Oxygen absorption efficiency of gas/liquid mixing device in closed recirculating aquaculture systems

Abstract: Control of dissolved gases, especially oxygen and carbon dioxide, is an essential component of closed recirculating aquaculture systems. The use of oxygenation in closed recirculating aquaculture systems increases the mass of fish that can be supported in a given water flow by removing oxygen as a limiting factor to a system's carrying capacity. In general, oxygenation technique in aquaculture is relatively mature with well established operational principles, techniques, and equipment. Since the 1970s, pure oxygen gas has been used as an economical means of providing supersaturated dissolved oxygen and thereby intensifying fish production in recirculating systems. Use of pure oxygen can lower fish production costs by supporting high fish and feed loading rates and reducing water flow requirements, which in turn reduces the size and cost of pumping, culture tanks, and water reuse equipment. Transferring pure oxygen rather than air into water can increase the mol fraction of gaseous oxygen and solubility of dissolved oxygen in water. The maximal dissolved oxygen concentration can be influenced by the ratio of gas to water flow rate within the gas transfer unit. When pure oxygen is in contact with water, the dissolved gases tend to come to equilibrium at saturation. If there is venting of off-gas from an oxygenation system, some of these other gases will be removed from the water. Choosing the appropriate oxygenation technology for recirculating systems depends on oxygen absorption efficiency (AE, mass of oxygen absorbed per mass of oxygen applied), oxygen transfer efficiency (mass of oxygen transferred per power required), ability to strip nitrogen and other gases, ability to treat flows containing suspended solids without plugging, system layout and intended location of the oxygenation process, and whether flow must be pumped through the unit or if gravity flow is adequate. The configuration of a recirculating system determines the appropriate type of oxygenation unit. Regulation of dissolved oxygen concentrations is typically carried out by means of gas transfer processes, and the gas transfer units commonly used to add pure oxygen within large recirculating aquaculture systems include U-tubes, oxygenation cones, and multi-staged low head oxygenators. The gas/liquid mixing device can inject pure oxygen into liquid, especially suited to applications in closed recirculating aquaculture systems with high carrying capacity, and it had recently been successfully used to provide supersaturate water with oxygen for increasing fish production in our pilot scale recirculating aquaculture system. In this study, AE of the gas/liquid mixing device was determined under conditions with different oxygen gas to liquid ratios (G/L) ranging from 0.333% to 3.333% (0.57-5.70 g/min) at different water temperatures from 30.5℃ to 19.2℃) in order to assess application of pure oxygen in a closed recirculating aquaculture system. The AE of the gas/liquid mixing device averaged 94.00%-36.05%. Among conditions with different GL values, the AE under the condition with the G/L value of 0.667% (1.141 g/min) was highest with 87.833% at 30.5℃, 90.451% at 26.3℃, 93.606% at 22.9℃, and 94.001% at 19.2℃, respectively. It was obvious that AE value declined with decreasing temperatures at a given G/L value, suggesting that water temperature was a key factor affecting the AE of the gas/liquid mixing device. The highest value of AE (94.001%) was obtained when G/L equaled to 0.667% (1.141 g/min) and water temperature at 19.2℃. Under this condition, the corresponding dissolved oxygen entering fish ponds was 13.36 mg/L.