Anaerobic digestion (AD) is a powerful technology that turns organic waste, like manure or food scraps, into valuable biogas. But this biological process is sensitive. When digesting nitrogen-rich wastes, it can produce high levels of ammonia, which is toxic to the very microbes that make methane. This “ammonia inhibition” stalls the digester, causing harmful acids to build up and methane production to plummet. A new study by Shuaishuai Ma and colleagues, published in the journal Carbon Research, explores a novel solution using two special carbon materials derived from a common industrial waste product.

The researchers turned to alkali lignin, a waste by-product from paper pulping, as their source material. Using a process called hydrothermal carbonization, they created two distinct materials: lignin-based hydrochar (LHC) and carbon quantum dots (CQDs). They then added these materials to AD systems suffering from severe ammonia inhibition (a high concentration of 4 g/L ammonia nitrogen).

The results showed a clear rescue effect. In the experiments, the high ammonia levels had cut methane yield from a normal 244.90 mL/g VS down to just 172.01 mL/g VS. But adding the new materials brought the system back to life. A small dose of CQDs (1 g/L) increased methane production by 24.25% over the inhibited control. The LHC, when optimized, performed even better. An LHC prepared at 250°C, added at a concentration of 3 g/L, boosted methane yield by an impressive 30.53%.

This wasn’t a simple case of the materials “soaking up” the toxic ammonia. The study found that the LHC had very limited ammonia adsorption. Instead, the materials acted as electrical conduits, enhancing a process called Direct Interspecies Electron Transfer (DIET). In a healthy digester, different types of microbes must “hand off” electrons to each other to process waste into methane. Ammonia disrupts this. The new carbon materials acted as “wires” to reconnect the microbial community. The CQDs, with a graphene-like structure, served as a low-resistance pathway for electrons. The LHC worked as a “redox hub,” improving the system’s overall electron transfer capacity and even promoting the formation of other natural electron shuttles, like humic acids.

This electrical boost had a profound effect on the digester’s microbial ecosystem. The additives helped enrich bacteria that are known to participate in DIET, such as Chloroflexi and DMER64. They also supported the initial waste-degrading bacteria like Fibrobacter and ammonia-tolerant methanogens like Methanosarcina. Most tellingly, the additives helped revive Methanosaeta—a key methane-producer that is notoriously sensitive to ammonia. Its comeback showed that the inhibited methanogenic pathways were fully restored, stabilizing the entire system.

The researchers also found they could “tune” the materials for even better performance. By increasing the preparation temperature of the LHC to 250°C, they dramatically improved its redox capacity—making it 72% more powerful as an electron mediator than the 190°C version. This study provides a clear, promising strategy: using one type of waste (lignin) to create a high-tech solution that overcomes a major bottleneck in biogas production.

Source: Ma, S., Wang, H., Gao, X., Bian, C., & Zhu, W. (2025). Mitigating ammonia inhibition in anaerobic digestion with lignin-based carbon materials synthesized by hydrothermal carbonization. Carbon Research, 4(20).