The mechanism of bacterial selection was investigated in aerobic granular sludge wastewater treatment systems (i) to determine why biological nutrient removal (BNR) gets deteriorated during granulation start-up under wash-out dynamics, and (ii) to identify the operation factors governing bacterial dynamics, PAO-GAO competition, and BNR performances in mature systems operated at steady-state. Bacterial community dynamics and nutrient removal performances were analyzed in a SBR started under wash-out conditions and inoculated with activated sludge. Subsequently, bacterial population shifts were followed with stepwise changes in acetate-propionate (Ac/Pr) ratio and concentration. Finally, a multifactorial experimental design was used to screen for main effects on PAO/GAO competition within a set of six parameters, i.e. COD concentration, Ac/Pr and COD/P ratios, pH, temperature, and redox conditions during starvation. Eight SBRs were run over 50 days at a fixed sludge age (15 days) and with fixed 4-h cycles comprising anaerobic feast (100 min) and mixed starvation (120 min) phases. During wash-out, insufficient sludge age and anaerobic contact time between acetate and biomass led to predominance of Zoogloea spp. (54±8%) and deteriorated BNR. BNR populations proliferated again after a substantial granular sludge bed (>5 gVSS•L-1) had formed and enabled full anaerobic acetate uptake. Experimental campaigns on PAO/GAO competition with mature granules showed that Accumulibacter (47%) and dephosphatation (80-100%) were favored with alkaline pH (7.5), temperature below 20°C, and Ac/Pr of 50:50%. Efficient nitrification-denitrification was obtained at pH 7.5 and above 25°C, and enhanced BNR was obtained with at least 750 mgCOD•L-1. With a fixed 2-h starvation phase, dephosphatation was only complete with full aeration, whereas with alternating aerobic-anoxic starvation conditions, the phase length had to be extended for full BNR. In conclusion, pH was identified as the most dominant factor for bacterial selection. Furthermore, control of duration of anaerobic and starvation phases is required for optimal performance.