Mexico City Loves Street Food. Its Sewer System Does Not.

When the heavens opened up over Mexico City this year, chaos ensued in the streets.

Water flooded homes, roads, sidewalks, metro stations and even the airport, in what officials called the worst rainy season in at least 40 years.

Contributing factors to the frequent flooding, experts said, including climate change, an antiquated drainage system, rapid urban growth and the ground slowly sinking under the city.

But one ingredient, at least, is in many citizens’ direct control: trash, especially grease, especially cooking waste.

“Imagine a layer of fat in your arteries, like cholesterol,” said Ricardo Munguía, who oversees the hydraulic infrastructure for Mexico City’s water and sewer agency. “It’s a big problem.”

“We as citizens are to blame,” said Gloria Pantaleón Heredia, a 79-year-old resident of northern Mexico City, where heavy rains gathered into deep pools during the rainy season. She faulted a nearby taco stand for not being careful with its grease runoff, and her neighbors for not picking up debris that blocks drains.

“If we were conscientious,” she said, her own broom nearby, “we’d say, ‘I’m going to sweep my part of the sidewalk and pick up the trash.’”

This hazard isn’t unique to Mexico City. London has often dealt with fatbergs — greasy masses weighing as much as 140 tons — as has New York.

But experts said that Mexico City was distinctly vulnerable to clogging. Parts of the capital, once a water-rich valley that was drained to build a vast city, are sinking as much as 20 inches per year because of excessive extraction of groundwater. (The sinking has also been exacerbated by droughts.)

Drainpipes, on average five decades old, face growing strains above ground, too. The metropolitan area has exploded over the decades to 23 million people now, with some areas built without the proper infrastructure.

“And it’s raining more, so it worsens the drainage that was obviously designed for a city from 50 years ago,” said Mauricio González González, the general director of urban services for the Cuauhtémoc municipality in Mexico City.

Then, of course, there is Mexico City’s famous food.

The city has over 10,000 registered taco shops, with many informal, temporary street stands that likely aren’t in any books.

Nor is it just their products — like carnitas (pork slow-cooked in its fat) or suadero tacos (confit-style beef) — that produce enormous quantities of grease. There are churros (fried dough), chicharrón (fried pork skin), gorditas (fried stuffed corn cakes) and more.

Every time someone eats suadero, they should feel some responsibility for the flooding problems, joked Omar Arellano-Aguilar, an expert in environmental risk assessment at the National Autonomous University of Mexico.

Everyone could do a better job at raising awareness of what residents can do, he said, and the government in “Mexico City fails on many occasions to have a preventative policy.”

While businesses are required by law to dispose of their cooking waste properly — using a special collection service, for example, or taking it to a collection center — officials said many simply dump it down a sink or storm drain.

“It causes a very large blockage and it’s difficult to wash,” said Mr. Munguía, the sewer agency official, pointing to a dredging truck that uses a high-pressure hose and a suction tube to clear pipes.

Trash, he said, can at least be scooped out of the manholes. But fat does not dissolve in water and requires more effort to dislodge.

Many parts of Mexico City, particularly those with flooding problems or many restaurants, have been warning people to be careful about how they dispose of fat and trash. Cuauhtémoc launched a program in September to collect more used oil from markets.

“It’s complicated because of the frequent irregularity of businesses,” said Mr. González González, who noted the difficulty of catching someone in the act.

Ignacio Rodríguez, 47, who sells chicharrón at several street markets throughout the metro area, said he strains his oil and puts pork lard in the trash every day — but he knows others who do not.

Fines, he said, would force them to dispose of waste properly, too.

“The municipalities have to be strict,” he said.

The rainy season’s end, in October, kicked off the busiest time of year for Mr. Munguía and his crews, who clean sewer lines, clear tree roots and install new pipes in the old systems. The city, he said, tripled its fleet of vehicles this year, gaining 40 new dredging and cleaning trucks.

The crews also take on some of the worst blockages.

One recent morning in Mexico City’s historic center, when Cuauhtémoc municipal workers removed a manhole cover, a repulsive smell wafted out.

With a shovel, they scooped up chunks of debris. Then they ran a high-pressure water hose into the sewer. Half an hour later, they dislodged the true culprit: a nearly two-foot block of fat.