Author(s)

T. Karches & K. Buzas

Abstract

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. Keywords: CFD, LMA, residence time, settling tank design. 1 Introduction Computational fluid dynamics (CFD) in wastewater treatment processes has a significant role in recent years. Several studies demonstrate that CFD is an effective tool in design (Greenfield [8]), optimization (Bratchley et al. [3]) and operation (Brouckaert and Buckley [4]) Furthermore CFD studies revealed that with an extensive knowledge of fluid flow not only the hydraulic performance of e.g. an oxidation ditch could be improved (Yang et al. [17]) but we could have a closer look into the biological processes, too (Bartrand et al. [2]) For a few decades researchers used to focus only on the biology and chemistry in wastewater treatment, and hydraulics was mostly neglected.

Keywords

CFD, LMA, residence time, settling tank design