The article is based on work in FHF project 910827 and describes the chemical properties of water based on the complete carbon balance, and how this affects degassing in seawater, brackish water and freshwater.

Photo: Searas

The hidden cost of inadequate degassing

Why the carbon balance can limit feeding, fish welfare and production capacity in closed aquaculture facilities.

Published 22 September 2026 - 06:57

Authors: Eldar Lien (Searas), Helene Bjørnevik Bjelland (Searas), Morten Aga (Searas), Sara Queralt (Searas), Alexandr Bylinkin and Eivind Farestveit Larsen (Searas)

Summary

Public accounts from both listed and unlisted companies show a general trend for closed aquaculture facilities: actual production is significantly lower than designed capacity. The reasons behind the figures in annual accounts are varied and complex, but a common factor is often water quality and the ability to remove CO₂ during the treatment process. Through an FHF project, Searas has developed a sensor to measure total inorganic carbon. This work has provided an in-depth understanding of carbon dynamics in water and increased understanding of what occurs during degassing.

Elevated CO₂ levels result in reduced performance in post-smolt, lower growth and/or increased mortality. New insights into carbon balance and degassing offer considerable potential to increase production capacity in closed facilities. This potential can be realised through better control of the carbon balance, documented degassing capacity, new degassing technology and more precise water quality management.

Understanding carbon in closed facilities

In a closed aquaculture facility, carbon is added in the form of feed, which is converted into proteins in the fish and faeces. One consequence of this is that the fish consume oxygen in the process and produce CO₂ as a by-product that must be managed. This by-product is in the form of inorganic carbon (TIC), where CO₂ is only part of a broader equilibrium governed, among other factors, by pH (Figure 1).

Figure 1: Carbon balance in a closed system

In a fish tank, TIC is distributed among three chemical forms: dissolved carbon dioxide (CO₂*), bicarbonate ions (HCO₃⁻) and carbonate ions (CO₃²⁻). These species exist in a dynamic pH-dependent equilibrium described by the following reaction chain:

CO₂(aq) + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ ⇌ CO₃²⁻ + 2H⁺

Under typical operating conditions, bicarbonate and carbon dioxide are dominant and account for the majority of the inorganic carbon content. Put very simply, there is always a pH-controlled equilibrium where:

CO₂ ⇌ HCO₃⁻

The distribution of bicarbonate and carbon dioxide is determined by, among other factors, pH, salinity and temperature. The distribution can be illustrated as in figure 2 at a given TIC value and different pH levels, for freshwater and seawater.

Figure 2: TIC controlled by pH and salinity, where the figure shows the proportion of free CO₂ available for degassing

A system will always move towards this pH-dependent equilibrium. If the system is out of balance, HCO₃⁻ will be converted to CO₂ and vice versa. Under the relevant water chemistry conditions, the characteristic half-life for restoring carbonate equilibrium is estimated at approximately 22 seconds. This means that a substantial part of the equilibrium is not restored until after about one minute.

The importance of TIC in closed aquaculture facilities

The importance of TIC and the carbon balance

Through water replacement and water treatment, the aim is to balance the carbon budget. Mechanical filtration and protein skimming remove organic carbon, while degassing removes inorganic carbon. To maintain stable CO₂ levels in the system, it is essential to control the TIC balance as follows:

TIC from feed [kg] ≈ TIC out [kg]

In operational water treatment, TIC is mainly removed through water exchange and the conversion of inorganic carbon to CO₂, which is then degassed. When degassing water, only free CO₂ can be removed during the degassing process. The amount of TIC available in the form of CO₂ depends on pH and salinity, see Figure 2. When using degassing towers or column degassers, the available CO₂ is removed in an almost instantaneous process. After CO₂ stripping, the system is out of balance and corrects this by converting HCO₃⁻ to CO₂. After approximately one minute, a large proportion of the carbon dioxide removed through degassing has been replenished by the conversion of bicarbonate to carbon dioxide. Time is an important factor for effective degassing.

When water is degassed over a longer period, as in Searas’ AquaDUCT, the system’s drive towards equilibrium can be turned to an advantage. With a retention time of 30–60 seconds, CO₂ converted from bicarbonate will be stripped from the water, resulting in significantly increased efficiency in TIC removal (Figure 3). The time taken for degassing is absolutely essential for the CO₂ level experienced by the fish in the tank.

Figure 3: Degassing depends on time. Rapid degassing primarily removes CO₂ that is already available. A longer contact time also enables the removal of CO₂ regenerated from bicarbonate.

To obtain an accurate figure for how much CO₂ is removed from the system through degassing, the actual CO₂ removal in kg per hour from the degasser exhaust must be measured, as the carbon system’s drive towards equilibrium means that measurements before and after degassing provide only part of the picture. Measuring the degasser’s mass removal rate in kg CO₂/hour, combined with data on feeding, TIC, water flows and CO₂ in the water, provides a substantially better basis for establishing a complete carbon balance.

The figures below show measurements from a stocking in an RAS facility with feeding from 600 kg/day to 1,200 kg/day, and a pH of 7.3. CO₂ and TIC measurements are real-time measurements in the fish tank, while CO₂ measurements in the exhaust are real-time measurements of the CO₂ quantity, measured in kg per hour, in the ventilation duct from AquaDUCT.

Figure 4 shows that the CO₂ level increases in a controlled manner at the start of the period and subsequently remains within a relatively stable range, while the CO₂ removed from the system through degassing increases as biomass and feeding increase (figure 5). Stable CO₂ values indicate an equilibrium between TIC in and TIC out (figure 6). The measurements indicate that degassing during this time interval keeps pace with the increasing carbon load, with no signs of persistent accumulation that limits feeding.

Click on the figures to enlarge them

Figure 4: CO₂ levels in the fish tank over the last 2 months with increasing biomass and feeding.

Figure 5: CO₂ removal rate, measured in kg CO₂/hour, for one of four AquaDUCT units during the same period as shown in Figure 4.

Figure 6: TIC values, expressed as mg CO₂ equivalents per litre, measured during one production batch. For comparison, natural seawater typically contains around 100–105 mg CO₂ equivalents per litre in the form of total inorganic carbon. The level varies depending on factors such as salinity, temperature and alkalinity.

Degassing and production capacity

As an illustrative rule of thumb, the fish’s respiratory CO₂ production can be estimated at approximately 0.4–0.5 kg CO₂ per kg of feed. Using 0.5 kg as a simplified basis, the ratio between CO₂ production and feeding is approximately 1:2. Over a production year, even a moderate limitation in degassing capacity can result in significantly lower production and poorer utilisation of the facility’s biomass capacity.

If degassing capacity is the limiting factor, one additional kg of CO₂ removal per unit of time can therefore free up capacity for approximately two kg more feed during the same period. The actual effect on production will depend on factors including fish size, temperature, salinity, system design and other bottlenecks.

During stress events, the fish’s respiration and CO₂ production increase. Measurements using AquaSENSE during observed stress events have shown short-term CO₂ production of up to five times the normal level.

Facts about TIC measurements

What the operations manager should know

CO₂ constitutes only part of total inorganic carbon, TIC.
Acceptable CO₂ levels do not rule out the accumulation of TIC in the system.
When pH falls, a greater proportion of TIC is converted into free CO₂.
When degassing does not keep pace with carbon production, the facility must compensate through reduced feeding, more water or increased energy use.
The facility's carbon balance affects production capacity, operating costs and biological risk.

What the measurement technology can show

The total amount of inorganic carbon (TIC) in the water.
How much CO₂ is actually removed through the degasser (kg CO₂/hour).
Whether the carbon balance is in equilibrium or TIC is accumulating in the system.
How close the facility is to its degassing capacity limit.
How changes in pH affect the risk of increased CO₂ levels.
How much reserve capacity the facility has as biomass and feeding increase.
How efficient the degasser is under different operating conditions.
Development trends that provide early warning of increased biological risk and the need for measures.

Increased production costs through increased degassing

When degassing capacity is limited, farmers have different strategies depending on whether the facility is a RAS plant or a flow-through system with reuse.

In a RAS facility, insufficient degassing capacity can be managed by;

i) Increase the volume of water entering and leaving the tank, and replace lost alkalinity with buffer

ii) Increase pH to “hide” CO₂ by manipulating pH through the addition of bases to convert CO₂ into bicarbonate

iii) Reduce feeding

What the various options have in common is that they are costly and result in a significant increase in production costs.

In a flow-through system with reuse, insufficient degassing capacity can be addressed by;

i) Increasing the volume of water entering the tank

ii) Reducing feeding

These measures have a significant impact on production costs through;

Increased energy costs from pumping more water,
Reduced growth and fish welfare in tanks due to lower temperatures resulting from increased intake of cold water
Increased costs of heating more water if the facility is operated with an energy system
Reduced growth as a result of reduced feeding

Improved fish welfare through increased degassing

High CO₂ levels affect fish welfare and may result in reduced growth and increased mortality. Prolonged exposure may cause fish to use more energy to maintain physiological balance, reducing their capacity to cope with other pressures and stress events. Over time, the strain becomes greater than the fish’s ability to compensate, weakening its health and resilience. High and fluctuating CO₂ levels may be one of several risk factors associated with the development of nephrocalcinosis. However, the relationship is multifactorial and is influenced by, among other factors, salinity, mineral balance, smoltification, osmoregulation and other conditions in the production environment. HSS is often observed in the same production environments and is assumed to share several of the same underlying risk factors. Both conditions may affect post-smolt performance after transfer to sea. Challenges with high CO₂ levels occur as biomass grows and gradually approaches the limit of degassing capacity.

Greater understanding of TIC and actual measurements of increased CO₂ production during stress indicate that the risk profile is more serious than initially assumed. This is illustrated by actual data from a three-month stocking period with increasing TIC values and a stable pH of 7.4 (see Figure 7), where TIC values towards the end of the period range between 130 and 180 mg/l. The sustained increase in TIC indicates that carbon is being added faster than it is removed, and that operational degassing capacity is not keeping pace with the load. Inorganic carbon accumulates in the water as the biomass grows.

Figure 7: Data from a facility with increasing TIC values.

During a stress event caused, for example, by a crowding operation over a period of 1–2 hours, CO₂ production may increase to as much as 3–5 times normal production as a result of increased respiration. This will lead to increasing TIC values and decreasing pH. Decreasing pH will shift the carbonate equilibrium so that a larger proportion of TIC is present as free CO₂.

Table 1 illustrates a critical risk scenario in which the fish may rapidly be exposed to very high CO₂ levels. The scenario analysis below illustrates the consequences of a short-term increase to three and five times normal CO₂ production, respectively. The result of this stress event may be a rapid and substantial increase in CO₂ exposure, even without any further build-up of carbon in the system.

Table 1: Scenario during a stress event triggered by crowding, showing CO₂ values at various pH levels.

Scenario	TIC	pH 7,4	pH 7,2	pH 7,0
Normal TIC / pH-fall alene	180 mg/L	17 mg/L	26 mg/L	38 mg/L
3× CO₂-produksjon	220 mg/L	21 mg/L	32 mg/L	46 mg/L
5× CO₂-produksjon	260 mg/L	25 mg/L	38 mg/L	55 mg/L

Facilities controlled solely according to CO₂ values may underestimate the risk because, as the scenarios show, high TIC values can cause a sudden increase in CO₂ even with a moderate fall in pH. Measures to manage such a risk would be:

i) Controlling the facility according to TIC values rather than CO₂

ii) Sufficient capacity to remove TIC in order to avoid the build-up of carbon reserves

Conclusion

In a closed aquaculture facility, carbon does not disappear simply because the CO₂ measurement appears satisfactory. The carbon may still be in the water as bicarbonate and carbonate, ready to be released as CO₂ when the load increases or pH falls. Therefore, control based on CO₂ alone is not sufficient.

When degassing does not keep pace with carbon production, the farmer must pay in one or more ways: through reduced feeding and growth, increased water consumption and energy use, or higher biological risk. Inadequate degassing is therefore not merely a water quality issue. It is a direct limitation on the facility's production capacity and profitability.

Closed aquaculture facilities should therefore be managed according to a documented carbon balance:

how much carbon is added through feeding
how much TIC is present in the water
how much CO₂ is actually removed from the system
what reserve capacity the facility has when biomass increases or the fish are exposed to stress

The most important conclusion is simple: A facility that does not know its carbon balance does not know its true production capacity either.

By combining continuous TIC and CO₂ measurements with sufficient degassing capacity, the producer can move from reactive management of high CO₂ levels to proactive control of feeding, biomass, risk and costs. This enables higher and more stable production, lower operating costs and better protection of the fish.

The question is therefore not whether good carbon control costs money. The question is how much inadequate carbon control is already costing the facility.

For further information about the FHF TIC project, AquaSENSE™, AquaDUCT™ and AquaWARE™, please contact the project team.