Methodology to determine residence time distribution and small scale phenomena in settling tanks

Hydraulic residence time is a crucial parameter of any wastewater treatment tank planning, operation and optimisation. Calculation of the residence time distribution at every point within the tank gives information about the deadzones and short circuits and well-operating zones. Using the advective-diffusive transport equation supplemented with a scalar source term local mean age (LMA) can be detected if the flow field is given. Combining LMA with a tracer study the exact residence time can be determined. This method supports problem detection in a small wastewater treatment plant (2 dead zones and one shortcircuiting path were detected) and also gave us better understanding of the operation of settling tanks.