A waste‑derived material best known for improving soil health may hold the key to making wastewater treatment dramatically more energy‑efficient, according to a comprehensive new review published in the journal Biochar.
The study, led by researchers at Suzhou University of Science and Technology in China, reveals that biochar can act as an "electron bridge" that significantly boosts the performance of anaerobic ammonium oxidation, or anammox. This microbial process removes nitrogen from wastewater naturally.
Unlike conventional treatment methods that require massive amounts of energy for aeration and external carbon sources, anammox bacteria convert ammonium and nitrite directly into harmless nitrogen gas. The process reduces aeration energy needs by 50 to 60 percent, eliminates the need for added organic carbon, and lowers operational costs by up to 90 percent compared to traditional nitrification‑denitrification systems.
However, anammox bacteria grow extremely slowly, doubling every 10 to 12 days, and are highly sensitive to environmental changes. These limitations have prevented the technology from being widely adopted in municipal wastewater treatment plants. The new review suggests that adding biochar to anammox reactors may be the solution.
"Biochar serves as a conductive interface that physically links microbial electron donors and acceptors, or as a redox‑active mediator that transiently accepts and donates electrons," the authors write. In plain terms, the material acts like a molecular bridge, helping bacteria pass electrons between one another more efficiently, much like copper wire conducts electricity better than rubber.
The researchers identified three distinct mechanisms through which biochar enhances electron transfer in anammox systems.
First, biochar stimulates bacteria to secrete more extracellular polymeric substances (EPS) – a sticky, protein‑rich biofilm that surrounds microbial cells. This biofilm contains C‑type cytochromes, proteins that act like biological wires, shuttling electrons across cell membranes. Biochar addition increased EPS secretion by 30 to 40 percent and boosted the electron transfer capacity of the biofilm by nearly 74 percent.
Second, the electrically conductive surfaces of biochar enable direct interspecies electron transfer (DIET). This is a process where microorganisms exchange electrons through physical contact or conductive materials, bypassing slow and energy‑intensive chemical intermediaries. Biochar's carbon‑rich structure, especially when produced at high temperatures above 500°C, forms graphitic, conductive networks that act like microscopic power lines connecting different bacterial species.
Third, redox‑active chemical groups on biochar surfaces, particularly quinone and phenolic compounds, can reversibly accept and donate electrons. This allows biochar to function as a reusable electron shuttle, similar to how a ferry carries passengers across a river. Biochar produced at lower temperatures (300–400°C) is especially rich in these electron‑donating groups.
The practical benefits are striking. In one study cited by the review, biochar‑amended anammox reactors showed 5.6‑fold higher levels of hydrazine synthase genes, 8.7‑fold higher hydrazine dehydrogenase genes, and 9.4‑fold higher nitrite reductase genes compared to controls without biochar. A fixed‑bed column reactor integrating biochar and anammox sludge achieved a maximum total nitrogen removal efficiency of 90.5 percent.
The review also highlights that biochar properties can be tuned by selecting different feedstocks and pyrolysis temperatures. For instance, cattle manure biochar demonstrates higher electron exchange capacity than sawdust‑derived biochar due to its abundance of oxygen‑containing functional groups and redox‑active metals. Metal‑modified biochars, such as those infused with iron or zinc, show even more pronounced enhancement effects.
"Future research should integrate machine learning with mechanistic investigations to improve the prediction of biochar performance and guide the rational design of biochar materials," the authors note. By using artificial intelligence to predict how different biochar formulations will perform, researchers hope to optimize electron transfer pathways and accelerate the development of scalable anammox systems.
The researchers also emphasize the importance of sustainability. Biochar production requires energy, typically at 300–700°C, and future studies should carefully weigh this energy input against the savings achieved through reduced aeration and carbon addition during the anammox process. Emerging technologies such as microwave‑assisted pyrolysis and hydrothermal carbonization could further reduce the energy footprint of biochar production.
While the evidence is compelling, the authors caution that much remains unknown. The relative contributions of the three electron‑transfer mechanisms – EPS‑mediated transfer, DIET, and MIET – likely vary depending on biochar properties, aging state, microbial community structure, and reactor configuration. Direct confirmation of these pathways will require advanced techniques such as electron‑flux measurements, isotope tracing, inhibitor experiments, and spatially resolved electrochemical analyses.
Nevertheless, the review paints an optimistic picture. By serving as an inexpensive, tunable, and persistent electron bridge, biochar could finally unlock the full potential of anammox technology, making wastewater treatment cheaper, greener, and more sustainable for communities around the world.
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Journal Reference: Zhao, W., Li, W., Zhu, Y. et al. Biochar as an electron bridge: mechanistic insights into enhanced anammox performance in wastewater treatment. Biochar 8, 131 (2026).
https://doi.org/10.1007/s42773-026-00650-8
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
