What component is required on a sump pump's discharge pipe to ensure previously pumped discharge does not return?

The correct component required on a sump pump's discharge pipe to prevent previously pumped discharge from returning is a check valve. A check valve is designed to allow fluid to flow in one direction only. When the sump pump expels water, the check valve opens to let the flow through; however, once the pump stops, the valve automatically closes to block any backflow of water back into the sump pit. This is crucial in maintaining the efficiency of the drainage system and ensuring that the sump pump does not have to work harder by re-pumping the water that has already been expelled.

A pressure relief valve, although important in systems that experience variations in pressure, is not specifically meant for preventing backflow and is typically found in pressurized systems. A backflow preventer is aimed at preventing contamination of potable water supplies, which is not the primary function needed in this scenario. A flow meter measures the rate of fluid flow, but it does not influence the direction of flow or stop backflow, making it irrelevant to the requirement stated in the question.