Abstract Heat shocks are a favoured method used to trigger hydrogen production in dark fermentation. However, these treatments are often restricted to a very small volume, which is then inoculated into synthetic medium. Moreover, heat shocks are usually applied with an oven. This study explored the feasibility of applying heat shocks to the entire seed sludge using a microwave, a method not previously investigated. The experiments were conducted on a technical scale with 5 litre tank reactors. The highest productivity was achieved with an oven (0.95 mol H 2 /mol hexose), but the microwave was not significantly different and had the advantage of heating the sludge 6.4 times faster. The study also emphasized the repeatability of heat shocks, with microwave-assisted experiments showing low variation coefficients averaging 29%. The approach is already economically sustainable with high-temperature heat pumps with a COP of 4.3, enhancing its practical relevance. Highlights - The entire reactor content of 3.5 litre was heat shocked. - Heat shocks economically boosted dark fermentation hydrogen yield. - Heat pumps with COP of 4.3 would allow energetic net gain due to heat shocks. - Oven and microwave-based heat shocks are not significantly different. - Microwaves heat sludge 6.4 times faster than ovens. - Heat shocks inhibited methane production completely. - Heat shocks increased particularly Clostridium sensu stricto 1 and 11. - Hydrogen could be enriched from sucrose, but not from lipids or proteins.

The quest for improved biohydrogen formation: are heat shocks the solution?