A liquid treatment system (2; 2'; 102; 202) comprising: - at least one ultra-violet light treatment lamp arranged within an elongated protective UV-transparent sleeve (8; 8') provided along a central longitudinal axis A, said sleeve (8; 8') having an outer surface (10; 10') and an essentially circular cross-sectional shape; and - an elongated reactor (12; 12'; 12") configured to receive said sleeve (8; 8'), whereby an elongated liquid treatment chamber (4; 4') for receiving liquid to be treated, is provided between an inner surface (14; 14') of the reactor (12; 12'; 12") and the outer surface (10; 10') of the sleeve (8; 8'); wherein said liquid treatment system further comprises - at least one elongated cleaning device (16; 16') provided side by side with the sleeve (8; 8') within the liquid treatment chamber (4; 4') and along at least a part of the length of the elongated sleeve (8; 8'), wherein said at least one cleaning device is compressed towards the outer surface (10; 10') of the sleeve (8; 8') by the reactor (12; 12'; 12"), and wherein at least one of the sleeve and the reactor is configured to rotate around the longitudinal axis A such that the at least one cleaning device (16; 16') will be touching and cleaning the outer surface (10; 10') of the sleeve (8; 8') over essentially the whole circumference of the sleeve (8; 8'), wherein the reactor (12; 12') comprises for each cleaning device (16; 16'): o a first part (18; 18') having a cross sectional shape being essentially a part of a circle, which circle is centered around the longitudinal axis A, and o a second part (20; 20') configured for holding the cleaning device (16; 16') and connected to the first part (18; 18'), wherein an inner surface (14a; 14a') of the first part (18; 18') and an inner surface (14b; 14b') of the second part (20; 20') together encircle the outer surface (10; 10') of the sleeve (8; 8'), wherein the inner surface (14b; 14b') of the second part (20; 20') is provided at a greater distance from the outer surface (10; 10') of the sleeve (8; 8') than the inner surface (14a; 14a') of the first part (18; 18'). - at least one ultra-violet light treatment lamp arranged within an elongated protective UV-transparent sleeve (8; 8') provided along a central longitudinal axis A, said sleeve (8; 8') having an outer surface (10; 10') and an essentially circular cross-sectional shape; and - an elongated reactor (12; 12'; 12") configured to receive said sleeve (8; 8'), whereby an elongated liquid treatment chamber (4; 4') for receiving liquid to be treated, is provided between an inner surface (14; 14') of the reactor (12; 12'; 12") and the outer surface (10; 10') of the sleeve (8; 8'); wherein said liquid treatment system further comprises - at least one elongated cleaning device (16; 16') provided side by side with the sleeve (8; 8') within the liquid treatment chamber (4; 4') and along at least a part of the length of the elongated sleeve (8; 8'), o a first part (18; 18') having a cross sectional shape being essentially a part of a circle, which circle is centered around the longitudinal axis A, and o a second part (20; 20') configured for holding the cleaning device (16; 16') and connected to the first part (18; 18'), wherein an inner surface (14a; 14a') of the first part (18; 18') and an inner surface (14b; 14b') of the second part (20; 20') together encircle the outer surface (10; 10') of the sleeve (8; 8'), wherein the inner surface (14b; 14b') of the second part (20; 20') is provided at a greater distance from the outer surface (10; 10') of the sleeve (8; 8') than the inner surface (14a; 14a') of the first part (18; 18').