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Results:• Head and tail of the signal are well reproduced; numerical velocity field in the sludge blanket isclose to the on-site velocity field,• However, the signal is more dispersed within the experiment, which can be explained by thedispersion at the entrance ,• Probes n°2-5-8 located in the middle of the process give the best agreement between experimentaland numerical results.FIG.53. Comparison between CFD model and radiotracer experiment – Probe D1.Results:• Sludge in the bottom of the tank is of high concentration and possesses a longer residence timethan in other region (between 120 and 150 minutes),• Experimental results show heterogeneity in different angles and periodic pulsations in the signal ofthe tank which can not be reproduced by a 2D axial CFD simulation.ConclusionFor tracer experiments:• Short circuit is observed both for water and sludge: an important fraction (from 10 to 30 % of theflow) is transferred directly to the recirculation circuit without entering the clarifier.• The repartition of water and sludge is very heterogeneous in the clarifier. Tracers are observed inall the system but with very high variations of concentration.• Low quantities of labeled sludge are transferred to the outlet (less than 1% of the injectedactivities) showing the good efficiency of the clarifier.• It has been possible to measure reliable radial velocities of sludge and water particles inside theclarifier for various functioning conditions.• It has been possible to measure the mean residence time distribution of water and sludge inside theclarifier for various functioning conditions.For the CFD model:• Rheology modeling using CFD simulation is not reliable, in particular for solid phase and scraperflow motion,• Comparison with tracer studies gives relatively good agreement in region where the scraper effectis negligible (clarified zone & middle probes).52