In this study, the effect of agitation rate, particle size, and shape of solids on the viability of a mesophilic culture of acidophilic bacteria was examined in stirred tank reactors (STRs) fitted with the Rushton turbine (RT) and the pitched blade turbine (PBT) impellers. In the absence of solids, the hydrodynamic shear levels generated as a function of impeller type and speed (1.0-3.35 m/s) were found to have a limited adverse effect on the bacterial cells, i.e., only a less than or equal to 16% loss in the viability of the bacterial population over a mixing period of 4 h. However, in the presence of solids (20% w/w) and at an agitation rate of 3.35 m/s, an extensive loss in the viability of bacterial population was observed apparently due to the action of solid particles on the bacterial cells. The rate and extent of the deactivation (i.e., loss in viability) of bacterial cells were shown to depend on the impeller type, the size, and shape of solid particles. The RT impellers caused more extensive loss in the viability of bacterial population than the PBT impellers under the same experimental conditions. The first-order deactivation rate of bacterial cells was found to increase with decreasing particle size down to a certain size (- 63 + 45 mum), below which a reverse trend was noted. The findings have also indicated that the spherical particles (ballotini) deactivate the bacterial cells at a faster rate than the irregular particles (quartz) in shape. (C) 2004 Elsevier B.V. All rights reserved.