The sea already holds more carbon than the atmosphere and all land plants combined. The trick, as any chemist will tell you, is getting it to hold more, durably, without breaking the bank. In Hamburg, a startup called Planeteers is trying to do just that, not with complex new chemistry but by accelerating a natural one. Their bet is that industrial emitters will pay to turn their captured CO₂ into a harmless component of mineral water and let the ocean take it away [PERPLEXITY SONAR PRO BRIEF].
The chemistry of a simpler sink
Most carbon removal discussions quickly turn to geology: pumping CO₂ deep underground into porous rock. Planeteers is looking the other way, toward the water. Their process reacts captured CO₂ with crushed limestone, converting it into bicarbonate ions,the same stuff that gives mineral water its fizz and stabilizes ocean chemistry. This solution can then be discharged into seawater, where the bicarbonate is stable for millennia [PERPLEXITY SONAR PRO BRIEF]. The unit economics, in theory, hinge on the abundance of the inputs: CO₂, seawater, and limestone, a cheap and widely available mineral.
The company's early focus is on point sources with concentrated CO₂ streams, like wastewater treatment plants. In 2024, it reported testing pilot reactors at such facilities [PERPLEXITY SONAR PRO BRIEF]. The logic is straightforward. These sites already handle massive liquid flows; adding a bicarbonate stream is an operational fit rather than a reinvention. The announced goal is to begin permanent CO₂ storage at Hamburg's Köhlbrandhöft wastewater treatment plant by 2028 [PERPLEXITY SONAR PRO BRIEF].
A team built for industrial scale
This is not a lab experiment spun out by academics alone. The founding quartet was assembled to bridge science, heavy industry, and company-building.
| Founder | Role & Background |
|---|---|
| Prof. Dr. Jens Hartmann | Scientific founder; geoscientist at University of Hamburg focusing on aquatic geochemistry and CO₂ removal [ResearchGate.net]. |
| Florian Brinkmann | Co-founder; geologist with over seven years at Deutsche Erdöl AG, including on a carbon-storage project [PERPLEXITY SONAR PRO BRIEF]. |
| Florian Birner | Co-founder & Managing Director; engineer with experience constructing large-scale plants at Linde and Bosch [PERPLEXITY SONAR PRO BRIEF]. |
| Dr. Frank Rattey | Co-founder & Managing Director; brings engineering and entrepreneurship experience [PERPLEXITY SONAR PRO BRIEF]. |
This blend is reflected in the backing. Investors include BonVenture and the Smart Energy Innovationsfonds, but also Carmeuse Ventures,the corporate venture arm of a major global limestone producer [Carmeuse Ventures, December 2025]. That's not just a check; it's a strategic vote on the core input.
Traction in a nascent market
Carbon dioxide removal (CDR) is a market that runs on future commitments. Planeteers has secured early validation from some of its most important potential customers: the buyers of removal credits. The company has been selected as a CDR supplier by organizations including AirMiners, Frontier, Shopify, and Stripe [PERPLEXITY SONAR PRO BRIEF]. These are not revenue announcements, but they are crucial endorsements that place Planeteers in the pipeline for future offtake agreements.
Financing has followed to build the team and technology. Reports from 2024 describe a seed round as a "high single-digit million" euro amount [Startup City Hamburg, July 2024]. Another source characterized a summer 2024 raise as a "mid-seven-figure" euro sum [WHU]. The company's headcount has been reported at different points as 25 [Hamburger Morgenpost, June 2025] and 18 [ZKG, October 2026], with a founder recently stating the team has grown to almost 30 [Florian Brinkmann - Planeteers | LinkedIn].
Navigating the permitting deep
The most credible risk for Planeteers isn't the chemistry; it's the logistics and legality of using the ocean as a waste product sink, even for a benign one. Ocean alkalinity enhancement is a nascent field, and regulatory frameworks for large-scale discharge are complex and vary by jurisdiction. Public acceptance is another unknown. The planned 2028 project at the Hamburg treatment plant is explicitly noted as being subject to financial and regulatory hurdles [PERPLEXITY SONAR PRO BRIEF].
The company's answer appears to be a focus on partnership and existing infrastructure. By working with municipal wastewater plants, they tap into permitted discharge points and a public utility partner. The backing from Carmeuse provides not just limestone but industrial credibility. The bet is that by aligning with established industrial flows,wastewater out, limestone in,they can navigate the red tape more smoothly than a project trying to build a standalone ocean pipeline.
The next twelve months
For Planeteers, the path to 2028 runs through the next year. The key milestones are less about lab breakthroughs and more about real-world engineering and paperwork.
- Finalizing the Hamburg pilot design. Moving from concept to detailed engineering for the Köhlbrandhöft project is the immediate technical hurdle.
- Securing binding offtake. Converting supplier status with Frontier and Shopify into a firm purchase agreement for tons removed would be a major traction signal.
- Expanding the partnership model. Proving the wastewater plant wedge works in Hamburg could open doors to similar sites in other port cities.
Doing some back-of-the-envelope math illustrates the scale of the ambition. If a single large wastewater plant emits 10,000 tons of CO₂ annually, mineralizing it would require roughly 23,000 tons of limestone. That's about 40 standard rail cars of rock. The cost battle will be won or lost in the logistics of moving and reacting those materials cheaply at the source.
Planeteers is not competing with direct air capture startups for the same niche. Its natural incumbent is the status quo of simply venting CO₂ from industrial sites. To win, it must prove that its process of turning gas into dissolved bicarbonate is not just permanent, but simpler and more cost-effective than the alternative: doing nothing at all.
Sources
- [PERPLEXITY SONAR PRO BRIEF] Planeteers company brief
- [Startup City Hamburg, July 2024] Planeteers secures seven-figure financing | https://startupcity.hamburg/news-events/news/planeteers-secures-seven-figure-financing
- [WHU] Fünf Fragen an Planeteers | https://www.whu.edu/de/news-insights/whu-magazin/artikel/fuenf-fragen-an-planeteers
- [ResearchGate.net] Jens Hartmann profile
- [Carmeuse Ventures, December 2025] Discover our partnership with Planeteers | https://www.carmeuse.com/eu-en/carmeuse-ventures/planeteers
- [Hamburger Morgenpost, June 2025] Hamburger Start-Up will den Klimawandel stoppen: So soll es klappen | https://www.mopo.de/hamburg/eimsbuetteler-nachrichten/hamburger-start-up-will-den-klimawandel-stoppen-so-soll-es-klappen/
- [ZKG, October 2026] Planeteers uses natural processes to decarbonise the industry | https://www.zkg.de/en/artikel/planeteers-uses-natural-processes-to-decarbonise-the-industry-4169887.html
- [Florian Brinkmann - Planeteers | LinkedIn] Founder profile update