A method for providing a liquid-foam mixture by mixing water with at least one additive (4, 35) by means of a liquid mixing system, in which the following steps are carried out: - providing a water pressure source for discharging the water, - providing at least one first additive reservoir unit with a first additive accommodated therein, - providing an air compressor unit for providing compressed air, comprising a compressor and a drive device, wherein the drive device is drive-connected to the compressor, - providing a first mixing device with a first mixing chamber for admixing at least the first additive to the water discharged from the water pressure source, - providing a second mixing device with a second mixing chamber for optionally admixing compressed air to the liquid-foam mixture discharged from the first mixing device, and a connecting line connecting the two mixing devices (8, 10) to one another, - providing a discharging unit for discharging the liquid-foam mixture with a connection device and a discharge line, by means of which discharge line the connection device is line-connected to the mixing devices (8, 10), - providing a water line network with at least one first water feed line, by means of which first water feed line the first mixing device is line-connected to the water pressure source, - providing an additive line network with at least one first additive line, by means of which first additive line the first additive reservoir unit is line-connected to the first mixing device, - providing a compressed air line network with a first compressed air line, by means of which compressed air line the second mixing device is optionally line-connected to the compressor, - feeding the water from the water pressure source to the first mixing device via the first water feed line, - feeding at least the first additive from the first additive reservoir unit to the first mixing device via the first additive line, - mixing the water with the at least one additive by means of the first mixing device and forwarding the liquid-foam mixture from the connection device to the discharging unit, characterized in - that the drive device of the air compressor unit is formed by a water engine or a water turbine comprising at least one water inlet and at least one water outlet, - that a second water feed line is provided, and the at least one water inlet of the water engine or the water turbine is line-connected to the water pressure source via the second water feed line, and - that the water discharged from the water pressure source is fed to the water engine or the water turbine via the second water feed line, and the water engine or the water turbine is driven by pressurized water of the water pressure source. - providing a water pressure source for discharging the water, - providing at least one first additive reservoir unit with a first additive accommodated therein, - providing an air compressor unit for providing compressed air, comprising a compressor and a drive device, wherein the drive device is drive-connected to the compressor, - providing a first mixing device with a first mixing chamber for admixing at least the first additive to the water discharged from the water pressure source, - providing a second mixing device with a second mixing chamber for optionally admixing compressed air to the liquid-foam mixture discharged from the first mixing device, and a connecting line connecting the two mixing devices (8, 10) to one another, - providing a discharging unit for discharging the liquid-foam mixture with a connection device and a discharge line, by means of which discharge line the connection device is line-connected to the mixing devices (8, 10), - providing a water line network with at least one first water feed line, by means of which first water feed line the first mixing device is line-connected to the water pressure source, - providing an additive line network with at least one first additive line, by means of which first additive line the first additive reservoir unit is line-connected to the first mixing device, - providing a compressed air line network with a first compressed air line, by means of which compressed air line the second mixing device is optionally line-connected to the compressor, - feeding the water from the water pressure source to the first mixing device via the first water feed line, - feeding at least the first additive from the first additive reservoir unit to the first mixing device via the first additive line, - mixing the water with the at least one additive by means of the first mixing device and forwarding the liquid-foam mixture from the connection device to the discharging unit, characterized in - that the drive device of the air compressor unit is formed by a water engine or a water turbine comprising at least one water inlet and at least one water outlet, - that a second water feed line is provided, and the at least one water inlet of the water engine or the water turbine is line-connected to the water pressure source via the second water feed line, and - that the water discharged from the water pressure source is fed to the water engine or the water turbine via the second water feed line, and the water engine or the water turbine is driven by pressurized water of the water pressure source.