Experiments were carried out on full scale clarifiers. Two types of clarifiers were studied, circular scraper-type and radial suction tubes clarifiers. Sludge tracing experiments were made with radioactive gold.

For the first type of clarifier, the results obtained show a short circuit concerning 10 to 30% of the sludge and a large distribution of the remaining sludge arrivals materialized by a succession of peaks that appear with increasing delay. Depending on cases studied, 40 to 50% of the sludge remained more than 5 hours in the system.

For the suction clarifier, a perfect sludge distribution with time was observed for thin sludge bed (90% of the sludge recycled within 2 hours), but this result is rapidly affected by an increase of the sludge bed depth (60% within 5 hours).