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Open AccessFeature PaperArticle

The Impact of Local Hydrodynamics on High-Rate Activated Sludge Flocculation in Laboratory and Full-Scale Reactors

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BIOMATH, Department of Data Analysis and Mathematical Modelling, Faculty of Bioscience Engineering, Ghent University, Coupure links 653, 9000 Gent, Belgium
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Research Group of Sustainable Energy, Air and Water Technology, Department of Bioscience Engineering, University of Antwerp, 2020 Antwerpen, Belgium
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CAPTURE, Centre for Advanced Process Technology for Urban Resource recovery, Coupure links 653, 9000 Ghent, Belgium
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Waterboard Brabantse Delta, Bouvignelaan 5, 4836 AA Breda, The Netherlands
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AM-TEAM, Oktrooiplein 1-Box 601, 9000 Gent, Belgium
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Author to whom correspondence should be addressed.
Processes 2020, 8(2), 131; https://doi.org/10.3390/pr8020131
Received: 18 December 2019 / Revised: 9 January 2020 / Accepted: 13 January 2020 / Published: 21 January 2020
(This article belongs to the Special Issue Modelling and Optimization of Chemical Reactors)
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.
Keywords: water resource recovery facility; sludge settling; flocculation state; jar test; CFD water resource recovery facility; sludge settling; flocculation state; jar test; CFD
MDPI and ACS Style

Balemans, S.; Vlaeminck, S.E.; Torfs, E.; Hartog, L.; Zaharova, L.; Rehman, U.; Nopens, I. The Impact of Local Hydrodynamics on High-Rate Activated Sludge Flocculation in Laboratory and Full-Scale Reactors. Processes 2020, 8, 131.

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