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4.00E-033.50E-03ExperimentalModel3.00E-032.50E-032.00E-031.50E-031.00E-035.00E-040.00E+00151101151201251301351401E(t)451501551601651701751801851901Time (min)FIG.42. Experimental and model RTD curves of the aeration tank.FIG.43. Model of the aeration tank. V1 = 80 m 3 , V2 = 101 m 3 , V3 = 100 m 3 , V4 = 100 m 3 ,V5 = 97.7 m 3 , V6 = 106.7 m 3 .The model consists of five perfect mixers in series with back mixing and connected with a plugflow reactor at the beginning. After the injection point, the incoming wastewater passes through anarrow duct before it enters into the series of tanks through small holes. Because of this reason, a plugflow reactor is used between node no. 1 and node no. 2. Back mixing ratio of the tanks connected inseries is found to be 2.7.The volume of the aeration tank was 567.5 m 3 and the volumetric flow rate during theexperiment was 2.08 ± 0.08 m 3 /min. It gives the theoretical mean residence time of 272.8 min. Theexperimental mean residence time of the unit was estimated as 271.9 min with a very small deadvolume (0.32%). Therefore, almost negligible amount of dead volume was estimated in the aerationtank. This is due to the vigorous mixing process inside the aeration tank.Due to high gas flow rate, tracer experiments conducted in aeration tank have shown that waterand sludge have generally similar flow behavior. Radiotracer investigation in aeration tank has shownthat the fluid flow can be modeled either by perfect mixing cells in series or by perfect mixing cells inseries with back mixing. The number of mixing cells found by RTD modeling was J=5, in fact numberJ is a function of both gas and water flowrates as well as of the geometrical configuration. The resultsof this experiment showed that the aeration tank was achieving the designed residence time and wasworking efficiently as far as residence time is concerned.(c) Secondary clarifierThe experimental RTD curve obtained at the output of the secondary clarifier is given in theFig. 44. Figure 45 shows the best model found using RTD software simulation.47