Abstract: Apparatus for disinfecting fluid. The apparatus includes a disinfection tank including a heat exchanger extending from an inlet to an outlet for heating preheated fluid to a second temperature to thereby disinfect the fluid. A preheat heat exchanger is provided for heating the fluid to a first temperature using heat recovered from disinfected fluid provided at the outlet and a control system, including one or more sensors for monitoring operation of the apparatus, is used for determining if signals from the sensors fall outside predetermined operating ranges and either generating an indication of a fault or stopping operation of the apparatus.