Visualizing flow and dose delivery in UV reactors using 3-dimensional laser induced fluorescence

Application of UV is becoming increasingly popular for drinking water disinfection since it effectively inactivates Cryptosporidium parvum oocysts and Giardia lamblia cysts at relatively low doses. Guidance and regulations for UV disinfection require that utilities verify dose delivery by validation testing. UV reactor validation is currently accomplished using the biodosimetry method, which determines a reduction equivalent dose (RED) value from the inactivation of a test microorganism. However, biodosimetry has several limitations including: (1) inability to measure the dose distribution delivered by the reactor, (2) difficulty of directly extrapolating the result to the REDs of other microbes that have different inactivation kinetics, and (3) relatively high costs and time required for analysis, which prevents real-time measurement of dose delivery. To address some of these deficiencies, recent research has investigated the use of fluorescent microspheres as non-biological surrogates to measure dose distributions. While dose distributions measured in this way provide RED estimates for microbes with different inactivation kinetics, this technique also does not provide spatial information on dose delivery within the reactor. Computational fluid dynamics (CFD) models can predict spatial information, estimates of the dose distribution, and RED for different microbes. But the accuracy of these models has not been experimentally verified. The objective of this currently on-going, Water Research Foundation-funded study is to develop an innovative laser-induced fluorescence (LIF)-based method to measure real time, three-dimensional mixing behavior and hydrodynamics within UV reactors. Both pilot- and lab-scale UV reactors equipped with low-pressure UV lamps were constructed for the LIF system. Conservative tracer tests were performed using the lab-scale reactor with UV lamps off, in which 3D instantaneous distribution of fluorescent dye within the reactor was captured real time. The images were further analyzed to successfully visualize the structure of eddy in the UV reactor and quantitatively analyze dye distributions in UV reactors. Reactive tracer tests will be conducted using the lab-scale reactor in which tracer dye decayed in response to UV lights such that distribution of tracer dye within the reactor was quantitatively correlated with UV dose distribution within the reactor. Further experiments using the lab-scale reactor are currently being performed and experiments using a pilot-scale UV reactor will be performed. Preliminary validation of this novel method by comparing the results to dose delivery measured using biodosimetry performed with Bacillus subtilis will be also performed. The newly developed LIF technique is expected to provide a highly innovative and unique approach for UV reactor optimization and validation as well as greater insight into the performance of UV reactors. 2010