Secondary clarifier: The secondary clarifier allows more solids to settle out from the liquids. Photo courtesy City of Lincoln, Nebraska
Secondary clarifier: The secondary clarifier allows more solids to settle out from the liquids. Photo courtesy City of Lincoln, Nebraska

By Gabe Rivin

Thanksgiving is a notorious day of overeating, as family members stuff themselves with turkey and pie. The stomachaches can be brutal.

Pumps and pipes transport waste from cities to wastewater-treatment plants and move the waste throughout the plants. Photo credit: Gabe Rivin

But something that often isn’t discussed over the feast – understandably – is what happens afterward, when the toilets are flushed and the stomachaches abate.

In other words, what happens to our poop after it goes down the toilet?

With Thanksgiving just around the corner, North Carolina Health News toured a wastewater treatment plant that serves Chapel Hill and Carrboro. The goal: to see where all those digested meals will end up.

And though it’s not exactly dinner-table conversation, the fate of human waste might surprise you – from its transformation into farm fertilizer to its use in air conditioning systems on UNC-Chapel Hill’s campus.

Step 1: It hits the sewers

The toilet handle goes down, the water spins, and it’s all gone. At least from the bathroom.

Everything flushed down a house’s toilet travels through a small sewage pipe called a service lateral. That pipe leaves the house and connects to a larger municipal pipe, or sewer main. (This is all assuming that the house is connected to a municipal system, though about half of North Carolinians use off-the-grid septic systems.)

From here, municipal sewers carry our waste toward a wastewater-treatment plant.

Step 2: It hits the headworks

The raw, untreated sewage enters the headworks, a structure that screens out solid objects like floss and flushable wipes. These objects belong in a garbage can, not the toilet, according to John Kiviniemi, manager of wastewater treatment and biosolids recycling at Orange Water and Sewer Authority and the leader of the day’s tour.

Wastewater-treatment plants screen out solid objects that, Kiviniemi said, should not have been flushed. Photo credit: Gabe Rivin

“The only thing that should go in a toilet is what comes out of you, as well as toilet paper,” he said. “That’s one thing we’ve told people as part of our public-education outreach: ‘Don’t use toilets as a trash can.’”

The headworks include screens that catch solid materials and a system that transfers these materials to a dumpster. That Q-tip you flushed instead of throwing in the garbage? A city employee made sure it was removed from wastewater and that it landed in a trash bin.

Step 3: It’s off to the settling tanks

Next the waste heads to large tanks known as primary clarifiers. Here organic solids slowly settle to the bottom and separate from the liquids. OWASA covers these tanks, which helps stifle smells.

From here, the liquids and solids take different paths.

Step 4a: The solids get turned into fertilizer and gas

This building houses belts that thicken solid waste, a process that prepares it for the adjacent anaerobic digesters. Photo by Gabe Rivin

At OWASA’s wastewater-treatment plant, the solids are shuttled to another building, where moving belts thicken them. The thickened solids are then transferred to a series of anaerobic digester tanks. These tanks turn human feces into a material that’s usable as farm-grade fertilizer.

The tanks are swimming with anaerobic bacteria: microorganisms that survive without oxygen. These bacteria break down the waste over about 40 to 50 days. In the process, they kill off harmful pathogens and leave behind nutrient-rich materials called biosolids, which can be used as fertilizers.

OWASA gives away these biosolids to farmers in Orange, Alamance and Chatham counties. It even applies the biosolids for free to farmers’ fields.

The utility also makes sure the biosolids reach a local company, which uses them to produce salable compost.

“These things have traditionally been looked at as waste byproducts to get rid of,” Kiviniemi said. “Now people are realizing they’re a resource, and there are markets opening for the use of these products.”

The bacteria also produce methane while they break down the waste. OWASA burns this gas in order to power a boiler, which keeps the anaerobic digester hot enough. It’s a self-powered system.

OWASA also burns, or “flares,” its excess methane into the open air. The utility hopes to one day avoid this by supplying the gas as a source of energy, with a possible partnership with UNC-Chapel Hill, Kiviniemi said.

Step 4b: The liquid gets a first treatment

Back to the liquids.

Liquids in a treatment tank, where bacteria consume organic matter and nutrients. Photo credit: Gabe Rivin

After the liquid and solid wastes separate (step 3), the liquids are pumped to a series of treatment tanks. These rectangular tanks are full of cloudy, brown, frothy liquids. Sprinklers spray water over the tanks, which helps get rid of the froth.

These tanks are full of bacteria that break down organic matter and nutrients in the liquids, including nitrogen and phosphorous. This cleans the water and prepares it for the next step.

Step 5: The liquids settle again

The secondary clarifier allows more solids to settle out from the liquids. Photo courtesy City of Lincoln, Nebraska

The liquids are shuttled to another series of settling tanks, known as secondary clarifiers. In these wide circular pools, the remaining solids are given time to settle to the bottom. These solids will ultimately be pumped to the anaerobic digesters.

By now, the liquid is a clear water. It pours over the edges of the clarifier, where it flows toward its next destination: filtration.

Step 6: The water gets one last filter

The water travels through a series of raised troughs. Then, like a waterfall, it drops into a bed of sand. This sand captures the last remnants of solids.

Step 7: The water gets disinfected

The water may be clear, but it’s not yet free of harmful pathogens.

The liquids fall into a bed of sand, which acts as a final filter. Photo credit: Gabe Rivin

In OWASA’s plant, the filtered water passes through closed channels with ultraviolet lights.

“The UV lights inactivate the pathogenic organisms,” Kiviniemi said. “It disturbs the replication ability of those organisms … so they are no longer pathogenic.”

Other wastewater-treatment plants use chlorine to disinfect the water. But after doing so, the water has to be de-chlorinated, since chlorine can be toxic in aquatic ecosystems, Kiviniemi said.

Step 8: Goodbye to the liquids

The water is now harmless and clear. And it’s ready to leave the plant.

Most of OWASA’s treated water is released into Morgan Creek, which ultimately leads to Jordan Lake, a major drinking water source in the Triangle. Photo credit: Gabe Rivin

OWASA sends this water to a couple places. One is UNC-Chapel Hill and UNC’s hospital system. The university uses the water to irrigate athletic fields, including the field at Kenan Stadium. It also uses the water in cooling towers, which provide university buildings with air conditioning.

Still, OWASA releases the majority of its water into Morgan Creek. This creek ultimately leads to Jordan Lake.

And that lake is a reservoir that supplies water to a number of communities in the Triangle. What once was feces and urine, then, ultimately flows as drinking water – through kitchen taps and bathroom showers.

Of course, the drinking water in residents’ taps has gone through its own lengthy treatment process. But that’s a story for another holiday.

For now, it may be best to think about turkey and pies.

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Gabe Rivin

Gabe is our former environmental health reporter from 2014-2016. He is a former editor of The Cooperative Business Journal, and a former reporter for Inside Washington Publishers, where he covered federal...