the present invention is directed to an assembly and a method of treating rainwater to produce potable water . a compact and easy - to - install apparatus is achievable as a result of a simple , yet highly effective sanitisation method . rainwater is entrained with a gas , and the entrained rainwater is irradiated with an ultraviolet light to fully sanitise the rainwater . the uv light is used during the irradiation of the entrained water and is also used to form a gaseous bactericide which is used to entrain the rainwater . none of the disadvantages of the prior art systems apply to any rainwater treatment systems which use this method . all of the connections between the component parts may be already provided for in this rainwater treatment unit , and consequently both installation and maintenance costs will be significantly reduced in comparison to the rainwater treatment systems which are known from the prior art .