This paper deals with an improvement of settling tank performance. New equipment is proposed and its usefulness is confirmed. In combined sewerage systems that are employed in big cities, storm waters often exceed the capacity of the treatment plant, so that they are discharged with only simple treatment. Recently, water pollution caused by this flush-over water has caused environmental problems. To meet this problem, the enhancement of the settling tank capacity is an important and urgent issue for urban sewage treatment plants. The objective of this paper is to develop a new method to enhance the treatment efficiency without building additional treatment plants. In the paper, the characteristics of a rectangular settling tank are investigated using the numerical solution of a mathematical model to show the capacity limit of the conventional settling tank. The newly developed settler is an improvement of the lamella settler, which arranges inclined parallel plates in the vertical direction, contrary to the usual horizontal arrangement. In this method, the separated clear water is removed directly by suction from the top end of the parallel plates. For the removal of clear water, the right and left edges of the plates are closed, to make a tube with rectangular cross section. These are the unique and original points of the equipment. Since each settling tube acts as a small settling tank, the treatment capacity is proportional to the number of settling tubes. Furthermore, this system is less restricted by the surface area of the tank, as the tubes are arranged vertically. The usefulness of the method was confirmed by laboratory experiments and numerical analysis. It was also shown from a numerical estimation that the new system can enhance the capacity of the settling tank to more than five times that of the conventional one.