Seeing Our Future: Water Reuse with Seth Denizen

Denizen’s focus on this question stems from his longstanding research in global dry lands, particularly Mexico City and the nearby Mezquital Valley, a 380-square-mile arid region that produces much of the region’s food. The valley has been irrigated by wastewater from Mexico City for more than a century. In recent years, concerns over the way wastewater could impact public health have come into focus. “If cities want to resolve water reuse issues, Mexico City-Mezquital Valley hydrological region is the place to look,” Denizen says. “Contaminants in the soil have been accumulating there longer than anywhere else, so it’s a way of seeing our future and finding solutions that the rest of the world can use and learn from.”  

What started as a trickle of water in 1901 is now a 38-mile-long canal that manages the wastewater of 22 million people, sending about 60 cubic meters per second to 100,000 hectares of land, according to Denizen. Today there is conflict between farmers who have been fertilizing their fields for free with untreated wastewater for generations and national water regulators who want to treat the water as in the United States.  

In most U.S. cities today, wastewater is sent to treatment plants to remove sediments and bacteria like E. coli and salmonella, among other contaminants. “Once the water is treated, it goes back into a river, flowing to whomever lives downstream,” Denizen says. “Everyone has a responsibility to treat their water, because at a large scale, we’re all reusing water by drinking what has been treated by our upstream neighbors.”  

The wastewater system in the U.S. also requires a huge investment. “Not only are wastewater treatment plants enormously expensive, but they also require a lot of resources to operate effectively,” Denizen says. Everyone from operators and mechanics, to designers and environmental scientists, to universities that provide education for those jobs, and so on, has a part in making the system work well.

By contrast, Mexico City and the Mezquital Valley farm their wastewater, which is rich in vital plant nutrients nitrogen, potassium, and phosphorous — nutrients that U.S. farmers need to buy as synthetic fertilizer.

“So the question really is, do they reproduce the same wastewater system we have here, or do they try to make their current system more sustainable?” Denizen asks. 

Untreated wastewater poses threats to farmers, laborers, crops, and consumers. “Urban wastewater from a city of 22 million contains a lot of stuff,” Denizen says. “It has heavy metals from light industry, oil and gasoline that wash off the roadways, toilet waste, hospital waste, microplastics from laundry, caffeine, DEET from mosquito spray, and more.” Antibiotics are of particular concern, as they can produce antibiotic resistance in soil, leading to multidrug resistant superbugs.  

“There was a possible solution where the farmers would use concentrated sludge leftover from wastewater treatment as fertilizer,” Denizen says, “but the sludge is full of heavy metals, which are more toxic the more concentrated they are in the environment.” Without access to safe wastewater and without being able to safely utilize the sludge, farmers are left with a soil that will one day become too toxic to farm.

“Right now, the problem in Mexico City and the Mezquital Valley is framed such that there’s this irresolvable conflict and opposition between two parties,” Denizen says. “But if we can understand the problem in another way, is there a third strategy?” He’s quick to point out that he doesn’t see his role as finding a perfect-fit solution, but rather opening lines of inquiry to explore new fields of solutions. 

One such inquiry involves looking earlier in the system. “We often think about end-of-pipe solutions to wastewater, or what tools and technologies farmers should use to deal with contaminated water from cities,” Denizen says. “We need to flip the question around and ask what cities can do to create high-quality, nutrient-dense water that will fertilize crops and not endanger our own food supply and the people who grow it.”  

Denizen believes this process of water reuse is promising, building climate resilience, and resolving water scarcity issues. “In order to create a fertile soil in our agricultural areas, which provide our food, all we need to do is clean up our cities,” he shares. “We should all be excited about the possibility of living in cleaner, healthier urban environments.

In 2024, Denizen took a WashU landscape architecture studio to Mexico City and the Mezquital Valley to study its wastewater agriculture system. He emphasizes the importance of site-specificity, teaching designers to eschew their assumptions and instead experience a place firsthand to truly understand it. “We visited farmers whose lives and incomes depend on the environmental solutions we’re examining,” he says. “We were able to taste the pulque from the agave farms, visit landfills that were producing heavy metal contamination, and see the largescale urban compost facilities the city is building.”  

As graduate students explore solutions in their studio work, Denizen — who has nurtured relationships with community partners in Mexico for years — is able to take their ideas back to the people he works with for further consideration. At the same time, those partners can come to St. Louis to offer new perspectives on our environmental problems. Denizen invited soil scientist and research collaborator Christina Siebe to visit St. Louis and help his students understand urban soil in the city. “Creating this long-term, bidirectional flow of information can be incredibly beneficial,” Denizen says.  

Denizen also emphasizes the importance of drawing as a way of thinking for his own research as well as his students’ work. “As designers, we have this basic commitment to making visible the things we’re trying to understand,” he says. This is evident in his book, “Thinking Through Soil,” co-authored with Visiting Assistant Professor Montserrat Bonvehi-Rosich. The volume features drawings of soil characters, or processes that affect soil formation. They include historical, political, chemical, pharmaceutical, and industrial influences, and illustrate a physical change to the soil. “Soil is always so radically specific,” he says. “The problems in this region would be vastly different if the valley happened to have a sandy soil.”  

In October 2025, Denize launched a three-day symposium and design charrette with UNAM called Destino Mezquital: Repensando el drenaje de la Ciudad de México (Rethinking the drainage of Mexico City). “The Spanish word ‘destino’ means both destination and destiny,” Denizen says. “By invoking both meanings, we claim that the Mezquital Valley is both the literal destination of the wastewater, and also the site of a shared destiny with Mexico City as their environmental futures are inextricably intertwined.”

With his collaborators and with support from WashU’s Global Incubator Seed Grant, Denizen is working to publish a series of recommendations, images, and maps aimed to catalyze a conversation between designers and public policy experts. “I hope that the larger next step is to come back to the question of what the world can learn from this place, produce knowledge, and make it accessible so that others can learn from it and apply it to their conditions,” Denizen says.  

Denizen believes that landscape architects are uniquely well-positioned to address complex questions of social and ecological change like the global South’s water reuse systems. “We have the tools and the sensibility to take on those challenges,” he says. “And in the face of the massive climate shifts that we are going to face this century, we are going to have to learn how to navigate the social and political dimensions of environmental change in order to protect our basic sense of collectivity and environmental justice as the material basis of our lives gets renegotiated in fundamental ways. I think this is the new mandate for landscape architecture in the 21st century.”