Online monitoring of an ultraviolet (UV) facility not only can ensure disinfection efficiency, but also conserve energy. In this work, a tri-parameter online monitoring system was developed by using three micro-fluorescent silica detectors (MFSDs) installed into a UV disinfection reactor. The detector signals were computed by theoretically derived mathematical equations to determine three key operational parameters: the lamp output attenuation coefficient (N), water UV transmittance (T), and quartz sleeve fouling coefficient (F). Results indicate that the N, T and F parameters could be monitored well by the MFSDs. The monitored lamp output exhibited a significant dependence on water temperature; the monitored water transmittance was verified by a UV-Vis spectrophotometer; and the monitored sleeve fouling coefficient decreased from 1.00 to 0.66 after an 18-day operation with a synthetic hard water. A long-term 30-day experiment, where the UV reactor was directly connected to a tap water faucet to simulate practical conditions, demonstrated the good applicability of the developed monitoring system. This monitoring system in real time can be used to adjust the number of operating lamp modules, alarm any accidental breakdown of the lamps, and trigger auto-cleaning of the quartz sleeves, thus having potential applications to open-channel and enclosed UV disinfection facilities. (C) 2013 Elsevier B.V. All rights reserved.