Substrate ratio as an ecological selector in co-fermentation: Steering dissolved organic matter dynamics and microbiome assembly for enhanced short-chain fatty acid production from food waste and sludge

Co-fermentation of waste activated sludge (WAS) with food waste (FW) enhances short-chain fatty acid (SCFA) production, yet the regulatory role of the mixing ratio remains unclear. This study reveals that the FW-to-WAS ratio functions as an ecological selector, steering dissolved organic matter (DOM) dynamics and microbiome assembly to direct acidogenic efficiency. Increasing the FW proportion raised total SCFA yield and shifted metabolism toward butyrate dominance (>72%). A balanced 1:1 ratio optimally structured the DOM pool, increasing bioavailability and reducing humification, thereby shaping a specialized bacterial consortium. Microbial analysis showed selective enrichment of acidogenic Bacillota (e.g., Clostridium) and suppression of methanogens under the resulting low-pH, high-SCFA environment. Metagenomics confirmed upregulated hydrolysis/acidogenesis genes and inhibited methanogenesis pathways at this ratio. Semi-continuous operation demonstrated the long-term stability of the 1:1 system, sustaining high SCFA production for over 90 days. These findings establish a DOM-mediated microbial selection mechanism and offer a substrate-ratio-based strategy to program carbon flux toward efficient SCFA recovery from organic wastes.