The metabolic patterns of the complete nitrates removal in the biofilm denitrification systems supported by polymer and water-soluble carbon sources as the electron donors

[Display omitted] •Using water-soluble carbon spent shorter HRTs for the complete removal of nitrates.•The optimal denitrification rates reached 0.728 (NaAc) and 0.561 (PCL) g N/(L·d).•The observed biomass yield of both carbon sources was 0.09 g VSS/g COD.•NaAc biofilter had a proper ratio of nitrate removal and COD degradation microbe.•The potential denitrification mechanism of two carbon sources were proposed. In this study, two denitrification bio-filters adopted polycaprolactone (PCL) and sodium acetate (NaAc) as polymer and water-soluble carbon sources respectively. With the increasing influent nitrate concentrations, NaAc bio-filter always had shorter HRT to achieve complete nitrate removal. Furthermore, the optimal volumetric denitrification rate in NaAc bio-filter was 0.728 g N/(L·d), which was higher than 0.561 g N/(L·d) in PCL bio-filter. For nitrates removal, the costs of bio-filters supported by NaAc and PCL were 24.93 and 120.25 CNY/kg N respectively. Although Proteobacteria in PCL bio-filter was abundant with 83.98%, NaAc bio-filter had better denitrification performance, due to the appropriate ratio of nitrate removal microorganisms and organic matters degradation organisms. The total abundance value of the denitrification genera is NaAc (16.06%)