New ocean carbon removal technique proves successful

The new technique is 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, PML was commissioned to provide independent and impartial monitoring and analysis of a trial carried out off St Ives Bay (Cornwall, UK) in September 2022 by Canada-based carbon removal company 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 wastewater flow at a nearby wastewater treatment plant in St Erth. The treated water was then released four miles offshore through the existing outflow.

Planetary Technologies investigated whether this form of ocean alkalinity enhancement (OAE) is a safe and effective way of deacidifying seawater, which could then draw down atmospheric carbon dioxide (CO2). The company said that seawater naturally absorbs CO2, so the process - which changes the pH of the water - is designed to enhance its carbon removal capacity.

The PML and PML Applications researchers found 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 per cent. The alkalinity and pH returned to normal levels quickly after stopping the addition, showing it can be easily reversed if needed.

It was also found that near the offshore discharge site, lower CO2 and higher pH levels were detected up to a few meters away, confirming the alkalinisation worked but was limited by the small scale of the trial.

In a statement, 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 Vassilis Kitidis, senior scientist at PML and the paper’s lead author, said: “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.”

In terms of mitigating climate change, the researchers said that emission reductions remain paramount, but ocean alkalinity enhancement and other carbon removal solutions are widely accepted as having a key part to play.

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

The research paper, published in Communications Earth & Environment, can be read in full here.