Transfer of bacteria via vapor in solar desalination units -

Abstract

Microbial contamination of drinking water still poses a major health threat to many communities worldwide. In the complete absence of literature on the transfer of bacteria in solar stills, the objective of this research was to investigate whether bacteria in water subjected to low temperature solar desalination finds its way into the distillate as a result of cross contamination or transfer through water vapor, and whether bacterial destruction in solar stills takes place due to the effect of temperature or solar U.V. radiation or both. To achieve this objective, passive solar still units were constructed, then solar distillation experiments were conducted by spiking a pure bacterial culture (Escherichia coli ATCC 25922 or Klebsiella pneumoniae ATCC 13883 or Enterococcus faecalis ATCC 51922) in two different sources of water (low mineralized water vs. highly mineralized water) and exposing it to sunlight for 24 hours. Next, to eliminate the role of U.V. radiation and study bacterial transfer under low temperatures, experiments were repeated in the laboratory in darkness and given a regular heat source. To do this, distillation experiments at low temperature ranges (30-35ºC, 40-45ºC and 50-55ºC), which are similar to water temperatures reached in solar stills under natural sunlight, were conducted on the three bacterial species in the two water types. Results show that bacteria indeed get transferred with the vapor in a solar still when not exposed to solar UV radiation. The trends observed were adequately explained by a zero-modified Hurdle Poisson model. The numbers of viable bacterial colonies transferred were highest in E. faecalis E. coli K. pneumonia and were temperature dependent with highest transfers occurring at the 40°C range. Moreover, salinity had a synergistic interaction effect with increased temperature on bacterial survival. While solar desalination can effectively inactivate bacteria in water, there exists a mode of bacterial transfer in the humid medium of the s