Is there an alternative for desalination?

Other desalination alternatives using MOFs include membrane distillation, capacitive deionisation, forward osmosis, and photocatalytic purification.

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The World Economic Forum (WEF) has listed water scarcity as one of the largest global risks in the upcoming years, as current water supply can only meet future water demands partially. Two-thirds of the 7.7 billion people on earth – around 4 billion people – face severe water scarcity at least one month per year. 0.5 billion live under such conditions throughout the whole year. While the overall amount of potable water would fulfil the global demand, globally unequal distribution and increased demand from the industry led to water stress in large areas of the world. The WEF expects the demand to be 40% higher than the available water in 2030. Given these figures, water treatment and purification technologies will become one of the most important future technologies. Furthermore, as around 97% of the global available water is saline, efficient desalination technologies could provide sufficient water.

Current desalination options

Several options are available to turn sea or brackish water into potable water based on either membrane separation or evaporation and re-condensation. Of all the used technologies, reverse osmosis (membrane-based process) and multi-stage flash (thermal-based process) are the most commonly used technologies.

Reverse osmosis desalination

Water is led under pressure through semi-permeable membranes to extract ions, molecules, and other particles from it. High pressure is required to overcome osmotic pressure (water molecules rather bind with the removed ions or molecules, and thus energy is needed to remove these). Because of osmotic pressure, reverse osmosis requires up to 100 bar to work. As the membranes have pores of 0.1 nm, it is important to filtrate the water and remove larger impurities beforehand. To do so, a multi-step filtration process is used. This includes sediment filters for particles and several filters and membranes (e.g. activated carbon) to capture chemicals such as chlorine. In the end, the water needs to be sterilised – often using ultraviolet light. The concentration of minerals in the brine outflow is significantly higher than in the inflowing water. Depending on the desalination plant, the base water quality and the used process, the energy consumption of a reverse osmosis plant is between kWh/m3 to 6.6 kWh/m3.

Distillation/evaporation desalination

By evaporating and re-condensing seawater, the water is purified and desalinated. There are different methods including solar energy or reducing the boiling point under vacuum to decrease waste heat. The most energy-efficient option of distillation methods are Multi-effect distillation plants with an energy consumption of down to 1.5 kWh/m3. The more commonly used multi-stage flash process consumes significantly more energy. You find more details in this article.

But at what cost?

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Both technologies have their drawbacks:

High energy consumption

Environmental concerns: (a)disposal of highly concentrated brine, and (b) source of energy mainly from fossil fuels Regarding energy consumption, excess heat from the multi-stage flash process is recuperated and pressure from the reverse osmosis process is led back into the system. This has already greatly improved early systems. Currently, drinking water production is at around 95 million m3 drinking water per day. At the same time, an excess of more than 140 million m3 of concentrated brine has to be disposed. Environmental concerns are partially catered for in long-term underground storage systems or disposal into the sea at environmentally acceptable doses. There is, however, a larger concern about the amount of produced brine. If you are interested, read this article by National Geographic on “Desalination plants produce more waste brine than thought”.

Alternatives to current solutions

Aqualife Global, an EPFL spin-off, has recently entered the market with an evaporation-based desalination plant. This plant requires less energy and has a higher through-put than current desalination methods. Another approach could be capillary-driven desalination as presented by a group of the University of Nevada and of Oregon State University. Metal-organic frameworks (MOFs) have certain properties that could improve the current available desalination methods. Their hollow framework of pores allows for surface adsorption/desorption or molecular sieving applications in gaseous or liquid media.

Reverse osmosis/membrane-based technologies with MOFs

Schematic description of reverse osmosis desalination using metal-organic frameworks in the membrane

MOFs – such as UiO-66, MIL-125 and MIL-101 (Cr) – showed comparable, or an even improved rejection rate of NaCl compared to currently used RO membranes. Using MOFs in the membranes increased the water flow by up to 44%. ZIF-8 and MIL-121 have shown the potential to be used in mixed matrix membranes (MMM). MMM are polymeric membranes with zeolites or silica as adsorbents. Lately developed MMM feature MOFs instead or additionally to the other two adsorbents. ZIF-8 was able to remove NaCl and other pollutants such as ethanol, various dyes (e.g. Blue 21, reactive black 5, rhodamine B dye), pharmaceuticals or borons. MIL-121 has a high adsorption rate of LiCl, NaCl, MgCl2, and CaCl2. In a test, MIL-121 generated 1.6 ml/g-1 water from water with up to 35 000 ppm of salt. They then cleaned the material of the salt with water at 80° C for further use.

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