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3. Conclusions• The aeration tank behavior was normal. The model consisting of five perfectly mixing cells inseries fits well with the design.• The primary and secondary clarifiers had large dead volumes. Necessary remedial action wasrequired to be taken by the plant operator in this regard.4.7. RADIOTRACERS FOR DIAGNOSING THE PERFORMANCE OF A SECONDARYCLARIFIERRadiotracer tests were used to diagnose the performance of a secondary clarifier (Fig. 46).Characteristics of the clarifier were:• Diameter 23.6 m• Surface 437 m 2• Volume 1000 m 3 FIG.46. Scheme of the secondary clarifier.The objectives of radiotracer tests were:• study of the behavior of water and solid phases for different conditions of sludge bed andrecirculation flow-rate• validation of a CFD model.Seven radiotracer tests were conducted:• Water tracing experiment : 2 tests• tracer : Tc-99m, 50 mCi (1.85 GBq)• recycling flow rate 250 m 3 /h• Sludge tracing experiment : 5 tests• tracer : Au-198 , 50 mCi ( 1.85 GBq)• 3 conditions of the sludge bed• 2 recycling conditions 250 and 500 m 3 /hThe injection point was at the outlet of the aeration channel which is the inlet of the clarifier.The location of the detection probes was:• 9 probes (D1 to D9) were placed on the rotating arm of the scrapper (1 turn/30 min) (Fig. 47)• 1 probe (D10) was placed inside the Clifford baffle at the entrance of the clarifier• 1 probe (D11) was placed on the recirculation circuit• 1 probe (D12) was paced at the water outlet of the clarifier (discharge to the river).49