Kai Nip, Staff Photographer

Water filtration is a practice that dates back to the time of Hippocrates, and over time, researchers have worked to create more efficient and reusable filters. This year, Menachem Elimelech, professor of chemical and environmental engineering at Yale, led his research group to discover “the holy grail” of water filtration: a chlorine-resistant membrane. 

On Oct. 5, the journal Nature Sustainability published a study by Elimelech’s team on their development of a multilayer polyester membrane that is resistant to degradation by chlorine. This discovery could provide a more sustainable, lower-cost alternative to conventional membranes.

“The exciting component is that we’ve been able to develop a membrane that not only has performance, that is up to reverse osmosis standards, meaning high salt rejection and high water permeability, but we’ve done so using chemistry that is also chlorine resistant,” said Ryan DuChanois GRD ’23, a doctoral candidate in chemical and environmental engineering and a member of Elimelech’s team. “To our knowledge, that hasn’t been done before.”

Reverse osmosis, or RO, is a type of water filtration system that relies on the use of a semipermeable membrane to purify water of dissolved ions, the paper explains. An optimal RO membrane selectively allows the passage of water while blocking the passage of contaminants. As a result, when pressure is applied to wastewater through an RO membrane, clean water is produced. 

The mechanism hinges upon the choice of material that is used for the RO membrane, according to the paper. Polyamide has conventionally been used due to its ability to withstand a greater range of temperatures and pH levels in comparison to its alternative, cellulose acetate.

However, polyamide is intolerant to chlorine, which is used to treat wastewater before the filtration process. But such treatment prevents biological growth on the surface of the RO membrane, according to Jaehong Kim, chair of the Department of Chemical & Environmental Engineering. Kim added that current state-of-the-art RO membranes therefore degrade when exposed to chlorine, which increases operational costs for water filtration facilities.

Elimelech worked with Xuan Zhang — a professor at the Nanjing University of Science & Technology — on the research team that worked to find a good candidate for RO that would be resistant to deterioration by chlorine. Using swimsuits as inspiration, they found their ideal material: polyester.

“I focused on conventional polyamide matrix for nearly four years (from 2013), and finally realized that it was structurally unstable towards chlorine in any case,” Zhang wrote in an email to the News. “Therefore, I decided to find some other candidates from 2017. Polyester is an easy option to access, because some other research groups have reported the fabrication of polyester membranes for ultrafiltration or nanofiltration. Polyester as an RO remained challenging at that moment.”

DuChanois said that past attempts to create RO polyester membranes have been unsuccessful. But by forming “multiple, dense polymer layers on top of a porous support,” DuChanois said that their team was finally able to make a polyester membrane — with a base similar to swimsuits — that was successful and possibly the first of its kind.

This membrane is just as effective as commercially available membranes at rejecting salt and allowing water to pass through, according to the paper. It is also resistant to degradation by chlorine under a wide pH range, from pH 4 to 8, and according to Zhang, it performs better than current RO membranes. 

“Since [the membranes] can tolerate chlorine, they would last longer than conventional membranes because they won’t be as prone to biological fouling,” DuChanois said. “And they would improve the sustainability of treatment processes, because you wouldn’t have to remove any chlorine that’s already in the water before you treat the water with reverse osmosis.”

These membranes could be used for desalination and wastewater reuse, according to DuChanois, because they would not only be efficient but also remain intact for longer — helping to reduce waste and costs in the field of water filtration.

The team is currently working on simplifying the process of fabricating the membrane so that it may be scaled up for larger production at a lower cost.

“This research is showing a potential to resolve the problem that the scientific community was not able to solve for [the] past couple of decades,” Kim wrote. “If this research further advances and becomes translated for commercial use, the impact on [the] desalination industry will be immense.”

Polyamide has been the most commonly used RO membrane material in water filtration since its introduction in the early 1980s, according to the paper.

Alexa Loste | alexa.loste@yale.edu
Ben Scher | ben.scher@yale.edu