Which type of valve allows water to flow in one direction only?

The correct choice is the type of valve known as a check valve. This specific valve is designed with a unique mechanism that permits fluid flow in only one direction, effectively preventing backflow. The primary function of a check valve is to maintain the correct flow direction in piping systems, especially in sewage treatment where it is critical to avoid contamination and ensure efficient wastewater transport.

Check valves typically contain a hinged disk or a spring-loaded mechanism that opens when fluid flows in the intended direction and closes to block any reverse flow. This property makes check valves essential in protecting pumps and other equipment from damage that can occur due to backflow.

In contrast, the other types of valves listed provide different functionalities. For instance, ball valves, gate valves, and butterfly valves are primarily used to start, stop, or regulate the flow rather than control the direction of flow. While they can be employed in various processes within wastewater treatment, they do not inherently prevent backflow as a check valve does.