In a review published in Biochar, authors Wenya Zhao, Wenqi Li, Yuheng Zhu, Yidi Li, Mabruk Adams, Hanbo Chen, and Chongjun Chen examine how biochar functions as an electron bridge to improve anaerobic ammonium oxidation during wastewater treatment. The anammox process is an energy-efficient biological method that converts harmful ammonium and nitrite into harmless nitrogen gas under oxygen-free conditions. Compared to traditional wastewater treatment methods, this natural process cuts aeration energy needs in half and eliminates the necessity for external organic carbon, offering significant cost savings for municipal facilities. However, the bacteria responsible for this reaction grow extraordinarily slowly, with doubling times taking nearly two weeks, which limits real-world application. The researchers evaluated how adding carbon-rich biochar overcomes these growth bottlenecks by improving cellular communication and accelerating fundamental chemical reactions.

The findings reveal that biochar acts as a versatile electron conduit through three main physical and chemical mechanisms. First, biochar stimulates the bacteria to produce up to forty percent more extracellular protective matrix, which enriches vital protein components and lowers electrical resistance across the biofilm. This structural shift transforms the surrounding matrix into a conductive highway, helping electrical charges move rapidly outside the cell wall. Second, the conductive surfaces of biochar physical link different bacterial cells, enabling direct interspecies electron transfer without requiring liquid intermediaries. Third, surface functional groups on biochar act like rechargeable micro-batteries, continuously accepting and donating electrons to drive conversion pathways.

These combined mechanisms produce dramatic operational improvements across various reactor setups. Biochar amendments consistently increase nitrogen removal rates between five and thirty-two percent while improving overall nitrogen removal efficiency by up to 34.6 percent. Furthermore, biochar integration reduces startup times for complex reactor systems by as much as thirty-six percent. Low-temperature biochar prepared between three hundred and four hundred degrees Celsius exhibits optimal electron-donating capabilities, whereas higher preparation temperatures yield greater physical conductivity. To optimize material selection, the authors propose combining machine learning tools with long-term environmental assessments to design tailored biochar materials for scalable, low-carbon wastewater facilities.

Source: Zhao, W., Li, W., Zhu, Y., Li, Y., Adams, M., Chen, H., & Chen, C. (2026). Biochar as an electron bridge: mechanistic insights into enhanced anammox performance in wastewater treatment. Biochar, 8, Article 131.