A counter top reverse osmosis water purification system, suitable for home use, is connected to a water supply and contains a closed fluid treatment circuit extending to a water outlet. The closed fluid circuit flows through a plurality of replaceable water treatment modules mounted on a flowboard and each having a specific water treatment function, such as the removal of a particular material from the water by the use of reverse osmosis, filtration, carbon adsorption, ion exchange or the addition of a chemical to balance the desired water conditions. Preferably the circuit also includes traversing a radiation device, for example an ultraviolet light, for the purpose of sanitizing the water.