Microwave assisted acidification for green hydrogen and organic acids production

Background The integration of anaerobic digestion into bio-based industries can create synergies, which help to make anaerobic digestion self-sustaining. Two-stage digesters with separate acidification stages allow to produce green hydrogen and short-chain fatty acids, which are promising industrial products. Heat shocks can be used to foster the production of these products. However, the practical applicability is oft not addressed sufficiently. The here presented work aims to close this gap. Methods Batch experiments were conducted in 5 litre double-walled tank reactors incubated at 37 °C. Short microwave heat shocks of 25 min duration and exposure times of 5 - 10 min at 80 °C were performed and compared to oven heat shocks. Pairwise experimental group differences for gas production and chemical parameters were determined using ANOVA and post-hoc tests. High-throughput 16S rRNA gene amplicon sequencing was performed to analyse taxonomic profiles. Results Heat shocking the entire seed sludge, the highest hydrogen productivity was observed at a substrate load of 50 g/l with 1.09 mol H 2 /mol hexose. With 1.01 mol H 2 /mol hexose, microwave assisted treatment was not significantly different from oven-based treatments. The study emphasised the better repeatability of heat shocks with microwave-assisted experiments, showing low variation coefficients averaging 29 %. Microwave pre-treatment indicates a high predictability and a stronger microbiome shift to Clostridia than the oven. The pre-treatment of heat shocks supported the formation of butyric acid up to 10.8 g/l in average and a peak of 24.01 g/l at a butyric/acetic acid ratio of 2.0. Conclusion Results: show the suitability to heat shock the entire seed sludge rather than just a small inoculum, which makes the process more relevant for industrial application. A microwave-based treatment has proven to be a promising alternative to oven-based treatments, which ultimately might facilitate the implementation into industrial systems. The approach becomes economically sustainable with high-temperature heat pumps with a coefficient of performance (COP) of 4.3.