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Solar Water Disinfection (SODIS)

Discussion in 'Resource Conservation - Water, Air, Earth, Etc.' started by arbru125, Apr 22, 2012.

  1. Apr 22, 2012
    arbru125

    arbru125 Sustainable Newbie

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    "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.
     
  2. Apr 22, 2012
    Wannabefree

    Wannabefree Little Miss Sunshine

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    Interesting read. Thanks for the information! And :welcome
     
  3. Apr 24, 2012
    GreenLinnet

    GreenLinnet Sustainable Newbie

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    Wow, solar power can disinfect water? That's so awesome! Why don't they do this in those like starving countries where they have no water? More people should know this! Even if it isn't super effective, couldn't they, like, boil it and use solar power?
     
  4. Apr 24, 2012
    nalym

    nalym Sustainable Newbie

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    This was very interesting! It got me thinking about a few things... I was wondering if you could clarify a little bit, or if we could talk about some implications of this process if it were ever applied.

    In order to supply water to developing countries, the bottles would either need to be really huge or established in extreme numbers. Did you find any information on how realistic that would be? It seems like the huge bottles would not be able to correctly purify water because it is not circulating (I assume). I just wonder how this could possibly provide clean water on such a large scale. Then you have to consider how the water gets to these bottles in the first place - that kind of setup would probably take a lot of time and money. Unless you mean to use this as a personal technique that individuals with a tin roof can practice.

    Was there any way to tell what specific microbes/pathogens/bacteria were in this lake water? If there was no way to test that, I totally understand. But it'd be interesting to see if there was a specific pathogen that did not respond to this method, or responded particularly well.

    I was a little unclear on this - how does this method actually kill bacteria? Does it have to do with heat (assumed because you compared to boiled water) or is this related to solar rays? What kind of studies did you find based on this?
     
  5. Apr 24, 2012
    arbru125

    arbru125 Sustainable Newbie

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    #nylam,

    According to the Swiss SODIS website (the official page for the foundation), any size PET water bottle is usable in theory, though huge or opaque bottles are out of the question. The light loses its intensity after penetrating the water approximately four inches, meaning that thicker containers wouldn't be useful. Also, opaque bottles would scatter the light, making it ineffective. Circulation of the water plays a minimal role, as the recommended container is thin enough for even penetration. Our country has an ample supply of transparent PET water bottles that would be otherwise discarded or recycled. Perhaps recycling companies could form a partnership with SODIS to provide them with the materials they need.
    SODIS is recommended for personal use, i.e.: by individuals, families, or providers for a small community. By donating each household enough bottles to hold several gallons in total, one could essentially provide them with gallons of fresh water per day, which is an astounding thought in itself. For larger scale operations, perhaps as needed in urbanized areas, a more industrialized method of disinfection and purification would likely be needed.

    The bacteria that I cultured from the reclaimed water could have been anything, to be honest. I took proper precautions with sterilization and technique when handling and disposing of the samples on the chance that I might have cultured a dangerous pathogen. Most likely, though, it was harmless. There was no way for me to tell using the tools made available to me in my school's science department. Gram staining could have provided at least some insight, though.

    According to the World Health Organization, SODIS works as the product of three biological, chemical, and physical processes:

    1) The UV-A and UV-B radiation coming from the sun directly interferes with the cell walls and plasma membranes of bacterial cells, bringing their metabolic activities to a halt and killing them off.
    2) UV-A rays, specifically, have the potential to react with dissolved oxygen in the water and create hydrogen peroxides or oxygen radicals, which will in turn assist in killing the bacteria. These chemical changes are not detrimental to the health of those drinking the disinfected water, as they appear in such trace amounts.
    3) Heat from the sun also raises the temperature within the bottle, speeding up any chemical reactions, and perhaps aiding in enzymatic denaturization of the bacteria.

    The heat condition (boiled water) was just a reference point for the effectiveness of SODIS. Heat-treatment is the most basic method of disinfection, and has been a reliable method of sterilization since its first uses in the Roman Empire. Ideally, since solar radiation is the primary contributing factor, a UV sterilization lamp would have been used, but due to the cost, boiling was determined to be an appropriate condition. There's a multitude of scholastic articles on SODIS, ranging from practicality to mechanics, available online.
     

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