Effects of biochar application on anaerobic digestion processes

The use of biochar as an additive in anaerobic digestion (AD) is a potential pathway to enhance AD while promoting biomass-to-biochar value chain within a circular bioeconomy. Biochar, a novel biomaterial, serves for managing residual biomass following the cascading use principle, prioritizing material use over sole energy generation. AD, as a well-established method for biogas production and organic substance management, plays an important role in the global carbon cycle. Therefore, this thesis investigated the effects of biochar on AD, considering different process conditions, types of biochar, and effects on the resulting digestates. Biochar effects on AD processes were investigated under different experimental conditions towards a better understand of enhancing AD. Batch tests were conducted to explore the impact of various biochar dosages on AD, under both normal and severe substrate loading conditions. The obtained result that effects occur only at high biochar dosages and severe conditions, was critically compared with previous studies, considering variations in experimental conditions and observed effects. Chapter 2 extensively studied these investigations, aiming to identify biochar effects on AD under different experimental conditions and providing practical implications for achieving comparable effects in real-world AD settings through the application of realistic biochar dosages. In the pursuit of valorizing biomass and optimizing biochar's efficacy in acid-stressed AD, Chapter 3 tested various biochar types under the most promising experimental condition identified in Chapter 2. Residual woody biomass and low-value grass-type biomass were selected to produce biochar at different pyrolysis temperatures. Biochar properties and their relationships were determined to assist in understanding the role of biochar in acid-stressed AD. The potential environmental implications of different biochar types in AD were discussed to provide advice on biochar selection. For instance, grass-derived biochars are promising for acid-stress mitigation due to their buffer capacity, and carbon-rich woody biochars have high carbon sequestration potential. The study also explored the utilization of the resulting digestates with respect to heating values and heavy metal concentrations, thereby completing the value chain. Thus, this study added value in a broader biomass and carbon management system. To close the material loops and complete the AD-based biomass-to-biochar value chain, the resulted biochar-enriched digestates were investigated in Chapter 4. The dewaterability of the whole digestates was investigated to check the potential value-added post-effects of using biochar in AD. After the solid-liquid separation, the nutrient (P, S, K, N) concentrations in liquid fraction were also analyzed to determine the effects of using biochar in AD on nutrient fluxes. The dewaterability indicated the incineration potential and transport efficiency of the digestates, while the increased phosphorus and potassium concentrations in liquid fraction provided insights into the potential for further nutrient recovery from it. This study contributed to a better understand of post-effects of application biochar in AD, thus closing the loop and aiding in the establishment of a circular bioeconomy based on AD and biochar. To explain the effects observed in Chapters 2 and 3, further investigations were conducted in Chapter 5, focusing on exploring relationships between specific biochar properties and changes in the microbial community. The biochars were further characterized and the enrichment of specific microbes collected at the end of digestion was analyzed using qPCR. Chapter 5 critically examined the controversial explanations of biochar effects on AD in previous studies. No significant impacts on the microbial community were observed in this study, and the selective enrichment of microbes by biochar could not be conclusively confirmed, possibly due to variations in experimental conditions compared to previous studies. Nevertheless, the effects of biochar on AD within this thesis were primarily attributed to the physicochemical properties of biochars, which induced a short-term adjustment of the AD environment.