In past decades, secondary water supply (SWS) systems in high-rise residential buildings have seen a rapid development in China, and ultraviolet (UV) treatment has been selected as a principal disinfection technology. However, several special problems of the SWS systems, such as large daily fluctuations in water flow rate (Q) and seasonal variations in water temperature, may strongly impact the disinfection efficiency, but these issues have never been addressed before. In this study, the practical application of UV technology in an SWS system was investigated in detail. A tri-parameter monitoring system, which had earlier been developed based on three micro-fluorescent silica detectors, was installed in a UV disinfection reactor for the SWS system in a high-rise residential building. Combined with an initial fluence validation by biodosimetry, the lamp output attenuation coefficient (N), sleeve fouling coefficient (F), water transmittance (T), Q, and output fluence (U) were monitored online for about 6 months. During the long-term test period, the daily average N increased by ca. 50% due to the seasonal variation in water temperature; the F decreased by ca. 70% due to the sleeve scaling; and a large daily fluctuation of Q induced an obvious fluctuation of U. The average U over the whole operation period was as high as 169 mJ/cm(2), alerting an enormous loss of energy. Through online monitoring of the N, T, F, Q and U, this study revealed the current serious problems for UV disinfection of SWS systems, and proposed potential solutions accordingly. (C) 2014 Elsevier B.V. All rights reserved.