The augmentation of biochar (BC) during anaerobic digestion (AD) has been identified as a potential strategy for improving the AD of complex feedstocks. This study evaluates the influence of oak wood biochar 450 °C and fermentation conditions during the AD of the invasive aquatic plant, water hyacinth (WH). Factorial 2² design of experiments (DOE) allowed the evaluation of the effect of the crucial processing conditions, inoculum-to-substrate ratio (ISR) and biochar load. Further optimisation was performed to identify the best processing conditions for the AD of WH, at an ideal ISR of 1. The contour plots suggested that methane yield is favoured at biochar loads of ≤0.5%, whereas the production rate is favoured by increasing biochar loads. However, biochar addition offered no further improvement or significant effect on the digestion of WH. The subsequent AD of WH samples collected from different locations in India and Uganda exhibited variable biochemical methane potential (BMP) yields. BC addition had little effect on BMP performance, and in some cases, it even reduced the BMP. This study concludes that the amendment potential of biochar is influenced by digestion conditions and the substrate, particularly when working with complex substrates.

Biochar addition in anaerobic digestion has been repeatedly reported to improve methane production, however, this ability is not well understood. This work aims to understand and correlate the most important factors influencing anaerobic digestion performance using principal component analysis along with quantitative and qualitative descriptive analysis to evaluate the variations of methane production with the addition of biochar. Reports from the literature using biochar produced from several feedstocks under variable pyrolysis conditions and therefore different compositions were carefully gathered and compared with their own non-biochar controls. Woody-derived biochars, produced at 450–550 °C, containing an ash content of 3.1–6.3%, and an O:C ratio of 0.20, were responsible for having the greatest positive effect. The amount of biochar added to the digesters also influences anaerobic digestion performance. Increasing biochar loads favours the production rate, although this can be detrimental to methane yields, thereby, biochar loads of approximately 0.4–0.6% (w/v) appear to be optimal. This work provides a guide for those interested in biochar augmentation in anaerobic digestion and identifies the main interactions between the variables involved.

The transition to renewable energy requires scalable and sustainable hydrogen production technologies. Dark fermentation (DF) can generate biohydrogen from diverse biomass feedstock, but its efficiency remains limited. Immobilising anaerobic consortia offers a route to improve performance. This study reports on the immobilisation of whole cells in hybrid biochar–alginate beads (BAB) compared with control alginate beads (CAB) during DF. Biochar from oakwood and water hyacinth, pyrolysed at 450 and 600/650 °C, were incorporated into BAB. BAB increased biohydrogen production rates by 1.4–2.6-fold relative to CAB, driven by enhanced microbial attachment, synergistic interactions, and improved mass transfer. High-temperature biochar generated the strongest effects, raising hydrogen yield by up to 23% and shortening the lag phase by 94%. Biochar properties, including porosity, surface area, inorganic content, electrical conductivity and buffering capacity, likely support these effects. These results establish hybrid biochar-alginate support as a promising platform to accelerate DF and advance biohydrogen as a sustainable biofuel.