Graphitic biochar-anammox achieved by multi-heme-based extracellular electron transfer.

Area of Science:

• Environmental Microbiology
• Biotechnology
• Wastewater Treatment

Background:

• The anammox process offers carbon-free nitrogen removal but is limited by reliance on traditional nitrification and denitrification.
• These linked processes can generate greenhouse gases and increase organic carbon demand, diminishing the anammox process's environmental benefits.

Purpose of the Study:

• To develop an innovative biochar-assisted anammox system to overcome the limitations of conventional nitrogen removal methods.
• To investigate the role of biochar's graphitic defects in facilitating extracellular electron transfer (EET) for ammonium oxidation.

Main Methods:

• Engineered a biochar-assisted anammox system utilizing biochar with high graphitic defect density.
• Employed metagenomic and in vitro assays to elucidate microbial interactions and electron transfer mechanisms.
• Quantified nitrogen removal efficiency and greenhouse gas emissions under varying conditions.

Main Results:

• Biochar produced at 800 °C for 4 hours (BC800-4 h) demonstrated superior electron accepting capacity, supporting complete ammonium oxidation.
• The system achieved 62% nitrogen removal without external nitrite, significantly reducing N₂O emissions by 28%.
• Identified a cooperative microbial network involving suspended anaerobic ammonia-oxidizing bacteria (AnAOB) and biofilm-forming bacteria on biochar.

Conclusions:

• Biochar facilitates hydroxylamine-dependent ammonium oxidation by AnAOB through interspecies EET, enabled by graphitic defects.
• A partitioned microbial community structure is crucial for efficient electron transfer to biochar.
• This biochar-anammox synergy presents a promising strategy for sustainable and low-emission nitrogen removal in wastewater treatment.