Abstract
High rate activated sludge (HRAS) processes have a high potential for carbon and energy recovery from sewage, yet they suffer frequently from poor settleability due to flocculation issues. The process of flocculation is generally optimized using jar tests. However, detailed jar hydrodynamics are often unknown, and average quantities are used, which can significantly differ from the local conditions. The presented work combined experimental and numerical data to investigate the impact of local hydrodynamics on HRAS flocculation for two different jar test configurations (i.e., radial vs. axial impellers at different impeller velocities) and compared the hydrodynamics in these jar tests to those in a representative section of a full scale reactor using computational fluid dynamics (CFD). The analysis showed that the flocculation performance was highly influenced by the impeller type and its speed. The axial impeller appeared to be more appropriate for floc formation over a range of impeller speeds as it produced a more homogeneous distribution of local velocity gradients compared to the radial impeller. In contrast, the radial impeller generated larger volumes (%) of high velocity gradients in which floc breakage may occur. Comparison to local velocity gradients in a full scale system showed that also here, high velocity gradients occurred in the region around the impeller, which might significantly hamper the HRAS flocculation process. As such, this study showed that a model based approach was necessary to translate lab scale results to full scale. These new insights can help improve future experimental setups and reactor design for improved HRAS flocculation.
Highlights
Proper separation of the sludge from the bulk flow is critical for the overall functioning of sewage treatment plants operating an activated sludge (AS) process
The DSS values found for High rate activated sludge (HRAS) (122 ± 55 mg TSS L−1 ) were considerably higher compared to those reported for conventional activated sludge (CAS) (
This can be attributed to the fact that the floc properties of the sludge produced in high rate systems are different compared to those in low rate systems
Summary
Proper separation of the sludge from the bulk flow is critical for the overall functioning of sewage treatment plants operating an activated sludge (AS) process. The efficiency of the collisions is influenced by multiple factors such as hydrodynamic effects, short range forces, and floc surface properties [2] As flocs grow, they become more sensitive to break-up by fluid shear. The existence of dispersed solids in AS systems may result from failure of the aggregation process (due to collision frequency or efficiency limitations) or break-up of the flocs due to excessive hydrodynamic shear. To distinguish between these flocculation issues, a test developed by Wahlberg et al [4] can be employed, determining dispersed and flocculated suspended solids (DSS and FSS). This test was originally designed for troubleshooting in a final clarifier, but proved to be useful in pinpointing flocculation issues in several related applications [5,6,7]
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