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| Fig. 1: Cities along U.S.-Mexico border. (Courtesy of the EPA. Source: Wikimedia Commons) |
Access to clean water is a pressing issue for thousands of residents living in the rural colonias of Texas. Colonias are impoverished, unplanned residential communities along the U.S.- Mexico border (Fig. 1) that lack access to basic infrastructure, including running water, sewage systems, and solid waste disposal. [1] More than 384,700 people live in Texas colonias, constituting one of the largest groups of individuals in the United States living without basic sanitation. [1] For many, brackish groundwater is the only available source of drinking water. Many of these colonias, such as those located near El Paso, Texas, receive sunlight for the majority of the year, making solar distillation a feasible option to provide communities with access to clean water.
Solar distillation is a simple process that removes water impurities through evaporation and condensation. Solar stills contain a basin to hold impure water, an angled glass or plastic cover, a condensation collection trough, and a collection receptacle for pure water. Designs include the hemispherical solar still, pyramid solar still, double and triple basin solar stills, and tubular solar still, among others. [2] Solar distillation harnesses solar energy and the greenhouse effect to accelerate the process of evaporation; incident short-wave solar radiation passes through the still cover and becomes trapped, heating the water in the basin to the point of evaporation and leaving behind particulate contaminants. [3] The evaporated water condenses as it makes contact with the cool underside of the cover, which is angled to allow the condensed water to trickle into a collection receptacle without dripping back into the basin. [2] The resulting clean water is free of minerals and microorganisms, making it safe to consume.
Conventional methods of water desalination use technologies categorized into two groups: membrane distillation, which includes reverse osmosis and electrodialysis, and thermal (phase-change) distillation, which includes multi-stage flash, multi-effect distillation, and vapor compression. [4] These desalination processes all require high amounts of energy to separate clean water and particulate impurities. Both membrane and thermal distillation methods share a common minimum energy required to complete this separation process; the minimum work needed is equal to the difference between free energy of the incoming feed (seawater) and the energy of outgoing streams (pure water and discharge brine). [4] Using the van't Hoff formula for normal seawater of salinity equal to 33,000 ppm at 25°C, the theoretical minimum work needed for the desalination process has been calculated as 0.77 kWh/m3. [4] The actual energy used is often much greater than this, with desalination plants using 5 to 26 times the work of the theoretical minimum. [4]
Assuming that 384,700 people live in colonias in Texas, with each person consuming the minimum 5 liters of clean water per day needed to survive, the use of a conventional desalination process would require an approximate minimum work of 1,481,095 kWh/m3 each day. [5] This does not include the energy that would be needed to maintain the desalination plant itself. It is clear that desalination plants requiring this quantity of energy input are not feasible in Texas colonias, which lack access to basic infrastructure, electricity, and running water.
In contrast to the energy-intensive technologies of conventional desalination methods, solar stills use passive distillation and require no source of electricity to function. [4] The passive nature of solar stills makes them quite inexpensive to build and operate; they have a low capital cost and do not require the use of fossil fuels to function. [4] These qualities make solar distillation a viable, feasible alternative to sophisticated methods of water desalination for use on a community scale in impoverished colonias that lack infrastructure and economic resources.
Solar distillation is a method of desalination that can be implemented with relative ease in colonias along the Texas-Mexico border. In colonias that receive sunlight throughout most of the year, solar stills provide a reliable, energy-efficient process of purifying brackish water. Small solar stills could be implemented for families, while larger solar stills could be installed for neighborhood use. Access to clean water would decrease the spread of waterborne illness and disease, effectively raising the quality of life for thousands of people living in colonias.
© Maya Navar. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. Ura, "Trying to Help Thousands in Colonias Obtain Health Coverage," New York Times, 22 Feb 14.
[2] T. Arunkumar et al., "Experimental Study on Various Solar Still Designs," ISRN Renewable Energy 2012, 569381 (2012).
[3] M. N. Bahadori, "Solar Desalination for Domestic Applications," in Water Conservation, Reuse, and Recycling: Proceedings of an Iranian-American Workshop (National Academies Press, 2005), p. 67.
[4] A. Al-Karaghouli and L. L. Kazmerski, "Energy Consumption and Water Production Cost of Conventional and Renewable-Energy-Powered Desalination Processes," Renew. Sust. Energ. Rev. 24, 343 (2013).
[5] P. H. Gleick, "Water In Crisis: Paths To Sustainable Water Use," Ecol. Appl. 8, 571 (1998).
