Advancements in Water Ultrafiltration Technology

Strained water resources across the world necessitate innovative ways for power plants to more efficiently use, and reuse, the water that is available. At the U.S. Department of Energy’s Argonne National Laboratory, researchers working on materials for water-energy systems have developed a suite of membrane technologies that tackle this issue at the molecular level.

Water that is used for power plants often must undergo ultrafiltration as part of its pretreatment process. The membranes used for this process are typically designed to filter particulates of about 1 nanometer (nm) to 50 nm in size. These could be anything from colloids and larger organic molecules like proteins, to viruses and bacteria, all of which can lead to problems down the line as the water makes its way through the power plant.

The membranes typically used for ultrafiltration offer considerable durability against poor water quality, are compact, and do not require the use of chemicals to clean the water. All filters, however, must address two serious problems: fouling and energy-efficiency challenges.

Fouling is the physical blocking up of the filter, much like bits of food getting caught in a kitchen sink drain. Ultrafiltration membranes prevent fouling further downstream within a power plant, but the membranes themselves inevitably become fouled. This limits the lifetime of the membrane, as the pores within it are blocked, eventually requiring the membrane to be removed and cleaned, or replaced altogether, costing valuable time and resources.

The smaller the pores get, the more energy it takes to push water through them. Commercial membranes used for ultrafiltration do not have uniform pore sizes either. There is a gamut of pore sizes within any given membrane. In order to ensure that a power plant is filtering out the necessary particulates, one has to use a membrane with a smaller average pore size than the smallest substance one is trying to filter out. The smaller pore sizes then dramatically increase the energy required to push water through the membranes.

Membrane Improvements

At Argonne, scientists have developed several membrane technologies to address these exact issues, focusing on the interface between the water and the ultrafiltration membrane. They have developed technologies for both passively and actively addressing fouling, as well as technologies for improving the energy efficiency of ultrafiltration membranes.

1. Seth Darling is director of the Institute for Molecular Engineering at Argonne and the Advanced Materials for Energy-Water Systems Center. He said “Innovating these technologies for the recycling and reuse of water is a boon for all future uses of water.” Courtesy: Argonne National Laboratory

“We’ve been able to both manipulate the material of the membrane, so things are less likely to adhere to them, and degrade what does manage to adhere,” said Seth Darling (Figure 1), director of the Institute for Molecular Engineering at Argonne (IME@Argonne) and the Advanced Materials for Energy-Water Systems Center (AMEWS), one of the U.S. Department of Energy’s Energy Frontier Research Centers.

Using atomic layer deposition (ALD), a process commonly used to coat microprocessors and other circuitry components, researchers have successfully applied different coatings to ultrafiltration membranes that directly address the issue of fouling.

“ALD has been commercialized for high-volume manufacturing of computer chips for years,” explained Darling. “Now Argonne has modified it to treat water.”

By exposing ultrafiltration membranes to chemical vapors, the membranes can be imbued with the particular qualities of those vapor molecules in the form of metal oxide coatings. This process increases the efficacy of the membrane’s ability to filter by imparting crucial improvements to these membranes due to the properties of the coatings.

The coatings can provide, for instance, a significant element of hydrophilicity, in order to prevent fouling. Using a hydrophilic coating can attract water molecules to the surface of the membrane, thus forming a protective layer of water across the membrane. This layer of water prevents oils from interacting with the membrane and subsequently prevents fouling. These types of anti-fouling coatings represent “passive” anti-fouling techniques.

ALD can also be used for “active” anti-fouling coatings. As Darling explains: “You can coat membranes with a catalytic metal oxide, such as titanium dioxide, that can accelerate the degradation of the foulants.”

Unfortunately, titanium dioxide normally only catalyzes these reactions when exposed to ultraviolet light. Throughout their research, Argonne scientists determined that the titanium dioxide would need to be “sensitized” in order to absorb more than just ultraviolet light to catalyze the degradation of foulants.

“We added nitrogen in order to get the coating to absorb visible light as well,” explained Darling. “The photocatalytic coating allows us to then degrade organic and biofouling agents. It’s essentially antimicrobial and self-cleaning.”

Pore Size Is Important

It stands to reason that by depositing additional layers of material onto the surface of these membranes, and on the surfaces of the pores themselves, this would decrease the size of the pores, thus decreasing the flow of water through the pores.

“One would think this would decrease efficiency,” said Darling. “While ALD does slightly narrow the pores, by using hydrophilic coatings, the flow through the pores actually increases.”

In addition to applying these metal oxide coatings to ultrafiltration membranes, Argonne has developed a technology for embedding them within the physical structure of the membrane polymers themselves. In a patented Argonne process called sequential infiltration synthesis (SIS), the ultrafiltration membranes are exposed to the same chemical vapors, but for longer periods of time.

In this process, the metal oxides actually sink into the spaces within the molecular structure of the membrane and change it from within. By penetrating the material of the membrane itself, the metal oxides are more tightly integrated into the membrane. These embedded metal oxides then impart increased temperature, chemical, and even mechanical stability to the membranes.

“Theoretically, most coatings can eventually be eroded or removed,” Darling explained. “With SIS, however, we changed the membrane itself. We’re not just painting the wall; we’re changing what the wall is made of.”

The technologies that Argonne has developed to combat fouling directly correspond to increasing the efficiency of ultrafiltration membranes by significantly extending their lifetimes. Ultrafiltration membranes that last longer and both passively and actively work against fouling need to be cleaned and replaced less frequently. This not only saves energy, but eliminates the labor and operational costs inherent in the necessary maintenance shutdowns to clean and replace the membranes.

Efficiency Enhancements

In addition to developing technologies to combat fouling, researchers within IME@Argonne and AMEWS are also developing techniques for addressing the issue of membrane efficiency as it relates to the energy required to move water through the pores of the membrane themselves. These efforts revolve around the idea of making membranes that have drastically more consistent pore sizes than current commercial ultrafiltration membranes.

More consistent pore sizes would allow power plants to use membranes whose pores more accurately reflect the particulates they are trying to filter out, as opposed to using membranes whose average pore size is dramatically smaller than the particulates. This would allow power plants to use less energy pumping water through the membranes.

Using a combination of polymer science, SIS, and ALD, researchers at Argonne created a nanostructured film, consisting of one material studded with uniform dots of another material.

“You then remove the dots,” explains Darling, “and you’re left with precise holes that are all the same size.” Further applications of the ALD process allow for precise modifications to the size of the pores, “one atom at a time.”

More often than not, power plants sited at water sources merely dump their intake water back into that same water source after it has made it through the power plant’s water system. As it returns to the water source, it is generally of degraded quality. As viable water sources become scarcer, this form of water usage becomes less and less ideal. The more a power plant can recycle the water within its water system, the more effective it can be in the long term.

“The traditional approach of ‘once-through’ water use will become increasingly less sustainable,” Darling explains. “Therefore, innovating these technologies for the recycling and reuse of water is a boon for all future uses of water.”

Argonne: Here to Help

Although Argonne is a federally funded laboratory, it regularly works with a range of partners/sponsors, including private industry, federal agencies, and state and local governments. Often, Argonne researchers help organizations solve technical problems they are unable to solve themselves because they do not have the necessary scientific and technical expertise or facilities and tools. In addition, organizations license Argonne technologies—exclusively or non-exclusively—to solve inefficiencies or unlock new possibilities in their respective systems.

In some cases, organizations engage in cooperative research and development projects with Argonne. Often, these efforts revolve around further developing technologies that were invented at Argonne so they can be effectively applied to the organization’s business or technical needs.

Organizations with interest in learning how they might tap into Argonne’s expertise, facilities, and tools to unlock technical challenges and seize opportunities—including technologies to assist with ultrafiltration in power plants—should contact partners@anl.gov. ■

J.D. Amick is an associate with Argonne National Laboratory.