Dark fermentation (DF) of kitchen waste (KW) is a promising technology for the production of renewable biohydrogen. It can be both a method of obtaining clean energy and a sustainable waste management. Despite its potential, this process requires further research to improve efficiency. The aim of the research was to test the effect of thermal pretreatment of the inoculum on H₂ and volatile fatty acids (VFAs) production in the DF process. The process was carried out at 37 °C, in batch mode. The digested sludge from the Group Wastewater Treatment Plant in Lodz was used as inoculum. KW from households was used as substrate. The inoculum was pre-treated at 70 °C for 15 and 30 min. Two control reference experiments were also used. The first without the inoculum, and the second without heating the inoculum. The thermal pretreatment inhibited methane production and increased hydrogen production. After the thermal pretreatment, the amount of CO₂ produced during the process decreased compared to the bioreactor without inoculum pretreatment. Additionally, the main VFAs in the samples with pretreated inoculum were acetic and butyric acids, which are associated with hydrogen production in the biochemical pathways of the DF process. However, the time of thermal pretreatment had no significant effect on H₂ production.

The production of biohydrogen from food waste (FW) by dark fermentation (DF) is a promising technology for commercialisation, as it is both a clean fuel and a suitable means of sustainable waste management. The described experiments compared the biohydrogen production yields obtained after the use of inoculum from two different sources: digested sludge from the wastewater treatment plant (WWTP) in Lodz and sludge from the anaerobic treatment of dairy industry wastewater (DIW) (unconcentrated and double-concentrated). In addition, the effect of different temperatures (70, 90 and 121°C) of inoculum pretreatment on the biohydrogen production in DF was tested. The process was carried out batchwise at 37°C. The highest yield of hydrogen production was obtained after the inoculum pretreatment at 70°C. In addition, a higher amount of hydrogen could be obtained by using sludge from the WWTP as the inoculum (96 cm3 H2/gTVSFW) than unthickened sludge from the DIW (85 cm 3 H 2/g TVSFW). However, after thickening the sludge from the dairy industry, and at the same time balancing the dry matter of both sludges, the hydrogen production potential was comparable for bothsludges (for the WWTP sludge – 96 and for the DIW sludge – 93 cm 3 H 2/g TVSFW). The kinetics of hydrogen production was described by modified Gompertz equation, which showed a good fit (determination coefficient R2 between 0.909 and 0.999) to the experimental data.

The possibility of hydrogen (H₂) production from sizing waste, specifically starch-based substrates, was investigated through dark fermentation. Modified starch substrates produced less (up to 54% without heating and 18% after heating) H₂ than natural ones. However, heating modified starch samples led to 18% higher H₂ production than unheated ones, suggesting that high temperatures activate more favorable metabolic pathways. The highest H₂ production (215 mL/g_{TVS_substrate}) was observed with unheated natural starch, where the classic butyric–acetic fermentation pathway predominated. This variant also generated the highest CO₂ levels (250 mL/g_{TVS_substrate}), confirming the correlation between H₂ and CO₂ production in these pathways. Modified starch substrates shifted fermentation towards fatty acid chain elongation, reducing CO₂ production. The proportion of CO₂ in the fermentation gases correlated strongly with H₂ production across all variants. A decrease in total volatile solids (TVS) indicated effective organic matter conversion, while varying dissolved organic carbon (DOC) levels suggested different degradation rates. Nitrogen analysis (TN) revealed that the differences between variants were due to varying nitrogen processing mechanisms by microorganisms. These results highlight the potential of sizing waste as a substrate for bioH₂ production and offer insights for optimizing the process and developing industrial technologies for bioH₂ and other valuable products.

This study presents an innovative approach to the production of hydrogen from liquids following hydrothermal treatment of biowaste, offering a potential solution for renewable energy generation and waste management. By combining biological and hydrothermal processes, the efficiency of H₂ production can be significantly improved, contributing to a reduced carbon footprint and lower reliance on fossil fuels. The inoculum used was fermented sludge from a wastewater treatment plant, which had been thermally pretreated to enhance microbial activity towards hydrogen production. Kitchen waste, consisting mainly of plant-derived materials (vegetable matter), was used as a substrate. The process was conducted in batch 1-L bioreactors. The results showed that higher pretreatment temperatures (up to 180 °C) increased the hydrolysis of compounds and enhanced H₂ production. However, temperatures above 180 °C resulted in the formation of toxic compounds, such as catechol and hydroquinone, which inhibited H₂ production. The highest hydrogen production was achieved at 180 °C (approximately 66 mL H₂/g_TVSKW). The standard Gompertz model was applied to describe the process kinetics and demonstrated an excellent fit with the experimental data (R² = 0.99), confirming the model’s suitability for optimizing H₂ production. This work highlights the potential of combining hydrothermal and biological processes to contribute to the development of sustainable energy systems within the circular economy.