Commercial and climate imperatives for integrating carbon removal activities into existing industrial processes and value chains are becoming ever more apparent. Opportunities to integrate carbon removal can help businesses grow and diversify, increase operational efficiencies, and adhere to evolving regulations. Simultaneously, climate stabilization demands rapid, large-scale carbon removal, positioning established industries as vital contributors. Forward-thinking industry leaders are beginning to strategically invest in a variety of carbon removal methods that align with their operational capabilities.

This series explores the economic and environmental incentives for integrating carbon removal into the wider industrial landscape. Through this series, we examine the potential for carbon removal integration into specific industries, identifying synergies with existing processes, along with the challenges, potential scale, and critical needs to advance opportunities. This report assesses the potential to integrate carbon removal and storage technologies into desalination activities.

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Desalination is the process of removing salt and impurities from seawater to create freshwater. In addition to helping improve drinking water supplies in areas that most need it, desalination facilities also provide synergistic opportunities for both removing CO₂ from the atmosphere as well as storing it in concentrated form.

Desalination plants continually pump, filter, and process significant quantities of seawater, creating possibilities for the integration of water-based carbon removal approaches. They also produce a large quantity of brines, which can be used to carbonate and store CO₂.

The most modern and energy-efficient desalination plants utilize a reverse osmosis (RO) process to separate a seawater input into two outflows — drinking-quality freshwater, and brine with a salinity approximately double the input seawater.¹ Globally, coastal seawater desalination plants process over 150 million cubic meters (m3) of seawater every day.² This volume of seawater contains over five million tons of salt (sodium chloride, or NaCl), as well as a significant quantity of magnesium and calcium ions, which can be converted into other products through electrochemical reactions.³

Worldwide, the scale of the desalination sector is growing at rates of 6%-12% each year,⁴ with the majority of seawater desalination capacity currently deployed in the Middle East and North Africa. The desalination industry is expected to grow in volume and expand geographically in response to the impact of rising global temperatures and extreme weather events on dwindling freshwater supplies amidst increasing freshwater demand.⁵

With projected global shortfalls in freshwater resources of up to 40% by 2030, public and private sector interest in desalination is growing, and modern technologies to reduce impacts from brine discharge are being developed and deployed.⁶ Carbon removal and storage approaches that require brines as feedstocks represent further options to optimize resource use while mitigating the effects of high-salinity outputs.

In this report, we analyze the potential for three electrochemical carbon removal approaches to integrate into coastal desalination value chains (see Exhibit 1).⁷ We further comment on potential carbon storage and mineral extraction opportunities using brines given the global need for carbon management and resource optimization.

How desalination can support carbon removal

Carbon removal using desalination brines can be carried out using electrochemical techniques, while carbon storage can be done by using brine contents directly and converting them to valuable products.⁸ Below is a list of current techniques available to support carbon removal using desalination plants.

• Direct Ocean Capture (DOC), also referred to as CO₂ stripping,⁹ refers to the direct removal of CO₂ already present in seawater in the form of dissolved inorganic carbon. DOC technologies extract CO₂ as a concentrated stream, analogously to how direct air capture (DAC) separates CO₂ from ambient air. Electrochemical processes can be used to divide water or brines into acid and base streams. Low pH levels in the acid stream convert dissolved inorganic carbon into dissolved CO₂, allowing it to be captured. The base stream is then re-combined with the CO₂-depleted acid stream to produce an outflow with a lower carbon content and slightly higher pH, able to uptake new CO₂ in the future.

• Electrochemical Ocean Alkalinity Enhancement (eOAE), also known as electrochemical alkalinity production (EAP),¹⁰ refers to the use of an electrochemical process to divide salt water into an acid and a base stream, returning the base stream to the open water system. By removing acid from the open water system, the local pH of the ocean is slightly increased, which raises its capacity to absorb CO₂ from the atmosphere. Acid byproducts will then need to be sold, utilized, or properly stored.

• Direct Air Capture (DAC) can be carried out with desalination brines by using electrochemical processes to divide brine into acid and base streams and then using the base stream as a solvent to react with atmospheric CO₂.¹¹ These processes actively extract CO₂ from the atmosphere by using air contactors to concentrate CO₂ within the base stream. Net carbon removal is achieved by either extracting and storing the CO₂ as a concentrated gas stream, releasing it in the form of stable bicarbonates after neutralizing the acid stream with an alkaline feedstock, or mineralizing it with an alkaline feedstock to form solid carbonate species. Applications where solid carbonates are formed during this process are also referred to as electrolytic seawater mineralization.¹²

• Carbon storage is possible by reacting the calcium and magnesium ions in brines with concentrated CO₂. This involves shifting the pH of the brine, which allows for solid carbonate minerals to precipitate out.¹³ These minerals can then be used in other products, including concrete.¹⁴

Each of these processes are currently being developed through pilot-scale and first-of-a-kind projects in coastal facilities, operating at technological readiness level (TRL) 6 or above.¹⁵ Such projects are essential to demonstrate these technologies and measure their safety and effectiveness. Further pilot and demonstration projects directly incorporated into desalination facilities are needed to confirm synergies with the industry and accelerate deployment, but commercial-scale projects are already on the horizon.

Synergies from integrating carbon removal

Key synergies of integrating electrochemical carbon removal projects into coastal desalination plants include:

• Desalination wastes as feedstocks: Brine is a valuable feedstock for eOAE, DAC, and for carbon storage in regions without economic access to geological carbon sequestration. Electrochemical processes are more energy efficient when using concentrated brine rather than seawater as an input.¹⁶ These brines have a higher salinity than seawater, which reduces the energy cost associated with the electrochemical acid/base separation process and reduces the volume of water processed by half, thus reducing the energy intensity per ton of CO₂ removed.

• Sharing physical infrastructure: Utilizing the pumping and pre-treatment infrastructure of an existing desalination plant can present energy savings of up to 30% compared to the operation of standalone CDR projects.¹⁷ Membrane-based RO desalination requires water to be pumped from the ocean into and through a plant at high pressure, which carries a significant energy cost. Desalination operators can benefit from an additional productive use of capital assets, reduce their carbon footprint, or derive a potential revenue stream through the sale of carbon credits.

• Improving process efficiencies: Current pre-treatments to incoming sea water to remove impurities, such as biological material and calcium and magnesium ions, to protect RO membranes from scaling and fouling are also important steps for carbon removal processes. Removing the same impurities improves equipment lifetimes and process efficiencies for membranes used for electrochemical carbon removal, representing an opportunity for shared operating expenditures between the desalination and carbon removal activities.

  Carbon removal processes involving the precipitation of solid carbonates can be used as pre-treatment steps for membrane-based RO desalination. A key pre-treatment step for desalination plants involves the softening of water by removing calcium and magnesium ions from the incoming seawater. Some approaches being developed for electrochemical carbon removal involve the precipitation of CO₂ in the form of solid calcium or magnesium carbonates, which removes these ions from the water stream. If the carbon removal process is performed before seawater is passed to the desalination plant, instead of afterward, this softening effect can reduce the energy demand of the desalination process. This presents a trade-off between energy savings through softening for desalination when performing the carbon removal process first, or increased salt concentration in brines when performing the carbon removal process second.

• Reducing environmental impacts: Carbon removal methods that reduce either the salinity or the total volume of discharged brines can also mitigate marine impacts. Responsible brine discharge is a significant challenge for desalination projects, as local spikes in salinity are detrimental to aquatic ecosystems.¹⁸ eOAE projects can use waste brine as an input and convert the salt within it into alkalinity, reducing overall salinity. Reduction in salinity may reduce harm to marine life and make outputs easier to disperse, while delivering the additional environmental benefit of reversing ocean acidification. The extent of salinity reduction and possible positive environmental impact depends on location, the fraction of salinity converted into alkalinity within the carbon removal process, and the dispersal method. Pilot studies are currently testing different dispersal methods for alkaline water from eOAE projects, including monitoring of the resulting pH gradients near the dispersal site and any potential positive or negative impacts on local aquatic ecosystems.¹⁹

• Synergistic growth: Additional desalination activities are vital as the global need for fresh water rises due to human population increases and shifting climate conditions. As the desalination industry grows, hundreds of new coastal desalination plants are being constructed each year.²⁰ In some cases, securing the social license to move forward may require increased attention to environmental concerns, including addressing marine impacts as well as carbon impacts.

  This new construction presents not only an increase in the total available capacity for carbon removal processes due to the increased volumes of seawater being processed each year, but also a significant opportunity to include carbon removal infrastructure directly into the design of new plants, rather than adding carbon removal systems as retrofits in the future. For plant retrofits, carbon removal startups often focus on modular systems that are space-efficient and can easily be integrated into existing brine outflows without significantly disrupting desalination plant uptimes.

Current annual volumes of seawater treated by coastal desalination plants contain the equivalent of ~4 Mt of CO₂ in the form of dissolved inorganic carbon.²¹ Integration of DOC projects into the desalination process would allow for approximately half of this CO₂ to be purified and captured.²²

A larger amount of carbon removal could be realized using eOAE and DAC approaches. A theoretical upper bound on the potential capacity for carbon removal is based on converting all of the salt within the seawater processed by all coastal desalination plants into alkalinity. Upper-bound capacity estimates reach as high as 1 GtCO₂/y.²³ However, both renewable energy availability and environmental constraints will be limiting; energy requirements for electrochemical processes currently range from less than 0.5 up to 3 MWh/tCO₂.²⁴ Further, to allow for the release of alkalinity within safe pH ranges,²⁵ some water will likely be needed for dilution of the return flow, reducing the total amount of CO₂ removed per quantity of seawater processed.²⁶ RMI estimates that under ideal conditions, as much as 200 MtCO₂/y of CDR could be achieved through eOAE using the quantity of seawater processed in coastal desalination plants today. This is based on the current rate of brine and seawater use for alkalinity generation and outflow dilution by CDR startups.²⁷ Desalination industry growth rates of 6%–12% could significantly increase this capacity in the coming years.²⁸

In practice, the total capacity will be limited by access to low-carbon energy and the need for offtake of high volumes of hydrochloric acid or chlorine, which may be limited by the availability of alkaline mineral feedstock needed to neutralize that acid. For context, the IEA forecasts that the global supply of electricity generated from renewable sources will reach 17,000 TWh by 2030.²⁹ Even If 1% of this total were to be allocated to electrochemical carbon removal approaches, the capacity would only reach up to 85 Mt/y. More sophisticated models of future carbon removal potentials should consider these material limitations. Optimal deployment of the technology will require locations with significantly decarbonized grid electricity supplies able to meet process demands, or dedicated construction of facilities to generate renewable electricity as part of project deployment.

The technical potential of integrating carbon removal into desalination plants is significant, though it will be constrained by several factors outlined below. Successful scale-up will involve cross-cutting efforts in the provision of low-carbon energy and offtake of byproducts. Current key barriers to large-scale deployment include:

• Availability of low-carbon energy (all electrochemical approaches). The carbon removal approaches discussed here make use of electrochemistry. Both the electrochemical process and the pumping of water through the system can be powered by low-carbon electricity. With current technologies, these processes can require up to 7 to 10 gigajoules (GJ) or 1.9 to 2.8 megawatt-hours (MWh) of electrical energy per ton of CO₂ removed, which is equivalent to two to three months of the electricity consumption of an average American household.³⁰ DOC projects require the pumping and processing of larger volumes of seawater than eOAE projects per tCO₂ removed, increasing overall energy demand.³¹ DAC projects may require additional electrical energy to power fans to pull air through the system and directly extract CO₂.

  While efficiency improvements are still expected as technologies improve, current high energy needs represent a significant obstacle to scale. The amount of electricity needed per cubic meter of treated seawater ranges significantly based on the amount of alkalinity produced, but is larger than the electricity demand of the desalination process itself.³² Establishing desalination carbon removal projects with net negative CO₂ emissions is significantly easier if the desalination process itself is powered by renewable electricity, and may not be possible otherwise. Project implementation will be most successful in areas where low-carbon energy is available or dedicated renewable generation can be included in the project plan. Because the electrochemical systems used in these approaches can be turned on and off rapidly, systems can improve project economics by ramping up and down based on the availability of excess low-carbon electricity. Some electrolytic approaches piloted by carbon removal companies in these areas also produce green hydrogen as a coproduct, which can serve as a partial offset of the energy consumed.³³

• Acid or chlorine offtake (eOAE, DAC). eOAE can be thought of as removing acid from, rather than adding alkalinity (base) to, the ocean. Many approaches yield an acid byproduct — typically in the form of hydrochloric acid at a concentration of 1-5% (v/v) — which needs to be safely stored, neutralized, or otherwise utilized in a way that does not result in the acid being re-released into the ocean as it would result in reversal of the carbon removal process. Some fraction of HCl may be sold into the existing market for acids, but the market is too small to offtake the volumes of HCl that would be produced at scale,³⁴ and produced acid streams are typically too low in concentration to be valuable. At scale, carbon removal companies employing eOAE will likely need storage solutions or partnerships with providers of alkaline materials, such as mine operators, to cost-effectively neutralize or utilize their acid coproducts. Some eOAE approaches instead produce the base output through a chlor-alkali process, yielding hydrogen and chlorine gas instead of a liquid acid stream. In these cases, the chlorine gas must be transported safely and either utilized or stored.

• Environmental safety testing (all approaches). All carbon removal approaches described in this report involve the re-release of processed water back into the ocean environment and must adhere to environmental safety standards. While removing CO₂ from the ocean is environmentally beneficial because it reverses ocean acidification,³⁵ interventions that result in significant local changes in pH, salinity, or dissolved inorganic carbon concentrations can be harmful to local aquatic ecosystems. This limits the concentration at which alkalinity can be safely released. Safe limits vary based on project geography, as local natural pH and dissolved carbon levels can naturally vary significantly. Pilot projects are determining these safe bounds and testing different solutions involving pre-dilution of the alkaline stream and gradual dispersal, enabling rapid diffusion into the ocean environment. Such safety tests are required for successful permitting for future electrochemical carbon removal projects.³⁶ It is possible that reduction in effluent salinity will improve the overall environmental impact of desalination plants when integrating eOAE projects, making the inclusion of carbon removal a beneficial addition.

• CO₂ transportation and storage (DOC, DAC). Approaches producing a concentrated stream of CO₂ need infrastructure to safely and reliably transport their CO₂ product to a site performing permanent storage or durable utilization. Integrating storage into remaining brines through mineralization may provide a potential on-site solution, assuming the minerals can then be transported off-site for use.

• Regulatory clarity (all approaches). Carbon removal projects that release materials directly into the ocean may be considered and permitted as waste disposal.³⁷ Knowledge-sharing from successful and safe pilot projects and the implementation of policies enabling water-based carbon removal approaches will be key for enabling the rollout of these technologies at scale. Established permits compatible with project integration can significantly accelerate deployment.

• Measurement, reporting, and verification (all approaches). All carbon removal projects need reliable and comprehensive measurement, reporting, and verification (MRV) to demonstrate efficacy, and enable any crediting of durable, additional carbon removals.³⁸ Approaches that leverage the surface area of the ocean for carbon removals require measurement and monitoring techniques that adequately capture the spatial scales involved. MRV solutions for these approaches are being developed alongside measurement and modeling of positive and negative environmental impacts beyond carbon sequestration.³⁹

Electrochemical carbon removal technologies have already advanced from lab-based research to pilot projects at scales up to thousands of tons of removals per year in geographies including North America, Asia, and Europe.⁴⁰ Current deployments include both standalone facilities and projects integrated into desalination facilities.

Enthusiastic actors within the desalination field, including project developers and relevant policymakers, should act now to identify synergistic project opportunities. A larger number of pilot and demonstration projects will enable further testing for environmentally safe practices, development of cost-effective and robust engineering solutions for electrochemical treatment steps, and proof of concept of the economic viability of water-based carbon removal projects.

If the environmental safety of the release of alkaline outputs from eOAE projects is established alongside robust MRV demonstrating downstream carbon removals, eOAE could become a significant carbon removal technology and simultaneously address issues related to brine discharge.

Reversing ocean acidification presents an environmental benefit beyond the reversal of climate change, with benefits for ecosystems and the human food supply.⁴¹ Additionally, eOAE may present a promising solution for the environmental safety and permitting challenges faced by desalination plants disposing of highly saline brines. This may be crucial to enabling access to freshwater for communities facing limited clean water supplies.

Further research and innovation on improving efficiencies should be carried out and piloted without delay, as new approaches, including those seeking to modify desalination membranes directly to remove CO₂ from seawater without a follow-up electrochemical step, may prove effective at a lower energy cost.⁴²

Project developers will need to identify sites with a suitable combination of desalination development, available low-carbon energy, and favorable environmental conditions. Alongside environmental safety studies, clarity on regulations regarding the uptake and release of seawater and processed brines affecting both desalination plants themselves and the potential deployment of water-based carbon removal deployment, is needed to highlight and expand beneficial siting opportunities.

Finally, solutions are needed for the transportation and/or storage of CO₂ itself for projects producing a concentrated CO₂ stream, or of acid or chlorine coproducts for approaches that release alkalinity and retain acidity. In the case of using brine mineralization to store CO₂, transport and further use of solid carbonate products, for example as building materials, will need to be addressed. The large global potential capacity of eOAE projects can only be unlocked if hundreds of megatons of acid or chlorine globally can be stored safely on land or used in projects that prevent project reversals. Local partnerships with producers of alkaline minerals may prove key in finding an energy- and carbon-efficient solution for neutralizing acid coproducts. There is potential for synergies with other mineral-based carbon removal approaches not discussed in this report, as some feedstock pre-treatment processes require reaction with acids to activate minerals into more amenable forms⁴³.

As discussed in our accompanying reporthighlighting the opportunities for integrating carbon removal into the wider industrial landscape, ⁴⁴ the world will need to deploy multiple gigatons per year of carbon removal by 2050 in order to reach climate goals.⁴⁵ The rapidly growing and developing desalination industry has a significant opportunity to become part of the accelerated deployment of carbon removal technologies worldwide. Early actors can benefit from partnerships with existing carbon removal technology developers to gain an advantage in finding and deploying efficient bespoke or modular solutions and become a part of this growing market. Desalination project developers, technology startups, environmental scientists, and coastal communities should work together to deploy projects and share their findings to validate the efficacy of these approaches and determine any positive or negative environmental impacts before realizing this potential. Local and national governments can support this growth with supportive policy environments and by requiring net-zero or net-negative operations of desalination projects.

Acknowledgements

RMI would like to thank the following for contributions to this report:

• Charithea Charalambous, Third Derivative
• Jennifer Yin, Isometric
• Edward Sanders, Equatic
• Suzy Schadel, RMI
• Nicolas Sdez, Pronoe
• Tara Bojdak, Captura

RMI would also like to acknowledge and express gratitude for funding support from the Grantham Foundation for the Protection of the Environment.