"How effective is Solar Water Disinfection (SODIS) at eliminating waterborne microorganisms?"
Overview
Water-borne illness is a significant problem in developing countries, and is responsible for 3.4 million deaths annually (WHO). In industrialized nations like the US, water treatment facilities are integral in providing citizens with clean, dependable drinking water. However, because of the difficulties and costs of such an operation, many people in the developing world struggle with finding clean, potable water on a daily basis.
Solar water disinfection, commonly known as SODIS, is recommended as a cheap, efficient, accessible, and safe method of water disinfection in areas where potable water is not a commodity (SODIS). The process begins by filling a sterile and transparent bottle composed of PET a type of plastic with minimally turbid water. Depending on cloud cover, the bottle is placed on a reflective surface in the sun, such as a tin roof, for anywhere from six hours to two full days. After exposure, the water is pathogen-free and safe to drink.
SODIS is an effective method for treating water where fuel or cookers are unavailable or exorbitantly expensive. Even where fuel is available, SODIS is a more economical and environmentally friendly option. The application of SODIS does have limitations, such as the availability of bottles or the turbidity of the water supply. These limitations are minimal in comparison to alternative water disinfection methods used in industrial nations.
Other methods for water treatment and safe storage exist, such as chlorination, different filtration procedures or flocculation/disinfection. The selection of the adequate method should be based on the criteria of effectiveness, the co-occurrence of other types of pollution (turbidity, pollutants), treatment costs, labor input and convenience, and the users preference (WHO).
In the scope of its global potential, the mechanisms of SODIS are astoundingly simple. Due to PETs chemical composition, solar radiation fully penetrates through it and is absorbed by the water. Chemical leaching is a minimal threat in PET plastics, which is why bottles composed of PET are used over PVC-composed containers. Additionally, unlike glass bottles filled with UV-absorbing iron oxide or harmful PVC bottles, PET allows for the full spectrum of solar radiation to pass through (EAWAG). In the sealed PET bottle, a combination of raised temperatures and consistent UV-A and UV-B irradiation kills off any potentially threatening microorganisms, leaving the once dangerous water sterile and safe to drink.
The implications of SODIS as a renewable source of drinking water are astounding. The abundance of PET water bottles in the developed world would not only solve the problem of supplying countries in need with the materials required for SODIS, but also creates an outlet for waste that could otherwise damage the ecosystem (Wegelin). The availability of clean drinking water is the most drastic factor in increasing the quality of living in developing countries. With no more than water bottles and proper instruction, the application of SODIS could theoretically eliminate or drastically reduce the problem of clean drinking water on a global scale.
Experimentation
In an effort to investigate the efficacy of SODIS, an experiment was conducted to determine its effectiveness in eliminating waterborne microbes.
Hypothesis:
If solar water disinfection (SODIS) is an effective method of eliminating water-borne bacteria, then solar-treated water will show a considerably lower count of bacteria than untreated water because most if not all microbes will be killed by the treatment.
Methods:
Water samples were collected from a community pond filled with water reclaimed by the city of Glendale, AZ and applied to one of three conditions.
In preparation for solar water disinfection, 900mL of sample water was placed in a sterile 1L Fiji water bottle. The bottle was capped, agitated, and filled with 100mL more of sample water to ensure proper aeration. The bottle was sealed and placed on its side in direct sunlight for six hours. Another 500mL of water was boiled in a glass beaker for 10 minutes.
Using sterile technique, 24 petri dishes were filled with nutrient agar and allowed to cool completely. One mL of each sample untreated, SODIS-treated, and heat-treated was spread on a petri dish and incubated for 30 hours at 37 Celsius. At the end of the incubation period, colonies were counted by eye and recorded for analysis.
Results:
Bacterial Colony Counts of Pond Water
Untreated: SODIS: Heat-treated:
1) 130 40 0
2) 170 60 1
3) 95 65 0
4) 115 30 1
5) TMTC (200+) 75 12
6) 95 10 0
7) 120 14 0
8) 160 60 10
Average: 136 44 3
% Decrease: 68% 98%
Conclusion and implications:
SODIS is not the most effective method of water purification. Solar disinfection resulted in a 68% decrease in bacterial development while boiling resulted in a 98% decrease. Heat treatment is still superior in the elimination of bacteria, though it must be considered that fuel for fire is not always safe, available, or practical. Additionally, it must be considered that error in experimentation could account for the difference in efficacy, as SODIS is a delicate process and populations are always thoroughly instructed before using it as a means of water disinfection. Then again, perhaps error in this experiment is more representative of true field use of SODIS, where rigid sterilization techniques and precise measurements cannot be used.
If SODIS can be improved or somehow supplemented, it could become an accessible and efficient method of water disinfection. Already, a 2010 study conducted by the World Health Organization suggested that when supplemented with trace additions of hydrogen peroxide, SODIS is more effective in not only eliminating waterborne pathogens, but prevents them from reproducing within the disinfected bottle, creating a reliable method for producing and storing drinking water. Further research and development is needed for the widespread success of solar water disinfection, but with what has been demonstrated in the scientific community and the field thus far, SODIS shows great promise for combating the global threat of unclean drinking water.
Overview
Water-borne illness is a significant problem in developing countries, and is responsible for 3.4 million deaths annually (WHO). In industrialized nations like the US, water treatment facilities are integral in providing citizens with clean, dependable drinking water. However, because of the difficulties and costs of such an operation, many people in the developing world struggle with finding clean, potable water on a daily basis.
Solar water disinfection, commonly known as SODIS, is recommended as a cheap, efficient, accessible, and safe method of water disinfection in areas where potable water is not a commodity (SODIS). The process begins by filling a sterile and transparent bottle composed of PET a type of plastic with minimally turbid water. Depending on cloud cover, the bottle is placed on a reflective surface in the sun, such as a tin roof, for anywhere from six hours to two full days. After exposure, the water is pathogen-free and safe to drink.
SODIS is an effective method for treating water where fuel or cookers are unavailable or exorbitantly expensive. Even where fuel is available, SODIS is a more economical and environmentally friendly option. The application of SODIS does have limitations, such as the availability of bottles or the turbidity of the water supply. These limitations are minimal in comparison to alternative water disinfection methods used in industrial nations.
Other methods for water treatment and safe storage exist, such as chlorination, different filtration procedures or flocculation/disinfection. The selection of the adequate method should be based on the criteria of effectiveness, the co-occurrence of other types of pollution (turbidity, pollutants), treatment costs, labor input and convenience, and the users preference (WHO).
In the scope of its global potential, the mechanisms of SODIS are astoundingly simple. Due to PETs chemical composition, solar radiation fully penetrates through it and is absorbed by the water. Chemical leaching is a minimal threat in PET plastics, which is why bottles composed of PET are used over PVC-composed containers. Additionally, unlike glass bottles filled with UV-absorbing iron oxide or harmful PVC bottles, PET allows for the full spectrum of solar radiation to pass through (EAWAG). In the sealed PET bottle, a combination of raised temperatures and consistent UV-A and UV-B irradiation kills off any potentially threatening microorganisms, leaving the once dangerous water sterile and safe to drink.
The implications of SODIS as a renewable source of drinking water are astounding. The abundance of PET water bottles in the developed world would not only solve the problem of supplying countries in need with the materials required for SODIS, but also creates an outlet for waste that could otherwise damage the ecosystem (Wegelin). The availability of clean drinking water is the most drastic factor in increasing the quality of living in developing countries. With no more than water bottles and proper instruction, the application of SODIS could theoretically eliminate or drastically reduce the problem of clean drinking water on a global scale.
Experimentation
In an effort to investigate the efficacy of SODIS, an experiment was conducted to determine its effectiveness in eliminating waterborne microbes.
Hypothesis:
If solar water disinfection (SODIS) is an effective method of eliminating water-borne bacteria, then solar-treated water will show a considerably lower count of bacteria than untreated water because most if not all microbes will be killed by the treatment.
Methods:
Water samples were collected from a community pond filled with water reclaimed by the city of Glendale, AZ and applied to one of three conditions.
In preparation for solar water disinfection, 900mL of sample water was placed in a sterile 1L Fiji water bottle. The bottle was capped, agitated, and filled with 100mL more of sample water to ensure proper aeration. The bottle was sealed and placed on its side in direct sunlight for six hours. Another 500mL of water was boiled in a glass beaker for 10 minutes.
Using sterile technique, 24 petri dishes were filled with nutrient agar and allowed to cool completely. One mL of each sample untreated, SODIS-treated, and heat-treated was spread on a petri dish and incubated for 30 hours at 37 Celsius. At the end of the incubation period, colonies were counted by eye and recorded for analysis.
Results:
Bacterial Colony Counts of Pond Water
Untreated: SODIS: Heat-treated:
1) 130 40 0
2) 170 60 1
3) 95 65 0
4) 115 30 1
5) TMTC (200+) 75 12
6) 95 10 0
7) 120 14 0
8) 160 60 10
Average: 136 44 3
% Decrease: 68% 98%
Conclusion and implications:
SODIS is not the most effective method of water purification. Solar disinfection resulted in a 68% decrease in bacterial development while boiling resulted in a 98% decrease. Heat treatment is still superior in the elimination of bacteria, though it must be considered that fuel for fire is not always safe, available, or practical. Additionally, it must be considered that error in experimentation could account for the difference in efficacy, as SODIS is a delicate process and populations are always thoroughly instructed before using it as a means of water disinfection. Then again, perhaps error in this experiment is more representative of true field use of SODIS, where rigid sterilization techniques and precise measurements cannot be used.
If SODIS can be improved or somehow supplemented, it could become an accessible and efficient method of water disinfection. Already, a 2010 study conducted by the World Health Organization suggested that when supplemented with trace additions of hydrogen peroxide, SODIS is more effective in not only eliminating waterborne pathogens, but prevents them from reproducing within the disinfected bottle, creating a reliable method for producing and storing drinking water. Further research and development is needed for the widespread success of solar water disinfection, but with what has been demonstrated in the scientific community and the field thus far, SODIS shows great promise for combating the global threat of unclean drinking water.
