Wastewater isn’t just waste – it’s packed with hidden value. Stanford researchers are developing cutting-edge resins – porous beads that together act like a coffee filter – not only to remove contaminants but also to recover valuable products, like ammonia for fertilizer. With global demand for drinkable water projected to exceed supply by 40% by 2030, the project has the potential to make water treatment cheaper, more efficient, and even profitable.

“Amidst efforts to reduce the emissions, energy, and cost of chemical manufacturing, selective resins can enable circular economies that convert pollutants into products by using wastewater as a feedstock,” said William Tarpeh, assistant professor of chemical engineering and one of the project’s principal investigators. “Wastewater treatment plants are increasingly being reconceptualized as water resource recovery facilities that achieve multiple co-benefits at once.”

Funded by the Stanford Woods Institute of the Environment’s Environmental Venture Projects program, the project is aimed at advancing resin technology to more precisely remove contaminants. Tarpeh and his fellow investigators have developed specialized resins – similar to the beads of a Brita filter – that can separate out harmful chemicals and compounds. In the future, the team could potentially design resins to remove perfluoroalkylated substances (PFAS), often referred to as “forever chemicals” due to their persistence in the environment and bioaccumulative nature. These chemicals, commonly found in heat-resistant household items like nonstick cookware, have made their way into water supplies around the country.

A microscopic image of resin beads developed at Stanford for use in water treatment plants. | Eric Appel Lab; edited by Madison Pobis

Beyond improving filtration, the team is working to expedite the development of new purification technologies by streamlining the design of resins, therefore reducing manufacturing costs for water treatment facilities. This approach offers the potential for a new revenue stream. For example, ammonia and phosphorus could be extracted and sold for fertilizer and pesticides, respectively. Since resins are already widely used in water treatment plants, this technology can be seamlessly integrated into existing infrastructure with minimal disruption, removing a barrier to adoption.

Collaborating across campus

Tarpeh’s lab specializes in extracting valuable resources from wastewater and improving filtration methods to make the process more efficient and affordable for wastewater treatment plants. With growing concern over hard-to-remove contaminants like PFAS, Tarpeh saw an opportunity to develop a better resin. Traditional resins are useful for discarding basic contaminants, like heavy metals, but they lack the selectivity needed to remove PFAS or recover valuable compounds. Resins work by swapping unwanted ions – such as calcium, which causes water hardness – with harmless ions like sodium. While effective for making water drinkable, existing resins struggle to capture and recover beneficial elements. To achieve that, they need an upgrade in selectivity.

However, designing and testing resins can take months, if not years. To accelerate the process, Tarpeh turned to Stanford scholars in other departments for expertise in different fields and fresh perspectives.

Resins are a type of synthetic polymer designed to remove pollutants and unwanted materials, so to refine this resource recovery technology, Tarpeh brought in a polymer expert Eric Appel, an associate professor of materials science and engineering. Appel’s research focuses on engineering synthetic polymers – long string-like molecules – that can be easily tuned to have many different properties, including to mimic natural biological polymers like proteins. “We can design many different polymers inspired by organisms that naturally filter water or biological receptors that bind to chemicals in water,” said Appel.

Appel tailors polymers’ chemical properties, such as their ability to stick to molecules like PFAS, to enhance their effectiveness for a given task. In this case, that meant increasing the resin’s selectivity or its ability to distinguish between different elements in wastewater. However, testing these resins to ensure they target the right nutrient or contaminant is slow and expensive since each one must be evaluated individually.

That’s where Polly Fordyce, associate professor of bioengineering and genetics, comes in. Fordyce specializes in developing microfluidic platforms – a method of shrinking biological and chemical processes to a miniature scale. As an expert in miniature experiments, Fordyce developed a method to test hundreds of polymers simultaneously.

Instead of using traditional tubes and flasks full of liquid, the team can work with microscopic resin droplets that are a billion times smaller in volume. This microscale approach reduces time, cost, and materials while making it possible to explore thousands of polymer combinations that would otherwise be too resource-intensive to test.

Fordyce compared this miniaturization approach to the shrinkage of computers, which transitioned from room-sized machines in the mid-1900s to handheld devices small enough to fit in a pants pocket today.

“We’re trying to speed the pace of screening and discovery,” Fordyce said. In the same way as integrated circuits shrunk the space required to do parallelized electronic calculations, microfluidic devices can shrink the volumes required, too.”

Developing a climate solution: Wastewater as a resource

Leveraging their interdisciplinary expertise, the team is now able to quickly and more affordably create and test a large range of new resins. This enables them to more effectively design resins for improved purification and resource recovery.

“Turning wastewater pollutants into valuable chemical products can help achieve sustainability goals, enable circular economies, and mitigate pollution all at once,” Tarpeh said.