There is provided an air treatment apparatus comprising an air flow generator that is arranged to generate an air flow through the air treatment apparatus, a moisture source that is arranged to introduce water vapour into the air flow, a body that is use surrounds both air flow generator and the moisture source, and a water supply system that is arranged to provide water to the moisture source. The water supply system comprises a water tank assembly that is separable from the body, and the water tank assembly comprises a water tank and a water pump for pumping water from the water tank to the moisture source. The water tank comprises a tank opening and the water tank assembly further comprises a removable tank cap that is arranged to fit over and thereby occlude the tank opening, and wherein the water pump is provided on the removable tank cap.