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

The check valve is specifically designed to allow water (or any fluid) to flow in one direction only. This is critical in various plumbing and sewage systems, including septic systems, because it prevents backflow, which could lead to contamination or system failure. The internal mechanism, usually a flap or ball, opens with forward flow and seals tightly to block reverse flow, ensuring that fluid does not travel back into areas it shouldn't, such as a septic tank or treatment system.

In contrast, the gate valve is primarily used for on/off control rather than for preventing backflow. Globe valves are typically used for throttling flow rather than for just allowing one-way flow. Plug valves can also regulate flow, but they do not provide the one-directional flow feature as effectively as a check valve does. Hence, the check valve is the most appropriate choice when the requirement is to ensure water flows in only one direction.