on the characteristics of granule-based system. Performance of a biological wastewater treatment system, in terms of organic matter removal and energy yield, is usually governed by two main interrelated factors: microbiological processes and hydrodynamics. The hydrodynamics determines the resultant mass transport processes and accordingly the final performance of a given reactor. Since the microbial granules are usually formed in the special condition, the hydrodynamics in reactor are widely recognized to play an important role in the self-immobilization and stability of microbial granule.In this study, based on the steady-state microbial granule-based system, the hydrodynamic behavior of the reactors was well documented. The characteristics of some certain microbial granules and reactors were comprehensively investigated. In this paper, many methods have been used, e.g., tracer test, mathematic model, computational fluid dynamics analysis and three dimensional reconstructions. Main contents and results are as follow:1. Aerobic granules were successfully cultivated in sequencing batch reactors (SBRs) fed with synthetic wastewaters, VFAs-rich wastewater and industrial wastewaters. When the influent was kept at high initial Ca concentration (40 mg L^-1), the granules was more steady compared with the granules without Ca accumulation, and the Ca-rich granules had more rigid structure, a higher density, settling velocity and strength. The granule size increased continuously due to its high shear strength, and the average diameter was 4.2 mm. The simultaneous nitrification and denitrification (SND) process was happened in the Ca-rich granules for their big size. However, their bioactivity reduced and the ash content increased after the Ca accumulation inside them, after which scaling and granule deactivation might occur. The NaNO₂-oxidizing aerobic granules were cultivated using the SND granules as seed sludge. The NaNO₂-oxidizing activity of granules was high but the cells growth was very slow. The features of the aerobic granules are high porous and 63% of them are permeable.2. The CH₄-producing and H2-producing granules have been cultivated in the upflow anaerobic sludge blanket (UASB) reactor, and the parameters and the characteristics of the granules have been obtained. The density of the anaerobic granules is very high and the pores in them are small. Therefore, the fluid collection efficiency of anaerobic granules is extremely low and more than 75% of them are impermeable. Then the mass transfer in the anaerobic granules should be controlled by the molecule diffusion.3. The flow in the SBR is usually turbulent. Combined with the image analysis and mathematic model, the important hydrodynamic parameters, e.g., gas hold up, mixing time and regime transitions, were described in this study. The mixing and the dispersion behavior are all related to the structure of the reactor. The mixing is more rapid and the dispersion coefficient is higher in the reactor with higher height and diameter ratio (H/D). Furthermore, the gas hold up is also higher in the higher H/D reactor. In this paper, the shear force model was established for the SBR, and the calculation of the shear force in the SBR could be quantified. After calculation, we found that the detachment by cell decay was significant higher than by shear. Among the shear forces act on the granular surface, the contribution of the fluid, gas bubble and collision shear is different. In the SBR system, the gas bubble and collision shear are main shear force source on the granular surface.4. The flow pattern in the UASB reactor is controlled by the gas bubble. Compared with the single-zone axial-dispersion model, a two-zone axial-dispersion model was found to be more appropriate for simulating the dispersion characteristics of this reactor, suggests that the dispersion in the UASB reactor was non-uniform. Since the gas and liquid flow velocity are very low, the UASB reactor is potentially dispersion-controlled, and the dispersion coefficient decreases along the axial of the UASB reactor. An increasing-sized CSTRs (ISC) model was developed to describe the hydrodynamics of such a bioreactor. Simulation results demonstrate that the ISC model is better to describe the hydrodynamics of the UASB reactors than the other models. This hydrodynamic model might be also useful in simulating other similar reactors.5. A three-dimensional computational fluid dynamics (CFD) simulation was performed with an Eulerian-Eulerian three-fluid approach to visualize the flow pattern in reactors. The finite volume method was used as the numerical technique. In SBR, the granular velocity contours are similar with the liquid velocity contours. There are two big circle flows in the reactor. The feature of the gas phase velocity distribution is that the velocity decreases wavyvily along the axial of the SBR. In UASB reactor, the sludge volume fraction decreases along the reactor height, and the velocity magnitude decreases along the UASB reactor height when the flow pattern is fully developed.6. The three-dimensional structures of aerobic granular sludge were identified using the fluorescence in situ hybridization (FISH) and confocal laser scanning microscope (CLSM) images. There are a few large pores and large amounts of small pores in the granule. The reconstructed results indicate that the fractal-cluster model could predict the distribution of the primary particles in the microbial granules. The growth rule of the microbial granule is similar with the transfer from the diffusion-limited-aggregation (DLA) model to the reaction-limited-aggregation (RLA) model. Simulation of a sphere with one straight pore passing through its centre reveals that large pores controlled the advective flow through a granular sludge, and the big pore is a requirement to permit the advective flow happen. Using sphere-tube model, the size of the big pores was predicted to be at least more than 50-250μm when the granular size was in range of 1-5 mm. The new porous media model can be used to describe the micro-hydrodynamics in the microbial granule. Combined with the biochemical process, the nitrification process in an aerobic granule can be predicted that the depth of the reaction is approximately 200μm. Compared with the oxygen penetrating depth measured by microelectrode, the diameter of cluster was predicted to be more than 200μm.7. Employing the CFD simulation and the gas hold up experiment, we can find that the bigger granule will disturb the flow field more significantly. Since the von Karman vortex street is usually happen after the bigger granule, then the unsteady moving of the granular is more significantly. Therefore the change of the macro-hydrodynamics with the sludge concentration should be more significant with bigger granule.