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.
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.
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.
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).
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.