Boosting performance and unraveling DIET induction in food waste anaerobic digestion: The role of ethanol pre-fermentation across solids loading gradients.

Food waste anaerobic digestion is susceptible to acidification, and its treatment process encompasses a wide concentration range, from wet to dry. To investigate the enhancement efficiency and mechanism of ethanol pre-fermentation (EP) at varying total solids (TS) concentrations, this study systematically compared its performance in wet (TS 5%), semi-dry (TS 15%), and dry (TS 20%) systems. The results indicate that the enhancement efficiency and mechanism of EP are highly dependent on the TS concentration. In the semi-dry system with TS 15%, EP significantly increased methane production by 1514%, successfully restoring stability to a system on the brink of failure. Mechanistic studies reveal fundamental differences in EP's mode of action: at low solid concentrations (TS 5%), stability improvement primarily arises from fermentation pathway optimization and favorable mass transfer conditions; at higher solid concentrations (TS 15% and 20%), all physicochemical data, thermodynamic analysis, and shifts in microbial communities support a plausible hypothesis: the electroactive functional microbes enriched (e.g., Petrimonas, Syntrophomonas, and Methanosarcina) by EP may have activated the direct interspecies electron transfer (DIET) pathway, effectively overcoming mass transfer limitations through an efficient syntrophic network. This study reveals for the first time that solid concentration is the key parameter determining the enhancement mechanism of EP, challenging the conventional understanding of its universal mode of action. It provides a new theoretical framework for precisely enhancing the digestion processes of organic waste with varying solid concentrations.