Surface properties of sludge and their role in bioflocculation and settleability

Area of Science:

• Environmental Science
• Water Treatment Engineering
• Microbiology

Background:

• Sludge properties significantly influence wastewater treatment efficiency.
• Extracellular Polymeric Substances (EPS) play a crucial role in sludge characteristics.
• Understanding the impact of operational parameters like Sludge Retention Time (SRT) is vital for optimizing treatment processes.

Purpose of the Study:

• To investigate the effect of Sludge Retention Time (SRT) on the physicochemical properties and Extracellular Polymeric Substances (EPS) of sludge.
• To determine the relationship between SRT, EPS composition, and sludge properties such as hydrophobicity, surface charge, and settleability.
• To elucidate the governing factors for bioflocculation and sludge settleability.

Main Methods:

• Laboratory-scale sequencing batch reactors (SBRs) were operated at varying SRTs (4, 9, 12, 16, 20 days).
• Synthetic wastewater containing glucose and inorganic salts was used as feed.
• Sludge surface properties (hydrophobicity, surface charge) and EPS composition (protein:carbohydrate ratio) were analyzed.

Main Results:

• Higher SRTs (16 and 20 d) resulted in more hydrophobic and less negatively charged sludge surfaces.
• The protein:carbohydrate ratio in EPS increased with SRT up to 12 d, then stabilized.
• Total EPS content was independent of SRT, but higher EPS correlated with poorer sludge settleability (higher Sludge Volume Index - SVI).

Conclusions:

• Sludge surface properties (hydrophobicity, surface charge, EPS composition) are key drivers of bioflocculation, not just EPS quantity.
• EPS content is more critical for controlling sludge settleability.
• A transition in sludge properties occurs between SRTs of 9 and 16 days.