A liquid ring pump for treating a corrosive effluent gas stream from a processing chamber which is reactive with, or soluble in, a service liquid of the pump to form corrosion products, the pump comprising: an annular pumping chamber (56, 90) which is cylindrical around a central pumping chamber axis for receiving the gas stream and a service liquid; a rotor having a rotor axis which is offset from the central pumping chamber axis, the rotor having a plurality of rotor blades which, on rotation of the rotor, cause liquid in the pumping chamber to form a ring having a centre coincident with the central axis of the pumping chamber and compression of effluent gas conveyed from an inlet to an outlet of the pumping chamber; wherein the liquid ring pump comprises a drive chamber in fluid communication with the pumping chamber allowing circulation of the service liquid in the drive chamber and the pumping chamber, and wherein the pump comprises a magnetic drive assembly for driving the rotor, the magnetic drive assembly comprising a magnetic follower received in the drive chamber configured to be magnetically coupled with a magnetic drive outside the drive chamber such that when the magnetic drive is driven by a motor the magnetic follower imparts rotation to the rotor and wherein the magnetic follower is fixed relative to the rotor, and wherein the pumping chamber, drive chamber, magnetic follower and rotor comprise one or more materials which are resistant to the effluent gas stream and the corrosion products generated when the gas stream is treated by the service liquid, characterized in that the axial alignment of the magnetic drive and magnetic follower is arranged so that in use the magnetic drive imparts an axial thrust on the magnetic follower causing the rotor to cooperate with a thrust plate for setting the axial alignment of the rotor in the pumping chamber. an annular pumping chamber (56, 90) which is cylindrical around a central pumping chamber axis for receiving the gas stream and a service liquid; a rotor having a rotor axis which is offset from the central pumping chamber axis, the rotor having a plurality of rotor blades which, on rotation of the rotor, cause liquid in the pumping chamber to form a ring having a centre coincident with the central axis of the pumping chamber and compression of effluent gas conveyed from an inlet to an outlet of the pumping chamber; wherein the liquid ring pump comprises a drive chamber in fluid communication with the pumping chamber allowing circulation of the service liquid in the drive chamber and the pumping chamber, and wherein the pump comprises a magnetic drive assembly for driving the rotor, the magnetic drive assembly comprising a magnetic follower received in the drive chamber configured to be magnetically coupled with a magnetic drive outside the drive chamber such that when the magnetic drive is driven by a motor the magnetic follower imparts rotation to the rotor and wherein the magnetic follower is fixed relative to the rotor, and wherein the pumping chamber, drive chamber, magnetic follower and rotor comprise one or more materials which are resistant to the effluent gas stream and the corrosion products generated when the gas stream is treated by the service liquid, characterized in that the axial alignment of the magnetic drive and magnetic follower is arranged so that in use the magnetic drive imparts an axial thrust on the magnetic follower causing the rotor to cooperate with a thrust plate for setting the axial alignment of the rotor in the pumping chamber.