CFD simulation and optimization of the UVC-LED reactor in aquaculture water treatment system

High fish stocking density has posed a huge hidden risk of diseases on sustainable aquaculture with environmentally friendly practices. The ultraviolet (UV) radiation has been widely used to disinfect the surfaces, water body, and air for the water treatment in an ever-increasing Recirculating Aquaculture System (RAS). The current UV light source is commonly applied to a low-pressure mercury lamp with a wavelength of 254 nm, particularly in the disinfection of aquaculture water body during industrialized production. However, the ultraviolet light light-emitting diode (UVC-LED) has been expected to replace the mercury lamp with high energy consumption and easy pollution in recent years. In this study, a disinfection model of UVC-LED photo-reactor was established to optimize the reasonable structure suitable for the circulating water aquaculture system using the computational fluid dynamics (CFD) simulation. The flow field of the reactor was evaluated using the momentum and mass conservation, and the light field distribution in the reactor was obtained using the radiation transfer equation by P1 approximation. Microorganisms were assumed as small particles during water treatment, of which movement in the independence of the flow field. A discrete phase model (DPM) was selected to simulate the flow of microorganisms in water. The residence time of microorganisms in the flow field was first calculated to integrate the radiation intensity within the duration, where the radiation dose was obtained by microorganisms through the reactor. Then, the effective radiation dose of the reactor was optimized using the microbial disinfection response curve from a parallel light test. E. coli was cultured. A microbial killing experiment was carried out in a UVC-LED photoreactor at the flow rates of 2 and 2.5 L/min. The experimental killing rate of the reactor was collected to compare with the calculation. The results show that the calculation model performed well to predict the disinfection effect of the reactor, where the error of logarithmic inactivation rate between simulation and experiment was less than 8%. The rectifying zone was significantly determined the flow field in the reactor. Furthermore, the structure of the rectifying zone was greatly contributed to tailor tailoring the water flow in the reactor for the better disinfection while the high flow of the system. The equivalent reducing radiation dose increased by 19.6% in the reactor with 10 cylindrical flow channels, compared with the original. The emissivity of the opaque surface was adjusted to calculate the radiation under the different inner wall reflection coefficients, both of which followed the Kirchhoff's law. The inner wall reflection coefficient presented a significant effect on the high radiation intensity, especially in the water environment with a high light transmittance. Two models of the reactor with different inner wall materials were constructed under the ultraviolet Transmission (UVT) 80%, UVT 85%, and UVT 90% transmittance, where the red of high reflectivity model increased by 47.74%, 55.02%, and 66.87% respectively, compared with low reflectivity model. Consequently, the CFD modeling can be expected to characterize and optimize the disinfection performance of the UVC-LED photoreactor. This finding can provide a theoretical reference to design a new UV disinfection photoreaction system in an aquaculture water treatment system.