Abstract: A metering pump includes a motor compartment with a drive compartment defining a drive chamber. The drive chamber has an inlet and an outlet for a main liquid. A drive piston is moveably arranged in reciprocating translational motion in the drive chamber. A controller for controlling the movement of the drive piston under the action of the main liquid. A metering compartment defines a metering chamber. The metering chamber has an inlet and an outlet for an auxiliary liquid. The outlet for the metering chamber is separated from the outlet of the driving chamber. A metering piston is arranged in the metering chamber. The metering piston is driven by the drive piston. A frustoconical sealing member is arranged with its small base facing the drive compartment. An extension of the metering piston has a diameter smaller than a body of the metering piston. The extension and the frustoconical sealing member form a suction valve. A suction-delivery portion is arranged on the metering piston.