There is provided a mixing device. The mixing device comprises a first branch (7) to provide a first substance from a source (3). The mixing device further comprises a reservoir (9) for storing a second, additive substance, the reservoir (9) comprising an inlet (18) for receiving the first substance from the first branch (7). The mixing device further comprises a swirl generator (18A) configured to impart swirl in the first substance entering the reservoir (9) from the first branch (7). The mixing device further comprises a second branch (5) to output a mixture of the first substance and the additive substance from an outlet (17) of the reservoir (9), or the reservoir comprising a vent outlet (17) to output a mixture of the first substance and the additive substance.