Hydrogen is a potential clean energy that can replace fossil fuels due to its clean, renewable, and non-polluting characteristics. It offers higher energy efficiency compared to gasoline and diesel, making it an attractive option for various sectors, including transportation, industry, and residential areas. The researches on hydrogen production from organic wastes are increasing as organic wastes generation continues to rise. Among various methods of hydrogen production, hydrogen fermentation provides the advantage of simultaneous hydrogen production and waste disposal. It also yields volatile fatty acids (VFAs) widely used in different fields, while minimizing greenhouse gas emissions (CH4, CO2).

  Organic wastes, including food waste and livestock manure, consist of 30-50 % (by dry wt.) lignocellulose. The presence of lignocellulose structures in these organic wastes can inhibit the biodegradation rate and hinder hydrolysis step, resulting in low biogas production. In order to solve this issue, various pretreatment methods, such as biological, mechanical, chemical, and thermal hydrolysis, have been applied prior to HF to enhance biodegradability by breaking down lignocellulose. Among these various pretreatments, thermal hydrolysis process (THP), which disintegrate lignocellulosic waste using steam of high temperature and pressure, has been widely applied due to its significant performance improvement.

  THP exhibits certain drawbacks, including high heat and electricity requirements, as well as the potential generation of recalcitrant substances, such as melanoidins, through Maillard reaction and carbonization at high temperatures. Previous studies investigating the application of THP in HF primarily focused on improving hydrogen production. However, the energy balance of the HF process and the underlying mechanism of Maillard reaction, carbonization remained unknown. Further research is necessary to understand these aspects and address the knowledge gaps.

  The main objective of this study is to investigate the effects of thermal hydrolysis temperature of food waste on hydrogen production and net energy gain. The investigation focuses on assessing the influence of THP on biogas production through biochemical hydrogen potential (BHP) test. Additionally, an analysis of structure change of lignocellulose was performed to enhance our understanding of how THP impacts HF. Furthermore, an energy analysis is performed to evaluate the net energy gain of HF depending on THP of food waste using continuous stirred-tank reactor (CSTR) test.

  Food waste was treated with THP at temperatures of 120, 140, 160, and 180 ℃. The highest hydrogen production was exhibited in the case of food waste treated at 140 ℃, with a 3-fold increase compared with untreated food waste, as determined by BHP test. Decrease in cellulose crystallinity and the morphological change of lignocellulose indicated an improvement of accessibility for microorganisms, which resulted in hydrogen production increase. However, at temperatures of 160 and 180 ℃, changes in functional groups and formation of humic-acid like substances were observed, suggesting the occurrence of Maillard reaction and carbonization, which can lead to a decrease in hydrogen production.

  In CSTR test, food waste pretreated at 140 ℃ (THP HF) and untreated food waste (Control HF) were used as substrates. The application of THP contributed to the improvement of organic matter removal rate by 1.1-fold, and the enhancement of butyrate-toacetate ratio (Bu/AC) in THP HF, resulting in a 1.6-fold increase in hydrogen production. The application of THP led to significant alteration in the bacterial communities, contributing to enhance process stability. However, the net energy gain of THP HF was lower than Control HF, due to the higher energy input associated with THP.

  In conclusion, THP proved to be effective in enhancing hydrogen production from food waste by increasing surface area through the depolymerization of lignocellulose, while also improving the stability of the reactor. However, at high temperatures, the production of refractory substances led to a decrease in hydrogen production. Despite the positive effect of THP on hydrogen production, the net energy gain decreased due to higher energy input required for pretreatment step. Nevertheless, the energy efficiency of THP HF can be improved by increasing the heat recovery rate of the THP process to 95 %.