Welcome Prof./Dr. Zhimin Qiang, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, China to be keynote speak!

Prof./Dr. Zhimin Qiang, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, China

Title: Optimal design and energy-saving operation of UV reactor for disinfection of secondary water supply
     The discovery of anti-chlorine pathogenic microorganisms (e.g., Cryptosporidium and Giardia) in water sources and the detection of chlorinated disinfection byproducts (DBPs) in drinking water bring about a big challenge for the traditional chlorine disinfection process. Because of its high efficiency in inactivatingCryptosporidium and Giardia and little DBPs generation, UV technology has been increasingly employed for drinking water disinfection all over the world. Nonetheless, the efficiency of UV disinfection during practical applications depends on the specific configuration of a UV disinfection reactor, reactor operation and maintenance, and water quality and flow rate.
     To ensure a high efficiency of the UV disinfection reactor in practical applications, an accurate evaluation of the reactor performance is necessary. In this study, computational fluid dynamics (CFD) simulations were adopted to evaluate the UV reactor performance and the key steps included: 1) establishing a geometric model based on the UV reactor configuration and generating meshes across the entire computational domain; 2) selecting an appropriate turbulent model for hydrodynamics modeling and setting velocity boundary conditions; 3) selecting an appropriate radiation model and setting radiation boundary conditions; and 4) activating a discrete phase model, setting the delivery pattern of simulating micro particles, compiling a user-defined function to integrate UV doses along the particle trajectories.
     First of all, the accuracies of various radiation models were evaluated with an in-situ fluence rate distribution measurement platform, and the results indicated that the fluence rate distribution in a UV reactor could be predicted accurately with a calibrated discrete ordinates (DO) radiation model (calibration factor = 0.8). The microbial inactivation efficiencies predicted by the CFD simulation procedure developed in this study (i.e., with Realizable k-e turbulent model, calibrated DO radiation model and Lagragian approach of the discrete phase model) agreed well with those measured by experimental tests.
     Then, the above CFD simulation procedure was employed to optimize the configuration of a 3-lamp UV disinfection reactor, with a focus on the lamp arrangement and the adjustment of inlet/outlet internal diameter. Both theoretical deduction and CFD simulation proved that the reactor minimum UV dose (Dmin) was superior to the traditional reduction equivalent dose (RED) as an indicator of the reactor performance during the optimized configuration of a UV reactor. In general, a reactor with normal positioned lamps (i.e., NOR reactor) had a larger Dmin value and thus a better performance than that with reverse positioned lamps (i.e., REV reactor). The NOR-0.4-75 (0.4 = ratio of the distance between the lamp and reactor axes to the reactor radius, 75 = inner diameter of the reactor inlet (mm)) reactor had the largest Dmin, mainly because the influent flow was well dispersed by the lamps located right below the reactor inlet. The UV reactor with an optimized configuration achieved a 47-100% increment in the maximum disinfection flow rate, corresponding to an energy saving efficiency of 32-50%.
     The application of UV disinfection to secondary water supply (SWS) systems has become a prevalent practice in China’s cities in recent years. However, the highly fluctuating flow rates render the regular operation of a UV disinfection reactor (i.e., lighting all the lamps all the time) a considerable waste of energy. Therefore, an economical running strategy based on periodical adjustments of the lamp output power (LOP) was developed for a UV disinfection reactor employed in the SWS systems. The daily 24 h were divided into 6 time periods according to the daily flow rate variation pattern, and a critical flow rate (Qc) was determined for each period for the individual lamp mode verification. The lamp mode assessment results indicated that for a constant LOP, the best reactor performance was always achieved through the lamp switching mode rather than through the power adjusting mode. Sequential combination of the verified lamp modes with proper LOPs for the 6 periods resulted in an economical running strategy for the UV disinfection reactor (Figure 1). This strategy could reduce the reactor operation cost by 32% without any adverse effect on the disinfection efficiency, which corresponded to a daily electrical energy cut of 4.8 kWh.
Figure 1 The economical-running strategy with periodically adjusted LOPs and the predicted hourly REFs of the UV disinfection reactor with the regular- or economical-running strategy.