Coastal Outfalls: A Key to Safe and Scalable Ocean Alkalinity Enhancement

As the urgency to achieve Net Zero goals intensifies, scaling up carbon dioxide removal (CDR) technologies to industrial levels becomes paramount. Ocean Alkalinity Enhancement (OAE) emerges as a promising CDR method, leveraging the ocean's vast capacity to absorb and store atmospheric CO2.

However, implementing OAE presents practical challenges, particularly concerning the necessary piping infrastructure to introduce the alkalinity into the environment safely. A strategic solution to this issue lies in co-locating OAE operations with existing coastal industries. This co-location accelerates the deployment of OAE technologies in a framework known to scientists and governments and has existed for over a century: wastewater dispersion.

Most coastal industries at scale use outfalls or pipelines to disperse their return flows rapidly. When introducing an alkalinity-enhanced return flow for OAE, the alkalinity will be rapidly diluted in the “mixing zone” near the outfall’s dispersion point. A better understanding of the dilution rates and retention times after coastal outfalls disperse the return flows into the environment is key to preventing undesirable chemical interactions.

Alkalinity addition in seawater can trigger the precipitation of calcium carbonate (CaCO₃) and is commonly known as “secondary precipitation” or “runaway precipitation.” The precipitation of calcium carbonate consumes alkalinity, reducing the performance of the alkalinity addition for CDR; therefore, it should be avoided. Secondary precipitation is a transient phenomenon, so it needs time to occur, from minutes to hours. Coastal outfalls reach considerable dilution rates in seconds to minutes. Thus, the precipitation kinetics after alkalinity addition must be interpreted based on the dilution rates of coastal outfalls.

This work evaluates the precipitation time after different alkalinity additions for CO2 removal. The most common alkalinity types mentioned in the literature for OAE via coastal outfalls were considered: hydroxides and carbonates. Moreover, the impact of different agitation levels, salinity, and temperatures was assessed, simulating a wide range of global coastal environments.

The results and conclusions inform and promote responsible OAE deployment and give a realistic view of what ocean CDR could look like at scale across various geographical contexts. Aligning OAE with coastal industries represents a pragmatic approach to advancing our climate goals while optimizing resource utilization in the fight against climate change. Ultimately, this work triggers the conversation of how to interpret laboratory results in the context of coastal outfall dilutions.