Disclosed is a liquid mixing and pumping system (10) and a waste water treatment works incorporating it. The system is for a reservoir (20) operatively containing liquid (46) and includes a mixer (28), operatively submerged in the liquid and rotatable about a vertical rotational axis (30). By rotation of the mixer by drive means (32), a reduced pressure zone is induced at the mixer and about the rotational axis. Peripherally around the vanes (44), outward flow of the liquid away from the mixer is induced. The system includes also at least one transfer pipe (34) defining an outlet (38) exposed to the reduced pressure zone and an inlet (36) in a source (22) of the liquid. The reduced pressure zone induces a pressure head which causes transfer of liquid from the source into the reservoir. The system serves a dual purpose, i.e. mixing in the reservoir (20) and pumping into the reservoir (20).