A water treatment and disinfection apparatus is provided, which has separate flow channels to allow entry and exit of separate streams of water through the apparatus, with an ultraviolet light source emitting rays through first and second ultraviolet transparent sleeves having an annular space therebetween to form an inner flow channel and an annular space between the second sleeve and an ultraviolet resistant pressure vessel. The apparatus of the invention is specially suited for disinfection of water in reverse osmosis water purification systems, whereby pre-reverse osmosis (untreated) water and post-reverse osmosis (treated) water are directed inside the apparatus of the invention to be separately and simultaneously disinfected by ultraviolet rays and again separately exit the apparatus.