Factors affecting the growth rates of ammonium and nitrite oxidizing bacteria

► Two bench scale SBR were run under varying SRT in alternating and oxic conditions. ► The kinetics of ammonium and nitrite oxidizing bacteria were investigated. ► No correlation between SRT and μ max was found in tested range of SRT (2 to 5 d). ► μ max of AOB and NOB were higher in alternating than in oxic conditions. ► Decay was shown to be slower in anoxic conditions. The maximum specific growth rates of both ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) were investigated under varying aerobic solids retention time (SRT a) and in the presence/absence of anoxic (alternating) conditions. Two bench SBRs, reactor R1 and R2, were run in parallel for 150 d. Reactor R1 was operated in aerobic conditions while R2 operated in alternating anoxic/aerobic conditions. The feed (synthetic wastewater), temperature, hydraulic retention time and mixing were identical in both reactors. The SRT a in both reactors was, sequentially, set at four values: 5, 4, 3 and 2 d. Kinetic tests with the biomasses from both reactors were carried out to estimate the maximum specific growth rates ( μ max) at each tested SRT a and decay rates, in both aerobic and anoxic conditions. The kinetic parameters of nitrifier were estimated through the calibration of a two step nitrification–denitrification activated sludge model. The results point to a slightly higher μ max,AOB and μ max,NOB in alternating conditions, while both μ max,AOB and μ max,NOB were shown not to vary in the tested range of SRT a (from 2 to 5 d) at 20 °C. They were relatively high when compared to literature data: 1.05 d −1 < μ max,AOB < 1.4 d −1 and 0.91 d −1 < μ max,NOB < 1.31 d −1. The decay coefficients of both AOB and NOB were much higher in aerobic (from 0.22 d −1 to 0.28 d −1) than in anoxic (0.04 d −1 to 0.16 d −1) conditions both in R1 and R2, which explained the higher nitrification rates observed in the alternating reactor.