A radiation source module for use in a fluid treatment system comprising an open channel, the radiation source module being configured to be disposed on the open channel comprising: - a housing having an inlet, an outlet and a fluid treatment zone disposed between; the fluid treatment zone comprising a first sidewall and a second sidewall interconnected by a floor surface, the first sidewall, the second sidewall and the floor surface configured to receive a flow of fluid through the fluid treatment zone; - a series of radiation source assemblies disposed in the fluid treatment zone, secured with respect to the first sidewall and the second sidewall; and - a module motive coupling element (145, 150) connected to the housing and configured to be coupled to a module motive element to permit the radiation source module to be installed in and extracted from the fluid treatment system; - the housing further comprises a top surface interconnecting the first sidewall and the second sidewall; the top surface, surface portion, first sidewall and the second sidewall combining to define an enclosure having substantially closed cross-section to receive the flow of fluid; characterized in that - the upper edge of the inlet is at a distance below the top surface, the housing being designed so that it acts as a small open channel fluid treatment system, the fluid level being unconstrained by any physical structure of the radiation source module and lower than the fluid level in the open channel upstream of the housing, - and in that each radiation source assembly is supported at both ends, one end being supported by the floor surface and one end being supported by the top surface. - a housing having an inlet, an outlet and a fluid treatment zone disposed between; the fluid treatment zone comprising a first sidewall and a second sidewall interconnected by a floor surface, the first sidewall, the second sidewall and the floor surface configured to receive a flow of fluid through the fluid treatment zone; - a series of radiation source assemblies disposed in the fluid treatment zone, secured with respect to the first sidewall and the second sidewall; and - a module motive coupling element (145, 150) connected to the housing and configured to be coupled to a module motive element to permit the radiation source module to be installed in and extracted from the fluid treatment system; - the housing further comprises a top surface interconnecting the first sidewall and the second sidewall; the top surface, surface portion, first sidewall and the second sidewall combining to define an enclosure having substantially closed cross-section to receive the flow of fluid; characterized in that - the upper edge of the inlet is at a distance below the top surface, the housing being designed so that it acts as a small open channel fluid treatment system, the fluid level being unconstrained by any physical structure of the radiation source module and lower than the fluid level in the open channel upstream of the housing, - and in that each radiation source assembly is supported at both ends, one end being supported by the floor surface and one end being supported by the top surface.