Thermophilic dark fermentation for hydrogen and volatile fatty acids production from breadcrumbs

Food waste poses significant environmental challenges, with Japan generating 100 million kg of bread waste annually. This study explores the potential of thermophilic dark fermentation to convert bread waste into hydrogen (H2) and volatile fatty acids (VFA). A 3-L continuous stirred-tank reactor (CSTR) was operated for 269 days using breadcrumb as feedstock. The highest H2 production rate of 7.0 L-H2 L- 1 d- 1 was achieved on day 52 with the production of acetate and butyrate. However, H2 production fluctuated significantly, correlated with the shifts of microbial community structure. Initially, Thermoanaerobacterium saccharolyticum, Thermocoprobibacter melissae, and Anaerocolumna jejuensis coexisted, but An. jejuensis outcompeted others during the deterioration phase, resulting in acetate accumulation without H2 production. Metabolic potential analysis using the phylogenetic investigation of communities by reconstruction of unobserved states 2 (PICRUSt2) revealed that carbohydrate degradation was primarily carried out by An. jejuensis, while protein and lipid degradation involved diverse microbial members. The low functional diversity in carbohydrate degradation was a potential cause of instability. This study demonstrates the feasibility of using bread waste for H2 and VFA production, and provided insights into microbial dynamics essential for stable reactor performance.