Pioneering Ocean Carbon Removal technique proves successful in pilot study

1 July 2024

Scientists from Plymouth Marine Laboratory (PML) and its commercial subsidiary PML Applications, have confirmed the viability, and potential scalability, of a novel “ocean” or “marine” carbon dioxide removal (oCDR or mCDR) technique designed to remove carbon dioxide from the atmosphere by enhancing the alkalinity of treated wastewater before it is discharged out at sea.

In a first-of-its-kind study, the PML team was commissioned to provide independent and impartial monitoring and analysis of a field trial carried out off St Ives Bay (Cornwall, UK) in September 2022 by Canada-based carbon removal specialists Planetary Technologies.

The trial - which followed a series of lab-based tests and modelling - involved adding a diluted form of the alkaline mineral magnesium hydroxide to the wastewater flow at the nearby wastewater treatment plant in St Erth. The treated water was then released four miles offshore through the existing outflow.

Planetary Technologies was seeking to investigate whether this form of ocean alkalinity enhancement (OAE) (which is likened to adding an “antacid” to seawater) is a safe and effective way of deacidifying seawater, which will then draw down atmospheric carbon dioxide (CO₂). Seawater naturally absorbs CO₂ so the process - which changes the pH of the water - is designed to enhance its carbon removal capacity. 

The analysis provided by PML and PML Applications – and now published in the peer-reviewed journal ‘Communications Earth & Environment’ (“Magnesium hydroxide addition reduces aqueous carbon dioxide in wastewater discharged to the ocean”) - confirms that:

• Adding magnesium hydroxide to the wastewater increased its alkalinity (the pH increased from 7.4 to 7.8) and reduced dissolved CO₂ levels by up to 74%
• The alkalinity and pH returned to normal levels quickly after stopping the addition, showing it can be easily reversed if needed.
• Near the offshore discharge site, lower CO₂ and higher pH levels were detected up to a few meters away, confirming the alkalinization worked but was limited by the small scale of the trial.

Dr Vassilis Kitidis, Senior Scientist at PML and the paper’s lead author, said:

“This pilot study successfully demonstrated the potential for using alkaline materials like magnesium hydroxide to enhance the ocean's natural ability to absorb excess carbon dioxide from the atmosphere. We took a phased and gated approach to the project – examining all of the evidence to make sure the trial could be carried out safely at each stage. I’m excited by the results and how this science might now be taken forward sustainably – and alongside much-needed cuts in greenhouse gas emissions - for the benefit of society and the environment".
 
Will Burt from Planetary Technologies - which is currently carrying out trials in other geographic locations including Halifax, Canada - said:

“This is hugely significant – a major milestone for ourselves but also for the growing carbon removal industry. The process has previously been studied in the lab but it has never been demonstrated in the field before. It was vital to us that the project underwent rigorous scientific scrutiny and we’re very pleased that the published study - which is the culmination of two and a half years’ work - confirms the success of the trial. It’s a proof of concept and a really important step in terms of showing that OAE can be delivered using the existing wastewater treatment process".

During the trial in 2022, concerns were raised by the local community about potential environmental impacts. Dr Kitidis highlights that environmental considerations must remain front of mind during the planning and deployment of any ocean-based carbon dioxide removal technology or process:

“While this study demonstrates the CDR potential of alkalinity enhancement using magnesium hydroxide, it is imperative to consider potential ecosystem impacts, especially in terms of the scaling up of any such process. During the pilot, the alkalinity was only added for a few hours per day and we were very confident there would be no adverse environmental effects, based on our investigations. Magnesium hydroxide is a well understood mineral which is used in many household products and the levels at which it was being used in the trial were nowhere near anything that might have had a negative effect on marine life.  We carried out monitoring throughout the trial and confirmed that Planetary Technologies adhered to all of its environmental permits".

In terms of tackling global warming and mitigating climate change, emission reductions remain paramount, but ocean alkalinity enhancement and other carbon removal solutions are widely accepted as having a key part to play. Carbon dioxide removal is, for example, explicitly included in the UNFCCC Paris Agreement.

In 2023, PML, alongside a group of international ocean policy and conservation experts from North America and Europe released recommendations for ocean carbon dioxide removal (oCDR) research and there is ongoing discussion globally around regulation for the practice and the requirements for monitoring, reporting and verification (MRV).

Ocean carbon removal expert Sam Fawcett from PML Applications’ Centre for Coastal Technologies - which carried out the monitoring of the St Ives trial - said:

“There is huge potential for the increased use of the ocean and marine environment for carbon dioxide removal but this has to be done responsibly.  From initial modelling to chemistry analysis and environmental impact assessment we use our expertise to ensure that projects such as this are underpinned by high quality science. This is essential in order to accurately confirm effectiveness, understand any potential knock-on consequences, optimise techniques and ultimately to build trust".
 

Related information

Full paper: Magnesium hydroxide addition reduces aqueous carbon dioxide in wastewater discharged to the ocean
 

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