A fluid treatment system with an UV lamp in a reactor

A fluid treatment system (2) comprising at least one ultra-violet, UV, light treatment lamp (6) arranged within an elongated protective UV-transparent sleeve (8) provided along a central longitudinal axis A, and a reactor (12) configured to receive said sleeve (8), whereby a fluid treatment chamber (4) for receiving fluid to be treated, is provided between an inner surface (14) of the reactor (12) and an outer surface (10) of the sleeve (8), wherein said reactor (12) extends along the axis A, between a first end (12a) and a second end (12b) of the reactor and wherein said treatment chamber (4) comprises an inlet (5a) for receiving fluid to be treated inside the treatment chamber (4) and an outlet (5b) for expelling treated fluid out from the treatment chamber (4), whereby the inlet (5a) is positioned such that fluid is introduced into the treatment chamber (4) from the first end (12a) of the reactor (12) and the outlet (5b) is provided closer to the first end (12a) than to the second end (12b) of the reactor (12), and wherein the inlet (5a) is provided closer to the sleeve (8) than the outlet (5b) is, wherein the treatment chamber (4) is one single open treatment chamber (4) which is surrounding the sleeve (8).