The purpose of this study was to examine if it is possible to purify water from microbes using simple means like PET-bottles and sunlight energy. The water was collected from water sources in Tanzania. At the same time, the water supply and water quality at Hekima Girls Secondary School was investigated as well as in Tanzania and the Kagera region. My main two questions were: “is it possible to purify water from microbes with help from sunlight?” and “can one build a cheap solar powered ozone generator for water purification?”. The methods that were used in this study were specifically picked in purpose of purifying water with solar disinfection and ozone. Both methods were initiated by filling PET-bottles (600ml and 1.5L) with water from nearby water sources like Lake Victoria and the stream by the school. Subsequently one was placed in the sunlight for 5h while the other was placed in the shadow for 40 min. A solar panel was giving electricity to an ozone generator that pumped ozone (50-100 mg/h) down the bottle that was placed in the shadow. Moreover, samples were taken in intervals and put on agar plates to study the assumed decrease in bacteria growth during the purification process. Temperature and UV radiation was also measured.
My hypothesis for the experiments was that ozone purification would be much more efficient than purifying with the help of direct sunlight. Ozone is a reactive gas which is widely used in the aquarium world, reduced the need to maintain their aquariums so often from algae and bacteria. I also believed that the sunlight would kill the bacteria and that both methods would work for water purification. After sunlight or the ozone treatment, the water could be stored in the PET bottle for example in a ground cellar or come to direct use. That a 10 W solar cell could run an ozone generator, I assumed was possible, since Tanzania has a lot of intense sunlight exposure.
For sunlight purification, I would recommend at least 6h of continuous exposure in the sun as there was still little growth in the 600ml PET bottle after 5h of UVA radiation, especially when taking water from a source with high bacterial activity. 6h of continuous UV exposure in the sun is what "Swiss Federal Institute of Aquatic Science and Technology" recommends. The same applies for a 1.5 L bottle. The result of sunlight purification of water from Lake Victoria shows that, although the bacterial content went down after 2-3 h of exposure to UV light, the bacteria regrew in the warm water. This happened due to slightly less sunny conditions towards evening. Therefore, it is probably best to expose the water in the sun from morning to afternoon to ensure as high UV radiation as possible, while UV peak hours are around noon.
Sunlight purification with UV radiation is nothing I can fully recommend because it is still very easy for bacteria to grow in the warm water. It is therefore difficult to guarantee a 100% sterilization of the water.
My experiments show, that ozone is very toxic and the treatment time is thus very short when you want to purify water. 40 minutes of continuous ozone feed was required to get a decrease in bacterial activity. Ozone must also be generated in place as it is very reactive and disappears within a very short time interval. However, this can be seen as a disadvantage since ozone is not suitable for long-term treatment of water, for example, in water pipes. In addition, ozone did not kill eggs or larvae during my treatment which resulted in some agar plates containing larvae, which may mean that a filtration process would have been necessary before ozone treatment.
In summary the results of the experiments show that it is possible to purify water by lowering the bacterial content, using the sun's UV radiation for 5 hours and by using ozone for 40 minutes. My attempts, on the other hand, did not achieve drinking water quality. In order to accomplish such a result, one would probably need to combine my purification methods with some form of filtration and give the experiments longer time to treat the water.
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