Improving anammox activity and reactor start-up speed by using CO2/NaHCO3 buffer

•CCBS, HEPES, and PBS were investigated for improving the performance of anammox sludge.•Anammox activity and reactor start-up speed were improved with CO2/NaHCO3 buffer.•Dominant anammox bacteria was Candidatus Kuenenia with the relative abundance of 71.8%.•Reactors could be started and enriched with anammox bacteria within microaerobic conditions. Anammox bacteria grow slowly and can be affected by large pH fluctuations. Using suitable buffers could make the start-up of anammox reactors easy and rapid. In this study, the effects of three kinds of buffers on the nitrogen removal and growth characteristics of anammox sludge were investigated. Reactors with CO2/NaHCO3 buffer solution (CCBS) performed the best in nitrogen removal, while 4-(2-hydroxyerhyl)piperazine-1-ethanesulfonic acid (HEPES) and phosphate buffer solution (PBS) inhibited the anammox activity. Reactors with 50 mmol/L CCBS could start up in 20 days, showing the specific anammox activity and anammox activity of 1.01±0.10 gN/(gVSS·day) and 0.83±0.06 kgN/(m3·day), respectively. Candidatus Kuenenia was the dominant anammox bacteria, with a relative abundance of 71.8%. Notably, anammox reactors could also start quickly by using 50 mmol/L CCBS under non-strict anaerobic conditions. These findings are meaningful for the quick start-up of engineered anammox reactors and prompt enrichment of anammox bacteria. [Display omitted]