Separating the UV and thermal components during real-time solar disinfection experiments: The effect of temperature

To understand the real effect of temperature during the solar disinfection (SODIS) process when using real water, a series of experiments were conducted to separate both the UV and thermal inactivation components of SODIS independently at real-time experiments using natural water that might contain nutrients. Three flat disinfection reactors were set up simultaneously, one subjected to SODIS (UV radiation + temperature), one subjected to natural UV radiation but with controlled temperature below 15 °C (only UV radiation), and the third one in the dark subjected to the same temperature profile followed by the SODIS reactor under the sun (only temperature). E. coli and Enterococcus spp were the analysed microorganisms. Strong climatic conditions were achieved (>850 W/m2 global irradiance, 45 W/m2 average UV irradiance), and maximum water temperatures reached 39.8-48.9 °C. Microbiological results show strong synergy of UV and temperature when the latter is above 45 °C, as expected. Below 40 °C, the optical (UV-only) and SODIS experiments followed the same disinfection kinetics. But in the band 40-45 °C, under not so strong or non-continuous UV conditions, the SODIS process slowed down in comparison with the optical process (UV-only), and the thermal reactor experience microbial growth. Thus, this study confirms that there are certain temperatures (those in the range around the optimum microbial growth temperature) that might impair solar disinfection.