Using sewage discharged from wastewater treatment plants to sequester CO2 and stop it from reaching the atmosphere

Wei-Jun Cai is the Mary A.S. Lighthipe Chair of Earth, Ocean, and Environment in the College of Earth, Ocean and Environment.
Wei-Jun Cai is the Mary A.S. Lighthipe Chair of Earth, Ocean, and Environment in the College of Earth, Ocean and Environment.
Using sewage discharged from wastewater treatment plants to sequester CO2 and stop it from reaching the atmosphere

UD Prof. Wei-Jun Cai proposes adding alkaline materials to sewage discharge

With anthropogenic carbon dioxide (CO2) emissions continuing to pummel the environment, increasing atmospheric CO2 and contributing to global warming, the Intergovernmental Panel on Climate Change has said that simply reducing anthropogenic CO2 emissions will not be enough to avoid a global warming catastrophe. Novel solutions are needed to help safely, permanently, and cost-effectively sequester atmospheric CO2.

Wei-Jun Cai, the Mary A.S. Lighthipe Chair of Earth, Ocean, and Environment in the University of Delaware’s College of Earth, Ocean and Environment, recently published an article in “The Innovation”, a scientific journal, suggesting a new approach to treating sewage. The paper, which is an idea at this point and not peer-reviewed research, calls for an Ocean Alkalinity Enhancement (OAE) approach, in which alkaline materials could be applied to the sewage discharged from wastewater treatment plants to help sequester CO2 and stop it from reaching the atmosphere.

OAE involves adding materials with high alkalinity to increase the speed of removing CO2 from the atmosphere and decrease ocean acidification.

The paper is co-written by Nianzhi Jiao, a researcher at Xiamen University in China.

Materials with high alkalinity include calcium hydroxide, sodium hydroxide — more commonly known as lye — and calcium carbonate, which is found in minerals such as limestone and dolomite, and silicate minerals such as olivine.

As an expert in coastal CO2, Cai said that as he started to read the literature on Carbon Dioxide Removal (CDR), it became clear to him that he could pull on past experiences to come up with a potential solution. A study Cai conducted a few years ago measured the pH of waters in the Delaware River around Philadelphia. (The term pH is a “measure of hydrogen ion concentration in a logarithmic scale” and the measure of how acidic or alkaline a solution is, with 7 being a neutral compound. Numbers below 7 indicate acidic conditions and numbers higher than 7 indicate alkaline or basic conditions.)

“When the Schuylkill River flows into the Delaware River, the pH is very low, possibly because of the sewage discharge from Philadelphia,” Cai said. “Inside the sewage effluent, however, the pH was even lower. So that gave me the idea that we may increase the pH of the sewage to try and achieve carbon dioxide removal. After the water is treated at the plant, we are discharging the sewage into the environment anyway so I think this would be a prime candidate for this experiment.”

Previous papers have theorized about carrying out OAE by adding alkaline materials to the open ocean, but Cai said adding highly alkaline materials directly to the open ocean could have unintended, negative environmental consequences. Because the ocean is already saturated with respect to calcium carbonate, adding alkaline materials directly to the open ocean would cause calcium carbonate to precipitate out and fall through the water column and reach deeper into the ocean, which would make the OAE ineffective and also harm biological life.

In addition, because some of the materials, such as calcite and dolomite, are already saturated in the surface ocean, they simply wouldn’t dissolve. It is also not cost-effective to transport highly alkaline materials and apply them to the open ocean.

The coastal water and the wastewater effluent have low pH and high concentrations of organic acids, so they provide a favorable condition for minerals such as calcite, dolomite and olivine to dissolve at meaningful rates and to capture carbon.

This process also allows the highly alkaline materials such as sodium hydroxide to be added to the coastal waters without the fear of causing calcium carbonate precipitation, as they will lock in the carbon and deliver strong bases when they reach the open ocean and thus will allow OAE to be more effective and lessen their environmental impacts.

Once river plumes and ocean currents bring the alkalinity enhanced materials out to the vast ocean, nature will do the job to sequester CO2 from the atmosphere. “The key here is we want to lock CO2 into the ocean for a long time,” Cai said. “If you take out the CO2, but then the next year, it goes back to the atmosphere, that wouldn’t be good. We need to transport it and lock it in the deep ocean as bicarbonate.”

Once the carbon is locked into the deep ocean as bicarbonate, it can be stored for hundreds or thousands of years without the fear of returning to the atmosphere as CO2, buying time for fossil fuel emission reduction.

Cai said that this approach could have a clear environmental benefit, and as the wastewater effluent is heading out to the ocean anyway, it makes sense to add alkaline materials before it gets there.

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However, he said that there needs to be a lot more research on the idea before it becomes a reality.

“It’s an idea and we need both laboratory and field experiments to figure out the details on how do we add alkaline material to the wastewater and how effectively the river plume and ocean current will take them out,” Cai said. “The latter question will also need regional and global numerical models. So both field experiments and modeling simulations would be needed to further study this. Right now, it’s a good idea, but it needs to have a lot of studying involved.”



More from: University of Delaware | Xiamen University



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