The development of energy- efficient recirculating aquaculture systems (RAS) for Atlantic salmon is one of the main future goals of aquacultural engineering. RAS facilities are on its way to have more and more automated technologies. However, CO2 degassing units normally are not regulated or automatised. They just running on full power even if they have strip out very little CO2 from the production water. Energy prices increased a lot over the last years and an automation of CO2 Degassing units could help to significantly reduce energy costs in fish production. Degassing performance can be measured through sensors and displayed on the operating computer of the control panel of the RAS unit. This could be used to regulate the fan, pump, and blower performance to transport the high CO2 air more economically (save energy) out of the production facility. Dissolved CO2 sensors have their limitations (low accuracy, pression and response time). However, it is possible to measure CO2 directly in the outflowing air of the degassing unit. The air-based measurements can be a proxy for degassing performance. Air based non-dispersive infrared gas analysers are much faster, precise, and reliable and perhaps a good online control option for aquaculture CO2 degassing units.
Material and methods
CO2 removal in brackish and seawater RAS is theoretically less efficient compared to freshwater. We compared the degassing performance in fresh (0 ppK) and brackish water (12 ppK) at the Nofima Centre for Recirculation in Aquaculture (NCRA), Sunndalsøra on the main experimental degassing units (AquaOptima AS, Norway). A CO2/Air water vapor compensated CO2 meter (Li-850; LI-COR Biosciences, United States) was verified at the in- and out-flow of the degassing tower with high precision automated titration water-based carbonate data (TL7000; VWR International AS, Germany) and dissolved CO2 measurements (OxyGuard International, Denmark). Dissolved CO2 measurements (Oxyguard & TL7000) were done before the biofilter, after the biofilter and after the degassing tower. The LI-COR system (up to 2 measurements per sec) could provide fast, precise, and robust measurements in the air leaving the degassing units. Degasser design is system-specific and can vary in hydraulic loading rate, packing height and Gas:Liquid ratio. Therefore, degassed air CO2 concentrations “needs to be calibrated” to the dissolved CO2 levels in the water as a proxy for degassing performance (system-specific).
As expected, removal of dissolved CO2 from water and CO2 increases in air leaving the degasser were proportional and no differences between salinities could be observed. In our system, “removing (delta- in and out- flow)” 1 mg L-1 dissolved CO2 in water, increased air CO2 “removal (delta out- and in- flow)” by about 52 µatm. Also as expected, CO2 removal increased with an “absolute” increasing dissolved water CO2 levels into the degasser from the fish tanks. In our system, an increase of 1 mg L-1 dissolved CO2 out of the fish tanks into the degasser increased the “removal” CO2 concentrations” in air by about 29 µatm. The air “removal (delta out- and in- flow)” values can be used to predict the incoming concentration of dissolved CO2 into the degassing unit.
Under constant operating conditions, we observe an increase of 100 µatm in our off-gas. Using Figure 1 A, this corresponds to a CO2 removal of about 1.2 mg L-1 in water. Using Figure 1 B, we estimate that our CO2 concentration in water entering the degasser is about 3.2 mg L-1. The calculated CO2 removal rate is about 36% which fit perfectly with the manual measured data.
Air based measurements showed a good correlation with dissolved CO2 removal from water and allowed estimating dissolved CO2 concentrations entering the degasser. Having online measurements in milliseconds of CO2 concentrations in air could make the online LICORE unit reliable, and fast enough (which is a huge benefit compared to submerged dissolved CO2 sensors) for controlling the degassing unit (blowers and pumps). In future automated CO2 degassing unit controlled by online CO2 measurements can likely contribute to energy savings in RAS production.