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FIG.78. Experimental RTD curves in two tests.The tracer movement for a normal assumed flow regime should go from the inlet to the probeD 1 , after to the probe D 2 and at the end to the probe D 3 . Fig. 78 shows that the response signalrecorded by detector D 3 comes before the signal recorded by detector D 1 . This indicates that asignificant part of the wastewater flow is bypassing directly from the inlet to the outlet. The flowregime in this aeration tank was found abnormal and modification of the inlet position and installationof baffles were recommended to improve the efficiency of this WWTP’s unit.4.12. 99M TC AS A TRACER FOR THE LIQUID RESIDENCE TIME DISTRIBUTIONMESUREMENT IN ANAEROBIC DIGESTER: APPLICATION IN A SUGARWASTEWATER TREATMENT PLANT4.12.1. BackgroundDifferent chemical and radioactive tracers have been employed for diagnosing thehydrodynamic of anaerobic processes: K 82 Br, HTO, K 131 I, rhodamine-WT and lithium chloride totrace wastewater, and 113m In (InCl 3 ), 140 La and 198 Au for the sludge. However, for the countries whichdo not have nuclear reactor facilities for the production of such radioisotopes, the only possibility toextend the application of radiotracer techniques in sanitary engineering is the employment ofradioisotope generators.Among the commercial radioisotope generators the 99 Mo/ 99m Tc generator, due its generalizationin nuclear medicine for the production of labeled compounds, seems to be an attractive approach.However, the use of this tracer in anaerobic process must be preceded by the study of the physicochemicalbehavior of the eluate: the pertechnectate ion ( 99m TcO 4 - ).The validation of 99m Tc-pertechnetate as radiotracer to measure the RTD of the liquid phase inanaerobic digester has to be done. For this purpose, a new digester developed by the Cuban ResearchInstitute of Sugar for the treatment of the effluent of the sugar factory has been selected. The anaerobicdigestion processes need physico-chemical conditions easy to work: temperature around 30°C, pHbetween 6.8 and 7.8, absolute anaerobic medium, volatile fatty acids concentration around 1 g·L -1 .Previous investigations have shown that the lithium tracer was adsorbed slightly onto thepacking medium and held up within the biomass. Similar observation has been made for rhodaminedye diffused into the biofilm during the influx of tracer, and out-diffused slowly. The consequenceswere as much as 100% error in retention time. Therefore, when attempting to model reactorhydraulics, extreme care must be exercised in the choice of tracer compound and in the ultimateinterpretation of RTD functions obtained.70