Removing Carbon Dioxide
Tech Talk 33
Did you know that for every 1 lb of oxygen consumed by fish they exhale 1.38 lbs of carbon dioxide? Carbon dioxide does cause problems in recirculating systems without aeration or degassing. This can be the case, for example, where pure oxygen is used in place of aeration. Carbon dioxide must be removed, or it can build up to dangerous levels…dangerous to the fish and to humans if the fish are raised in a closed building.
Here are some numbers to keep in mind. Oxygen is about 20.9% of the air and, because it is only slightly soluble in water, it becomes saturated at a level of about 9 ppm at 68°F (20°C). Carbon dioxide is .033% of the air and is saturated in water at about .5 ppm (the ratio is higher because it is more soluble than oxygen). The comparative concentration of these two gases in blood is similar to that of water. Therefore, a lot of carbon dioxide in the water means there will also be a lot of carbon dioxide in the blood of the fish. An excess of 5 ppm carbon dioxide in the water will affect the ability of the fish to breathe.
If intensive aquaculture operations are being conducted outdoors, a splash aerator or aeration with air diffusers will drive the carbon dioxide into the air. If the operations are in a closed building, very high levels of carbon dioxide can accumulate in the air (we've seen levels exceeding 4,000 ppm in the air in closed aquaculture facilities!). It then has to be removed from the building.
Air ventilators can also remove a lot of heat along with the carbon dioxide. We suggest that carbon dioxide be stripped with a degassing column that is ventilated to the outdoors. Outdoor air can be drawn directly into the bottom of the degassing tower, forced up through the downflowing liquid, then directed back outdoors separate from the inlet. In cold weather, there will be a significant cooling effect on the water because it is being degassed through cold, dry air. A simple air-to-air heat exchanger will help.
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