Autotrophic growth and decay rates of ammonium and nitrite oxidizing bacteria (AOB and NOB, respectively) have a significant impact on the design and on the process performance of wastewater treatment systems where nitrification occurs. Literature data on the separate decay rates of AOB and NOB is scarce and inconsistent. In this study, batch experiments based on respirometric techniques were conducted to determine the NOB decay rates under different oxidation-reduction potential conditions, in order to widen the understanding of nitrite dynamics. The decay rate measured under anoxic conditions was 85% lower than under aerobic conditions, whereas under anaerobic conditions the decay rate reduction was 92%. A design and simulation tool was used to assess the impact of applying these results in differentiated areas of an activated sludge system. Simulations show a greater impact for systems with a sludge retention time under 10 days, for which up to a 16-fold increase in NOB biomass concentration and up to 86% and 80% reductions in ammonium and nitrite concentrations in the effluent were calculated. Therefore, this work demonstrates that considering different decay rates for autotrophic biomass under different ORP conditions avoids underestimating system performance and over dimensioning new activated sludge schemes.