Enhanced Ethanol Fermentation of Food Waste by Simultaneous Saccharification and Its Performance in Anaerobic Co-Digestion with Sludge

Food waste (FW) and waste activated sludge (WAS) are major components of municipal organic solid waste and are produced in large quantities annually. Anaerobic co-digestion of FW and WAS is a promising strategy to balance the carbon-to-nitrogen (C/N) ratio and improve the overall operational stability of anaerobic systems. However, the hydrolysis of complex organic components in FW often becomes the rate-limiting step for enhancing system performance. FW contains abundant carbohydrates that can be fermented by yeast to produce ethanol. Compared with the oxidation of other common volatile fatty acids (VFA), ethanol oxidation releases more energy, which favors the reduction of carbon dioxide to methane and thereby promotes direct interspecies electron transfer (DIET)-based anaerobic digestion. Yeast inoculation is a commonly used method for in situ ethanol production. However, yeast can only utilize reducing sugars, whereas carbohydrates in FW primarily exist as polysaccharides, such as starch and cellulose, which limits their effective utilization by yeast. To address the low ethanol yield during fermentation pretreatment of FW, this study employed simultaneous saccharification and fermentation to enhance ethanol production and investigated its effects on subsequent anaerobic co-digestion with WAS. The results showed that under optimal conditions (a saccharifying enzyme dosage of 50 U/mL, a yeast inoculation ratio of 2%, a pretreatment temperature of 30 °C, and a pretreatment time of 21 h), the ethanol concentration reached 41.1 g/L, accounting for approximately 27.2% of the chemical oxygen demand (COD) of FW. When the pretreated product was used as the substrate for co-digestion, the methane yield increased by 17.2% compared with that of the control (without pretreatment), while the volatile solids (VS) removal rate improved by 3.2%. Electrochemical analysis showed that ethanol-type fermentation pretreatment increased sludge capacitance and decreased internal resistance, indicating enhanced electrochemical activity and improved charge–discharge capability. Moreover, microbial community analysis revealed that DIET-associated microorganisms, including Methanothrix and Methanosarcina, were enriched. Economic analysis estimated that treating 1.0 t of FW and 3.7 t of WAS could generate a net economic benefit of approximately CNY 35.7. This study demonstrates that simultaneous saccharification and fermentation effectively enhances ethanol production from FW and promotes energy recovery and organic waste reduction via the DIET mechanism, providing a theoretical basis for the application of enzymes and functional microorganisms in anaerobic digestion.