Overcoming floc formation limitations in high-rate activated sludge systems

Area of Science:

• Environmental Engineering
• Wastewater Treatment Technologies
• Biotechnology

Background:

• High-rate activated sludge (HRAS) systems aim for energy-positive sewage treatment by maximizing organic capture.
• Effective solid-liquid separation in clarifiers is crucial for HRAS performance.
• Flocculation limitations often hinder organic capture in HRAS, impacting overall efficiency.

Purpose of the Study:

• To identify and overcome floc formation limitations in full-scale HRAS reactors.
• To analyze the impact of shear, sludge concentration, and chemical additives on flocculation.
• To develop an analysis toolkit for addressing solid separation challenges in HRAS.

Main Methods:

• Conducted orthokinetic flocculation tests under varied conditions (shear, sludge concentration, coagulant/flocculant).
• Utilized traditional and novel settling parameters and extracellular polymeric substances (EPS) measurements.
• Employed bio-augmentation with sludge from a long solids retention time (SRT) reactor.

Main Results:

• HRAS limitations were primarily due to insufficient collision efficiency, linked to short SRT and high colloid loading.
• Impaired flocculation, driven by EPS composition (low protein/polysaccharide ratio), limited collision efficiency.
• Bio-augmentation resolved collision efficiency issues but introduced floc strength limitations related to EPS amount and morphology.

Conclusions:

• Flocculation, not coagulation, was the main limitation in the studied HRAS system.
• EPS composition is critical for collision efficiency, while EPS amount influences floc strength.
• The proposed analysis toolkit aids in optimizing HRAS solid separation and organic recovery.