Turning Sea Water Desalination Systems into CO2 Ocean Vacuum Cleaners
© 2025 Kremesti Environmental Consulting Ltd – All Rights Reserved
Introduction
As global water demand rises and natural freshwater sources become increasingly scarce, desalination is emerging as a crucial solution for many sea facing regions experiencing water shortages. However, traditional desalination methods can be both energy-intensive and environmentally impactful, largely due to their extensive use of chemicals and high energy consumption. To ensure desalination remains a sustainable option for providing clean water, innovative approaches are needed to minimise the environmental impact of SWRO while maximising efficiency. This is where ‘green chemistry’ come into play, offering a transformative approach to desalination and industrial wastewater treatment.
What are ‘Green Chemicals’?
The concept of ‘green chemicals’ is revolutionising desalination by creating a self-sustaining plant that eliminates the need for external inputs. Instead of relying on outside sources for the chemicals needed in the desalination process, the plant produces these chemicals on-site, cutting down transportation costs and significantly reducing its carbon footprint. This innovation highlights the core of green chemistry in desalination: maximizing resource efficiency while minimizing environmental harm.
But it doesn’t stop there. The approach is not only self-sufficient but also recycles useful chemicals from rejected brine water, a byproduct of the desalination process as feedstock to produce valuable by-products part of an overall Circular strategy. This effectively reduces the environmental impact of discarded brine, turning a waste product into a resource.
Moreover, the minimisation of the environmental impact is achieved through several factors: eliminating the transportation of chemicals, utilising renewable energy to produce these chemicals on-site, and employing energy-efficient production methods. This integrated approach reflects the direction of the future of sustainable desalination, where system in the process works toward reducing environmental impact while maintaining operational efficiency.
Challenges with Traditional Chemical Use
Various chemicals play crucial roles across different stages of desalination , from pre-treatment to post-treatment, ensuring water quality and maintaining plant operations. However, they also present significant challenges, particularly in terms of supply chain logistics, cost management, and environmental impact. A large portion of the chemicals used in desalination plants, about 80%, consists of calcium carbonate, carbon dioxide, Cl2 and NaOH, primarily utilized in the remineralisation process to enhance the quality of desalinated water and disinfecting incoming raw water. Sodium hydroxide is the third most significant chemical, accounting for 11.6% of the total usage.
The reliance on these chemicals presents several challenges. Desalination plants are often located in remote or isolated areas, making the transportation and delivery of chemicals logistically difficult and costly. Transporting these chemicals not only adds to operational expenses but also increases the plant’s carbon footprint, which conflicts with sustainability goals. Stockpiling chemicals is not always feasible or cost-effective due to storage limitations and the risk of chemical degradation over time. For instance, supply chain disruptions affecting sodium hypochlorite can lead to plant shutdowns, impacting the reliability of the desalination process.
Chemical costs are another significant challenge. Prices can be volatile and unpredictable, driven by market conditions, supply and demand dynamics, and geopolitical factors. This volatility complicates financial planning and stability for desalination plant operations, as it affects operational budgets and makes cost forecasting difficult. Factors beyond production costs, such as transportation expenses, tariffs, and regulatory changes, can further influence chemical prices.
But what if we could overcome these challenges by producing chemicals on site? And what can be used to make chemicals on site?
The answer is easy: Concentrated Seawater or rejected brine, which is typically twice as concentrated as seawater.
Seawater contains primarily sodium and chloride while sulphate, magnesium, calcium, and potassium are also present in significant amounts and with smaller amounts of bicarbonate, carbonate, and carbon dioxide. Seawater contains almost all the essential elements we need. The challenge lies in developing economical and environmentally friendly methods to extract the required chemicals from seawater or rejected brine for use in the desalination process.
Innovative Approaches to On-site Chemical Production
A simple process to precipitate calcium carbonate from seawater was developed around 15 years ago by IDE in Israel. The goal was to enhance the performance of thermal desalination plants by removing scale from seawater. Later, they expanded their work to the precipitation of various sparingly soluble salts in a fluidized bed reactors from different water sources. In recent years, they’ve also explored carbon capture techniques, particularly those involving capturing carbon from the sea.
IDE decided to integrate these processes to assess their combined impact on the desalination plant. When they brought these sub-systems together, they discovered that this approach not only captures carbon from the air but also produces essential chemicals for the desalination plant. And there’s a bonus here… this process also enhances the plant’s overall performance, enabling improved boron removal (enhanced by higher operating pH), biofouling free operation of RO membranes, anti-scalant free operation, and a significant reduction in the plant’s size since there is no longer a need to construct a large second pass reverse osmosis stage designed for boron removal.
Boron Removal
While Reverse Osmosis (RO) is highly effective at removing salts and other impurities from seawater, boron, existing as boric acid, poses a challenge due to its small size and non-polar nature, requiring specific strategies for effective removal.
Boron in seawater exists primarily as boric acid (B(OH)3), a small, non-charged molecule, which is not easily rejected by standard RO membranes.
Improving Boron Rejection:
Double-Pass RO: A common approach is to use a two-pass RO system. The first pass operates at natural pH, and the second pass increases the pH of the permeate to convert boric acid to borate (B(OH)4-), which is negatively charged and more readily rejected by RO membranes.
pH Adjustment: Elevating the pH of the RO permeate from the first pass to above the pKa of boric acid (around 8.7) is crucial for effective boron removal in the second pass.
Introducing: Green chemicals production for a sustainable desalination process:
The on-site production of chemicals for desalination plants involves a series of innovative steps designed to extract and utilize essential components directly from seawater or rejected brine. The process begins with the intake of raw seawater or concentrated brine, which is directed into a fluidized bed reactor. In this reactor, calcium hydroxide (slaked lime) is added to precipitate calcium carbonate on the available particles or pellets. The fluidized bed reactor is a well-established technology where water flows upward through a bed of fluidized particles, promoting the efficient precipitation of calcium carbonate. The reactor operates at a high upflow velocity of about 80 to 120 meters per hour, which allows for a compact footprint and produces highly dry calcium carbonate crystals, eliminating the need for additional dewatering stages. The effluent from the reactor contains relatively low concentrations of suspended solids, meaning existing filtration technologies at the desalination plant can be used without additional treatment.
Next, the part of calcium carbonate pellets produced in the fluidized bed reactor is transported to the remineralization process while another part is transported to a calciner, where they undergo thermal decomposition at temperatures exceeding 900°C. This high-temperature process breaks down the calcium carbonate into calcium oxide (quicklime) and carbon dioxide gas. Various types of calciners, such as regenerative or rotary kilns, can be employed depending on factors like pellet size, energy consumption, and the desired purity of the carbon dioxide produced. IDE prefer to use an electrically operated calciner because it produces a cleaner carbon dioxide stream with carbon dioxide concentrations higher than 90%, which reduces the need for extensive cleaning or liquefaction, making the process both cost-effective and energy-efficient.
The chemicals produced on-site, including calcium carbonate, calcium oxide and carbon dioxide, are then utilized in the desalination plant’s post-treatment stage to enhance water quality. Calcium oxide is used in the fluidized bed reactor itself and for pH adjustment in desalination processes, replacing the more commonly used sodium hydroxide and thereby reducing chemical costs.
Using seawater for chemical production offers several advantages for the desalination process. Higher pH enables more efficient boron removal in the first stage, resulting in a smaller second stage for boron elimination. Additionally, elevated pH prevents or reduces biofouling, extending the intervals between membrane cleanings required to remove biofouling. Furthermore, reducing calcium levels produces softened seawater, allowing for antiscalant-free operation. These benefits collectively enhance the efficiency and sustainability of the desalination process.
The chemical production process described above can be integrated into existing seawater desalination plants by installing it on the rejected brine stream.
Additionally, Brine from SWRO systems can be treated using the Chlor-Alkali process to generate NaOH, Cl2 and H2 a renewable energy source. The Chlor-Alkali process is an industrially mature process used to produce Chlorine and NaOH.
Note that Seawater intake pH typically ranges from 7.5 to 8.4, while brine reject pH can vary, but often falls within the range of 8 to 9.5. A more basic reject stream contributes to countering ocean acidification.
Figure 1. Ocean acidification due to increased dissolution of atmospheric CO2 has a negative environmental impact on Crustaceans.
‘Green Chemicals’ Production – Paving the Way to Sustainable Desalination
Producing chemicals on-site for SWRO systems offers numerous benefits, such as:
- a steady supply of essential chemicals,
- cost savings from reduced transportation and procurement expenses, and
- a significant reduction in the carbon footprint associated with chemical production and transport.
This innovative approach minimises environmental impact and enhances operational efficiency, aligning with global sustainability goals and paving the way for more sustainable desalination practices. By developing on-site chemical production capabilities, desalination plants can not only meet their immediate chemical needs but also operate more sustainably and cost-effectively, ensuring a reliable supply of clean water while reducing their overall environmental footprint.
Conclusion
The time to solve climate change and global warming is NOW and this can be coupled with sustainable drinking water production. The power of chemistry can be used to accomplish this goal.
References
About The Author
Rami Elias Kremesti with Dr Boris Liberman CTO of IDE
Rami Elias Kremesti is a chartered scientist and water/environmental manager. He is the founder of Kremesti Environmental Consulting Ltd based out of the UK. He tutors a CBT course on Reverse Osmosis technology with CIWEM and enjoys consulting for clients and helping them improve their sustainability through water/waste water treatment and recycling. He is the author of three philosophical books and passionate peace and environmental activist.
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