The problem of CO2 accumulation in aquaculture systems

There is increasing awareness of the problems caused by the accumulation of respired CO2 in land-baed aquaculture systems that recirculate water. Degassing CO2 from water requires a significant amount of pumping energy, therefore, there is a need to optimise and economise CO2 degassing. Several fish farmers operating land-based aquaculture systems had commented to me that removing CO2 from saltwater appeared to be more difficult compared to freshwater systems. Maintenance of low CO2 concentrations are particularly important for shellfish aquaculture (e.g. abalone, pictured below) due the effect of carbonic acid on shell formation.

Shell forming organisms such as these abalone are sensitive to the acidification resulting from respired CO2 that accumulates in recirculating aquaculture systems. 

Shell forming organisms such as these abalone are sensitive to the acidification resulting from respired CO2 that accumulates in recirculating aquaculture systems. 

Most of the work carried out to date on CO2 degassing has focussed on freshwater, primarily because the majority of recirculation systems are for freshwater species. I wanted to find out whether there was a difference in degassing efficiency of the same device at different salinities, so I tested the CO2 removal efficiency of a cascade column and airlift in fresh versus saline water.

In a cascade column the water falls down over a large surface area and CO2 diffuses from the water into the lower-CO2 air. I measured the CO2 removal efficiency of this column in fresh and saline water.

In a cascade column the water falls down over a large surface area and CO2 diffuses from the water into the lower-CO2 air. I measured the CO2 removal efficiency of this column in fresh and saline water.

An airlift can be used to pump and mix water with air to degas dissolved CO2. Air is injected at the bottom of a tube and rises, dragging water with it. I measured the CO2 removal efficiency of this airlift in fresh and saline water.

An airlift can be used to pump and mix water with air to degas dissolved CO2. Air is injected at the bottom of a tube and rises, dragging water with it. I measured the CO2 removal efficiency of this airlift in fresh and saline water.

I measured the alkalinity and CO2 concentration of water entering and exiting these CO2 stripping devices, which allowed for the calculation of different measures of CO2 removal efficiency. The CO2 mass transfer coefficient did not differ substantially between salinities for either the cascade column or airlift, meaning that the same mass of CO2 was removed in fresh and saline water for a given influent CO2 concentration. However, the CO2 stripping efficiency differed between salinities. But how can there be a difference in the CO2 stripping efficiency between salinities while there is no difference in the mass of CO2 removed? To understand this we need to look at the definition of each measure of CO2 removal and CO2 chemistry. The diagram below explains the difference between CO2 mass transfer versus CO2 stripping efficiency.

CO2 mass transfer efficiency differs from CO2 stripping efficiency as a measure of CO2 removal efficiency. CO2 stripping efficiency takes in to account the re-formation of CO2 from the large pool of carbonates following degassing. The equilibrium reactions that re-form CO2 following degassing take 1-2 min to complete. Most mechanical CO2 degassing treatments only last for about 5-30 seconds.

CO2 mass transfer efficiency differs from CO2 stripping efficiency as a measure of CO2 removal efficiency. CO2 stripping efficiency takes in to account the re-formation of CO2 from the large pool of carbonates following degassing. The equilibrium reactions that re-form CO2 following degassing take 1-2 min to complete. Most mechanical CO2 degassing treatments only last for about 5-30 seconds.

The next important point to understand is that CO2 typically represents a small fraction of the total inorganic carbon (Ct) that exists in water. When CO2 is degassed from water, more CO2 re-forms from the pool of carbonates that make up the bulk of Ct. The formation of CO2 from bicarbonate is a relatively slow process (taking about 1-2 minute to complete) compared to the rate at which water passes through a degasser (a few seconds).

When dissolved CO2 gas pressure is lowered by degassing, CO2 will re-form from the bicarbonate pool. This back reaction is a relatively slow process (1-2 min), meaning that CO2 re-forms after exiting the degasser as the carbonate system re-establishes equilibrium of the different inorganic carbon components.

When dissolved CO2 gas pressure is lowered by degassing, CO2 will re-form from the bicarbonate pool. This back reaction is a relatively slow process (1-2 min), meaning that CO2 re-forms after exiting the degasser as the carbonate system re-establishes equilibrium of the different inorganic carbon components.

The main affect salinity has on degassing is determining how much CO2 re-forms from the carbonate pool following stripping of CO2 gas. More CO2 re-forms in the equilibration reactions in sea or saline water compared to freshwater. This means that while the mass transfer of CO2 gas inside the degasser is (essentially) the same between salinities, more CO2 reforms from the carbonate pool in salt water, lowering the effective CO2 stripping efficiency of the degasser for salt waters. The image below illustrates this.

This research has been published:

Moran, D. (2010). Carbon dioxide degassing in fresh and saline water I: degassing performance of a cascade column. Aquacultural Engineering 43, 29-36.

Moran, D. (2010). Carbon dioxide degassing in fresh and saline water II: degassing performance of an air-lift. Aquacultural Engineering 43, 120-127.

This work was made possible by a postdoc fellowship from the NZ Foundation for Research Science and Technology.