The enrichment of r-strategist ammonium oxidizing bacteria (r-AOB) to enhance nitritation and effective nitrogen removal was studied. A lab-scale three-compartment reactor system removing nitrogen from synthetic wastewater containing ammonium and COD (NH₄ ⁺:COD ratio at ca. 1:8) was used. A stable nitritation-denitritation process was achieved. The first compartment was anoxic for denitrification and the last two compartments were aerobic to oxidize ammonium into nitrite (nitritation). Recirculation between last and first compartment allowed for effective denitrification. Nitritation was achieved by enhancing the growth of fast growing r-AOB and setting a high NH₄ ⁺ concentrations in the first of the two aerobic compartments (named as r-AOB selector). The sludge was wasted adaptively to maintain NH₄ ⁺ concentration between 10 and 20 mg N/L in the r-AOB selector. The adaptive sludge wastage resulted in a short sludge retention time (SRT) of 4.2 days that led to full nitritation within 100 days of operation. Kinetic parameter estimation indicated that the maximum specific growth rate for AOB increased from 0.39 to 1.45 d⁻¹, while the NH₄ ⁺ half saturation constant increased from 0.51 to 5.23 mg N/L, indicating the transition from K-AOB (slower growing K-strategist AOB) to r-AOB. Mathematical simulation indicated that the SRT and NH₄ ⁺ concentration were the main factors determining the outcome of nitrifying species competition.