A biocidal purification reactor, including a main cylinder tube (1A;1B), at least one end cap (2A,2B,2C) closing at least one axial end opening of the main cylinder tube (1A;1B), at least two ports (3A,3B) communicating with the interior volume (4) of the main cylinder tube (1A;1B) in the vicinity of the respective axial ends of the cylinder tube (1A;1B), and at least one of said end caps (2A) or said main cylinder tube (1B) including a UV-light source (7A;7B).