The invention provides an immersible and portable module for irradiating fluids including a multiplicity of ultraviolet radiation producing lamp and a first header for receiving and maintaining in position the multiplicity of ultraviolet radiation-producing lamps, the first header having upstream and downstream ends and opposed sides. There is also a second header for receiving and maintaining in position the multiplicity of ultraviolet radiation-producing lamps, the second header having upstream and downstream ends and opposed sides. Fluid flow diverters are positioned between the first and second headers and located adjacent the opposed sides and are adapted to divert fluids flowing adjacent the opposed sides toward at least some of the ultraviolet radiation-producing lamps.