A fluid treatment system for placement in a flanged pipe fluid conveyance system includes: a first flanged opening and a second flanged opening in substantial alignment to define a flow axis aligned substantially parallel to a direction of fluid flow though the first opening and the second opening; and a third flanged opening comprising a first cover element. The first cover element has connected thereto at least one radiation source assembly comprising at least one elongate radiation source having a longitudinal axis substantially transverse to the flow axis. The fluid treatment system may be advantageously utilized to treat fluid such as water, e.g., municipal waste water, municipal drinking water and the like. The fluid treatment system can be readily "spliced" into existing into existing piping systems. This facilitates installation of the system and also allows for a significant lowering of manufacturing costs of the system.