Abstract: A modular water treatment unit for use with water softeners and media filtration systems to permit the connection of filtration, light irradiation and other components to the softener or filtration system without the need to assemble a custom plumbing system to connect the components. By simply rotating the modular treatment unit 180 degrees, the unit can be used to treat either the water on the inlet or the outlet side of the water softener or media filtration unit. The modular water treatment unit includes a modular treatment head that is removably connected to a treatment sump. The treatment sump houses filtration cartridges, UV irradiation or other components. The treatment head of the modular water treatment unit includes a bypass tube and a treatment tube that is parallel to the bypass tube. The bypass tube permits the passage of water without subjecting the water to filtration or irradiation.