Abstract: Apparatus for the treatment of water supplies with the gas of the type comprising, in sequence, the following elements: (a) a vacuum regulator 2 for providing a low pressure supply of the gas from a high pressure source; (b) a monitor system 3 to signal unwanted pressure changes; (c) a gas flow meter 4; (d) an output flow control valve 5 coupled with a vacuum relief valve 6 to ensure the gas pressure stays above a required level; (e) a differential control valve 8 serving to maintain a constant pressure differential between the output flow control valve and the mixing device, (f) a gas and water mixing device 9, is improved by arranging that at least two successive elements (a) to (f) are directly abutted for gas flow through aligned openings. Abutted elements are conveniently housed in monolithic blocks.