UD Prof. Wei-Jun Cai suggests adding alkaline materials to wastewater disposal

As anthropogenic carbon dioxide (CO2) emissions continue to pound 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 global warming catastrophe. New solutions are needed to help sequester atmospheric CO2 safely, sustainably and cost-effectively.

Wei-Jun Cai, Mary AS Lighthipe Chair in Earth, Ocean and Environment at the University of Delaware College of Earth, Ocean and Environmentrecently published an article in Innovation, a scientific journal, offers a new approach to wastewater treatment. The paper, which at this stage is an idea rather than a peer-reviewed study, calls for an ocean alkalinity enhancement (OAE) approach in which alkaline materials can be applied to wastewater discharged from sewage treatment plants to help to sequester CO2 and stop it from reaching the atmosphere.

OAE involves the addition of highly alkaline materials to increase the rate of CO2 removal from the atmosphere and reduce ocean acidification.

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

High-alkalinity materials include calcium hydroxide, sodium hydroxide—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 on CO2 in coastal areas, Cai said that when he started reading the carbon dioxide removal (CDR) literature, he realized he could use past experience to come up with a potential solution. A study conducted by Cai several years ago measured the pH of waters in the Delaware River around Philadelphia. (The term pH is “a measure of hydrogen ion concentration on a logarithmic scale” and a measure of how acidic or alkaline a given solution is, with 7 being a neutral compound. Numbers below 7 indicate acidic conditions, and numbers above 7 indicate alkaline or basic conditions.)

“When the Schuylkill River flows into the Delaware River, the pH is very low, probably due to the discharge of sewage from Philadelphia,” Kaye said. “Inside the wastewater, however, the pH was even lower. So that gave me the idea that we could increase the pH of the sewage to try to achieve carbon dioxide removal. After the water is treated in the plant, we release sewage into the environment anyway, so I think that would be a prime candidate for this experiment.

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

Also, since some of the materials, such as calcite and dolomite, are already saturated in the surface ocean, they simply would not dissolve. It is also not cost effective to transport highly alkaline materials and apply them in the open ocean.

Coastal waters and wastewaters have low pH and high concentrations of organic acids, so they provide favorable conditions for minerals such as calcite, dolomite, and olivine to dissolve at significant rates and sequester carbon.

This process also allows highly alkaline materials such as sodium hydroxide to be added to coastal waters without fear of causing calcium carbonate to precipitate, as they will lock in the carbon and supply strong bases when they reach the open ocean and thus allow OAEs to be more efficient and reduce their environmental impact.

After river currents and ocean currents carry the materials of increased alkalinity into the vast ocean, nature will do the work to remove CO2 from the atmosphere. “The key here is that we want to lock CO2 in the ocean for a long time,” Cai said. “If you take CO2 out, but the next year it goes back into the atmosphere, that’s not going to be good. We have to transport it and lock it in the deep ocean as bicarbonate.

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

Cai said this approach could have clear environmental benefits, and since wastewater is headed for the ocean anyway, it makes sense to add alkaline materials before it gets there.

However, he said the idea needs a lot more research before it becomes a reality.

“This is an idea, and we need both laboratory and field experiments to understand the details of how to add alkaline material to wastewater and how effectively the river plume and ocean current will remove it,” Cai said. “The latter issue will also need regional and global digital models. So both field experiments and modeling simulations will be needed to investigate this further. It’s a good idea at the moment, but it has to involve a lot of learning.”


Since: University of Delaware | Xiamen University

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

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