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source. The dose received is directly proportional to the time spent in handling the source. Thus thetime of handling should be as short as possible. The radiation intensity at a point varies exponentiallywith the thickness of shielding material. Thus a radiation worker has to use an optimum thickness ofshielding material against the radiating source.The most elementary means of protection is known as ‘TDS’ or ‘time, distance and shielding’:• Decreasing the time spent around a radiation source decreases the exposure• Increasing the distance from a source decreases the exposure• Increasing the thickness of shielding to absorb or reflect the radiation decreases the exposure3.4. TYPICAL RADIOTRACER APPLICATIONS IN A WASTEWATER TREATMENT PLANTSRadiotracer techniques are largely used in routine services for investigation of thehydrodynamics of wastewater in various processes and units of a WWTP. Possible radiotracersapplications in most of the units are:(a)(b)Measurement of flow rates by radiotracer dilution methodMass balances.The residence time distribution (RTD) method is common for many investigations in almost allprocesses. Radiotracers can be used for separate determination of RTD functions of liquid and solidphases and on the base of this information evaluation of mixing processes intensity. The separatelabeling of solid particles provides the information concerning the sediment distribution in tankvolume (zone of solid phase stagnation localization). The analysis of RTD functions indicate thepresence or absence of short-circuit (by pass flow) and rate of active volume of tank.The equalization tank has to ensure that all the incoming effluent is uniformly mixed (usually byair sparging) so that the outgoing effluent concentration is uniform. Since the size of these equalizationvessels are large (several thousand m 3 ), radiotracer offers a unique opportunity to check theperformance of the equalization tank by assessing its mixing characteristics, presence of dead(stagnant) zones and short circuiting. Experimental RTD response is analyzed using mathematicalmodels to asses the efficacy of the equalization tank.The volume of clarifier, V[m 3 ] and the flow rate of water Q[m 3 /h] have to be optimized forobtaining the wastewater residence time sufficient for sedimentation process realization (appropriaterelations between horizontal components of water flow velocity and vertical velocity ofsedimentation).Radiotracer can be used for determination of liquid and solid phases flow structure. The RTDfunction of both phases give a possibility to calculate the mean residence times of water and thesediment, evaluate the rate of dead and active volume of tank, and propose the appropriate flowmodel.Radioactivity mass balance (for solid phase mass balance) and RTD (for liquid flow -mixing,dead zone and short circuiting – characteristics) are most ideal methods to evaluate the performance ofvarious designs of clarifier under a variety of operating conditions (flow rate, mean residence time andsolid flock concentrations).After the primary clarifier the wastewater, if it is to be discharged, are directed to sand filterstation, where additional process of solid particles removal take place.Different types of sand filters i.e. fixed media or fluidized media are used and their performancein terms of the efficacy of solid-liquid separation can also be evaluated using radiotracers. Theradiotracers can be used for identification of the presence of unfavorable water channeling effect.35