Acidogenic fermentation of thermally hydrolyzed organic matter recovered from residual municipal waste: process performance and insights from batch and semi-continuous operation.

To enhance energy and material recovery from complex residual municipal wastes, the post-treated thermally hydrolyzed mechanically sorted organic fraction (PHSW) was evaluated through thermophilic acidogenic fermentation. Using a sequential approach, batch tests first assessed the influence of inoculum source and substrate-to-inoculum ratio on fermentative performance. Stabilized mixed sludge outperformed digested sewage sludge, increasing biohydrogen production by 46%, associated with a higher microbial diversity (Shannon index: 5.489) that favored complementarity between hydrolytic and acidogenic taxa. Despite thermal pretreatment, substrate conversion remained limited, with hydrolysis efficiencies ranging from 8% to 26%. Subsequently, semi-continuous stirred tank reactors were operated at 55 °C under different hydraulic retention times (HRT: 3, 5, and 8 days) to evaluate process performance. The highest specific hydrogen production rate was obtained at HRT of 3 days, reaching 19.6 mL H2·g-1 volatile solids·d-1 and a volatile fatty acids productivity of 3.31 g chemical oxygen demand·L-1·d-1, with butyrate as the predominant metabolite. Microbial analysis revealed a community shift from hydrogen-producers acidogens toward a syntrophic consortium, which was associated with the emergence of hydrogenotrophic methanogenic activity under mildly acidic conditions (pH 5.5-6.5). These findings highlight the potential of PHSW valorization through thermophilic acidogenic fermentation, supporting the transition toward a circular biorefinery model.