Disclosed herein is an apparatus (100) for field monitoring disinfection of a fluid system. The apparatus comprises an inlet (102) for connection to a fluid system undergoing disinfection, and instruments (108, 109) for detecting parameters indicative of turbidity T and disinfectant concentration D. A controller (112) is associated with the instruments (108, 109) and is configured to output a signal indicative of successful disinfection of the fluid system if detected values of disinfectant concentration D and turbidity T satisfy a predetermined criteria.