There is provided an air treatment apparatus comprising a body, an air flow generator housed within the body that is arranged to generate an air flow, an evaporator assembly removably disposed within the body and that is arranged to introduce water vapour into the air flow, and a water supply system that is arranged to provide water to the evaporator assembly. The water supply system comprises a water tank assembly that is separable from the body, the water tank assembly comprising a water tank having a tank opening. The tank opening is configured to allow the evaporator assembly to fit through the tank opening and the evaporator assembly is configured to fit within an interior volume of the water tank.