5.14.4 Solar distillation
The energy available in solar radiation can be harnessed to distil sea water. In a simple and inexpensive solar still system designed by the Technical University of Athens, for the island of Patmos, sea water is first pumped to a feed reservoir from which it flows by gravity, when required, into a large shallow basin divided into long narrow sections (Figure 36). Separating these channels are concrete strips, which provide access for maintenance. The interior surface of the entire basin is lined with butyl rubber sheet. Above each water-filled section is a double sloping glass roof supported by a light aluminium structure. Heat from the sun passes through the glass, causing evaporation from the sea water surface. The vapour condenses on the inside of the glass and runs down to channels at the edges of the sealed unit along which it travels to the freshwater storage reservoir. The salt concentration in the basin sections grows steadily stronger, and once every two days the resulting brine is run off to the sea, being then replaced by more sea water. Experience has shown that the 48-hour cycle avoids the formation of scale. As the sun does not shine every day, the designers have incorporated a second water channel in the concrete strips. These are fed from the upper surface of the glass panels, and from the concrete itself, when it rains.
The output of distilled water from the Patmos solar still averages three litres per square metre of water surface per day. The only running costs of the system are those for pumping sea water to the feed reservoir, and for general maintenance, which includes cleaning the glass panels.
If you have tried to drink distilled water you will know that water without any salts is insipid. It is also corrosive, due to the deficiency of ions which would be present if the water were in its natural state. Lime (Ca(OH)2), phosphates and bicarbonates are added to raise the alkalinity and make the water less corrosive. These chemicals also raise the level of total dissolved solids (TDS) to about 300 mg l−1, to give the water taste. If unpolluted brackish groundwater is available, this can be used instead to raise the TDS level. Often a combination of the two – addition of chemical salts and blending with underground water – is economic. Such a procedure is used in the water supply of Muscat in the Sultanate of Oman. After adjustment of TDS content, the pH is corrected if necessary. Finally, the water is disinfected and passed into transmission mains to feed service reservoirs (see Section 4).