The discharge pipe on a sump pump should have a _________ installed to prevent previously pumped discharge from returning to the pump.

The discharge pipe on a sump pump should have a check valve installed to prevent previously pumped discharge from returning to the pump. A check valve is a mechanical device that allows fluid to flow in one direction while preventing backflow. This is crucial in sump pump systems because it ensures that once water has been pumped out, it cannot flow back into the basin, which would necessitate additional pumping and could lead to inefficiency and potential pump damage.

In a sump pump application, the check valve is typically installed in the discharge line, just above the pump, so that when the pump turns off, the water that has been ejected does not return due to gravity or pressure fluctuations in the system. This function enhances the overall performance of the sump pump by ensuring it only has to engage when new water enters the sump basin, thus maintaining a more effective and reliable drainage system.

The other choices do not serve the same purpose: an air gap is primarily used for preventing back siphoning in potable water systems, a flood barrier is intended to prevent water ingress and isn't specifically tied to the pump's discharge function, and a locking mechanism does not relate to fluid control in this context.