Membrane and Desalination Technologies - TCE Moodle Website

Desalination of Seawater by Thermal Distillation and Electrodialysis Technologies 527
Distribution of desalination capacity
80%
60%
40%
20%
0%
63%
Middle East
11%
North America
North Africa
7% 7%
Europe
utilizing brackish water. Around 800 desalination plants are operated in the USA, producing
freshwater with a capacity of 225 MGD (about 1.4% of total water consumption). Most of
them are in Florida, California, and Texas.
The general process of producing useable water from the water with high salt content is
illustrated in Fig. 12.2. The desalination technologies are normally developed for treating
large quantities of water (e.g. 100–1,000 MGD) at a central location. However, some have
been recently adopted for small-scale water supply at home.
Modern desalination practices are broadly categorized into distillation or membrane processes.
The distillation process involves phase changes and usually utilizes thermal energy and
mechanical energy. It includes the multistage flash (MSF) distillation, the multieffect distillation
(MED or ME), and vapor compression (VC). The membrane process involves no phase
changes and the separation is mainly carried out using semipermeable membranes. This
requires mechanical or electric energy. The commonly used techniques are RO and ED (EDR).
About 70% of the world’s desalination capacity is dependent on the distilling process.
However, in USA, the distribution is different; RO and ED (EDR) occupy around 80% of
the market. The freshwater production capacity normally follows the descending sequence
of MSF > RO > ED (EDR) > MED > VC. Table 12.1 gives the key factors in the different
desalination operations. Other processes such as freezing and membrane distillation are
seldom used. Of the aforementioned five main desalination technologies, MSF, MED, and
VC thermal processes are generally used in the following applications:
l To treat highly saline waters (predominantly seawater).
l Where large volumes of product water are required.
l In locations where energy costs are low or where a waste heat source is available.
4%
Pacific
Location
Caribbean
2% 2%
(former)USSR
4%
Others
Fig. 12.1. Distribution of desalination capacity in the world.