A very simple and inexpensive solar disinfection process consists of simply filling clear plastic bottles with source water. The bottles are then placed on a sunlight-reflecting surface such as aluminum or corrugated iron sheets, possibly on a roof. These bottles are exposed to direct sunlight for anywhere from one hour to two days, depending on the conditions. The Sun’s rays work to kill microorganisms in water by irradiation with ultraviolet (UV)-A rays and also by raising water temperature to 50 degrees Celsius or higher.
Solar disinfection, does not, of course, treat chemical water quality problems such as arsenic, heavy metals, pesticides, etc. It also requires relatively clear water, because suspended materials and the natural color in water block the UV rays. Bottles larger than one or two liters should not be used, which limits the quantity of water that can be treated. And it is not recommended for days of continuous rainfall. However, it has potential to greatly reduce incidences of diarrhea and dysentery, and to minimize the terrible public health toll that they exact worldwide.
Solar stills harness the Sun’s energy to cleanse water of contaminants including salts, heavy metals, and microbes. These systems have been employed for hundreds of years and have evolved for many different uses—even the desalination of seawater.
The distillation process, in some ways, mimics the Earth’s natural water cycle. Unpurified water is stored in a container and subjected to the Sun’s warming rays. The Sun’s heat produces vapor, which rises from the source water surface. This evaporation produces a purified vapor and leaves contaminants behind in the liquid source. The clean vapor is then captured within the still, so that it can be condensed as purified water.
The remaining unpurified water, including contaminants, can be periodically flushed from the still and discarded.
Compact and even portable solar units are popular at the household level. They have few moving parts and low operational and maintenance requirements.
Solar stills may be a good treatment option for developing nations with an abundance of sunny days because they are inexpensive and require almost no investment or infrastructure.
Such systems are limited by the power of available sunlight, however, and are far more effective in warm, sunny climes.
Finally, it should be noted that distilled water contains almost no dissolved minerals, and that this can be harmful if distilled water is the only source of drinking water and one’s diet is lacking an alternate source of essential minerals.