Mitigating ammonia inhibition in anaerobic digestion with lignin-based carbon materials synthesized by hydrothermal carbonization

Abstract Ammonia inhibition poses a significant challenge to the efficient and stable operation of anaerobic digestion (AD) systems by leading to the inhibition of volatile fatty acid conversion and reduced methane production. This study explores the utilization of lignin-based hydrochar (LHC) and carbon quantum dots (CQDs) produced via hydrothermal carbonization of alkali lignin to alleviate ammonia inhibition in AD processes. The results showed that both LHC and CQDs help counter the decline in methane yield and production rate typically associated with ammonia inhibition. Notably, the addition of 1 g/L CQDs significantly increased methane production by 24.25% compared to the control group. While LHC showed limited ammonia adsorption, its primary impact was enhancing direct interspecies electron transfer (DIET) through improved redox capacity and promoting humic acid-like organics formation. In contrast, CQDs reduced charge transfer resistance, significantly enhancing system redox capacity. Optimizing the hydrothermal carbonization temperature of LHC to 250 °C further optimized its redox properties, boosting methane production by 30.53% at a concentration of 3 g/L. Microbial community and metabolic pathway analyses indicated that LHC and CQDs enriched hydrolytic and acidifying bacteria, as well as DIET-associated microorganisms, facilitating efficient volatile fatty acid production and conversion. This process enabled the sustained operation of both acetoclastic and hydrogenotrophic methanogenic pathways, effectively mitigating the adverse effects of high ammonia nitrogen concentrations. Graphical Abstract