Capturing carbon

Wastewater treatment could be a cost effective combatant of climate change

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The Boulder Wastewater Treatment Facility is one of many operations that contribute to the treatment of more than 100 trillion gallons of wastewater in the U.S. every year.
Courtesy of Cole Sigmon

In the U.S., we treat more than 100 trillion gallons of wastewater every year, enough to submerge a mile-wide swath from Boulder to Colorado Springs, one mile deep in wastewater. Treating that water requires 15 gigawatts of electricity, the equivalent of 6,000 wind turbines running full tilt.

But process innovations promise to reduce energy consumption in wastewater treatment, making it a contender in the fight against climate change.

Currently, treating wastewater uses a lot of energy, emits carbon dioxide and leaves behind sludge — leftover solids that are hauled to landfills. But researchers at the University of Colorado Boulder developed a new treatment process that captures CO2, generates energy and produces byproducts that could fight ocean acidification.

“Innovative technologies like this that address wastewater treatment, that address greenhouse gas emissions and address energy production all in the same technology, that’s totally where this industry needs to be headed,” says Cole Sigmon, wastewater treatment optimization specialist at Boulder’s Wastewater Treatment Facility.

This new method of treatment, called Microbial Electrolytic Carbon Capture (MECC), produces more energy than it uses, transforms carbon that would be emitted into something we can use, and treats wastewater in the process. The study was published in the July issue of the journal Environmental Science and Technology.

MECC works by running an electric current through wastewater and introducing minerals like calcium and magnesium. Electricity splits water into hydrogen and oxygen while carbon dioxide reacts with the minerals to form new compounds like calcium carbonate, which is basically limestone.

“Most minerals on the face of the Earth, in terms of quantity, will react with CO2 to form carbonates and bicarbonates as a natural mineral weathering process,” says Greg Rau, a senior researcher at the Institute of Marine Sciences at the University of California Santa Cruz and a co-author of the study.

“Our CO2 [in the atmosphere] will be absorbed in this way, only it will take a hundred thousand years for this to happen,” he says. “So our interest [in the study] has been ways of accelerating that weathering process.”

The compounds formed through MECC could be repurposed as construction materials for use in concrete, bricks and pavement. They can also serve as a buffer in wastewater treatment. Buffers lower wastewater’s acidity and could be used in coastal treatment plants to combat ocean acidification.

Excess CO2 in the atmosphere causes oceans to acidify. The Intergovernmental Panel on Climate Change estimates that between 2010 and 2015, wastewater released 60 to 63 million tons of CO2 into the atmosphere. When CO2 dissolves in water it becomes carbonic acid, the stuff that makes soda bubble.

“When the ocean becomes more acidic, all the marine life is going to suffer, especially the coral reefs, the crabs, the shrimp, they all make their shells out of calcium, which will be dissolved by the acidity in the ocean,” says Zhiyong “Jason” Ren, an associate professor of civil, environmental and architectural engineering at CU and senior author of the study. “Releasing this carbon buffer into the ocean is going to counter that pH drop, so that’s going to help alleviate the ocean acidification problem.”

Rather than waiting until excess carbon makes it into the ocean, researchers have been investigating methods of carbon capture and storage since the early 1970s as a way of slowing climate change by removing CO2 from the atmosphere. The process usually involves capturing emissions from large sources like coal-fired power plants where CO2 must be separated, purified, compressed and transported, often long distances, before finally being injected underground.

MECC captures carbon onsite and avoids the expense and energy of traditional carbon capture and storage. Avoiding one ton of CO2 emissions with MECC is estimated to cost $48, as compared with $70 to $270 at a coal power plant. By putting captured carbon to use, MECC sidesteps another problem of carbon storage: injecting CO2 deep into the Earth can cause minor earthquakes.

But MECC isn’t without complications. While keeping carbon dioxide out of the atmosphere is an undeniable benefit, carbon is actually necessary for wastewater treatment.

Bacteria remove nitrogen from wastewater, essentially by breathing it. But they need to be fed to do their job, and carbon is their food. New state and federal regulations coming down the line will require treatment plants to reduce nutrient levels in water even further, meaning a need for even more carbon in wastewater treatment facilities.

“There’s this competing market, so to speak, even within the wastewater treatment facility, for carbon molecules,” Sigmon says.

The Boulder treatment plant already divides carbon between feeding bacteria and sending it to anaerobic digesters, which process sludge and produce methane gas, which can be used to generate heat and electricity. MECC would introduce a third carbon competitor.

Nevertheless, Boulder’s plant supports the MECC research, providing wastewater samples and offering to host pilot studies.

“As a city who’s carbon conscious and energy conscious, innovative technologies like this are exciting because we know that we can’t continue doing things as we’ve done for the last 100 years,” Sigmon says.

There’s another element to MECC beyond carbon capturing: hydrogen.

MECC needs a jolt of electricity to get the process going and the rest is powered by chemical reactions in wastewater. This naturally occurring energy makes MECC more efficient than other forms of carbon capture.

The electricity that MECC requires is offset by hydrogen production. Splitting water releases hydrogen gas, which can be harnessed in fuel cells.

“We produce pretty pure hydrogen,” Ren says. “It can be stored and used for fuel cell vehicles. … So if you think [about] what’s the biggest competitor of Tesla … that will be fuel cell vehicles, which are actually more efficient and more powerful.”

Fuel cell vehicles have been criticized because although the cars themselves emit no greenhouse gases, hydrogen gas is generated using fossil fuels. MECC offers a renewable alternative.

Ren and Rau hope to move forward with research to get MECC out of the lab and into treatment plants. Ren wants to improve MECC efficiency, test different types of wastewater and work on scaling up from lab models to reactors that could handle the millions of gallons of wastewater that treatment plants receive every day. But first they need funding.

“It is always a big issue,” Rau says. “And so we’re working on that right now to try to locate partners and funding to do an expanded study of this proof of concept.”