Improvement of ferrous ion-dependent nitrate removal (FeNiR) efficiency with acetate as co-substrate

•With acetate as co-substrate the FeNiR rate kept at 0.51 kg-N/(m3·d) for 40 days.•With co-substrate the specific FeNiR activity of sludge kept at 0.73 mg-N/(g VSS·h).•With co-substrate the formation of iron encrustation was efficiently avoided.•The obligate heterotrophic denitrifier contributed to the stably improved efficiency.