Mixing patterns inside a simulated flat bottom digester were imaged using the non-invasive techniques of computer automated radioactive particle tracking (CARPT) and computed tomography (CT). Mixing/agitation was provided using gas (air) recirculation at three different flow rates (Q(g)) of 28.32, 56.64 and 84.96 l/h, corresponding to superficial gas velocities of 0.025, 0.05 and 0.075 cm/s, respectively. Better mixing was observed in the upper zone near the top of the draft tube. However, at the bottom of the digester there was a total stagnancy at all the three gas flow rates. The maximum value of the time-averaged axial velocity inside the draft tube, at a gas flow rate of 84.96 l/h, was observed as 34.4 cm/s. The turbulent kinetic energy was observed to be maximum (724 dyn/cm(2)) inside the draft tube, and decreases radially toward the wall of the digester. The present study showed that the CARPT and CT techniques could be successfully used to identify the flow pattern in the digester and to calculate velocity and turbulence parameters quantitatively. On the other hand, the increase in gas circulation rate from 28.32 to 84.96 l/h did not significantly reduce the dead zones inside the flat bottom digester. To achieve the desired mixing and reactor performance, the operating conditions and reactor configuration need to be optimized.