Abstract: A system for treating wastewater using the activated sludge biological process by retaining and degrading biodegradable coarse solids. A receiving well contains both a pre-separator stage and an activation chamber arranged one after the other in a direction of through-flow. The preseparator stage has a sand catcher located at the bottom for receiving undegradable solids. A partition is disposed within the receiving well between the pre-separator stage and the activation chamber. A lower section of the partition is a wall separating the sand catcher from the activation chamber. At least a portion of the upper section of the partition is a screen that is inclined with respect to a vertical line, into the activation chamber by greater than 0.degree. up to 30.degree. to widen a space of said pre-separator stage above the sand catcher.