  Numerous natural systems depend on the sedimentation of passive particles in
presence of swimming microorganisms. Here, we investigate the dynamics of the
sedimentation of spherical colloids at various E. coli concentration within the
dilute regime. Results show the appearance of two sedimentation fronts, a
spherical particle front and the bacteria front. We find that the bacteria
front behave diffusive at short times, whereas at long times decays linearly.
The sedimentation speed of passive particles decays at a constant speed and
decreases as bacteria concentration ($\phi_b$) is increased. As $\phi_b$ is
increased further, the sedimentation speed becomes independent of $\phi_b$. The
timescales of the bacteria front is associated with the particle settling
speeds. Remarkably, all experiments collapse onto a single master line by using
the bacteria front timescale. A phenomenological model is proposed that
captures the sedimentation of passive particles in active fluids.
