I toured New York City's largest wastewater-treatment plant, which collects poop from 1 million people. It was stunning.

The plant recently underwent a $5 billion renovation and expansion, so it looks shiny and new.

The Newtown Creek plant was originally built in 1967. Nearly three decades later, the city announced plans to expand the facility and make upgrades to comply with the US Clean Water Act, which requires at least 85% of pollutants to be removed from wastewater before it can be discharged into local waterways.

At the time, neighbors were opposed to the expansion because they had grown tired of the rotten egg smell (which no longer exists).

The designers opted for materials that make the building stand out — a notable difference from most wastewater-treatment plants.

"It's hard to hide a 110-foot-tall building," Richard Olcott, one of the plant's designers, told the Ornamental Metal Institute of New York. "So, rather than hiding it, we thought we should do the opposite: We should really show the thing off and make something everyone would look at and go, 'Wow! What's that?'"

The fist thing you see when you walk through the gates is the plant's massive pipe system, which controls for odors.

The plant usually accepts 250 million gallons of wastewater each day from toilets, sinks, and storm drains.

After wastewater travels through sewers to the treatment plant, it passes through screens that remove large chunks of garbage like bottles, rags, newspapers, and plastic cups. Pam Elardo, New York's deputy commissioner of wastewater treatment, said one the most common forms of trash is baby or facial wipes that claim to be flushable but aren't.

All that trash is then sent to a landfill.

From there, wastewater travels to sedimentation tanks, where heavier solids (like poop) sink to the bottom. Lighter particles, including grease and small bits of plastic, float to the top.

This separation makes the top particles easy to remove. The bottom ones — the "sludge" — are sent to a device that spins them rapidly. The centrifugal force helps sift out tiny particles like coffee grinds from the bulk of the waste.

The sludge then moves to large "bubbling tanks," where it's mixed with air. The process encourages oxygen-using bacteria to feed on pollutants in the water.

After about three to six hours, the mixture moves to "settling tanks," where heavy particles are further sifted out. Some return to the "bubbling tanks" to help stimulate the growth of more good bacteria that feeds on pollutants.

Once the treated water is ready to be disinfected, it goes to tanks containing sodium hypochlorite, a chemical found in bleach, for about 15 to 20 minutes. Then the water is discharged into local waterways.

But some sludge remains even after this long treatment process. Those particles are thickened and sent to "digester eggs."

The eggs are clad in stainless steel and light up at night. There are eight of them at the facility.

When the sludge arrives, it gets heated to around 95 degrees Fahrenheit in an oxygen-free environment, where it remains for 15 to 20 days. This creates more good bacteria that breaks down the sludge into water, carbon dioxide, and methane gas.

The egg shape helps conserve energy for churning waste.

Our tour group took an elevator to the top of these "digester eggs."

The eggs are connected by a set of glass walkways 110 feet in the air.

The walkways are lined with sturdy guardrails. At each intersection, you can see some of the equipment used to control the "digestion" process.

Most of the tour was odor-free, but there was certainly an unpleasant stench near the digesters.

I understand why Greenpoint residents used to complain about the rotten-egg smell.

If you look closely, you can see what's happening inside the digesters.

We didn't get to see the sludge up close, but were told it's black in color and has the consistency of pea soup.

After up to 20 days, the sludge is moved from the eggs to storage tanks.

Eventually, it gets transferred from a storage tank to a "sludge boat" that takes the waste to another plant in the Bronx. That plant removes liquid from the sludge, forming it into a "cake" that can be used as fertilizer.

By the end of the tour, the elevators had temporarily stopped working. (The staff had warned us earlier that the elevators were fickle.) So we exited down 11 flights of stairs.

Back on the ground, the guides explained that the facility is exploring more uses for its byproducts. For example, the plant is working on a pilot program to collect food waste, add it to the digester eggs, and produce methane gas that could power local homes.

But there are still some issues to sort out: Elardo said that although New York City's water quality is the best it has been in 100 years, facilities like Newtown Creek still don't filter for every known contaminant.

"We're designed to take human waste and industrial waste and make it clean," Elardo said. "There are a lot of exotic things that come our way that we shouldn't necessarily be spending a lot of time, energy, and money treating."

Turning the wastewater from sewage pipes into "100% H2O" could cost "hundreds of billions of dollars," she added.

That means microplastics — tiny bits of plastic less than 5 millimeters long — can linger in the treated water that enters local waterways.

The best way to treat microplastics, Elardo said, would be to cut them off at the source — the way the city already does with silver from dental facilities.

"In the old days, we got tons of silver in our system that actually contaminated the biosolids. There was a huge national program to make dentists pull the silver out at their offices," she said. "That's the kind of source control I'm talking about — either treat it at the source or eliminate the product."

By the end of my visit, I was struck by how the facility combines such a practical mission — cleaning people's poop — with stunning design elements.

Whenever I do the dishes or flush the toilet at home, I'll think of my water's journey toward those giant steel eggs.