Calcium scaling on the quartz sleeve of ultraviolet reactor: On-line measurement and model prediction

Quartz sleeve scaling significantly affects the efficacy of ultraviolet (UV) processes for water treatment. Investigating the scaling process and developing prediction methods are important for selecting and optimizing scaling control measures. However, these have been limited by the lack of an appropriate experimental system. Therefore, this study investigated the kinetics, mechanisms, and prediction methods of calcium scaling (a predominant scale compound encountered in drinking water treatment) on quartz sleeves. An on-line experimental system for sleeve scale UV transmittance (UVT_SS) was developed based on previously developed micro-fluorescent silica detectors. The scaling process was accelerated by increases in [Ca²⁺] (100-400 mg L^-1), alkalinity (100-400 mg L^-1), and pH (6.5-8.5). Furthermore, water temperature (20.0-30.0 °C), sleeve temperature (24.9-31.7 °C), and flow velocity (2.5-10.0 cm s^-1) exhibited diverse effects on calcium scaling. By delving into scaling mechanisms such as mass transfer, surface crystallization, and scale stripping processes, we proposed a prediction model. The extinction coefficient and stripping rate of calcium scale were estimated to be 0.80 μm^-1 and 0.155 h^-1, respectively. Model predictions were validated through both laboratory and field tests. This study provides important methods for ensuring sufficient dose delivery by a UV reactor, enhancing energy efficiency, and improving reliability in UV processes for water treatment.