Design, construction and use of solar stills for water purification: A study of their performance in two geographical regions in Swaziland

Abstract

Clean water for drinking and other domestic purposes is becoming a scarce resource the world over. The presence of pathogens such as Escherichia coli [E. coli] in contaminated water often leads to health complications and at times death, particularly when they originate from human or animal waste, and may give rise to gastro-intestinal diseases such as diarrhoea and cholera. Simple and affordable water purification devices are therefore essential in rural and peri-urban areas without municipal water supply. Two locally built solar stills, sometimes called solar distillers, were designed and constructed at the workshop of the Physics Department of the Faculty of Science and Engineering, University of Swaziland. They were installed at two locations in Swaziland with different climatic conditions, one at Dvokolwako in the Lowveld and the other, at Etimpisini in the Middleveld. Data on monthly variation of the amount of distilled water collected at each of the two locations were recorded over a period of one year. The effect of the ambient temperature on the water collection efficiency was also evaluated. The amount of distilled water collected at each of the two locations was found to increase with ambient temperature. The collection efficiency was found to be higher in the Middleveld in spring and summer [September to February], whereas it was lowest in winter [June and July]. The percentage increase in the amount of water collected in the Middleveld between July and September was approximately 75%. In the Lowveld, however, the variation in the collection efficiency was only about 19%. On average, a still can produce about 2.75 litres of water per day in the Middleveld, compared to 2.6 litres in the Lowveld, using a still with a base area of 1 m2. The raw and distilled water were tested at the Swaziland Water Services Corporation [SWSC], Mbabane for the presence of E. coli and total coliform, as well as the levels of pH and total hardness. Tests on cations and anions were carried out by the Department of Geological Survey and Mines, Mbabane. Results of the water quality tests showed that the solar stills had the capacity to eliminate E. coli and total coliform completely from the raw water and reduce the levels of pH, total hardness, anions and cations significantly. The anions were reduced by 22 % to 100 % and there was a 53% to 100% reduction in cations. All the parameters were found to be within the South African [SA] and SWSC standards for water quality. Improvements for future work have also been highlighted.