Environmental Engineering Reference
In-Depth Information
Fig. 4.9 Flow diagram of a
solar still (Freeze Desalination
2014
)
air mass circulation, the production capacity has only reached 20 l of water per
square meter per day of collector surface (Kunze
2001
).
Distillation is an energy intensive process compared to RO. In Table
4.3
we see
that distillation energy requirements consist of electricity to operate the plant and
heat to boil the raw water. When this heat energy is generated by electricity, the
electrical power requirements increase 5
10 times their original value. Due to this
-
heavy dependency on heat energy, 50
75 percent of distillation unit costs are made
up of heat energy costs (Winter et al.
2002
).
-
4.3.3 Electrodialysis
The third method of desalination is called electrodialysis. This involves pumping of
brackish water at low pressure between several hundred
at, parallel, ion-permeable
membranes that are assembled in a stack. Electrodialysis is used to transport salt
ions from one solution through ion-exchange membranes to another solution under
Table 4.3 Energy
requirements for reverse
osmosis and distillation
(California Coastal
Commission
1993
)
Desalination technology
Electricity requirements
5.60 kWh/m
3
plus
heat energy
Multistage
ash
2.80
-
4.00 kWh/m
3
plus
heat energy
Multiple effect distillation
2.00
-
12.00 kWh/m
3
plus
heat energy
Vapor compression
8.00
-
4.64
-
8.80 kWh/m
3
Reverse osmosis
—
single pass
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