U of T Engineering undergraduates partnered with Isla Urbana, a local NGO, to find a sustainable alternative to the waterproofing chemical product currently in use
Research from a multidisciplinary team of U of T Engineering students suggests that nopal mucilage — a material harvested from a species of cactus — could be used as a waterproofing agent for rainwater harvesting tanks.
The discovery points the way toward a more sustainable, low-cost and locally-available solution than the chemical-based coatings currently in use.
The project was part of APS 490Y Multi-disciplinary Capstone Design, a full-year course for fourth-year students looking to step outside their core discipline. It was proposed by partner organization Isla Urbana, a Mexico-based organization dedicated to eradicating water scarcity and providing clean water access to communities through sustainable rainwater harvesting.
“I was looking for a capstone that had both an environmental and social impact focus,” says Lily VanderWoude (Year 4 MechE).
“This one really stood out to me as something that I wanted to work on, in part because it was multidisciplinary.”
The capstone team consisted of Aymun Qayume (Year 4 ChemE), Yusra Chowdhury, Kareem Madanat and VanderWoude (all Year 4 MechE), along with their supervisor Professor Bradley Saville (ChemE).
Isla Urbana has long held experience in designing rainwater capture systems that can be implemented and maintained by community members using local resources. Since 2009, the organization has installed more than 40,000 diverse rainwater systems across Mexico.
One challenge the organization faces is that these rainwater harvesting tanks currently use a chemical agent to waterproof the inside of the basins. This agent is produced outside of Mexico, and is expensive and difficult to transport to rural and indigenous communities within the country.
To find a more local and sustainable solution, Isla Urbana selected the nopal cactus plant — also known as a prickly pear — for the team to test. Research already exists on the use of nopal as a reinforcer in building infrastructure. Furthermore, its wide availability in Mexico and its use in everyday life makes it easily accessible.
“They really wanted something that you could find in the backyards of local communities so that they could make the mixture themselves,” says Qayume.
“The goal is really to make these communities self-sufficient.”
As nopal can be incorporated into both the concrete mixture itself and also as a coating once the concrete has hardened, the team tested multiple formulations with varying ratios of cactus. They evaluated these formulations by curing small concrete blocks to serve as samples and running waterproofing assessment tests on each one, such as taking measurements of water absorption and permeability, along with physical characteristics such as porosity.
At one point, the volume of samples became a challenge. VanderWoude and Madanat joke that the blocks began to crowd them out of their own homes, with each student having to find room for 40 concrete blocks.
In addition to testing for waterproofing viability, they also had to be sure the nopal was not impacting the quality of the stored rainwater.
“Our main concern was to make sure that the nopal we’d be putting inside the water tanks wouldn’t affect the water’s drinkability,” says Qayume.
“Because nopal can eventually degrade, it could serve as a food source for bacteria or something potentially harmful into the water.”
The team addressed this concern by testing for microbial activity and other drinking water quality characteristics.
Their water quality testing narrowed down their formulations significantly, but in the end, three options still passed the assessments.
The study concluded that nopal is a credible candidate for further investigation as a waterproofing agent for ferrocement tanks, with a nopal mucilage-based coating being the most promising formulation, even outperforming the chemical product currently in use.
Based upon the results of the project, Isla Urbana is now planning to conduct field tests with the coating formulation.
“In Mexico, the nopal is an iconic plant found throughout the country, acting as a source of food, medicine and now hopefully we can confirm that it is a reliable raw material for constructing water storage tanks,” says Jesus Sotomayor, director of Isla Urbana’s non-profit organization, called Lluvia para Todos (Rain for all).
“If the field research comes back positive, it could mean an opportunity for some communities to start their own businesses producing the nopal as a sealant for use in various constructions. The results of this research strengthen the importance of integrating local resources and community knowledge into construction and other types of processes.”
Throughout the project, the student team had to get creative about supplies. They bought their cactus samples from a Mexican grocery store in Toronto and the chemical agent had to be flown in from Mexico.
They also had to work with limited equipment.
“One of the main constraints for this project was no fancy equipment, like we couldn’t even use a burner,” says Qayume.
“We had to work from the mindset that whatever we did had to be replicated in a resource-limited environment. That made us adaptable and it will hopefully help Isla Urbana implement our research.”
The team is quick to credit the many different departments and graduate students who helped them complete their research.
“There’s a lot of teamwork involved in a multidisciplinary capstone,” says Saville.
“Not just the capstone team but also the many PhD students in various departments and institutes who were integral in supporting this project.”
“It’s a highlight to be able to demonstrate our bright engineering students to the global community and it’s reflective of the impact we can and do have as a faculty.”
– This story was originally published on the University of Toronto’s Faculty of Applied Science and Engineering News Site on June 12, 2026, by Samantha Younan.