Membrane and Desalination Technologies - TCE Moodle Website

Desalination of Seawater by Thermal Distillation and Electrodialysis Technologies 529
Table 12.1
Comparison of major desalination technologies
Item MSF MED ED/EDR RO
Preferred water
source
Seawater – brine Seawater – brine Brackish Any
Final product 5 mg/L < TDS 5 mg/L < TDS TDS < 500 TDS < 500
salinity
< 50 mg/L
< 50 mg/L
mg/L
mg/L
Land usage Large area Large area Small area Small area
Capital costs High High Medium Medium
Operating
temperature
High ( 100 C) Moderate ( 70 C) Not applicable Not applicable
Energy cost High High Moderate Moderate
Feed-to-product
ratio
7.0–12.0:1 6.0–10.0:1 1.2–2.0:1 2.0–2.5:1
Pre-treatment Not required Not required Required Required
Influent
requirement
Not required Not required SDI < 12 SDI < 4
Quality of water Pure Pure TDS of 350–500 TDS of 20–50
produced
mg/L
mg/L
Susceptibility to
scaling
Low Low Low High
Bacterial Unlikely Unlikely Post-treatment Possible
contamination
needed
Waste High salt content and High salt content and High salt Low salt
corrosive
corrosive
content content
adjustment. Selection of these technologies is dependent upon the water quality of the influent
and the desalination process that is to be applied.
The cost of desalting saline water ranges from USD 0.4–2.2/m 3 of product water, which
depends on the quality of the raw water and the technologies applied. Table 12.2 shows a
comparison of capital and operational costs of desalination processes. The membrane technologies
are cost effective for low-salinity sources (brackish water), whereas distillationbased
technologies make sense only for higher salinity courses, such as seawater. The RO
process can be regarded as the optimum choice for desalination of low-salinity water,
regardless of plant capacity. The optimum choice for desalination of higher salinity feed
depends on the required capacity. The MSF process would be the technology of choice for
capacities more than 25,000 m 3 /d, whereas the MED process should be chosen for capacities
in the order of 10,000 m 3 /d. VC is good for capacities in the order of 3,000 m 3 /d (6). When a
desalination plant with a larger capacity is built, the capital cost per unit of product water is
lower. The daily operational cost, however, is not significantly affected by the size of the
plant. It is advisable to build a larger plant instead of several smaller plants. Cost analysis
must be conducted even before design is initiated. Computer models such as ARIMA model
can be used (7).