What Role Does CO2 Play in Wastewater Treatment?

Modern urban wastewater treatment plant.

Wastewater is a huge threat to the world’s water resources. According to a UN Water report, 80 percent of all wastewater — sewage, agricultural run-off and industrial discharge — flows back into surrounding ecosystems without ever being treated or reused. 

This can have a devastating impact on the environment and to wildlife, particularly in the form of fish kills. Wastewater and farm runoff can lead to nutrient pollution in rivers and lakes, which deprives fish of oxygen, writes Dr. Frederic Beaudry, an associate professor of environmental science at Alfred University in New York.

There are two kinds of wastewater treatment: municipal and industrial. Municipal wastewater is the kind produced from homes, offices and other forms of public use. Industrial wastewater is a by-product of industrial activity, and it is a particular problem for the environment. 

Take the construction industry as an example. Concreting produces a huge amount of wastewater that looks clean once solids have been removed, but isn’t. In reality, wastewater from construction is incredibly basic, registering a pH of 12 to 13.5 (out of 14). This is far more alkaline than municipal wastewater. It’s dangerous to wildlife and poses a significant threat to humans if left untreated.

Neutralizing the pH level, as well as removing any other harmful toxins or chemicals, is the only way industrial wastewater can be safely returned to the environment. It is an essential part of any industrial operation, and a complicated process. But it’s one that can be effectively solved using CO2.

Treating Industrial Wastewater Presents Several Challenges

Wastewater is considered treated when it can be returned to the environment without causing the kind of harm described above. 

In the vast majority of cases, this treatment occurs on-site. Doing so is essential, writes Trimuvate’s Elise Musumano. But it isn’t easy. “Properly operating a wastewater system in accordance with federal, state, and local regulations includes many requirements.” Permits must be obtained, plans submitted, reports sent and tests carried out regularly to ensure that wastewater is treated satisfactorily. Failure to do so can lead to violation notices, fines and even criminal proceedings.  

To make things more difficult, industrial wastewater treatment is much harder and more complicated that municipal wastewater cleanup, writes mechanical consultant Amin Almasi. This is because industrial wastewater typically contains a mix of organic and inorganic matter, as well as non-biodegradable and potentially toxic components.

Almasi outlines the three specific methods for treating industrial wastewater:

  • Physical. Physical treatments remove dissolved and undissolved substances without changing their chemical structures. Methods used include adsorption, filtering, flotation and sedimentation.
  • Chemical. Chemical treatments use chemical reactions to remove substances by changing their molecular structure. This includes oxidation or reduction and chemical precipitation.
  • Biological. Biological treatments use living organisms to remove substances from water. 

The most common treatment is a mix of physical and chemical processes, says writer and researcher Derek Lotts. Chemicals are used to adjust the pH (typically to between 6 and 8 on the pH scale) and to change the ionic charge. But that could soon be changing.

The Industrial Wastewater is Discharged from the Pipe into the Water.

Chemical Treatments Pose Too Many Problems to Be an Effective Long-Term Solution

Chemical treatments, while initially effective, come with a series of unwanted challenges and side-effects, Andrew Kleisinger and M. Jason Stusick of MPE Engineering write. “Challenges related to shipping, handling and storage of the large volumes of acid that are required are significant.” 

A strict process must also be created and maintained to avoid overdosing chemicals and the problems that come with doing so.

“It is important that enough of each chemical is added to the flow to produce the required effect, but excessive dosing can lead to unnecessarily high ongoing materials costs and cause high levels of the treatment chemical to be present in the outflow to the sewer, potentially exceeding regulatory limits,” says the team at Filtration+Separation

Using chemicals to treat wastewater also presents a significant ongoing cost to businesses in the form of acquisition, storage and workers needed. 

The risk of chemical contamination is also a problem. While the chemicals used in wastewater treatment are effective at adjusting pH levels, sulfates and chlorides can easily find their way into water sources and cause the kind of environmental damage discussed above.

It’s no surprise, then, that companies are looking at other alternatives.

“In today’s competitive industrial manufacturing landscape, where companies face mounting cost pressures, tougher environmental regulations, and other operational risks, greater importance is being placed on wastewater technologies that, more than just compliance, offer opportunities for financial paybacks including other direct and indirect benefits,” writes Industrial WaterWorld’s Jeff Gunderson.

CO2 Wastewater Treatment Offers an Effective, Safer and Environmentally Friendly Solution

An alternative treatment solution is CO2. When injected directly into wastewater, CO2 causes the formation of carbonic acid, which naturally adjusts the pH to a suitable level. This treatment method is easy to control, and the byproducts it creates are harmless. None of the CO2 is released into the environment.

Not only does the CO2 wastewater treatment method produce harmless byproducts, a study by researchers Dr. Godpower Chimagwu Enyi and Dulu Appah found that carbonic acid was much more effective at controlling pH levels than sulfuric acid (H2SO4). A further advantage of CO2, specific to this study in particular, was that 90 percent of the CO2 used was captured from an ammonia plant — i.e. it was free — whereas the sulfuric acid had to be purchased.  

CO2 and carbonic acid are also much safer than chemical alternatives, notes Tsugio Tachibanaki, general technical department manager at Watertech. “You can use CO2 gas safely because even if you inject too much gas, the pH will not drop below a certain value, like it can with sulfuric acid or other reagents.”

This value is around 6 on the pH scale. Further, the carbonic acid created as a result of the CO2 injection is much weaker than the sulfuric acid and chlorine used in chemical treatments. This means that the pH of wastewater falls more slowly and is thus easier to control. 

But not all CO2 wastewater treatment solutions are created equal. Traditional CO2 wastewater treatment solutions inefficiently inject carbon dioxide into the water and allow gas to escape back into the atmosphere. Hence, some of the environmental benefits of a CO2 solution are nullified.

TOMCO2 Systems‘s patented PSF system is designed to inject the gas molecule into a carbonic acid solution. Then, the carbonic acid flows into the water stream, allowing the CO2 to effectively control pH and return it to a safe level.

Images by: dmitrimaruta/©123RF.com, Alexey Morozov/©123RF.com, Jeremy Bishop