Abstract: A process plant comprises a tank (1), an outlet conduit (2), a reservoir of cleaning agent (12), or feedstock(s) of additive(s) (13), a loop conduit (3) for returning liquid into the tank, valve means (9) for controlling admission of liquid into the loop conduit and a pump (4) for driving liquid through the loop conduit. Returning of the liquid takes place through a jetting device (5) adapted for introducing jets of liquid into the body of liquid inside the tank in order to cause stirring to the body of liquid. The jet nozzle is adapted for powered rotation about a first axis and about a second axis perpendicular, or non-perpendicular, to the first axis. The jet nozzles may be embodied in a flat jet rotating around one axis only. Upon emptying the tank, the jetting device may serve for cleaning the tank by spraying liquid onto the tank walls. The invention provides a method for treating liquid as well as a process plant.