For the reason of operating reliability of wastewater treatment plants it is necessary to separate the grit transported with the wastewater and other mineral materials (approx. 60 l / 1000 m³ of wastewater) from the digestible organic material.
Due to the significant organics content in the classified grit longitudinal grit traps are today additionally aerated to at least partly avoid settling of organic material in the grit trap and cause floating material (grease) to rise to the surface. According to Kalbskopf, aerated grit traps are generally laid out on the basis of the residence time of wastewater in the grit trap. However, even aerated grit traps are unable to ensure reliable separation of grit from organics. This can only be achieved by well performing grit washing plants.