Removing CO₂ from water

There is increasing awareness of the problems caused by the accumulation of respired CO₂ in 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.

Black lip abalone

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 the air-lift.

While salinity did not effect the mass of CO₂ degassed for each device at equivalent incoming CO₂ concentrations, the CO₂ stripping efficiency did differ between salinities. This statement may at first appear a contradiction, but it makes sense if we look at what happens at the chemistry of CO₂ during the degassing process. First, it is important to define what is meant by CO₂ mass transfer versus stripping efficiency. The diagram below explains this.

The next important point to understand is that CO₂ represents a minor fraction of the total inorganic carbon (C_t) 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 C_t.

The formation of CO₂ from bicarbonate is a relatively slow process (taking about 1 minute to complete) compared to the rate at which water passes through a degasser (a few seconds). So what factors determine the amount of CO₂ that re-forms after degassing? Salinity and pH are two factors. Below is an example of how inorganic carbon ionizes into CO₂, HCO₃^- and CO₃^2-, and how the ionization fractions change as inorganic carbon is stripped away.

Notice in the above graph that when the C_t concentration decreases (as happens during CO₂ degassing), there is a noticeably more C_t present as CO₂ in fresh compared to saline water. This difference means that when water exits the degasser, more CO₂ reforms from the C_t pool in salty waters compared to freshwater. A pictorial illustration is given below.

Differences in the carbonate equilibrium chemistry of fresh versus salt waters may explain why it has been reported anecdotally that it is difficult to achieve low CO₂ concentrations in saltwater recirculation systems.

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

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This research has been published and is available as follows:

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

doi:10.1016/j.aquaeng.2010.05.001 ------- get PDF of paper

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

doi:10.1016/j.aquaeng.2010.09.001 ------- get PDF of paper

Removing CO2 from water

Removing CO₂ from water