with past droughts in california and regions of india, and with the current crisis in cape town, city planners, policy-makers, and engineers must converge on a solution for water management that meets needs for fair distribution of water supply on a tight timeline. these droughts will only become more extreme and occur with greater frequency with exacerbated climate change scenarios. desalination of ocean water is one option for particularly drought-prone areas but need not be the only option. the most advanced and efficient desalination processes involving reverse osmosis uses pressure to force water through a membrane, ideally blocking salt from also passing through the membrane. the cost of constructing such desalination systems have fallen in recent years due to falling prices for the individual components of the plant, but the energy input required for the process still remains a serious obstacle to implementation. countries in the middle east region, rich in oil and natural gas reserves, have plentiful resources for energy generation but lack sufficient water supply. in that context, desalination makes sense, and so it accounts for about 40% of the total water supply in the gulf countries. in most contexts, however, desalination may not be economically feasible, unless the case is particularly dire.

while prices are falling for the individual components necessary to construct a plant, desalination processes still face a hard lower limit in terms of cost. no matter how durable and inexpensive membranes and pumps for desalination become or how much energy can be recovered with more efficient processes, there is a bare minimum energy input required by the laws of thermodynamics. no matter what system is employed, the energy input required to desalinate water in an ideal scenario, with maximum efficiency, is dictated by the energy required to break the natural attraction between salt and water molecules. this quantity when summed over the billions of water molecules in a cup of water is not trivial, and that energy input is the obstacle to implementation of desalination solutions for water crises around the globe. for this reason, understanding a scenario in a holistic sense is important when considering such large infrastructure projects like desalination plants and alternatives like water rationing.

sarah fletcher is a 2017-2018 rasikbhai l. meswani fellow for water solutions and was recently recognized for her outstanding research by the american geophysical union (agu). her work primarily focuses on using modeling to identify how to best allocate resources and systems for water supply, using projected climate scenarios that may have a high degree of uncertainty. she recently was featured in a panel at the mit abdul latif jameel world water and food security (mit j-wafs) lab with other graduate students presenting work related to water supply and management in the face of climate variability. 

sarah fletcher: one of the big challenges with desalination is cost. we are talking about these investments that are hundreds of millions and potentially billions of dollars. with droughts, and in general planning for water supply for the future, there are so many uncertainties: we have short term variability, run-off, rainfall, demand growth, and climate change. you face this risk, where if you build it, then you’re going to find out down the road that you don’t actually need it. so, i think that’s perhaps why desalination is seen as extreme, but it does on the other hand provide a very reliable water supply independent from rainfall. this water supply you can turn on and off in a way that is harder to achieve with other options.

gavin winter: i know in the mit j-wafs panel and in one of your recent papers you mentioned a modular approach to building water infrastructure as a method to reduce the risk of overbuilding. in practice, what does this modular approach look like?

a: traditionally, you build large-scale desalination plants, and the advantage of that is that there are economies of scale. you build it big, and it’s cheaper per unit of water output, but you face a risk of overbuilding. on the other hand, you could take a modular approach, where you build a smaller plant up front. then, you can add either modularly within that plant or you could just build a series of small desalination plants. and if you build a series of small desalination plants you could potentially select several different strategic locations. this way, you can have flexibility in the amount of capacity you are bringing online, and you can have flexibility in the location of that capacity you are bringing online.

q: could a flexibility-in-design approach also encompass diversification of water sources?

a: certainly, having a diverse portfolio of options has an advantage. rather, than deciding to invest upfront in wastewater reuse or whichever technology, there is a distinct advantage in making sure that you have all these different options lined up and ready. then, you can pursue each of them as one deems fit down the road.

q: in the panel, you touched on a robust approach to providing water supply, which involves building large scale infrastructure projects that generate a lot of public hype. are there any contexts in which you can justify a robust approach over the more flexible approach that you just mentioned?

a: absolutely. i think one of the challenges with the flexible approach is that it’s prone to reliability outages. let’s say you took a smart flexible approach, you built a smaller amount of water supply capacity, and then the drought gets much more severe and you were not able to get the rest of the capacity online before you need it. water supply systems are impacted by natural variability and stochasticity in the environment, and that’s more difficult to manage. so, in cases dominated by natural variability, a robust approach might be better-suited. of course, it also comes down to a matter of preferences and how a society values each method. as a case study, a desalination plant in london was designed with the intention that it would be large and infrequently used. there was a full public awareness campaign before it was built. it is really just to protect against drought, and it is there if they need it. maybe they only use it once every five years, but they’re still happy to have it there.

q: do you see the possibility that external circumstances drive the prices up for water from more traditional sources like rainfall reservoirs, which in turn would make desalination more economical?

a: i could see it happening. however, it would have to be in a very specific set of circumstances. it would be much more expensive to do it cities that are not coastal cities. that is one major constraint. you can sort of think of desalination as putting an upper bound on the cost of water supply. so, if there are other cheaper options, like pipelines and reservoirs or wastewater treatment, then you often would expect them to be developed first. but you can imagine in some places not having access to resources like those, especially with the onset of climate change, the question comes down to whether you are willing to pay the premium in order to meet the water needs.

q: often, water is too cheap to justify a large capital expense for higher technology method ways to get water, at least until a crisis is imminent? i read that this is particularly the case in china, where there is not much incentive to invest until a crisis is happening and action is reactive rather than proactive. are you able to offer some insight on how to change this mindset with more informed policy?

a: that is a very challenging question to answer. you can certainly make an argument that it is easier to get projects done in the wake of some extreme events. then, perhaps there is some value to taking advantage of these big droughts to build desalination capacity. even if you aren’t expecting it, then you will at least have the will to continue to build out capacity. in terms of changing the mindset, that’s an enormous challenge: i think that public awareness campaigns and education can go a long way. climate change is an issue that more and more people are relating to, and they are understanding that these types of challenges are going to be encountered more and more often. i think that’s one strategy, but it is definitely going to be a long haul.

q: in your paper, you mentioned framing these types of investments more as drought insurance. do you think that would be an effective way at least in drought-prone areas to increase the incentive to get funding in off-periods for the projects?

a: yes. that is exactly what happened in the case of london’s desalination plant. i think that is a good example of a governmental organization having convinced the public successfully. the challenge is, of course, that it is much more feasible and practical to take this approach in wealthy countries like the u.s. or those in europe. however, not everyone has those resources, in which case it is a much more challenging question.