Nicolas Lazaro
Having access to clean water and sanitation is of vital importance. 1.7 billion children suffer from diarrhea every year and 525’000 a million die because of it. These numbers can easily be lowered through access to safe drinking water and improved hygiene practices. Poor countries lack the necessary infrastructure and funds to build a robust and safe water supply system. Reliable and cheap solutions for disinfecting contaminated water are needed.
Picture 1: SODIS being used in Indonesia (https://commons.wikimedia.org/wiki/File:Indonesia-sodis-gross.jpg)
This blog post will present the SODIS method: pathogens such as bacteria and viruses are killed by exposing PET bottles filled with contaminated water to UV radiation. In the first paragraph I will present the method and the steps needed to implement it correctly. I will then discuss each step in more detail revealing SODIS’ benefits and drawbacks. Finally, I will talk about the results it yielded and conclude by expressing my opinion .
Implementation:
SODIS is a method for disinfecting water, making use of the sun’s energy and PET bottles. It successfully kills most bacteria (>99.9%) and some viruses (up to 90%) by exploiting the germicidal properties of UV-A radiation (wavelength between 320 and 400 nm). It is easily applied by using the following 4 steps (Luzi et al., 2016) :
- Wash a plastic bottle
- Fill the bottle with water
- Expose the bottle to the sun
- Store water
Wash a plastic bottle:
The geniality of SODIS relies on its low cost, and the abundant availability of PET bottles. The disinfection of water depends on the exposure to UV-A light (Luzi et al., 2016) ; its ability to kill pathogens is lowered through suspended particles in the water and dirt in the PET bottles. Therefore, it is advised to clean them before use. This step could potentially increase the cost of SODIS as it requires users to have access to soap, or water to clean and rinse the bottle. But is an essential step worth taking.
Fill the bottle with water:
The goal is to have safe drinkable water at our disposal. If the water is kept in the same bottle after having been exposed to the sun, pathogens will not regrow. This allows for the storing of the water. The SODIS method works best when water is not very turbid since suspended particles reduce the disinfection effectiveness. Studies have postulated that the threshold for a successful implementation lies at 30 NTU (Nephelometric Turbidity unit (Luzi et al., 2016) . Even though most users do not have access to measuring tools, there is a simple way for estimating water turbidity: if the font of a newspaper headline is not readable through the mouth of a filled bottle, then the threshold has been exceeded. Picture 2 shows people estimating the turbidity applying this simple trick.
Picture 2: People estimating the turbidity of the water using the newspaper test (Luzi et al., 2016).
Expose the bottle to the sun:
Exposing the bottle for at least 6 hours to the sun will effectively kill most of bacteria in the water. The UV-A light does not directly affect the DNA or RNA of pathogens. Instead, it causes the formation of so-called Reactive Oxygen Species ROS which react with and damage the DNA or proteins of microorganisms (Luzi et al., 2016) . ROS are highly reactive chemicals formed from O2. They are also produced naturally in our body with the goal of killing cells (Hayyan et al., 2016). It is important to note that the exposure time increases when solar radiation decreases. The promoters of the SODIS method advise following these guidelines:
- 1 day of exposure to direct sunlight on mostly sunny days (less than 50% cloud cover)
- 2 full consecutive days of exposure to direct sunlight on mostly cloudy days (more than 50% cloud cover)
- On days of continuous rainfall, SODIS is not performing effectively and should not be used
UV-B radiation is known to kill viruses, but PET bottles absorb this light (Luzi et al., 2016) . This makes the SODIS method less effective against viruses. Yet studies have shown that it is effective against rotavirus, which causes most virus-related diarrhea. Furthermore, exposing the water to sun light will increase its temperature. When temperatures of 45 °C is reached, thermal inactivation (or pasteurisation) will play a role in disinfecting the water together with UV radiation (Luzi et al., 2016) .
Store water:
As I have already mentioned, storing the water in bottles until consumptions prevents recontamination. Studies have shown that storing for too long will cause some pathogens to recolonise the water. Promoters of the method advise not to store the water for longer than two days. Being able to safely store water also gives users more time to do other things such as running their own clay pot shop. This is because they don’t need to invest time in collecting water (Luzi et al., 2016) . This may improve their overall quality of life.
Results:
Even though it is not very effective against viruses, users still benefit from this method. Some studies have measured an 80% risk reduction for diarrheal diseases such as cholera in children below the age of 5. Other studies have shown that children living in households where SODIS is implemented, grow 0.8 cm taller and 0.23 kg heavier by the age of 5 (Luzi et al., 2016) Diarrhoeal disease can spread through many pathways: air, direct human contact, water, and food. Therefore, SODIS is more impactful in areas where the disease spread mainly through water.
My thought to conclude:
Even though PET bottles are abundantly available, SODIS is limited to implementing clear, not bigger than 2 liters and not broken bottles. The effectiveness is reduced in shaded areas or during cloudy days. High turbidity is also a limiting factor though a very easy turbidity test using new papers allows users to estimate whether the water is too dirty for SODIS. The reports from households implementing the method show promising improvements. Though if users also drink contaminated water, the beneficial effect of SODIS are nullified. Given the simplicity of the method I feel it is a good idea for people with difficult access to safe water to give it a shot. I like this method also because it recycles PET, turning polluting plastic into a life changing asset.
References
Hayyan, M., Hashim, M. A., & AlNashef, I. M. (2016). Superoxide ion: generation and chemical implications. Chemical reviews, 116(5), 3029-3085.
Luzi, S., Tobler, M., Suter, F., & Meierhofer, R. (2016). SODIS manual: Guidance on solar water disinfection. SANDEC, Department of Sanitation, Water and Solid Waste for Development: Eawag, Switzerland.
https://www.who.int/news-room/fact-sheets/detail/diarrhoeal-disease. Last consulted 16.05.2022
https://commons.wikimedia.org/wiki/File:Indonesia-sodis-gross.jpg. Last consulted on 30.05.2022