CO₂ removal in intensive aquaculture: efficiency bottleneck and open challenge

[ChimanaTech]

Carbon dioxide accumulation is a persistent constraint in intensive aquaculture systems, particularly in RAS, nurseries, and oxygen-supplemented trout and marine production systems.

Unlike low-density flow-through systems where oxygen is limiting, intensive systems maintain oxygen at non-limiting levels, making CO₂ the primary dissolved gas constraint affecting fish welfare and performance. Even relatively low concentrations (~10–15 mg/L depending on species and salinity) can cause physiological stress, making continuous CO₂ removal a critical design requirement.

Most systems rely on air stripping via packed columns, trickling towers, or bubble contactors. These are fundamentally limited by gas–liquid mass transfer physics: as CO₂ is removed, the driving force (ΔpCO₂ / concentration gradient) collapses rapidly. At the same time, the gas phase itself becomes saturated unless very high gas-to-liquid ratios (typically 3:1 to 10:1) are used.

In practice, this leads to:

• High water recirculation rates through degassers
• Significant pumping and blower energy demand
• Large ventilation loads in indoor systems (with associated heating/cooling penalties)
• Rapid diminishing returns with increased contactor size

Despite decades of use, improvements have been incremental: packing materials, higher airflows, better distribution, and partial pH manipulation. However, none have fundamentally removed the core limitation: rapid equilibrium approach in a buffered carbonate system under dilute driving-force conditions.

Open question to the community:

How do we significantly improve CO₂ removal efficiency in aquaculture systems under strong equilibrium constraints?

Specifically:

1. Can staged or regenerative contactor designs outperform single-pass systems?
2. Are there viable crossovers from chemical or process engineering?
3. Is there a fundamentally different way to maintain driving force without exponentially increasing gas flow?
4. Can carbonate chemistry itself be exploited more intelligently rather than treated as a limitation?

[rhjiang]

This will be a discussion in the FAW x Tech series!

Feel free to start discussing and sharing resources in this thread. Our actual discussion group meet-up will be on Sunday, 03 May, 5pm GMT+1 (London time).

RSVP here to get the meeting link on the day. Hope to see many of you there!

[rhjiang]

(I'm not knowledgeable about chemistry or aquaculture, so my takes may be naive!)

It seems like there's a fair amount of research into carbon "direct ocean capture" for climate purposes. Do any of the learnings from that field transfer to RAS settings? For example, the company Captura uses electrodialysis to split water into acid and base. The acid is dosed into a larger pool of seawater to lower its pH, which converts dissolved bicarbonate into free CO₂ gas that can then be extracted. The base is then added back to restore pH before the water is returned to the ocean. Maybe their seawater setting is quite different from RAS, and also their target scales are quite different. But does the core technology lend anything here?

[rhjiang]

https://capturacorp.com/technology/

[michaelstjules]

Seems like there's a substantial risk of this improving the efficiency of aquaculture, e.g. allowing higher stocking densities, and so increasing the number of animals farmed. By default, you might expect any measurable welfare improvements through water quality to be compensated for by higher stocking densities, reaching the same average welfare level as before, and the primary change is just higher stocking densities and more farmed animals.