Summary of Week 3 of Introduction to Household Water Treatment and Safe Storage (Coursera – École Polytechnique Fédérale de Lausanne)
This week considered disinfection processes that result in high levels of removal or inactivation of different classes of pathogens:
Heat kills enzymes, proteins and cells through different mechanisms. The main form of heat disinfection is boiling, which is the oldest method of household water treatment, practiced by 600 million people worldwide. Heat is effective against all viruses, bacteria and protozoa, even at as low a temperature of 60 degrees celsius. Therefore, by the time water boils, even a little bit, most pathogens would have been killed. With pasteurisation, the temperature of the water is brought up to 70 degrees celsius. The challenge is in knowing when the water has reached that temperature.
Ultraviolet (UV) radiation is most effective against bacteria, followed by protozoa cysts, viruses, followed by bacterial spores. UV-C generated by lamps are effective but need electricity. Turbidity can shield particles from radiation and a higher dose is needed at lower temperatures. Another form of UV radiation is solar disinfection, which has been researched extensively. Its most important mechanism is indirect inactivation by UV-A which generates reactive oxygen species. Its effectiveness is highly dependent on climate and other factors. Moreover, it takes a lot of time and treats small volumes.
Chemical disinfection, especially with chlorine, has been used for over a century. It is the 2nd more common method of household water treatment, behind boiling. Chlorine deactivates cell walls, DNA and enzymes, but its mechanisms are not fully understood. Bacteria are very chlorine, while viruses and protozoa are tougher. Chlorine is not effective against cryptosporidium. Other types of chemical disinfection are not typically applied at a household level, although bromine and silver are used.