"Reducing chemical usage is critical for minimizing the environmental impact of desalination"
A new research innovation offers a cost-effective and environmentally friendly alternative to conventional chemical treatments, addressing boron contamination that exceeds safe drinking water limits set by the World Health Organization. To gain deeper insights into this pioneering research and its broader contributions to the field, we had the privilege of speaking with Professor Menachem Elimelech, the Nancy and Clint Carlson Professor of Civil and Environmental Engineering and Chemical and Biomolecular Engineering at Rice University. Renowned for his lifelong commitment to advancing sustainable water solutions, Professor Elimelech continues to shape the global scientific and engineering landscape through his innovative research and leadership. In this interview, we explore his latest advancements in desalination. But before delving into his groundbreaking work, let’s take a moment to introduce Professor Menachem Elimelech.
Professor Menachem Elimelech is a globally recognized leader in environmental engineering, specializing in the water-energy nexus. His research focuses on energy-efficient desalination, wastewater reuse, and advanced materials for water purification and separation technologies. He is currently the Nancy and Clint Carlson Professor at Rice University, with joint appointments in the Departments of Civil & Environmental Engineering and Chemical & Biomolecular Engineering.
There is an effort to displace thermal technologies for brine management with membrane-based technologies; these remain at the pilot scale
Born in Israel to an immigrant family from Morocco, Professor Elimelech grew up in the city of Beer Sheva. He attended the Ben Shemen Youth Village, an agricultural boarding school, before earning his Bachelor’s and Master’s degrees from the Hebrew University of Jerusalem. He later pursued a Ph.D. in environmental engineering at Johns Hopkins University.
Throughout his career, Professor Elimelech has authored over 580 peer-reviewed publications and is among the most cited scholars in environmental and water quality engineering. His pioneering contributions to membrane-based desalination, water treatment, and nanomaterials have shaped both academia and industry. He has been honored with numerous prestigious awards, including the Eni Prize for Environmental Protection, and has been elected to national academies in the U.S., China, Australia, Canada, and Korea.
What are the most significant advancements in membrane technologies for desalination in recent years?
Advancements in the water sector are generally slow and incremental. Reverse osmosis remains the dominant desalination technology and is likely to be so for years to come. Thin-film composite polyamide desalination membranes have been the gold standard since the early 1980s. While extensive research and activity are ongoing, it's important to recognize that advancements are primarily incremental. There is an ongoing effort to displace thermal technologies for brine management with membrane-based technologies, but these remain at the pilot scale.
Conventional reverse osmosis membranes struggle to remove boron from seawater. Can you elaborate on the limitations of current methods and why boron removal is a particularly difficult challenge?
Boron removal is a significant challenge in desalination due to its small molecular size and neutral charge at typical seawater pH levels
Boron is toxic but ubiquitous in seawater. Boron removal is a significant challenge in desalination due to its small molecular size and neutral charge at typical seawater pH levels. Conventional reverse osmosis (RO) membranes are designed to remove charged ions and larger molecules, but boron, which exists as boric acid in seawater, can pass through these membranes more easily. Additionally, achieving the drinking water boron concentration standard (e.g., 0.5 mg/L in Israel) often requires more than single pass RO. For example, multiple-stage RO, which increase operational complexity and cost. The limitations of current methods include high energy consumption and the need for additional chemicals. These challenges highlight the need for more effective and sustainable solutions.
Recent research you were involved in highlights the use of carbon cloth electrodes for boron removal. What makes this material particularly effective, and how does it compare to conventional desalination techniques?
We leveraged bipolar membrane and boron-selective functional groups that introduce pH swing in our system, transferring boric acid to borate, and increase boron selectivity for the removal. These two reasons make our process particularly effective. Compared to conventional desalination techniques, such as multi-stage RO and ion exchange resins, this approach is chemical-free, energy-efficient, and environmentally friendly. The carbon cloth electrodes can be regenerated and reused, further enhancing their sustainability and cost-effectiveness.
The new electrode-based boron removal process is said to be more energy-efficient. Can you provide insights into how this method reduces energy consumption compared to traditional post-treatment techniques?
Our method reduces energy consumption from two aspects. The first one is that our system is highly selective for boron. The energy consumption of our system is proportional to the boron removal selectivity. The more selective the system is, the less energy is required. The second reason is that our system does not need pressurized water, saving a substantial amount of energy.
One of the key benefits of this new technology is its potential to reduce chemical usage in desalination. How does this impact the overall environmental footprint of desalination plants?
Reducing chemical usage is critical for minimizing the environmental impact of desalination plants. Traditional boron removal methods often involve chemicals like caustic soda or hydrochloric acid for pH adjustment and resin regeneration, which can lead to harmful discharges and increased operational costs. Our electrode-based process eliminates the need for these chemicals, reducing the risk of environmental contamination and lowering the plant's overall carbon footprint. This aligns with the growing demand for sustainable desalination technologies that prioritize environmental stewardship while maintaining high water quality standards.
A study led by your lab and published in 2023 challenges the long-accepted solution-diffusion model of reverse osmosis. What were the key flaws in this model that led you to reevaluate its validity?
I have always found the solution-diffusion model for water transport unintuitive. In fact, many of its core assumptions are quite strange. For example, it assumes that hydrostatic pressure remains constant inside the membrane and then suddenly drops to zero at the exits, which is unphysical. It also claims that membranes have no pores, that water molecules are dispersed as single particles within the membrane, and that water moves solely due to a difference in its concentration (a concentration gradient). Our 2023 study proved all of these assumptions to be incorrect. The solution-diffusion model fails to describe water transport in reverse osmosis membranes, whereas the pore-flow model accurately captures the underlying transport mechanism.
The research suggests that water moves through membranes in clusters rather than individual molecules. How does this new understanding impact the design and optimization of reverse osmosis membranes?
It is critically important to recognize that water moves through membrane pores via viscous flow, rather than by diffusing as individual molecules. Our research shows that frictional interactions and pore structure are the key factors governing water transport. This means that future membrane development should focus on optimizing pore connectivity, size distribution, and membrane-permeant interactions to enhance water flow while improving salt and pollutant rejection. By minimizing friction between the membrane and water, we can reduce the energy required for desalination, making water treatment technologies more efficient and cost-effective.
It is important to recognize that water moves through membrane pores via viscous flow, rather than by diffusing as individual molecules
Additionally, tailoring pore structures could improve the removal of specific contaminants, such as boron and small organic chemicals, which remain challenging to filter with current membranes. Ultimately, this new understanding enables the development of membranes with precise separation selectivity, making them not only more efficient for desalination but also adaptable to a variety of water treatment challenges.
With global freshwater demand predicted to exceed supply by 40% by 2030, what role do you see innovative desalination technologies playing in addressing the global water crisis?
Future membrane development should focus on optimizing pore connectivity and membrane-permeant interactions to enhance water flow
Innovative desalination technologies will play a critical role in bridging the gap between freshwater supply and demand. As traditional water sources become increasingly strained, desalination offers a reliable and scalable solution for producing freshwater from seawater, brackish water, and other unconventional water sources. While reverse osmosis (RO) membrane-based desalination remains the gold standard for seawater desalination, innovative technologies can find their niche in treating brackish water. For example, our study in 2021 has shown that electrodialysis outperforms RO in desalinating low salinity brackish water, offering advantages such as less fouling potential and less maintenance. These innovations not only expand the toolkit for addressing water scarcity but also provide tailored solutions for specific water sources and conditions.
What are the next steps in your research to further improve membrane technology and desalination processes?
We continue our research on brine management technologies (e.g., low-salt-rejection- reverse osmosis, LSRRO), which is a major issue for inland desalination plants because of the lack of options for brine disposal. We continue our work on developing ion selective membranes for selective separations. And we are expanding our work on the fabrication of specialized desalination membranes for LSRRO and ultrahigh-pressure operation. Lastly, we have just established the Rice Center for Membrane Excellence (RiCeME) to advance next-generation membrane materials and separation technologies for critical applications in energy, environmental sustainability, and chemical processing.