The coupling of mixotrophic denitrification, dissimilatory nitrate reduction to ammonium (DNRA) and anaerobic ammonium oxidation (anammox) promoting the start-up of anammox by addition of calcium nitrate

[Display omitted] •One nitrate-calcium-based anammox start-up pathway was discovered.•The effective start-up time of anammox induced by calcium nitrate was 50–60 d.•The coupling of anammox, DNRA and denitrification existed in Ca(NO3)2-added system. This study discovered one nitrate-calcium-based anammox start-up pathway. Compared with control, the start-up time of anammox was saved by 33.3%, and the average total nitrogen removal efficiency increased from 29.6% to 53.7% during the start-up. Besides, the continuous nitrite accumulation (1.18 mg/L) and a marked increase in the relative abundance of denitrifying and anammox bacteria were observed in the only Ca(NO3)2-added group. These results suggested that calcium nitrate induced partial denitrification to provide nitrite for anammox. Additionally, the role of dissimilatory nitrate reduction to ammonium (DNRA) in the Ca(NO3)2-added systems also deserved attention, for the contribution of DNRA to nitrate removal as well as the relative abundance of DNRA bacteria were both increased for the Ca(NO3)2-added groups. These results suggested that a mutualistic symbiosis among denitrification, DNRA and anammox exists in the calcium nitrate-added systems, which may explain the reason for acceleration of anammox start-up by adding calcium nitrate.