SODIS--solar water disinfection with PET water bottles

[big brown horse]

I just read about this in National Geographic and had to share. Basically you can disinfect water, say from a stream or lake, by filling up clean, labeless PET plastic water bottles and putting them on a shiny-ish metal surface (like a tin roof) for 6 hours in direct sunlight*.

Here is the link: http://blogs.ngm.com/blog_central/2010/04/high-marks-for-clean-water.html

another on the subject: http://www.sodis.ch/methode/anwendung/index_EN

and another:
http://en.wikipedia.org/wiki/Solar_water_disinfection

PET water bottle info: http://en.wikipedia.org/wiki/Polyethylene_terephthalate

*must be in the direct sun for 6+ hours to work.

 

[big brown horse]

The reason why glass bottles don't work is b/c glass filters out the sun's uva radiation needed to kill the parasites, viruses and bacteria.

eta: rather I should say some glass bottles don't work b/c some filter out the sun's uva radiation.

 

[big brown horse]

From wiki:

Cautions

If the water bottles are not left in the sun for the proper length of time, the water may not be safe to drink and could cause illness. If the sunlight is less strong, due to overcast weather or a less sunny climate, a longer exposure time in the sun is necessary.

The following issues should also be considered:
Bottle material: Some glass or PVC materials may prevent ultraviolet light from reaching the water.[7] Commercially available bottles made of PET are recommended. The handling is much more convenient in the case of PET bottles. Polycarbonate blocks all UVA and UVB rays, and therefore should not be used.

Aging of plastic bottles: SODIS efficiency depends on the physical condition of the plastic bottles, with scratches and other signs of wear reducing the efficiency of SODIS. Heavily scratched or old, blind bottles should be replaced.

Shape of containers: the intensity of the UV radiation decreases rapidly with increasing water depth. At a water depth of 10 cm (4 inches) and moderate turbidity of 26 NTU, UV-A radiation is reduced to 50%. PET soft drink bottles are often easily available and thus most practical for the SODIS application.

Oxygen: Sunlight produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) in the water. These reactive molecules contribute in the destruction process of the microorganisms. Under normal conditions (rivers, creeks, wells, ponds, tap) water contains sufficient oxygen (more than 3 mg Oxygen per litre) and does not have to be aerated before the application of SODIS.

Leaching of bottle material: There has been some concern over the question whether plastic drinking containers can release chemicals or toxic components into water, a process possibly accelerated by heat. The Swiss Federal Laboratories for Materials Testing and Research have examined the diffusion of adipates and phthalates (DEHA and DEHP) from new and reused PET-bottles in the water during solar exposure. The levels of concentrations found in the water after a solar exposure of 17 hours in 60C water were far below WHO guidelines for drinking water and in the same magnitude as the concentrations of phthalate and adipate generally found in high quality tap water.

Concerns about the general use of PET-bottles were also expressed after a report published by researchers from the University of Heidelberg on antimony being released from PET-bottles for soft drinks and mineral water stored over several months in supermarkets. However, the antimony concentrations found in the bottles are orders of magnitude below WHO[8] and national guidelines for antimony concentrations in drinking water.[9][10][11] Furthermore, SODIS water is not stored over such extended periods in the bottles.

Regrowth of bacteria: Once removed from sunlight, remaining bacteria may again reproduce in the dark. A 2010 study showed that adding just 10 parts per million of hydrogen peroxide is effective in preventing the regrowth of wild Salmonella.

Applications

SODIS is an effective method for treating water where fuel or cookers are unavailable or prohibitively expensive. Even where fuel is available, SODIS is a more economical and environmentally friendly option. The application of SODIS is limited if enough bottles are not available, or if the water is highly turbid.In fact, if the water is highly turbid, SODIS can not be used alone, additional filtering is then necessary.[6]

In theory, the method could be used in disaster relief or refugee camps. However, supplying bottles may be more difficult than providing equivalent disinfecting tablets containing chlorine, bromine, or iodine. Additionally, in some circumstances, it may be difficult to guarantee that the water will be left in the sun for the necessary time.

Other methods for household water treatment and safe storage exist, e.g. 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, chemical pollutants), treatment costs, labor input and convenience, and the users preference.

Definition of Turbid: Turbidity is the cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality.