Circulation is created in some parts of settling tanks. It can increase the mixing level,
decrease the effective settling, and create a short circuiting from the inlet to the outlet. All above-mentioned
phenomena act in such a way to decrease the tank's hydraulic efficiency, which quantitatively shows how

ow within the tank is uniform and quiet. So, the main objective of the tank design process is to avoid
forming the circulation zone, which is known as the dead zone. Prediction of the
ow field and size of
the recirculation zone is the first step in the design of settling tanks. In the present paper, the non-linear
k - e " turbulence model is used for predicting the length of the reattachment point in the separated
ow of
a Karlsruhe tank. Then, the recirculation bubble size, which is out of the capability of standard turbulence
models, is determined. Also, the effect of the separation zone size on the tank's hydraulic efficiency is
investigated.