Desalination: Is it better to remove the water or the salt?
This blog is based upon "The cost effectiveness of electrodialysis for diverse salinity applications" by Ronan K. McGovern, Syed M. Zubair and John H. Lienhard, published in Desalination.
For researchers of new desalination technologies, the question of whether and why salt removal makes economic sense is a very important and relevant one. At present, the dominant desalination technologies (reverse osmosis and multi-stage-flash) are based upon water removal, suggesting that water removal technologies enjoy some fundamental economic advantage. In the last 4 years a number of new micro-scale salt removal systems have been developed in research laboratories, including salt removal with wires, ion concentration polarisation and deionisation shocks. There have been suggestions that these technologies are best suited to treating brackish (lower than seawater salinity) waters, seawater or even highly saline brines, such as those that can flow back from the process of hydraulically fracturing shales for oil or gas. The reality is that there has been no fundamental analysis done to understand for which salinity these technologies are best suited. Such an analysis is therefore what my co-authors and I have chosen to do.
The key finding of our study is that salt removal technologies will struggle to be competitive in desalinating seawater or streams of higher salinity. To move a quantity of salt from a solution of low salinity to a solution of higher salinity is much like moving a ball up a hill (see Figure 1 below) - there is a theoretical minimum amount of energy required. When moving a ball up a hill, the minimum energy required depends on the change in height. When moving salt from one solution to another, the minimum energy depends upon the change in salinity - the greater the salinity difference, the greater the theoretical energy required.
Figure 1, Just as the energy required to move a ball up a hill depends on the difference in height the energy required to move salt ions to a more saline solution depends upon the difference in salinity.
In reality the actual energy required will be greater than the theoretical minimum - when moving the ball up the hill you'll encounter air resistance, while when moving salt (ions) you'll encounter electrical resistance. Another way of saying this is that the efficiency - the theoretical minimum divided by the actual energy - will always be less than 100%. Figure 2 illustrates the energy required to move salt from a feedwater solution into a saturated salt solution (a solution in which no further salt can be dissolved). As the feedwater salinity approaches saturation the theoretical energy required approaches zero - just as the theoretical energy to move a ball up a hill approaches zero as the change in height approaches zero.
Figure 2, The theoretical minimum and the actual energy required to move salt from a feedwater solution into a saturated salt solution.
Figure 2 also shows that the actual energy, unlike the theoretical minimum, is relatively independent of the feedwater salinity. In reality, the story is more complex, but by and large, the actual energy required (especially at high feedwater salinity) depends mostly upon the resistance of the membranes that allow salt to leave the feedwater stream while retaining the water. Now, if we consider, based on Figure 2, the efficiency of salt removal systems, which is the ratio of the theoretical minimum to the actual, we can see why the efficiency and thus the economics of salt removal at high salinity is so poor.
The most interesting aspect of this analysis is that it tells us there is a very fundamental reason why salt removal makes little sense at moderate and high salinities. Indeed, I believe this is largely why we have seen the dominance of water removal technologies for seawater desalination. As to why we haven't seen salt removal technologies dominate at low salinities? It is a good question and one that we discuss in detail in our paper. Ultimately I believe that the challenges faced by salt removal technologies at low salinity are issues with the technology and not fundamental limitations. I therefore hope that this analysis will encourage those developing new salt removal technologies in the lab to direct their focus on treating low salinity waters.
The author's copy of the manuscript may be downloaded here.