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FIG.56. RTD simulation for experimental RTD profiles obtained at outlet of the primary digester.FIG.57. The results of RTD analysis on the secondary digester.The digestion process was simulated with the perfect mixers in a series with exchange volume.The results are plotted in Fig. 57. In contrast to the small increase of the MRT of the primary digester,the MRT of the secondary digester has been remarkably increased (Table 7).TABLE 7. COMPARISON OF MRTs BETWEEN BEFORE AND AFTER CLEANING-UPMRT(day)ItemsBefore dredgingAfter dredgingprimary secondary primary secondarytheoretical 1.95 1.89experimental 1.37 0.4 1.6 1.6Effective volume ratio 70% 20% 85% 85%(d) Discussion of resultsAfter the cleaning of the digesters the variance has been decreased and the sludge dynamics wasactivated as a result of the increase of the effective volume. Particularly the MRT of the secondarydigester which has no mixing mechanism has been increased by 3 times. The dynamic behavior of theprimary digester is strongly affected by the status of the secondary digester due to the circulation flow.Circulation improves the mixing effect without changing the mean residence time of the system.55
The breakthrough time and the maximum concentration time at the outlet of the secondarydigester have been greatly delayed, thus the sludge has more chance to be decomposed by themicroorganisms in the digester than before.Radiotracer study on cylindrical digesters revealed the change of the hydrodynamiccharacteristics after removing the stagnant zone. It is evident that results from radiotracer study can beused as a reference in the diagnosis of the efficiency of a digester system during its normal operation.4.9. DIAGNOSIS OF OVAL SLUDGE DIGESTER(a) ProblemAlthough some natural mixing occurs in an anaerobic digester because of rising sludge gasbubbles and the thermal convection currents caused by the addition of heat, these levels of mixing arenot adequate to ensure stable digestion process performance at high loading rates. The long timeoperation of an anaerobic digester causes stagnant zone (or inactive volume), which reduces effectivereaction volume and treatment efficiency. Therefore it is important to locate and quantify stagnantzone in digester for its optimal maintenance and effective operation. Poor quality of digested sludge isa common and often problem in practice.Two oval sludge digesters in series were studied using radiotracer (Fig. 58). The scope ofradiotracer tests was to assess the existence and location of the stagnant zone by estimating of meanresidence time on the two stage anaerobic digester.Radiotracer study on the digester can:• Supply the information about the flow pattern of sludge without disturbance to the system.• Enables to estimate the effective volume of digester• Locate and quantify the stagnant zone inside digester.FIG.58. View of typical oval(left) and cylindrical (right) sludge digesters.(b) Radiotracer experimentA radiotracer experiment was carried out on a two-stage anaerobic sludge digester with ovalform. This system consists of a pair of digesters, primary and secondary digester. Each digester has avolume of 7000 m 3 respectively, and the flow-rate of the raw sludge in an inlet was 1500~1800 m 3 /h.Sc-46 radioisotope (1.85 GBq) was chosen as radiotracer taking account the relatively long residencetime of sludge inside the digester. ScCl 3 was irradiated in the nuclear reactor and dissolved withEDTA solution resulting in the 46 Sc-EDTA complex compound, which is a good tracer of water phaseof the sludge.56
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Radiotracer Applications inWastewat
Page 3 and 4:
The following States are Members of
Page 5 and 6:
TRAINING COURSE SERIES No. 49RADIOT
Page 7 and 8:
EDITORIAL NOTEThe use of particular
Page 9 and 10:
PRACTICAL RECOMMENDATIONS .........
Page 11 and 12:
The success of radiotracer applicat
Page 13 and 14: skimmed out. Flotation, as currentl
Page 16 and 17: 1.3.3. Primary clarifiersA clarifie
Page 18 and 19: esidence times of phases, mixing ch
Page 20 and 21: FIG.8. Secondary clarifier: periphe
Page 22 and 23: processes that occur in natural env
Page 24 and 25: The RTD and MRT are parameters that
Page 26 and 27: (b) Perfect mixers in series modelA
Page 28 and 29: and practically it has better sense
Page 30 and 31: The experimental RTD curve gives ma
Page 32 and 33: FIG.18. Experimental and CFD calcul
Page 34 and 35: Optical tracers can be divided in t
Page 36 and 37: Table 1b. Liquid tracers.- Field of
Page 38 and 39: TABLE 2. SOME WATER RADIOTRACERSIso
Page 40 and 41: The detection efficiency (ϕ) is de
Page 42 and 43: Since on-line radiotracer technique
Page 44 and 45: source. The dose received is direct
Page 46 and 47: 4. CASE STUDIESWastewater from indu
Page 48 and 49: FIG.26. Scheme of equalizer-clarifi
Page 50 and 51: FIG.29. Exp. RTD of S2 and S3 jet m
Page 52 and 53: FIG.33. Experimental RTD curves mea
Page 54 and 55: 1. Radiotracer testsRadiotracer Br-
Page 56 and 57: 4.00E-033.50E-03ExperimentalModel3.
Page 58 and 59: 3. Conclusions• The aeration tank
Page 60 and 61: FIG.50. Experimental response curve
Page 62 and 63: 4.8. DIAGNOSIS OF A CYLINDRICAL TWO
Page 66 and 67: The radiotracer was chosen to trace
Page 68 and 69: (a) D1 - D3 (b) D1 - D4(c) D1 - D6
Page 70 and 71: (d) ConclusionsThe diagnosis of dig
Page 72 and 73: • average gamma energy of 400 keV
Page 74 and 75: To estimate the MRT of the mud insi
Page 76 and 77: FIG.71. Experimental RTD curve and
Page 78 and 79: FIG.75. Biological aeration tank.FI
Page 80 and 81: 4.12.2. Radiotracer experiment(a) L
Page 82 and 83: FIG.80. The experimental RTD curve
Page 84 and 85: FIG.84. Experimental RTD curve of l
Page 86 and 87: FIG.88. Characteristic curves of se
Page 88 and 89: tracer background level detection.
Page 90 and 91: As seen from Fig. 94, the tanks-in-
Page 92 and 93: FIG.99. Experimental RTD curves wit
Page 94: The experimental RTD curves obtaine
Page 98: BIBLIOGRAPHYBORROTO J.I., DOMINGUEZ
Page 102: CONTRIBUTORS TO DRAFTING AND REVIEW
Page 106: I S S N 1 0 1 8 - 5 5 1 8
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