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(d) ConclusionsThe diagnosis of digester system was performed by applying radiotracer test. The existence ofstagnant zones in the two stage anaerobic digester quantified and located. The primary digester has anactive zone of 87% and a stagnant zone of 13%, while the secondary digester has 74% active zone and26% stagnant zone. Experience has shown that under normal operation the anaerobic digesters have anacceptable stagnant zone up to 20-30 % that means the tested anaerobic digester system needs nocleaning work at this moment. The stagnant zones were observed at the lower part of the digester,mostly located at the bottoms of the digesters.Based on these conclusions the constructed location of the dead zones inside the digester systemis shown in Fig. 63.FIG.63. The constructed location of stagnant zones in the digester system.4.10. RADIOTRACER INVESTIGATION OF AN ANAEROBIC DIGESTER(a) ProblemThe WWTP engineers wanted to investigate the operation of anaerobic digesters and betterknow the behavior of mud (< 40-60 μm) and sand (125 - 163 µm) in this installation. It was a questionin particular of better determining the rate of clogging of a digester what is important in term ofmaintenance of the installation. Such a study requires the knowledge of the transfer functions of mudand sand in the digester and, in particular, their temporal characteristics. The methodology of theradioactive tracing is particularly adapted to meet this aim because of:• the specificity of the traced particles (either mud or sand exclusively),• the sensitivity of the measurements,• the no intrusive character of the method which allows an in situ detection.To determine the temporal parameters of mud and sand flow, two tracing tests were carried out:• mud tracing,• sand (125-163 µm) tracing.61