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Updated: Mar 17, 2026
06:00
Determination of the Settling Rate of Clay/Cyanobacterial Floccules
Published on: June 11, 2018
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Characterizing shallow secondary clarifier performance where conventional flux theory over-estimates allowable solids
Glen T Daigger1, John S Siczka2, Thomas F Smith3
1Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI, USA
Summary
Shallow activated sludge clarifiers can maintain good performance at high surface overflow rates if mixed liquor suspended solids are controlled. Traditional solids flux analysis is insufficient for determining limiting solids loading rates in these systems.
Area of Science:
- Environmental Engineering
- Water Treatment Technologies
- Wastewater Treatment
Background:
- Activated sludge secondary clarifiers are crucial for wastewater treatment.
- Shallow clarifier designs present unique performance challenges compared to conventional deep units.
- Understanding performance limitations is key to optimizing wastewater treatment plant efficiency.
Purpose of the Study:
- To evaluate the performance characteristics of shallow activated sludge secondary clarifiers.
- To compare actual performance with predictions from conventional solids flux analysis.
- To investigate the impact of hydraulic and solids loading on clarifier performance.
Main Methods:
- Conducted a 2-year testing program at the City of Akron Water Reclamation Facility.
- Performed hydraulic and solids loading stress tests.
- Measured sludge characteristics, including zone settling velocity (ZSV) and total suspended solids (TSS).
- Calibrated computational fluid dynamic (CFD) models using collected data.
Main Results:
- Good performance was achieved at surface overflow rates exceeding 3 m/h with controlled mixed liquor suspended solids (MLSS).
- The limiting solids loading rate (SLR) was lower than predicted by traditional solids flux analysis.
- CFD modeling indicated influent mixing with the sludge blanket caused dilution and a 'thin' overlying layer.
Conclusions:
- Traditional solids flux analysis is inadequate for determining allowable SLR in shallow clarifiers.
- A combination of empirical testing and CFD analysis is effective for evaluating shallow clarifier performance.
- Optimized MLSS control is essential for maintaining high performance in shallow secondary clarifiers.
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