Reverse Osmosis and Other Treatment Technologies - Global Drinking Water Management and Conservation - page 58

4.3.2 Distillation

The second method of desalination is distillation. In the distillation process, raw
water is heated and then evaporated to separate dissolved minerals and to kill
harmful bacteria. The steam is then condensed and collected. The four most
common methods of distillation to produce freshwater for commercial or semi-
commercial applications are multistage flash, multiple effect distillation, vapor

compression, and solar distillation.
In multistage flash, represented in Fig. 4.6 , the raw water is heated and the

surrounding pressure is lowered. Since the pressure is lower, the boiling point of
water is reduced and the water into steam. This process continues over

and over again at lower and lower pressures until the waste content becomes too
high. In general, the electricity requirements of distillation are considerably higher
than the RO process. This higher energy input is indicative of the inef

“ ashes ”

cient con-

version of electricity to heat.
In multiple effect distillation, represented in Fig. 4.7 , the inef cient exchange of

energy to heat is reduced by reusing the heated wastewater. As raw water enters an
evaporator it is heated and turned into steam. The wastewater is then used to assist
in the heating of the next evaporator. This process continues until the wastewater
cools and can no longer provide suf

cient heat.
Vapor compression is similar to multiple effect distillation except the inef cient

exchange of energy to heat is reduced even further. In vapor compression, repre-
sented in Fig. 4.8 , the raw water is heated and turned into steam. This steam is then
compressed and used to assist in heating the next evaporator. This process con-
tinues until the steam is condensed back into water.
Solar distillation offers the lowest operating cost of the distillation methods since
the heat energy is provided with little cost by the sun. In solar distillation, represented

Fig. 4.6 Flow diagram of a multistage flash system (Water Industry Portal 2014 )