Using Fluorescent Microspheres as a Non-Biological Surrogate Indicator for Sequential Disinfection Performance.

Abstract

Fluorescent YG-microspheres (Polysciences Inc.) were evaluated to simulate Cryptosporidium inactivation in treatment systems that utilize multiple disinfectants. Experiments were performed in batch reactors including an ozone primary stage at pH 7 and a secondary free chlorine treatment at pH 6. The impact of exposure to the chemical disinfectants was accomplished by tracking the changes in fluorescence distribution using a flow cytometer. Microsphere survival ratios (N/No) were calibrated to replicate the inactivation of different Cryptosporidium strains by selecting an appropriate threshold in a histogram analysis. The threshold value corresponds to a boundary between the beads representing the viable and non-viable Cryptosporidium cysts. The results suggest that YG-fluorescent microspheres are adequate non-biological surrogate indicators for the evaluation of sequential disinfection performance. In addition, it was found that microspheres had collateral reactions with sodium sulfite, affecting the physical integrity of the particle, a phenomenon that does not occur with the organism cyst. Analysis of the data showed that dot/density plot, which display the bead morphology characteristics, should be performed along with the histogram analysis to ensure the correct microsphere survival ratio outcome. Lastly, microsphere structural tests showed that the sequential disinfection mechanism consists of a polystyrene surface damage caused by ozone. This polystyrene damage enhances the diffusion of the secondary disinfectant into the microsphere, where it degrades the dye available in the opened polymer layer.