SLUDGE RETENTION TIMES DISTRIBUTION IN CLARIFIER - A KEY POINT FOR POPULATION-DYNAMIC AND NUTRIENTS REMOVAL CONTROL

Experiments were carried out on full scale clarifiers. Two types of cl arifiers 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 c ircuit concerning 10 to 30% of the sludge and a large distribution of the remaining sludge arrivals materialized by a succession of peaks th at appear with increasing delay. Depending on cases studied, 40 to 50% of the sludge remained more than 5 hours in the system. For the sucti on clarifier, a perfect sludge distribution with time was observed for thin sludge bed (90% ofthe sludge recycled within 2 hours), but this result is rapidly affected by an increase ofthe sludge bed depth (60% within 5 hours).