Developments in fluence rate modelling for ultraviolet light emitting diode based water treatment reactors
Creators: name: Simons, Richard Mark
Publisher: name: Imperial College London
Year: 2016

description: Ultraviolet-C light emitting diodes (UV-C LEDs) are a promising technology for water disinfection. Over the last decade UV-C LEDs have shown dramatic improvements in output power, efficiency, lifetime, and cost. The low-voltage power supply and effective point source geometry of LED light sources allows for new applications and designs. Mercury gas discharge lamps have been used in UV disinfection for several decades, and as a result the industry has developed around the limited specifications of these lamps, such as their size and shape, high power requirements, and mercury content. The increased design freedom offered by LED sources is expected to revolutionise the UV disinfection industry. A model for the optical (fluence rate) distribution within a UV photoreactor was developed to match the freedoms afforded by the use of LED sources. The model development focused on design flexibility with regard to the shape of the reactor body to allow for the accurate modelling of unusual designs. Particular attention was paid to the modelling of internal reflections within the reactor. A Monte Carlo method was selected, where general solutions may be implemented to specific conditions within an arbitrary reactor design. The model focussed on the effects of optical design and did not include fluid dynamics simulations. A series of batch reactors, from 100 – 7500 mL in volume, were modelled for a range of internal reflectivity conditions, from wholly absorptive to specular or diffuse reflectivity. The model produced a 3D representation of the fluence rate distribution within each reactor; this distribution could be evaluated qualitatively to inform future reactor designs. The specific required inactivation energy (SRIE) was defined to allow for the quantitative comparison of different reactors. It was determined that the reactor designs with the highest efficacy have diffusely reflective internal surfaces: the scattering of radiation by diffuse surfaces increases the fluence rate uniformity, which is important for efficient disinfection. The parameter space available for investigation is vast; it was therefore not possible to find a globally optimised solution. However, variation across three parameters showed a broad range of near-optimum designs which resulted in a comparable disinfection performance. This suggests that relative improvements that would result from a global optimisation may be small. A cylindrical reactor of internal depth 90 mm and diameter 119 mm, with Lambertian diffusely reflective internal surface, illuminated by a single LED source (of arbitrary optical power) could achieve an SRIE of 6.56 mJ mL-1 for a treatment water of 90 % transmittance. For an LED source of 100 mW and peak emission 254 nm, this corresponds to a treatment time of 66 s for the 1000 mL volume of the proposed reactor, under a static batch treatment regime. This predicted performance of this device is suitable for application in home-scale point of use water disinfection.