A novel membrane desalination approach may provide potable water from seawater

Those uses require freshwater— and it turns out that this is actually relatively scarce stuff. Making up under three percent of Earth's water and largely tucked away in glaciers, you can see where this is going, especially in a warmer, drier future. The truth is that even today, billions of people across the globe do not have access to usable clean water, causing what is described as a global water crisis.

With that said, desalination, the act of removing salt from seawater or salty groundwater, is now in demand more than ever. Yet conventional methods used for decades continue to be highly energy-consuming and costly. For the first time, researchers from the University of Jordan, Arab Open University, and King Abdullah University of Science and Technology have designed and utilized a novel class of aluminum-based biporous membranes to effectively remove salt from seawater at temperatures much lower than conventional alternatives.

In the realm of desalination using evaporation, the researchers suggest that this membrane technology serves as a significant breakthrough in terms of cost, energy, and sustainability.

A naturally sustainable and energy efficient approach

Dr.Mohammed Rasool Qtaishat, one of the lead researchers behind the novel biporous membrane, tells IE, "the new technology allows water evaporation at ambient temperatures, [approximately] 25°C so that desalination can be achieved without heating or pressure requirements. This will save a lot of energy and provide potable water from the seawater at the lowest possible cost, and it is environmentally friendly."

And that's not all. The research revealed that the biporous membrane was stable over long-term desalination processes, proving that the method shows potential for operating sustainably should it be scaled up.

Current state-of-the-art

So what exactly is desalination?

Desalination is a general term for methods used to remove salt content from water sources such as seawater or salty groundwater. Desalination processes can be split into two groups: thermal or membrane-based, with the latter being the primary choice. However, membrane technologies such as reverse osmosis have rapidly developed since the 1960s, and new pilot plant installations have even surpassed the performance of thermal processes.

Several conventional technologies used in the past required heating the saline water before desalination. The problem with these methods is that they are energy-intensive, making them expensive and inefficient in terms of carbon use. Additionally, most conventional heating methods would generate carbon dioxide that further contributes to the growing menace of global warming.

Professor Qtaishat further explains to IE, "Anciently, desalination was achieved by evaporating the water and condensing it afterward, which requires heating and high energy. Recently, new techniques evolved that don't require evaporation, known as membrane desalination, such as reverse osmosis. Instead of heating and evaporation, high pressure is applied. This pushes the water through the membrane and leaves the salt behind. Again, the high pressure is energy intensive."

Breaking high energy and cost barriers for low-income countries

In order to reduce energy consumption, either in the form of heating or the application of external pressure, it is important to conduct desalination at minimal trans-membrane temperature differences or at ambient temperatures.

This is where membrane desalination (MD) comes in. The low operating temperature and pressure required, high desalination capability, and the unique ability to work using low-intensity energy sources could make this process a successful alternative to more traditional desalination processes.

The new class of aluminum-based biporous membranes uses a novel approach to desalination that is environmentally friendly, sustainable, and cost-effective. All these features could help with issues such as high energy costs and water shortages that low-income countries often face.

With the disastrous effects of unsustainable energy use on Earth's climate in mind, the new aluminum-based biporous membranes show the potential to offer some relief. The approach is the first attempt to reveal the possibility of designing a desalination membrane capable of purifying water with a negligible trans-membrane temperature difference on both theoretical and experimental levels.

A novel double-layered membrane approach

The novel biporous membrane is essentially made up of two parts: an active layer laid over a support layer. The active layer is manufactured from a fluorine-based low surface energy material which in basic terms means that it does not form molecular bonds easily. This layer behaves similar to the waxy coating of a lotus leaf, where water droplets roll off its surface.

The support layer has a non-deformable honeycomb pore structure comprised of a high purity aluminum oxide matrix known as anodisc. This anodisc support layer is typically used for both liquid and air filtration as it offers accurately controlled pore size distribution with high strength and flexibility.

The result? A biporous structure embodying a thin active layer with a large number of nano or sub-nano-meter pores (hydrophobic) in connection with a thick support layer having much larger micro-meter range pores (hydrophilic).

The active layer is hydrophobic to prevent the water from flowing through it and forms an interface large enough to cause an increase in vapor pressure. On the other hand, the support layer is hydrophilic to draw water into its pores.

The active layer is also crucially hydrophobic to prevent the transfer of salt and only allows water vapor to pass through and into the hydrophilic layer. The design of such a biporous membrane structure is based on the fundamental concept of the capillary effect — the primary force that moves liquid up a tube.

Leveraging The Capillary Effect with a twist

The conventional desalination processes involve heating the saline water and forming water vapor that distills separately, leaving behind salts. "Our new technology was inspired by the fact that water evaporates at a faster rate when its surface is curved, so a membrane was designed to keep the water surface shape as desired," explains the researcher.

The present research employs the concept of the capillary effect on water vapor pressure at the curved water-air interface of the pores in the hydrophobic active layer. The vapor pressure at a curved surface in a small pore is higher than with a flat surface. The increased vapor pressure results in a higher evaporation rate towards the hydrophilic layer of larger pores. Thus efficient evaporation can be achieved while also keeping both the saline and desalinated waters at ambient temperatures.

Compared to other commercially-applied and conventional membranes, the research demonstrated that water vapor flux and salinity removal with the novel aluminum-based biporous membranes yielded more efficient results. According to the research paper, 99 percent of the salt was removed at temperatures as low as 25°C with a water production rate of 40 liters of water per hour for each square meter of membrane.

A potential boost for achieving UN Sustainable Development Goal Number 6 - Clean water

Therefore, the proposed biporous membrane technology is a highly energy-efficient process that shows promise of being suitable for large-scale industrial applications.

This innovation could help address and achieve number six of the UN sustainable development goals - access to clean water and sanitation for all. This goal targets capacity-building support to developing countries in water and sanitation-related activities and programs, including desalination, water efficiency, wastewater treatment, recycling, and reuse technologies.

The aluminum-based biporous membranes show potential for tapping into the use of relatively more abundant seawater while committing to an eco-friendly and sustainable approach. Additionally, this innovation will significantly minimize the unseen effect of energy consumption and the greenhouse effect on global warming and climate change.

Diversifying the horizon of desalination technology to a new level

Of course, no scientific development comes without limitations.

Whilst the research exhibit a breakthrough in the membrane desalination (MD) process as a low-cost and high-energy-efficient desalination approach, the fact remains that the researchers still need to demonstrate that the results can be scaled up.

"Another thing is how it will react to different water contaminants and if a pretreatment will be required or not," explains Dr. Mohammad Rasool Qtaishat.

Scaling up of the membrane will come with a price, and Qtaishat tells IE that "another major challenge we are facing now is scaling up of the membrane. To this end, we have now a funded project by one million dollars from KAUST (king Abdullah university of science and technology) in the form of a transitional fund to develop the membrane from the lab scale to the commercial level."

The research work could be a milestone in sustainability optimization and environmental stewardship.

The use of the capillary effect has diversified the horizon of desalination technology to a new level. It has given a new direction to the quest of attaining viable ways of utilizing seawater. This could be a stepping stone towards adopting and employing technologies to help support and grow life on this planet. After all, where else could we go?