Upgrading volatile fatty acids production from anaerobic co-fermentation of orange peel waste and sewage sludge: Critical roles of limonene on functional consortia and microbial metabolic traits

•Co-fermentation of OPW and SS is feasible for VFAs promotion.•The solubilization and hydrolysis are facilitated but restrain the methanogenesis.•The positive roles of limonene on the OPW/SS co-fermentation are elucidated.•The functional bacteria for VFAs biosynthesis are significantly enriched.•The microbial metabolic traits associated with VFAs generation are unveiled.