backwash

Backwash is the flow of a liquid or gas that moves opposite to the main direction of a system. It occurs when pressure changes, turbulence, or reversals in the flow cause a portion of the fluid to move backward. Backwash can transport suspended materials in the opposite direction to the primary flow, potentially affecting equipment, surfaces, and nearby environments.

In natural settings, backwash commonly refers to the water that runs back to the sea after a breaking wave on a beach. This backwash can entrain and transport sand and other sediments, shaping beach profiles and influencing swash lines. In rivers, brief reverse flows can also occur under certain hydraulic conditions, but are less common in steady flow regimes.

In engineered systems, backwashing is a deliberate process used to clean filtration media. In drinking-water treatment, swimming pools, aquariums, and similar installations, the flow is reversed through the filter so that trapped solids are lifted from the media and carried away with the waste stream. After backwashing, normal operation resumes, typically followed by a rinse cycle to recollapse the media.

Backwash is also used colloquially to refer to the residual effects or consequences of an event, such as the lingering impact on a system or community.