A flow pattern in which the water being processed passes through the medium (such as a granular filter or an ion exchange bed) in a "piston-like" fashion instead of in turbulent or mixed flow patterns such as are found in other processes like ultraviolet light disinfection and electrodialysis.

In fluid mechanics, Plug Flow is a simple way of modelling flow of a fluid in a pipe or, more generally, a reactor. Essentially no back mixing is assumed with "plugs" of fluid passing through the reactor. This results in differential equations (DEs) that need to be integrated to find the reactor conversion and outlet temperatures.