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Published on: April 9, 2016
Enhancing bioflocculation in high-rate activated sludge improves effluent quality yet increases sensitivity to
Tim Van Winckel1, Nam Ngo2, Belinda Sturm3
1Center of Microbial Ecology and Technology (CMET), Faculty of Bioscience Engineering, Ghent University, 9000, Gent, Belgium; District of Columbia Water and Sewer Authority, Blue Plains Advanced Wastewater Treatment Plant, 5000 Overlook Ave, SW, Washington DC, 20032, USA; Department of Civil, Environmental and Architectural Engineering, The University of Kansas, KS, USA.
High-rate activated sludge systems struggle with effluent solids. This study reveals fines, shear, and surface overflow rate significantly impact solids, offering solutions for better clarifier design and operation.
Area of Science:
- Environmental Engineering
- Wastewater Treatment Technologies
- Sludge Management
Background:
- High-rate activated sludge (HRAS) systems aim for efficient organic capture through bioflocculation and solid-liquid separation.
- Poor and unpredictable effluent suspended solids (ESS) remain a challenge in full-scale HRAS applications.
- The influence of fines, shear, and surface overflow rate (SOR) on ESS is not fully understood, despite extensive research on biological flocculation.
Purpose of the Study:
- To investigate the impact of fines, shear, and SOR on ESS in HRAS systems, specifically in the absence of settleable influent solids.
- To compare ESS in full-scale HRAS step-feed (SF) and pilot-scale HRAS contact-stabilization (CS) configurations.
- To provide a comprehensive understanding of ESS drivers and inform the design of effective HRAS clarifiers.
Main Methods:
- Batch settling tests were conducted to analyze sludge settling characteristics.
- Controlled clarifier experiments were performed to simulate operational conditions.
- Continuous operation of SF and CS reactors allowed for real-world data collection and analysis.
- Sludge properties, including fines content, extracellular polymeric substances (EPS), and settling velocity distribution, were evaluated.
Main Results:
- Fines were found to contribute up to 25% of ESS in the full-scale SF configuration.
- The CS configuration enhanced bioflocculation, reducing ESS by up to 30 mg TSS/L.
- The CS configuration's feast-famine regime promoted high-quality EPS but resulted in a narrow settling velocity distribution, increasing sensitivity to SOR.
- A low shear environment (20 s⁻¹) for at least one minute prior to clarification optimized settling velocity distribution, irrespective of previous shear history.
Conclusions:
- Fines, shear, and SOR are critical factors influencing ESS in HRAS systems.
- The CS configuration shows potential for reducing ESS but requires careful management of settling characteristics.
- Maintaining a low-shear environment before the clarifier is crucial for achieving optimal sludge settling.
- This research provides actionable recommendations for managing fines and optimizing sludge settling velocity in HRAS clarifiers.
