The removal of suspended solids and attached pollutants is one of the main treatment processes in wastewater treatment. The thesis deals with various methods for enhancing of the removal of suspended solids in wastewater and the work emphasize on clarifiers in treatment plants and overflows in combined sewer systems. The thesis is structured around 4 case studies where the flow and suspension process is studied profoundly by both physical and numerical models. In order to be easily understood, each part includes its own abstract, introduction and conclusions as well as the study results.