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Highlights

The maximum SCCA concentration was achieved at a C/N ratio of 25 with 18 days of fermentation.

The highest VFAs and LA were produced at C/N ratios of 12.7 and 35, respectively, after 12 days of fermentation.

The unit energy value of SCCAs for drop-in fuel production was 50.5% higher than that of methane.

The dominant bacteria changed from Clostridium to Lactobacillus with increasing C/N ratio.

Abstract. Advancing technologies to produce short-chain carboxylic acids (SCCAs) from agricultural waste are expected to be more economical and efficient. This study presented a new attempt to enhance SCCA production from co-digestion of swine manure and corn silage with the carbon-nitrogen (C/N) ratio adjusted from 12.7 to 50.2. Results showed that the maximum SCCA concentration of 26,214 ±2948 mg COD L^-1 was achieved at a C/N ratio of 25 with 18 days of digestion, and the concentrations of lactic acid (LA) and volatile fatty acids (VFAs) were 8700 ±1060 mg COD L^-1 and 17,514 ±1888 mg COD L^-1, respectively. The LA content first increased and then slightly decreased with increasing C/N ratio, and the maximum LA concentration was 468.5 ±44.3 mg COD g^-1 VS_added (VS_added means added fermentation material is calculated as volatile solids) at a C/N ratio of 35 at 12 days of digestion. VFAs showed an opposite trend, with a maximum VFA concentration of 601.1 ±42.4 mg COD g^-1 VS_added found at a C/N ratio of 12.7 after 12 days of digestion. Potential energy analysis of the highest SCCA production showed that the unit energy value of SCCAs for drop-in fuel production was 50.5% higher than that of methane. Analysis of the microbial community showed that the dominant bacterial groups changed gradually from butyric acid-producing bacteria (Clostridium) to lactic acid-producing bacteria (Lactobacillus) as the C/N ratio increased from 12.7 to 50.2.