Sludge deterioration in a full scale UASB reactor after a pH drop working under low loading conditions

Area of Science:

• Environmental Engineering
• Wastewater Treatment
• Anaerobic Digestion

Background:

• UASB reactors are effective for treating industrial effluent.
• Malting plant effluent presents unique treatment challenges.
• Microbial community stability is crucial for reactor performance.

Purpose of the Study:

• To investigate the long-term performance of a UASB reactor treating malting plant effluent.
• To understand the impact of process upsets, specifically pH drops, on reactor stability and microbial community.
• To evaluate strategies for mitigating sludge washout and recovering granular sludge properties.

Main Methods:

• Operation of a full-scale UASB reactor for nearly two years.
• Monitoring of key performance indicators: COD removal, biogas production, methane content, organic loading rates.
• Microbial analysis using Fluorescence In Situ Hybridization (FISH) to identify filamentous bacteria.
• Sludge characterization, including filamentous presence and settlement properties.
• Bench-scale reactor experiments to assess sludge recovery.

Main Results:

• Initial high efficiency (80% COD removal, 77% methane) was achieved.
• A pH drop to 4.8 caused destabilization, leading to fluctuating COD removal and filamentous growth (Chloroflexi).
• Low specific organic load and high VSS content contributed to filamentous granule formation and poor settlement.
• Sludge recycle resulted in washout, while bench-scale tests showed sludge property recovery.

Conclusions:

• Low pH events can significantly disrupt UASB reactor performance by altering microbial communities.
• Filamentous growth, promoted by low pH and low specific organic load, impairs sludge settleability.
• Sludge washout can occur with aggressive dewatering strategies, but recovery is possible under different conditions.