a reactor housing with an inlet for ingress and an outlet for discharge of a fluid to be disinfected;

a reactor chamber which is formed in the reactor housing and is configured to receive the fluid to be disinfected so that the fluid flows through the reactor chamber along a flow path from the inlet to the outlet;

a UV irradiation device with a plurality of UV light emitting diodes (LEDs);

an elongated support element on which the UV LEDs are disposed and which is configured to extend at least partially into the reactor housing so that by means of the UV LEDs, UV light can be emitted into the reactor chamber in order to disinfect the fluid along the flow path; and

a quartz glass tube which is configured to surround the support element in order to prevent direct contact of a fluid to be disinfected with the UV LEDs in the reactor chamber;

wherein the support element is releasably and exchangeably mounted in the reactor housing and can be exchanged together with the quartz glass tube and has a coolant passageway which extends through the support element and through which a coolant can flow in order to cool the UV LEDs;

the coolant passageway extends from a coolant inlet to a coolant outlet which are both disposed at the same end of the support element;

the coolant passageway comprises a first section which extends from the coolant inlet and a second section which extends to the coolant outlet, wherein at least a portion of the first section runs inside the second section; and

an external coolant circuit is provided for cooling, which is connected to the fluid to be disinfected via a heat exchanger in order to remove the heat in this manner.