It is not for nothing that Earth is called a blue planet. Not only is 71 % of the surface covered by seas, the water they hold is so unfathomably large (in relation to what circulates in the hydrological cycle) so as to be considered infinite. See here.
Removing salt from water is rather easy - boil it and then condense the vapours in another vessel, - and in the bargain get salt, as well. The issue has always been of the cost – primarily energy.
Developments in the past decades have dramatically reduced these costs – present estimates range from 50 to 80 cents (Rs. 20-32) per cubic metre (1,000 litres) of water. (See 1, 2 and 3), though in India a figure of Rs. 50 is also quoted. Costs are most sensitive to the level of salt in water (the lower the salt content the cheaper it is to desalt - so treating brackish water is cheaper) and energy costs. Note that the above costs are that of desalinisation and don’t include those of distribution. Transporting water over long distances (which may also entail lifting it) can increase considerably the final delivered cost of water.
There are two major technologies for desalting water – reverse osmosis (RO) and multi-stage flash (MSF); the former is increasingly more popular. RO is modular in nature and capacity ranges are wide. Another point to the noted is that while the cost of desalination is going down that of conventional water is going up as fresh supplies come from deeper aquifers or water is transported over longer distances. (See above citations for details including on costs, technologies and other matters. See also the references in the sources cited above.)
Given that so much water is available, costs are falling, traditional sources are turning dearer and more difficult to tap, and that there are severe shortages in several coastal location (Chennai is the example that springs to mind) is there a case for desalting water on a large scale?
Apart from Chennai, several other cities on the lower east coast and in Gujarat would seem to hold immediate potential. At present Chennai is building several desalination plants for both domestic use and for industries. See 1, 2 and 3.
Another interesting possibility is that if coastal areas can develop their own independent water supplies it may relieve pressures on upstream water resources that currently supply water to these locations. In other words, water which is now required for downstream users can be saved and used upstream. Hence, desalted water may have a role in helping upstream water users too!!
Desalted water is undoubtedly more expensive, say 2-5 times (numbers are illustrative) the cost of conventional water and it is feared that overall costs of water would shoot up considerably if it were to be adopted on a wide scale. The purpose of this post is not to discuss the costs in any detail but I would suggest a perusal of the links cited above. Instead I wish to make some more general observations and clarify certain matters in this regard.
To begin with it is not as if all water supplies would be met from desalted water. Only incremental supplies will be. So if say, 10 % of water is to be met from desalination and it costs 5 times as much as conventional water overall total costs go up only 50 % and don’t become 5 times. (A scenario analysis using various assumptions on the cost of desalting water and its contribution to overall water supplies is encouraged)
However, comparing the marginal cost of desalted water with the average of conventional is not the right way to go about it. Marginal costs ought to be compared with the marginal cost of conventional water supplied. Since the latter is likely to be closer to Rs. 50 and not Rs.10 or so the difference between the two sources narrows down considerably. (The average cost of the RO is nearly the same as its marginal cost).
In short, desalination may be cheaper, relatively, than what appears at first glance.
It is also argued that the energy costs of RO are considerable but that is already included in the higher costs of desalting water and highlighting them separately is wrong and if done unthinkingly may end up in double counting.
Note, that as a practical matter residents of Chennai and industries around the city already pay Rs. 50 or more per cubic metre, the very high end of the cost of desalted water. And if one adds the opportunity cost of time, disease, additional investments in pumps and storage, desalination is not more expensive, probably a cheaper alternative. The extra burden, assuming all costs are to be recovered from users would hardly burn a hole in the pockets of the residents. Compare the monthly expenditure on water with items such as telephony or entertainment. (Tamil Nadu seems rich enough for its government to give free colour TV sets to the needy!!)
The other major concerns with desalination are its environmental impact.
One of them is the loss of marine life during the intake as organisms get sucked in and die. This is rather a minor problem to solve and is preventable by the suitable placement of intake pipes, meshes and beach-wells.
The major worry has been the effluents generated during purification. Note, however, that the common notion that hot water generated during the process can damage the marine ecosystem is not true. Hot water is not generated during RO but brine (highly concentrated salt solution) definitely is.
It has been argued that brine can be discharged at appropriate places and diluted with water to lessen its impact. Another suggestion is to solidify the wastes and dispose them in say, abandoned mines or such places.
But the best possible solution would be to sell it. After all salt is a major input for many chemical industries and maybe it can even be made good enough (after treatment) for human consumption. I am reminded about flyash (generated from thermal power plants) and which was such a problem many years ago. Now cement companies clamour to gain access to it to make blended cement. They are willing to pay for it.
It is also curious that papers such as that of the WWF cited above make no discussion of environmental costs of existing water supplies. After all costs are relative. Groundwater depletion, energy use by borewells, tankers plying all over the city, water transported over long distances, are all environmental costs associated with conventional supplies.
Overall it seems that desalination of water has a promising role to play. It ought to begin small but if economic and environmental costs are reasonable it can be expanded to more locations, and water conveyed inland.
Finally, desalination is not a substitute for demand-side measures. It is sometimes argued that we should rely on the latter rather than the former to solve our problems. Of curse, we should. But where supplies need to be augmented, desalting would be as good a bet as withdrawing water from the ground or bringing it from distant places. Nor is desalination likely to be relevant for the whole of India. It is also not a panacea for the myriad ills of India’s water system but it could play a considerable role in supplying clean water to select locations at low rates with minimal damage to the ecosystem.
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