This work monitored advection flow through a floe by bubble tracking. Close examination of the motion of a swarm of hydrogen bubbles that passed over a free-falling floe allowed the extent of advection flow to be estimated at 53% for the original activated sludge floe, and 12% for the flocculated floe. The interior permeability of the sludge floes was estimated from this information. The fluid force exerted on the falling floe was also considered.