Curious Iowa: How do wastewater treatment plants work?

As the Cedar Rapids Water Pollution Control plant undergoes a $318 million modernization project that is expected to be completed as early as the end of 2029, the plant is getting extra attention from passersby.

Curious Iowa, a series from The Gazette that answers readers’ questions about our state and how it works, has received a number of questions related to how the plant works and details about the plant’s ongoing upgrades.

Peter Larson, of Cedar Rapids, wrote in asking for an explanation of the “anatomy of a (wastewater) treatment facility,” while a group of friends from Cedar Rapids sent their combined list of questions, including “How long does it take to treat wastewater before it is discharged back into the Cedar River?” and “What happens to poop — does it become powder and used in fertilizer, or is it burned?”

We spoke to officials at the Cedar Rapids facility and Coralville’s Wastewater Treatment Plant to find out how these plants treat millions of gallons of water every day.

Wastewater treatment plants by the numbers

The City of Cedar Rapids is unique in that it has an advanced wastewater treatment plant that can handle industrial strength waste without it being pre-treated.

Roy Hesemann, Cedar Rapids utilities director, told The Gazette that industrial waste is sent down the pipes to the plant just as commercial and residential waste is. The water is treated and the liquids are returned to the Cedar River and the solids are primarily incinerated.

The Cedar Rapids plant serves all of Cedar Rapids, Marion, Hiawatha, Robins and Palo — which have a combined population of about 190,000. About 13 billion gallons of water are treated annually in Cedar Rapids, Hesemann said, averaging out to about 36 million gallons per day. Although, the plant is designed to handle 86 million gallons per day.

“From the time a drop of wastewater enters the facility, it takes approximately four to seven days to reach the outfall depending on the flow rate in our system at the time,” Lauren Michael, Water Pollution Control plant manager, said.

The facility has four operators and one supervisor monitoring and maintaining the plant every hour and every day of the year.

Coralville’s wastewater treatment plant is designed for commercial and residential waste. The plant serves a population of 23,000 people and about a billion gallons are treated each year.

“On our current flow numbers, every gallon that comes into our plant stays here for roughly two days before it gets discharged,” David Clark, Wastewater Department superintendent for the City of Coralville said, noting that about 2.5 million gallons are treated daily.

The Coralville plant is designed to treat 5.75 million gallons of wastewater every day.

Both cities’ stormwater sewers are separate from the sanitary sewers, although Clark said rain events can still have an impact on inflow of water.

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Both of the plants have a lab on site, although smaller wastewater plants may not.

Michael said having a lab allows them to be “more nimble.”

“We are constantly checking our biochemical oxygen demand, that strength that’s coming in, or checking ammonia on that effluent that we’re sending out to the river because ammonia can be super toxic to fish as well,” Michael said, adding that they also test for metals and E. coli.

How does water get into the wastewater treatment plant?

Water travels through about 105 miles of pipe in Coralville and more than 700 miles of pipe in Cedar Rapids to get to the respective treatment facility. Gravity is the key to getting water to the plants, and lift stations may be used to accomplish this.

When water arrives at the Water Pollution Control Facility, 7525 Bertram Rd. SE in Cedar Rapids, the main lift station brings the flow up 60 feet so gravity can start the treatment process.

First, water is screened. Clark said this mechanical process removes sand, rocks, eggshells, paper towels — anything that gets into sewer pipes.

“So there’s no (chemical) treatment, just removing the stuff we don’t want in there because it could fill up pipes, plug pipes, fill up tanks and reduce our treatment capacity,” Clark said.

Hesemann said a number of unique items have been removed through this process over the years.

“We always advise people to not flush the so-called flushable wipes because it really does mess up things, but we’ve also seen little kids’ toys that’s been flushed down the toilet … one of our sewer maintenance staff found a diamond ring one time,” he said.

A mechanical process removes items stuck to the bar screen and drops it onto a conveyor belt that moves debris to a dumpster. Michael said the dumpster box is emptied to the landfill once or twice a week.

Virtual Tour of Cedar Rapids Plant

View a virtual tour of the Cedar Rapids Water Pollution Control Facility at cedar-rapids.org/WPC/virtual_tour/index.html.

How is wastewater treated?

“Every plant’s a little bit different, but the concept is still the same,” Clark said. “Everybody's going to have screenings and grit removal, everybody's going to have secondary treatment. You might not have clarifiers in some places, depending on the process.”

In Cedar Rapids, after the water is screened, it goes through clarifiers, which are wide spots in the treatment process that allow heavier materials to settle to the bottom and fats, oils and grease to rise to the top. A machine arm skims the materials at the top and bottom of the clarifiers into a hopper in the center and those materials are moved for treatment as part of the solids process.

The liquids are then pumped onto roughing filters, which spin, distributing the water across plastic media. The water trickles down and microscopic organisms capture the water and use organic matter as food.

“We call ourselves glorified bug farmers,” Hesemann said. “We really are relying on natural biology to break down the organics and the materials in that wastewater stream so that by the time it’s through the process, there’s very little of that organic matter going back out to the river.”

Michael said these “bugs” are the plant’s “workhorses” and noted that because they regenerate, they are a cheaper option than using man-made filters.

The water is cycled through multiple times before moving onto the carbonated activated sludge (CAS) process, which Hesemann described as “bugs put on steroids.”

“We’re feeding pure oxygen into that area and the pure oxygen will accelerate the bugs’ ability to break down that material,” he said. The water goes through this process multiple times.

Then, the water goes through an additional nitrifying activated sludge (NAS) process where ammonia is reduced, water is disinfected and chlorine is neutralized before it is sent to the Cedar River.

Water from the Coralville facility is sent to the Iowa River.

“In disinfection we use ultraviolet light … so the bacteria that are still remaining in the water basically get killed by the UV light, or if they’re not killed, they get inhibited so they can’t reproduce,” Clark said.

How are solids reduced?

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After the solids have been separated from the liquids, both Hesemann and Clark described the material as resembling chocolate milk. In Cedar Rapids, most of the solids are incinerated whereas in Coralville, treated solids are applied to agricultural land. Regardless of the end goal, water needs to be removed from the solids.

One of the dewatering processes places solids on a gravity belt thickener. Liquid falls through the porous, rotating belt allowing the solids to dewater.

A low pressure oxidation process heats and pressurizes the solids, conditioning them for more efficient dewatering, Hesemann said.

After this, the Cedar Rapids plant moves the solids to the incinerator where a majority of the waste is burned.

“There’s 5 to 8 percent of inorganics, things like sands and things that just won’t burn in the incinerator,” Hesemann said. “We pump those out to our ash lagoons.”

Hesemann said that the incinerator uses renewable natural gas from biosolids.

“So right now we have one stream coming in from a couple of industries that we bring into a separate process, it’s called our anaerobic process … so we break that down and we generate gas,” Hesemann said.

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As part of its ongoing modernization project, the Cedar Rapids facility is adding an anaerobic digester complex to create and store biogas. Anaerobic digesters are oxygen-free tanks where bacteria break down organic matter, creating biogas.

Currently, a small portion of the plant’s industrial waste stream is being utilized as renewable natural gas. This addition will allow the facility to maximize the production of renewable natural gas.

“It’s doing the right thing by capturing that resource that’s already in the waste stream and just releasing the gas and making a better environment for everyone,” Hesemann said.

If the incinerator is down for maintenance, a period that lasts about 10 days, Hesemann said a secondary process is used. Lime is added to solid waste to bring up the pH above 12. He said that the biosolids can be applied as fertilizer “for soil stabilization and enhancement.”

In Coralville, solids are treated to meet federal regulations for agricultural land application. A variety of processes are used to break down, dewater and stabilize the solid waste before samples are tested.

“And once it meets the federal regulations we can land apply it on agricultural ground like corn, beans and hay,” Clark said, noting that before application it is moved to a storage tank that can hold 1.6 million gallons. Land application is done twice a year in the spring and fall.

How do wastewater treatment plants limit the smell?

“We don’t do any odor reduction. If your plant’s running properly, there’s really not a whole lot of odors,” Clark said of the Coralville plant, adding that Cedar Rapids is an outlier due to its status as an advanced wastewater treatment plant.

The Cedar Rapids plant has eight odor scrubbers on site.

“We capture, as much as possible, the air that comes off of our roughing filters, off of our storage tank, off of various different processes, and then run that through scrubbers,” Hesemann said.

Large industrial fans bring odorous air in through the scrubbers. Within a lava bed inside the odor scrubber, microscopic bugs live, grow and capture odorous compounds from the air stream, Hesemann said.

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“And then we’re also adding water so that it stays wet. It’s a moist media, so we’re not drying out that lava rock,” Hesemann said.

Reduced levels of odorous air exit the scrubbers. Hesemann said only certain parts of odorous air can be captured and cleaned, which results in some odors escaping the plant.

“We try to be very conscious of our neighbors and be good neighbors,” Hesemann said. “Again, that’s why we spent over $7 billion a few years ago on upgrading that equipment and trying to meet that need.”

Clark said that the water and wastewater industry is facing challenges replacing workers who have reached retirement age. He noted that Des Moines Area Community College has a training program for water and wastewater operators, although one could get hired as a laborer without a degree.

“If people are looking for a good career field, this is a good place to start,” he said.

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