Intake
THE FUTURE SOURCE WATER: SEAWATER
What we choose as raw material in water industry is as important as in food industry. The basic cost, design, engineering, equipment procurement, and construction spending on intakes and outfalls, are estimated to total 5 to7 percent of capital costs for desalination plants (GWI, 2006a). The choice of source water is the key affects all the elements & all the costs of intake.
Figure 6.2.1 Percentage of total capacity of desalination source water in Spain and US.
Spain’s data (Left) comes from (Lei et al., 2015). US’s data (Right) comes from (GWI, 2006b). Original data comes from Global Water Intelligence.
There’re six kinds of source water as figure 6.2.1. In last chapter, we identify their different TDS range. The water has 500~15000mg/L is called brackish water and most of them are groundwater (GWI, 2006b). As Figure 6.2.1, every country‘s desalination industry has different major water source depending on their unique geological features. For example, most of Spain’s territory is close to the sea. Therefore, most of its source water is seawater. The desalination process usually focuses on either seawater or brackish water. In normal circumstance, what feed water the area use depends on which water is easier to get.
Yet, when we consider this question in global scale, the distance can no longer be the main standard. We have to analyze the difference of these two feed water in several other aspects such as supply capacity.
SUPPLY CAPACITY
There’s no doubt if the time frozen, seawater will have more water to satisfy global need. As Figure 6.2.2, seawater has 96.5% of water preserve, yet, brackish water only has 0.94% (Shiklomanov, 1993).
However, water is not changeless. What global water demand is truly chasing is the regeneration capacity of water resources using right now. Therefore, when we assess these two feed water, regeneration capacity is important as well. As Figure 6.2.3, scientists estimated seawater will have 4000 years of reserved time. Yet, groundwater’s reserved time is estimated to 2 weeks to 10000 years. (NEP, 2008) It’s because the regeneration capacity of groundwater is harder to access. It is a more changeful water resource comparing to seawater.
Example 1
Chinese Beijing, Tianjin and Hubei districts is Chinese capital and its surrounding area. This is an area suffering from water stress from long time ago. Beijing, Tianjin and Hubei district is a perfect example supporting the low regeneration capacity of groundwater when overusing it.
Over all, the seawater might have more advantages than brackish water due to its stability and its large water preserve.
Environmental concerns
As a rather new industry, desalination truly has several environment concerns remain unsolved. In this intake section, both seawater and brackish water has problems when it comes to expanded production.
Seawater extraction design has to avoid mariculture areas or extracting small wild creatures including shrimp and shells. The damage those creatures cause is called impingement and/or entrainment. This is rather easy to achieve.
However, brackish water has a bigger disadvantage. In large demand, brackish water extraction might cause ground settlement and relating problems. As Figure 6.2.4, land subsidence resulting from removal of groundwater has affected areas in 45 states in US and ranges from regional lowering to ground failure and collapse (Galloway et al., 1999; NRC, 1991).
