Reactor hydraulics is one of the key factors for plant performance and plant control. The residence time distribution is a good but limited indicator of reactor hydraulics. A more detailed view is obtained by direct observations within the reactor. Two different techniques (conservative tracer, reactive tracer) are discussed to detect major anomalies in the flow field of a wastewater aeration tank. Experiments with conservative tracers give valuable information over a very limited period of time making the analysis of the flow field difficult. On the other hand, reactive tracers can be monitored long term which helps identifying the flow pattern provided that a high spatial and temporal resolution of the measurements is applied. Experimental data is used to determine the structure and the parameters of a compartment model that corresponds well with the measurements.

Two innovative tracer methods for the quantification of extraneous discharges in sewers have been proposed as routine applications within the scope of the European research project APUSS (Assessing Infiltration and Exfiltration on the Performance of Urban Sewer Systems). In order to improve the accuracy of wastewater hydrograph separation compared to conventional practice, the novel approaches use intrinsic physicochemical characteristics (stable isotopes, pollutographs) of the wastewater as a natural tracer for the differentiation of its constituting components. The methods were evaluated and tested under field conditions and have been validated in the course of a comparative experimental study. We analyse the state of development achieved and demonstrate the application of the protocols with data from a reference experiment conducted in a combined sewer catchment housing 5400 inhabitants. The article details the requirements for suitable boundary conditions and presents a comprehensive assessment of measurement uncertainties.