Solar earth-water distillation: an appropriate technology for Hong Yuan?

Abstract

Nomadic Tibetan pastoralists living on the high plateau in western China face difficulties in locating good quality drinking water. In Hong Yuan County, western Sichuan Province, health statistics support the hypothesis that the ingestion of contaminated water is resulting in significant health problems among cattle herders.

As little other water quality data was available, a water quality sampling programme was conducted in 1996. The results from this study showed that faecal bacteria are a significant health risk in surface water, with median faecal coliform densities of between 60 and 490 colnies/100mL.

Solar earth-water distillation has been suggested to be an appropriate technology for small-scale water purification amongst pastoralists. To evaluate this hypothesis, four designs of solar stills were tested in Hong Yuan during May - October 1996. The most productive design was a single-slope glass covered single-effect still. The average daily production of this still was 1.6L/m², when soil moisture was not limiting.

An empirical model was developed relating distillate to commonly measured weather parameters. The model is: E=0.2019PE+5x10-9(S+1)(W)

where E = distillate (mm), PE = pan evaporation (mm), S = total daily sunshine (s), and W= average windspeed (m/s). The correlation coefficient, R², obtained for this equation was 0.78, improved to 0.87 if the data used in analysis is summed over three day periods.

The average daily production between May and October predicted using 20 year mean weather data is 1L for a 1m² still. Using grass stills instead of soil stills improves the efficiency of production. Formal and informal discussions found solar stills were culturally appropriate, if plastic was used, allowing them to be portable and rugged. Therefore, solar distillation of soil moisture is a technically and culturally viable method for providing a safe supplemental drinking water supply for nomadic peoples in the study area.

The study reviews appropriate technology philosophy and evaluated solar earth-water distillation using this. From this viewpoint, it is suggested that drink bottles may be a more appropriate technology. This hypothesis needs to be tested. If drink bottles proved unsuitable in Hong Yuan, then the development of plastic solar stills is recommended as the next most appropriate technology available.