Bioaugmentation of nitrate-dependent anaerobic ferrous oxidation by heterotrophic denitrifying sludge addition: A promising way for promotion of chemoautotrophic denitrification.

•The NAFO activity bio-augmented by HDS was more than twice as primary.•Bioaugmentation was mainly ascribed to the NAFO activity of HDS itself.•The predominant bacteria in HDS were Thauera and Hyphomicrobium.•HDS was suggested as bioaugmentation or seeding sludge for promotion of NAFO process.