Solar Distillation and Disinfection
Purifying water is essential to human health. Unfortunately water purification typically requires significant infrastructure or energy input; and these requirements leave much of the globe without consistent access to potable water. If a region receives sufficient sunlight it may be able to use passive solar technology to purify water, even when local peoople are faced with limited infrastructure, energy, money, and/or materials.

Solar disinfection destroys microbial contaminants in water by exposing them to the sun. Ultraviolet light is a microbicide and most waterborne pathogens can be destroyed when water is simultaneously heated and exposed to solar UV. As long as pathogens are the only contaminant this is a powerful means of purifying water and it can be used with extremely little infrastructure or economic cost ¹ .

Distillate condensate trickles down
the glass face of the distiller.

Solar distillation devices include a wide variety of designs including simple desert survival stills made of clear plastic to large permanent arrays constructed from concrete and metal fixtures, most solar distillers are glass covered boxes. The basin of the solar distiller is filled with polluted water and the glass pane cover is set at an angle allowing sunlight to enter the basin with minimal reflection. Sunlight incident on the water in the basin heats it and increases it's rate of evaporation. Ultimately internal water vapor pressure approaches saturation. The glass cover, exposed to the external air currents remains slightly cooler than the rest of the basin facilitating the condensation of water vapor on the glass pane. The condensed water droplets run down the glass and are collected by a drip catch placed at the low end of the glass pane.

Solar distillation and disinfection field study.
Chiapas, Mexico.

Distillate water is not only free of bacteria but is also free of most other forms of water pollution. Solar distillers ability to remove salts, heavy metals and chemical toxins ² is the most important advantage of this technology relative to solar disinfection and slow sand filters ^{3, 4}, SODIS ⁵ , and other disinfection techniques (including boiling and the use of bleach.) ⁶, both of which are otherwise powerful low cost point of use technologies suitable for many relatively resource poor settings.

Solar Distillation in Mexico
In the Agua Autónomo project Science for the People is conducting field tests on several distiller designs. To this effect we have studied still performance along the northern and southern borders of Mexico in the states of Chiapas and Baja California Norte.

In a pilot study centered on San Cristobal de las Casas, Chiapas the wooden solar distiller prototypes were constructed locally by local craftsman for less than $45 US dollars a piece (2007). The basin was covered with a 2 mm thick piece of glass which had a solar collection area of ~.5 m². On clear days during the darkest part of the year, a single unit was found to be capable of producing over 1.5 L of distilled water per day.

Distiller research farm. Baja California Norte.

The lush highlands of Chiapas experience more than adequate rainfall for the needs of the people, but the surface waters contain pathogens and remain unfit for drinking without purification. In contrast, Baja California Norte is semi-arid with little rainfall and under the pressure of population growth the coastal aquifers are suffering from overdrafting and salt water intrusion.

San Cristobal, Chiapas is approximately 16.73 North latitude whereas SfP's Centro de Ciencia in Baja is about 31.74 North latitude, a full 15 degrees farther north. Thus distillers placed in Baja California Norte receive far less direct solar radiation than those placed Chiapas but both areas have ample sunlight for solar distillation year round.

Baja California Norte is an excellent location to investigate solar distiller performance in the field. Baja's latitude, semi-arid climate, solar resources and increasingly scarce and salinated water supplies mirror conditions in many areas of the world suffering from water poverty. An example of clear day solar distiller performance in late winter (February) Baja California Norte is shown in the graph below.

Solar Radiation and Solar Distiller Performance. Outset: The temperature difference between the water inside of the solar distiller basin and the external ambient temperature (purple) lags solar radiation (red) by about two hours. Inset: Distillate yield (blue) shows the performance of this .5 m2 distiller in mL/minute. Yield can be seen to highly correlated with the temperature difference between the external ambient temperature and the basin temperature.

References
1. SODIS METHOD; SODIS safe drining water for all, retrieved 12/16/2012
2. Foster, Amos, & Eby. Ten Years of Solar Distillation Application Along the US-Mexico Border Solar World Congress, International Solar Energy Society Orlando, Florida, August 11, 2005.
3. Huisman, L. Slow Sand Filtation; 1974; The World Health Organization
4. Slow Sand Filtration; Tech Brief, Vol 14. June 2000; National Drinking water Clearing House.
5. SODIS METHOD; SODIS safe drinking water for all, retrieved 12/16/2012
6. Household Water Treatment Options in Developing Countries: Solar Disinfection (SODIS); January 2008. CDC