Microbiological and structural aspects of granular sludge from autotrophic denitrifying reactors

Area of Science:

• Environmental Microbiology
• Wastewater Treatment Technologies
• Bioreactor Microbial Ecology

Background:

• Denitrification is crucial for tertiary wastewater treatment to remove nitrogen pollutants.
• Autotrophic denitrification utilizes inorganic electron donors like thiosulfate, offering an alternative to heterotrophic processes.
• Understanding microbial community dynamics is key to optimizing bioreactor performance.

Purpose of the Study:

• To investigate the microbial community evolution in autotrophic denitrifying bioreactors.
• To identify key microbial players, particularly autotrophic denitrifiers, during bioreactor operation.
• To assess the impact of microbial changes on sludge structure and integrity.

Main Methods:

• Fluorescence in situ hybridization (FISH) and catalyzed reporter deposition-FISH (CARD-FISH) for microbial identification and quantification.
• Gene cloning and sequencing to analyze bacterial diversity.
• Scanning electron microscopy (SEM) to evaluate sludge granule structure.

Main Results:

• Bacterial diversity evolved significantly over 110 days of bioreactor operation.
• The abundance of Thiobacillus denitrificans increased from 1% to 35% of total cells, becoming dominant.
• Archaeal diversity remained stable, primarily Methanosaeta soehngenii; sludge granule integrity decreased.

Conclusions:

• Thiobacillus denitrificans plays a dominant role in autotrophic denitrification with nitrate and thiosulfate.
• Changes in microbial composition correlate with a loss of sludge granule integrity, potentially affecting bioreactor stability.
• Further research is needed to mitigate sludge buoyancy issues in such systems.