Bioaugmentation with ammonium oxidizing bacteria (AOB) was tested for 620 d. A seeding reactor (R1), two seeded reactors (R2 at 21 degrees C; R3 at 15 degrees C) and an unseeded-control reactor (R4 at 21 degrees C) were operated in parallel (2.4 < SRT < 4 d). The effect of seeding on nitritation efficiency was found to be dependent on solids retention time (SRT), influent ammonia concentration to the seeded reactors and the temperature difference between the seeding and seeded reactors. Mathematical modeling and batch tests were used to characterize the AOB selected in R1 and the effect of the seeding on AOB kinetics in R2 and R3. The AOB kinetics of R2 and R3 reflected the kinetics of R1 but differed from those in R4. This behavior affected the efficiency of bioaugmentation to varying degrees in the reactors and required a specific approach to represent the experimental results through mathematical modeling. (C) 2012 Elsevier Ltd. All rights reserved.