A fluid treatment system (100, 200, 300, 400, 500) comprising: an open channel (115, 215, 315, 415, 515) for receiving a flow of fluid; a fluid treatment zone comprising a series of arrays (220a, 220b, 320a, 320b, 420a, 420b, 520a, 520b) of vertical elongate radiation source assemblies (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b) orientated such that: (i) a longitudinal axis of each radiation source assembly (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b) is transverse to a direction of fluid flow through the fluid treatment zone, and (ii) an end of each radiation source assembly (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b) is disposed above a predetermined maximum height of fluid flow in the open channel (115, 215, 315, 415, 515); a first baffle plate (140, 240, 340, 440, 540) disposed upstream of the fluid treatment zone and an intermediate baffle plate (145, 245, 345, 445, 545) interposed between each adjacent pair of arrays (220a, 220b, 320a, 320b, 420a, 420b, 520a, 520b) of radiation source assemblies (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b), each of the first baffle plate (140, 240, 340, 440, 540) and the intermediate baffle plate (145, 245, 345, 445, 545) extending downward into the open channel (115, 215, 315,415,515); a control gate (130, 230, 330, 430, 530) disposed downstream of said arrays (220a, 220b, 320a, 320b, 420a, 420b, 520a, 520b) of vertical elongate radiation source assemblies (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b), said control gate (130, 230, 330, 430, 530) being configured to control the height of fluid upstream of said first baffle plate (140, 240, 340, 440, 540) to said predetermined maximal height; wherein each radiation source assembly (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b) comprises a radiation source (128, 228, 328, 428, 528) having an arc length, i.e. the length over which it emits radiation, comprising a proximal arc end, i.e. an upper arc end, and the distal end, i.e. the lower end, of the first baffle plate (140, 240, 340, 440, 540) and the intermediate baffle plate (145, 245, 345, 445, 545) are in substantial alignment with the proximal arc end of the radiation source (128, 228, 328, 428, 528) below the predetermined maximum height of fluid flow in the open channel (115, 215, 315, 415, 515). an open channel (115, 215, 315, 415, 515) for receiving a flow of fluid; a fluid treatment zone comprising a series of arrays (220a, 220b, 320a, 320b, 420a, 420b, 520a, 520b) of vertical elongate radiation source assemblies (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b) orientated such that: (i) a longitudinal axis of each radiation source assembly (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b) is transverse to a direction of fluid flow through the fluid treatment zone, and (ii) an end of each radiation source assembly (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b) is disposed above a predetermined maximum height of fluid flow in the open channel (115, 215, 315, 415, 515); a first baffle plate (140, 240, 340, 440, 540) disposed upstream of the fluid treatment zone and an intermediate baffle plate (145, 245, 345, 445, 545) interposed between each adjacent pair of arrays (220a, 220b, 320a, 320b, 420a, 420b, 520a, 520b) of radiation source assemblies (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b), each of the first baffle plate (140, 240, 340, 440, 540) and the intermediate baffle plate (145, 245, 345, 445, 545) extending downward into the open channel (115, 215, 315,415,515); a control gate (130, 230, 330, 430, 530) disposed downstream of said arrays (220a, 220b, 320a, 320b, 420a, 420b, 520a, 520b) of vertical elongate radiation source assemblies (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b), said control gate (130, 230, 330, 430, 530) being configured to control the height of fluid upstream of said first baffle plate (140, 240, 340, 440, 540) to said predetermined maximal height; wherein each radiation source assembly (125a, 125b, 225a, 225b, 325a, 325b, 425a, 425b, 525a, 525b) comprises a radiation source (128, 228, 328, 428, 528) having an arc length, i.e. the length over which it emits radiation, comprising a proximal arc end, i.e. an upper arc end, and the distal end, i.e. the lower end, of the first baffle plate (140, 240, 340, 440, 540) and the intermediate baffle plate (145, 245, 345, 445, 545) are in substantial alignment with the proximal arc end of the radiation source (128, 228, 328, 428, 528) below the predetermined maximum height of fluid flow in the open channel (115, 215, 315, 415, 515).