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Potable water treatment
Potable water treatment

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4.14.1 Multistage flash distillation

In this process (Figure 34) saline water (screened first, if it is sea water) is distilled under reduced pressure in a series of sealed tanks. Due to the reduced pressure, the water evaporates suddenly or 'flashes' at a temperature lower than 100°C, typically 80°C. Pure water condenses on cooling coils in the tanks and is collected. As the temperature of the feed water falls in each succeeding tank (as the latent heat of evaporation is extracted from it) a correspondingly lower pressure has to be maintained for flashing off to occur.

Figure 34
Figure 34 The multistage flash-distillation process. Brine at (A) passes under pressure in the condenser coils of the flash chambers to heat exchanger (B), and as it flows in the reverse direction, water vapour flashes off and is condensed on the cooler brine-filled coils above. The condensate forms part of the freshwater outflow at (C). The brine, now at 60°C, passes into flash chambers D and E, which contain condenser coils fed with raw sea water. This is recycled into the concentrated brine of the last flash chamber, and the resultant liquid is partly run off as waste and partly recycled to A.

Multistage flash (MSF) units are often located alongside power generation plants in order to utilise the waste heat generated in them. MSF plants can suffer from scale deposition and corrosion. Scale deposits of, for example, CaCO3 and Mg(OH)2 can interfere with the transfer of heat between different parts of the process, and can increase the resistance to fluid flow due to an increase of surface friction. Scale deposition is usually prevented by the addition of scale inhibitors to the feed. These modify the crystal structure of the scale and prevent it building upon surfaces.

Sulphate-reducing bacteria, often present in sea water, can contribute to corrosion. Under anaerobic conditions these bacteria reduce sulphate ions to hydrogen sulphide, which in turn dissolves away iron, forming iron sulphide. This results in 'pitting' corrosion. Other bacterial species can oxidize the H2S to sulphuric acid, which is very corrosive. Control of all forms of bacterially induced corrosion consists essentially of either eliminating conditions suitable for their growth, or, if this is not practicable, using biocides to prevent them colonising the parts at risk.