Nitrogen removal performances and metabolic mechanisms of denitrification systems using different volatile fatty acids as external carbon sources

•Propionate system had the highest and acetate system had the lowest NRE.•Rhodocyclaceae, Thauera and Blvii28 were the main functional bacteria.•Using propionate and butyrate as carbon sources could reduce N2O emission.•The proportion of denitrification module was highest in propionate system.•Propionate was mainly degraded through methyl malonyl-CoA pathway.