Water security is becoming an urgent global challenge. Hundreds of millions of people already live in water-scarce regions, and the UN projects that by 2030 about half the world’s population will be living in highly water-stressed areas.
This will be a crisis even for developed countries like the U.S., where water managers in 40 states expect freshwater shortages within the next 10 years.
As the global population and GDP grow, so will the demand for freshwater. And, with the continuing rise of global temperatures, water shortages will only get worse.
Desalination processes are increasingly being relied upon to augment water supplies. In fact, global desalination capacity is projected to double between 2016 and 2030. But these processes are expensive and can be harmful to the environment.
The ultrahigh salinity brines that are the byproduct of desalination can be several times that of seawater salinity and its management options are especially challenging for inland desalination facilities such as those in Arizona, California, Florida, and Texas.
Over the past year, Columbia Engineering researchers have been refining their unconventional desalination approach for hypersaline brines – temperature swing solvent extraction (TSSE) – that shows great promise for widespread use.
TSSE is radically different from conventional methods because it is a solvent-extraction-based technique that does not use membranes and is not based on evaporative phase-change: it is effective, efficient, scalable, and sustainably powered.
In a new paper, published online June 23 in Environmental Science & Technology, the team reports that their method has enabled them to attain energy-efficient zero-liquid discharge (ZLD) of ultrahigh salinity brines – the first demonstration of TSSE for ZLD desalination of hypersaline brines.
“Zero-liquid discharge is the last frontier of desalination,” says Ngai Yin Yip, an assistant professor of earth and environmental engineering who led the study. “Evaporating and condensing the water is the current practice for ZLD but it’s very energy intensive and prohibitively costly.
We were able to achieve ZLD without boiling the water off – this is a major advance for desalinating the ultrahigh salinity brines that demonstrates how our TSSE technique can be a transformative technology for the global water industry.”
Yip’s TSSE process begins with mixing a low-polarity solvent with the high salinity brine. At low temperatures (the team used 5 °C), the TSSE solvent extracts water from the brine but not salts (which are present in the brine as ions).
By controlling the ratio of solvent to brine, the team can extract all the water from the brine into the solvent to induce the precipitation of salts—after all the water is “sucked” into the solvent, the salts form solid crystals and fall to the bottom, which can then be easily sieved out.
After the researchers separate out the precipitated salts, they warm up the water-laden solvent to a moderate temperature of around 70 °C. At this higher temperature, the solvent’s solubility for water decreases and water is squeezed out from the solvent, like a sponge.
The separated water forms a layer below the solvent and has much less salt than the initial brine. It can be readily siphoned off and the regenerated solvent can then be reused for the next TSSE cycle.
“We were not expecting TSSE to work as well as it did,” Yip says. “In fact, when we were discussing its potential for ZLD, we thought just the opposite, that the process would likely give out at some point when there is just too much salt for it to keep working. So it was a happy surprise when I convinced lead researcher Chanhee Boo to give it a try, for the heck of it, on a Friday afternoon and we got such great results.”
With a simulated (lab-prepared) brine feed of 292,500 part-per-million total dissolved solids, Yip’s group was able to precipitate more than 90% of the salt in the original solution.
In addition, the researchers estimated that the process used only about a quarter of the energy required for evaporation of water – a 75% energy savings compared to thermally evaporating the brine.
They reused the solvent for several cycles with no noticeable loss in performance, demonstrating that the solvent was conserved and not expended during the process.
Then, to demonstrate the practical applicability of the technology, the team took a field sample of high-salinity brine, the concentrate of irrigation drainage water in California’s Central Valley, where irrigation drainage water is difficult and costly to treat, and achieved ZLD with TSSE.
Conventional distillation methods require high-grade steam and are frequently supplemented with electricity to power vacuum pumps. Because TSSE requires only moderate temperature inputs, the low-grade thermal energy necessary can come from more sustainable sources, such as industrial waste heat, shallow-well geothermal, and low-concentration solar collectors.
“With the right solvent and right temperature conditions, we can provide cost-effective and environmentally sustainable concentrate management options for inland desalination facilities, utilizing brackish groundwater to alleviate the current and pending water stresses,” Yip notes.
In addition to managing inland desalination concentrates, TSSE can also be used for other high salinity brines including flowback and produced water from oil and gas extraction, waste streams from steam-driven electric power stations, discharges from coal-to-chemical facilities, and landfill leachate.
Yip’s group is continuing to investigate the fundamental working mechanisms of TSSE, to engineer further improvements in its performance. This work includes further testing with real samples from the field, as well as optimization of the overall process.
The study is titled “Zero Liquid Discharge of Ultrahigh Salinity Brines with Temperature Swing Solvent Extraction.”
Considering the global water situation towards the next half century, Rockström et al. (2014) stress that ‘two giants collide – water demand from nine billion people and water impacts from a rapidly growing economy’.
Ability to successfully approach this challenge will demand a broader water paradigm, connecting as essential components (1) green water in the soil with blue water in rivers and aquifers, and (2) land and water integration.
A key strategy will be to build resilience and sustain rainfall, by inter alia sustaining forested areas as critical zones for moisture feedback. Stewardship of the landscape has to be seen as the core strategy. Water is not only the bloodstream of the biosphere, but also the key to resilience in social-ecological systems.
In this situation, a core question for the next generation to raise is how further future stresses, caused by ongoing climate change, continued population growth, increasing human demands and intensified land use change, will continue to disturb the life support system and how the interactions between water flows, water functions and roles, and relevant ecological processes like food production, timber production and biodiversity will be reflected in the life support system.
Critical questions worth focusing on, will be how to keep the planet in the present favourable Holocene-like conditions with a relatively stable climate, and how to achieve this by safe navigation of the life support system (Figure 1).
What transformations will be needed?
What shifts in governance approaches in terms of scope, scale, and speed will be required?
How can that be done?
And how closely will the UN’s Sustainable Development Agenda reach this overarching goal?
The aim of this article is to identify some of the most essential components of the answers sought for the core questions raised in Figure 1. We will consider where the current understanding of the planet’s biological playing field, of its many green and blue water functions and its different ecological processes, will lead. We will at the same time try to identify essential near-future constraints in both the green and the blue water realms. The time horizon will be the next half century.
Water: core of the life support system
Earth is the only planet in the solar system where liquid water makes life possible. The other planets are either too hot (like Venus) or too cold (like Mars). And while the energy transformations from ice to liquid and from liquid to vapour are essential for spreading solar heat across the surface of the Earth, one of the three core green water functions is to keep the Earth’s temperature 30 °C warmer than it would be without H2O as a key compound characterizing this planet.
Water determines all processes on Earth of importance for human well-being, but is at the same time itself affected by all environmental processes influenced by human activities (Rockström et al., 2014).
Water’s hidden roles and functions
As the core component of the global life support system, water is active in terms of having both water flows (playing different water roles) and water functions, together constituting the global bloodstream.
In a recent article, Stockholm Resilience Centre scholars analyzed these hidden roles and core functions of water in the life support system (Falkenmark, Wang-Erlandsson, & Rockström, 2019).
During circulation, water plays three roles: in its controlling role, it is a key in sustaining all life, generating ecosystem services and functions in both terrestrial and aquatic systems; in its state role, it is a victim of change, responding for example to land-use change and pollution; and in its driving role, it may cause social shocks by floods and droughts, or as a driver of conflict. These three roles exist simultaneously and interact dynamically.
There is also a dense interaction with the land through which water passes, together generating the life support system. Through this interaction, a set of water functions are active in transfer of human water uses and activities, as a rule causing water partitioning effects.
There is a long series of such core water functions, active around the water cycle (Figure 2). The water-cycle circulation, together with the dense system interaction, stabilizes the life support system, making it water resilient.
Green water functions are three: regulatory, involving all the functions of soil moisture, evaporation and transpiration flows to regulate the Earth’s energy balance and climate system through for instance carbon sequestration and water’s ability as a greenhouse gas; productive, such as evaporation and transpiration to sustain food, biomass and bioenergy production; and moisture feedback, regulating the water cycle over land by evaporation.
Water also has five different blue water functions: water for societal supply, available to be withdrawn; water as a carrier of nutrients and pollution, and for transport; water as state, involving the function of water masses and storages; the productive function, for irrigation to produce food, and water to sustain aquatic growth; and the control function, regulating the Earth’s energy balance, sea levels and geological processes, such as subsidence.
Together, all these water flows, roles and core functions profoundly interact with the land system, forming a dense interactive network. This network constitutes the life support system of humanity, and is active through two kinds of ecosystem services: regulating ecosystem services, such as weather dynamics, albedo, carbon sequestration, land and aquatic habitats and water flow partitioning; and provisioning ecosystem services, such as provision of food, timber and energy.
This intricate global system thus forms a densely tied water-related network, crucial for building the water resilience of ecosystems, and essential for the ability to generate ecosystem services.
Dense interaction with ecosystems
Human land-use activities tend to generate water partitioning phenomena, disturbing human roles and functions and contributing to a rich variety of typical ecosystem disturbances. Water flow alterations change water roles and functions, reflected in ecosystem degradation. Over time, such processes will modify ecosystems in a variety of ways. A rich archive of historical evidence shows that past land and water mismanagement has been degrading water functions (Falkenmark et al., 2019).
The result is a long array of partial degradation in terms of severe ecosystem collapses and regime shifts. Desertification may develop after dryland crusting; groundwater rise and waterlogging may develop after alterations in the root zone; savannization may develop as a result of altered evapotranspiration reducing atmospheric water vapour and its condensation into new rainfall; monsoon change may develop in regions where large-scale irrigated agriculture has been increasing evapotranspiration; aquifer depletion may develop where groundwater withdrawals have been larger than natural groundwater recharge; and eutrophication may develop in systems overloaded with nutrients.
Water-related societal problems may build up step by step, eroding water resilience. Recent cases are the Somalian drought, the Cape Town water crisis, and the Arab Spring / Syrian conflict. In the end societies may even collapse, as did the ancient Mesopotamian and Mayan civilizations. Such degradations humanity will have to live with, persist in, adapt to or transform.
Global life support system
Still, today, the dominant worldview tends to disconnect human progress and economic growth from fundamental interaction with the biosphere (Rockström et al., 2014; Rockström, Falkenmark, Lannerstad, & Karlberg, 2012).
The sustainability agenda for humanity must urgently shift towards seeing people and nature as interdependent systems, in which much stronger emphasis is placed on managing water for social-ecological resilience and sustainability.
Thus, the twin objectives of global water resilience and human development require modernized water thinking, acknowledging that water is, as stressed by Rockström et al. (2014), the most important greenhouse gas and a prerequisite for all biomass growth.
It sets the pace of global cycles of carbon, nitrogen and phosphorus; it is an agent of transportation and a solvent of chemicals; and it provides the basis for social-ecological resilience. The global life support system, the central role of water in both ecosystems and society, and the intricate linkages between the components make a system approach essential.
Awareness is required of the risk for global change in response to links between driving forces and potential tipping elements, of relevance for long-term land use planning. Such tipping elements may originate from three types of driving forces (Rockström et al., 2014): water-use driven (water crowding), land-use-change driven (food production, deforestation, afforestation), and climate driven (global warming, droughts, floods).
Without planetary stewardship for water resilience, it is difficult to see how the world can eradicate poverty and hunger, two of the Sustainable Development Goals. This makes understanding water’s profound involvement in the interlinkages between societies, ecosystems and the Earth System (Folke, 2003; Rockström et al., 2012) essential. Water resilience, i.e. safeguarding a well-functioning water bloodstream, is a central prerequisite for global sustainability (Falkenmark et al., 2019).
At the core of the life support system are the functions of both blue and the green water and the ecosystem services generated by their interaction with the land they pass through, in other words the water, land and ecosystem processes in landscapes. (Figure 3). Manoeuvring this system to achieve human well-being involves governance and management of its components and of the social transformations involved, to secure provision of ecological services and social-ecological resilience.
This system approach involves a switch from our past conceptualization of ‘the environment’ (Falkenmark, 2008) – a negative approach – to highlight its constructive components, i.e. the ecosystem services that form its core.
Over time, the scale of water impacts has increased with human activities such as withdrawals, land use change, air pollution and climate change (Rockström et al., 2014). Rainfall patterns, such as the monsoon, have changed. The function of large-scale biophysical systems, such as the tropical rainforests, has also changed.
In other words, the key is to shift from yesterday’s focus on how to reduce the environmental impacts of human activities towards what is required to connect our societies with the biosphere supporting them. What is needed is a transition towards development of a safe, just, stable and resilient society within the biospheric playing field provided by the Earth System.
After this deep dive into the details of how water is active in generating ecosystems as an important component of generating resilience, we will now dive into two broad water-related problem areas: the giant task of learning to master the atmospheric water interaction of vast tropical drylands, inhabited by a rapidly expanding population of poor farmers; and the foreseeable challenge of securing household water supply in broad arid regions with multiplying populations.
Dryland water interaction with the atmosphere
Water-based categorization of landscape types
Agriculture is the largest water user on the planet, and the largest shaper of landscapes (Rockström et al., 2014). The two water functions involving the largest global amounts of water are both green: productive function (biomass) and moisture feedback (Falkenmark et al., 2019).
In view of the global importance of arid lands for many millions of people, Weiskel et al. (2014) developed a water-related categorization of landscape types, based on the dominant horizontal and vertical inflows and outflows of water through a landscape unit (Figure 4). Input to landscapes is rainfall and/or inflow.
In humid regions, landscapes are dominated by blue outflows (river flow), in drylands by vertical green water flows (rainfall and evaporation). There, development has to take a different form depending on green water practices, protecting or restoring green water storages in terms of soil moisture, and managing green water flow, i.e. precipitation and evaporation.
Sink region vulnerability
Agricultural and land use decisions involve direct impacts on vertical water flows, i.e. the amounts of water that enter and leave the atmosphere. Keys (2016) showed that the removal of vegetation in Mato Grosso, Brazil, altered the amount of rainfall over the land downwind, and even the seasonality of the rainfall.
Such moisture-recycling-related findings imply that vegetation changes in one location may cause large changes far away in rainfall, soil moisture, crop production and even blue water flows (Figure 5).
Upwind land cover is critical to sustain precipitation in water-stressed downwind areas and allow agricultural production there. This link between upwind land use and downwind rainfall is especially relevant in drylands, where water is a limiting factor for agricultural production, since most rain evaporates before reaching a river, so that irrigation is difficult.
Water-stressed areas are water constrained, and semiarid areas with aridity index P/EP (quotient between precipitation P and potential evaporation EP) less than 0.5 are therefore highly vulnerable to reductions in rainfall due to land use change. Keys et al. (2012) found that such water-constrained areas occupy 50% of the earth’s land surface. Dryland rainfed agriculture is therefore particularly susceptible to even small changes in precipitation.
The concept of the precipitationshed indicates how upwind terrestrial evaporation source areas contribute moisture for precipitation to downwind sink regions. Keys (2016) identified seven terrestrial, water-constrained, recycling-dependent sink regions with 500–600 mm rainfall and vulnerable to upwind land use change. Many of these areas are also priority areas for global development.
Two of Keys’s seven cases have particular interest because of size and population: the Sahel and north and east China. In the western Sahel there is high local recycling and no absolute contribution from the nearby Sahara at present, but greater future evaporation in the Sahara might have large effect on precipitation in the Sahel. The precipitationshed is vast, containing 83 countries and more than 900 million people.
The Chinese sink regions have the highest vulnerability of the seven regions, due to many potentially evaporation-altering land cover changes: expanded urbanization as well as irrigated cropland, deforestation in both East and South-East Asia and afforestation in northern China. They also have the largest populations, with eastern China exceeding 2 billion.
Keys’s analysis suggests that the food security of some of the most water-constrained rainfed agricultural regions could be very sensitive to distant land use changes. Although until now Integrated Water Resource Management (IWRM) has been used primarily for holistic management of runoff (blue) resources, the importance of moisture feedback (green) would make IWRM appropriate for landscape management as well.
As a base for societal water supply, blue raw water, although harvested from river runoff or groundwater, basically originates from precipitation, which has its source in land evapotranspiration elsewhere. In special cases, therefore, drought may severely complicate a city’s raw water supply.
Many megacities (population over 10 million) are on the leeward side of a continent, where the prevailing winds have passed over land, and therefore find themselves in a vulnerable situation (Keys, 2016), since much of their raw water depends on terrestrial moisture recycling.
The most vulnerable are eight terrestrial-moisture-recycling-dependent cities (including Shanghai, Sao Paolo, Buenos Aires, Karachi and Kinshasa), where around half of the water supply originates from precipitation over upwind land areas.
The arid-zone Achilles’ heel
Achieving Sustainable Development Goal 2 (to end hunger and achieve food security) will require massive, continent-scale efforts to secure food for the population, which for only semiarid and dry sub-humid regions is likely to approach 900 million by 2030 (Figure 6).
About 40% of sub-Saharan Africa is semi-arid or dry sub-humid; climate change and ecosystem degradation are creating major water shocks on a continent with a population projected to grow from 1 billion to up to 3 billion in this century.
Based on an overview of green and blue water resources in Africa (Schuol, Abbaspour, Yang, Srinivasan, & Zehnder, 2008), and on the particular blue water geography in sub-Saharan Africa (Vörösmarty, Douglas, Green, & Revenga, 2005), it is not surprising that agriculture remains mostly rainfed. And as a consequence of the aridity, there is only limited irrigation – in the drylands, most rain evaporates before reaching a river – and most sub-Saharan agriculture is rainfed subsistence agriculture with very low yields.
The drought proneness seriously complicates crop production (Falkenmark & Rockström, 2006; Rockström & Falkenmark, 2000), both the interannual droughts, which are becoming both more serious and more frequent in response to the ongoing climate change, and the frequent dry spells, with many days or even weeks without any rain during the rainy season.
During such dry spells roots are damaged, reducing their capacity to absorb green water for the production of crop biomass. As a consequence, yields become extremely low, typically of the order of only 1 t/ha out of a hydro-climatically possible yield of 7 t/ha (Garg, Karlberg, Barron, Wani, & Rockström, 2012).
In contrast to the irrigation-based Asian Green Revolution, a better idea to upgrade rainfed agriculture in dryland sub-Saharan Africa is a particular African Green Revolution (SIWI, 2016). An attractive possibility is to make better use of the rain by vapour shift, which helps the roots take up more of the infiltrated water in the soil and protects them from drought damage during dry spells.
This can be done by small-scale supplementary irrigation during dry spells, with rainfall harvested during rainstorms and stored in small local dams. An African Green Revolution would thus involve three parallel green-water dimensions, combining blue water strategies with farming systems that operate on green water in rainfed systems (Bill and Melinda Gates Foundation, 2006; Dile, Karlberg, Temesgen, & Rockström, 2013) in a sustainable ‘triply green revolution’ (Rockström et al., 2014): maximized use of green water for radically increased production; adequate attention to protection of critical ecosystem services, increasing sustainability; and in the long term improving the ability of landscapes to safeguard water resilience, i.e. the capacity to regenerate rainfall by moisture feedback and wetness in landscapes for ecological functions.
‘Triply green’ thus means a green water, green production, and green sustainability revolution. There are three key implications of the vapour shift for overall water management (Keys & Falkenmark, 2019): minimal impacts on the hydrological environment means more water can be used for biomass production for human use; minimal reliance on blue water means more can easily be used for other purposes; and the vapour shift means that more green water can support crop diversification, allowing both subsistence food production and some cash-crop production for sale on market.
Vulnerability to upwind land use change
Development of large drought-prone regions with rain-dominated lands will be critical in the present poverty- and hunger-ridden global zones. African dryland developers have their focus on vertical water flows and are working to overcome the climatic challenge of limited and variable precipitation by seeking ways to secure effective rootzone water uptake through protective irrigation with harvested rainfall.
Vapour shift is becoming a dominant component of agricultural development. Managers in more developed dryland regions have started to direct their vertical water flow focus on the origin of rainfall in terms in upwind land use, especially its dependence on land use changes there.
Seven such recycling-dependent regions with only 500 mm of annual rainfall, including eastern China and the Sahel, are particularly vulnerable to upwind land use changes. A large part of the water supply of many megacities is also vulnerable to upwind land use changes (Keys et al., 2012).
More information: Chanhee Boo et al, Zero Liquid Discharge of Ultrahigh Salinity Brines with Temperature Swing Solvent Extraction, Environmental Science & Technology (2020). DOI: 10.1021/acs.est.0c02555
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