Treating Wastewater To Store Ocean Carbon
Press Release – Science Media Centre
The proof-of-concept results show that, if scalable, the approach could help reduce ocean carbon oversaturation, especially in the Northern Hemisphere where the most wastewater treatment plants are located.
Treating wastewater with alkaline minerals could soak up over 18 million tonnes of carbon dioxide a year, new research suggests.
Oceans are absorbing increasing amounts of CO from the atmosphere, which is making them more acidic. In theory, increasing the alkalinity of treated wastewater that's discharged into the sea would expand the ocean's capacity to store CO. Chinese research published in Science Advances uses lab experiments to test how effective this could be. The proof-of-concept results show that, if scalable, the approach could help reduce ocean carbon oversaturation, especially in the Northern Hemisphere where the most wastewater treatment plants are located.
The SMC asked local, third-party experts to comment on the research.
Dr Lokesh Padhye, Associate Director of the New York State Center for Clean Water Technology; and Honorary Academic at the University of Auckland, comments:
'This study takes the idea of using wastewater treatment plants as 'alkalinity factories' a step further. By adding olivine during the aerobic treatment stage, the researchers sped up the rock weathering process by about 20 times compared to just dumping the same mineral straight into the ocean. This helped boost the wastewater's alkalinity and, at least in theory, the authors claim, could lock away around 19 million tonnes of CO each year. Still, lower than what we emit from burning fossil fuels each year.
'The lab results are impressive, but scaling this up to the real world comes with challenges. Mining, crushing, and shipping huge amounts of olivine would use a lot of energy and create emissions of their own. Plus, dumping highly alkaline water through effluent plumes could cause calcium carbonate to precipitate, and how much CO actually gets captured would depend a lot on local ocean conditions like temperature and water chemistry. There's also a tradeoff for regulators to consider: adding olivine seems to help remove phosphate from wastewater, which is beneficial, but it might complicate sludge management.
'In short, turning wastewater plants into carbon-capturing hubs is a clever idea with real potential, but it needs more real-world testing, and it should be seen as one piece for solving this complex puzzle, not a replacement for cutting emissions from fossil fuels and industry.'
No conflicts of interest.
Professor Matthew Watson, Member of the Product Accelerator Network, Professor of Chemical and Process Engineering, Director – Biomolecular Interaction Centre (BIC), University of Canterbury, comments:
'A potential carbon sequestration of 0.018 billion tonnes of CO per year sounds big, but it is important to put it in perspective.
'Man-made CO emissions are approximately 36.8 billion tonnes of CO per year. That's approximately 2,000 times the 0.018 billion tonnes of CO figure listed in the article. About 3% of this is associated with wastewater treatment plants, or 1.1 billion tonnes of CO per year (approximately 60 times the 0.018 billion tonnes of CO figure listed in the article). Forests sequester 16 billion tonnes of CO per year.
'The mass efficiency (I estimate to be 0.04 tonnes of CO removed per tonne of olivine) seems to be very low (details of approximate calculations below). And the authors don't mention anything about the unintended consequences of adding large quantities of finely ground olivine (which may contain asbestos, chrome, and nickel) into rivers and waterways.
'The idea of the article is certainly a step in the right direction and the results are interesting in that the activated sludge seems to catalyse (speed up) the CO removal with olivine.'
'Mass efficiency estimate:
'Globally we discharge about 150 billion tonnes/yr of wastewater and authors have identified about 1 in 6 of the WWTPs as being suitable, so ~25 billion tonnes/yr. At 2 wt. % olivine = 0.5 billion tonnes of olivine to remove 0.018 billion tonnes/yr of CO2. The mass efficiency seems to be very low.'
Conflict of interest statement: Watson is a founder of and has a beneficial ownership stake in Aspiring Materials.
Dr Ricardo Bello-Mendoza, Associate Professor, Department of Civil and Natural Resources Engineering, University of Canterbury, comments:
'New research estimates that adding alkaline minerals to wastewater treatment could help capture and store about 18 million tonnes of greenhouse gas carbon dioxide (CO), when biologically treated and alkalinity-enhanced wastewater is discharged into the ocean. The researchers used olivine rock, a magnesium-iron silicate mineral abundant globally, to increase wastewater alkalinity. That is, to improve its capacity to neutralise acids such as carbonic acid produced when CO dissolves in seawater. In New Zealand, olivine is used as an aggregate to make roads, and it has previously been studied for carbon sequestration in industrial applications. This study proposes using olivine in wastewater treatment plants (WWTP) as a more effective way to maintain alkalinity in the treated water and disperse it into the aquatic environment. This would make a WWTP an alkalinity factory for ocean alkalinity enhancement (OAE) and CO sequestration. Alkalinity enhancement was studied before the activated sludge (AS) process, which is widely used in wastewater treatment, including New Zealand. While the results are promising, further research is needed to assess their potential in practical engineering applications. The addition of alkalinity to the wastewater should be customised according to the specific hydrochemical conditions at each discharge site. Furthermore, a life cycle analysis of the technology is still necessary.'
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