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This work, for the first time, monitored advection flow through the floc interior using a bubble tracking technique. Three sludge flocs were examined, including two original activated sludge flocs and one flocculated floc. Observing the motion of a swarm of hydrogen bubbles passing over a free-falling floc allowed us to estimate the extent of advection flow as over 55% for all investigated flocs. Polymer flocculation reduced the floc interior permeability and yielded a weak advection flow.