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Residence time and local mean age of flare important parameters of the fl uid fl ow, with which we are able to detect the functioning and non-functioning zones. It is a novel approach to estimate the effect of dead-zones on chemical and biological processes. In this paper, we introduce a numerical tracer study, which is calibrated by fi measurements and which is applied in different cases such as settling tank and aeration tank of wastewater treatment. Using an advective-diffusive passive scalar transport equation, the age of the fl uid can be calculated. It also shows the short-circuits and dead-zones of the fl uid and this corresponds with our observations. This method helps eliminate potential hydraulic problems at the early stage of design. Settling tanks in the wastewater treatment have a key role in the separation of suspended solids and sludge fl ocks. The design of these tanks is based on the ATV-A131 standard (1), which takes into account only the global parameters of the fl ow, e.g. average residence time, but in many cases the inequalities inside the tank may have serious effect on effl uent quality. In this paper, we are fi rst and foremost interested in the detection of dead-zones, where the mixing is adverse due to low velocity, high residence time and fl uid age. In aeration tanks, for example, anaerobic zones may develop because microorganisms consume the oxygen. There is mass transfer between the fl ow and dead-zone because of the presence of turbulence; how- ever, this is much less than the effect of convective transport. On the other hand, hydraulically inactive zones are redundant and these increase the cost of operation. Computational fl uid dynamics is widely applied in wastewater treatment tank design (2-4)) because with the knowledge of hydraulic phenomena, the understanding of biochemical processes could be improved. One signifi cant linking parameter between the hydraulics and biology is residence time; therefore, it is essential to know the precise value for every point. Residence time dis- tribution (RTD) can be measured experimentally (5). In pulse experiment, the tracer is mixed in a small volume of fl which is introduced to the reactor in a very short time interval (Dirac-delta function) and the response function is measured at the outlet. The tracer should be a conservative material without any background concentration of the wastewater. The response function shows the changes in tracer concentration in the course of time, from which we can estimate the hydraulic behavior of the tank. RTD calculations can be performed numerically, and they can be compared to the results of the fi eld measurement. This paper deals with circular fl ow reactors, one aeration tank and one radial fl ow settling reactor. The fi rst is designed as a fully mixed reactor, but the RTD investigation demonstrated that there is a short-circuit, where the nutrients could fl ow through the reactor rapidly and did not have enough time for biodegradation. Circular radial fl ow settling tanks generally have problems with short-circuiting and the wind also could have an infl uence on the fl ow regimes.

Investigation of residence time distribution and local mean age of fluid to determine dead-zones in flow field