Fluid treatment system

A fluid treatment system having an inlet, an outlet, and a fluid treatment zone therebetween. The zone has an array of rows of radiation source assemblies. Each radiation source assembly has a longitudinal axis disposed at an oblique angle with respect to a direction of fluid flow. Each row has a plurality of radiation source assemblies in spaced relation in a direction transverse to the direction of fluid flow, to define a gap through which fluid may flow between an adjacent pair of assemblies. Preferably, all rows in the array are staggered with respect to one another in a direction orthogonal to the direction of fluid flow, such that the gap between an adjacent pair of radiation source assemblies in an upstream row of assemblies is partially or completely obstructed in the direction of fluid flow by a serially disposed radiation source assembly in at least one downstream row.