A review of the fluence determination methods for UV reactors: Ensuring the reliability of UV disinfection

Ultraviolet (UV) is a green and effective technique that has been widely applied in water disinfection. The reliability of UV disinfection is an important issue, in which the aim is to ensure the delivery of adequate real-time fluence in a UV reactor. Unlike chemical disinfection systems whose disinfection dose can be directly measured with disinfectant residuals, UV is a physical process and the determination of fluence is complicated in practical reactors. To date, several fluence determination methods have been developed, including conventional methods such as biodosimetry and model simulation, as well as emerging methods such as dyed microsphere method and the model-detector method. However, a systematic and comprehensive review of these methods is still needed to discuss the attributes and application scenarios of each method. In this review, we summarized the principal theories, procedures, applications, and pros/cons of these fluence determination methods. Further, the selection and application of appropriate fluence determination methods were discussed based on different purposes (e.g., feedbacks for reactor design, evidence for third-party validation, as well as on-site determination and long-term monitoring of fluence). Overall, this review could provide useful information and new insights regarding the application of current fluence determination methods to ensure the reliability of UV disinfection.