Effect of COD/N ratios and DO concentrations on the NOB suppression in a multi-cycle SBR

Abstract NOB could be suppressed in a multi–cycle sequential batch reactor with an integrated control of four step feeding, low DO (dissolved oxygen) concentrations and short sludge retention time, but which is the key control parameter of mainstream NOB suppression remained unclear. This study investigated the effect of COD/N ratios and DO concentrations on NOB suppression with different COD/N ratios and DO concentrations. Nitrite accumulate rate reached 93.8% at COD/N ratio of 4:1 and 1.5 mg DO/L, and ranged between 73.4% and 80.0% at COD/N ratios of 1.4:1 and different DO ranges between 0.3 mg/L and 1.1 mg/L. As COD/N ratios decreased to 0.8:1, nearly no nitrite accumulated at 0.6 mg DO/L. Statistical results showed that there was a significant relationship among COD/N ratio, nitrite accumulate rate and total nitrogen removal efficiency, while there was no correlation between DO concentration and nitrite accumulate rate or total nitrogen removal efficiency. Therefore, the main reason of NOB suppression was nitrite competition between denitrifiers and NOB, rather than oxygen competition between AOB and NOB.