Simultaneous nitrification-denitrification has been observed in aerobic granular sludge sequencing batch bubble column reactors fed with either acetate or propionate with elimination rates of up to 60-70%. Several in-situ tests under anoxic conditions were performed to evaluate the capacity of the two granular sludges to reduce nitrate (NO3-) and nitrite (NO2-). In absence of dissolved carbon (C), the intracellularly stored carbon acts as electron donor for denitrification. Therefore, the NO3- and NO2- removal could mainly be associated to organisms with internal C-storage such as glycogen accumulating organisms (GAO) and phosphate accumulating organisms (PAO). It was found that in both reactors the specific NO2- uptake rate was much higher than the specific NO3- uptake rate. These results agree with recent findings that some denitrifying PAO (DPAO) can only reduce NO2- but not NO3-. Moreover the propionate reactor was more efficient for NO2- removal than the acetate reactor. These results indicated that propionate as carbon source favored the growth of PAO with NO2- reducing capacity