In this research study, we are using CFD to better understand processes in a wastewater settling tank. Secondary sedimentation tanks belong to the most important objects at the wastewater treatment line. CFD plays an important part in better understanding the processes from a standpoint of operation and optimization. We conducted four CFD simulations based on a verified model of a secondary settling tank at the wastewater treatment plant in Nitra, SK. The four simulations contain four different adjustments to the inlet zone of the tank. We reached a reduction in sludge zone velocity to 6 from 12 mm/s in a representative measurement profile. We also analyze the influence of temperature-induced viscosity changes based on seasonal temperature change. Graphical abstract