Tech Talk 84
AES Numbers indicate how many pounds of fish an aeration device can support. They are a quick reference for sizing aeration systems. If your system parameters differ significantly from the AES Number standard conditions listed below or if you are designing an aeration system for a commercial facility, call our Technical Department at 407-598-1401 for assistance.
Numbers may seem overly conservative because they are based on heavy oxygen demand conditions. AES Numbers assume the following conditions: half-pound catfish or tilapia in tanks (nonsoil bottoms) at 80°F (27°C) at sea level, stocking densities from .1 to .7 pounds of fish per gallon of water (12 to 84 kg/m³ ), recirculating system with a .82 alpha factor, 5.2 mg/l (67 percent of saturation) dissolved oxygen (D.O.) concentration, feeding rate at 3 percent of standing crop weight per day, pelleted feed at 40 percent protein, continuous solids removal, less than 3 ppt salinity, and a low algal population. If you stock at or below the rate indicated by the AES Number, your DO level will remain above 5.2 mg/l under these water quality conditions.
We've chosen warm water for our AES Number standard conditions because the oxygen consumption rate goes up with the water temperature. Keeping your fish weight below the AES Number value will insure a D.O. above 67 percent of saturation, which is generally considered adequate (while being very cost-effective) for most species raised in recirculating systems. If 50 percent of saturation were acceptable, you could support about 50 percent more fish with your aerator than the AES Number indicates. At 82 percent of saturation, you could support about 50 percent less (see aeration Tech Talks for details). AES Numbers are not practical to use where heavy algal populations are present because algae oxygen consumption (plant respiration) at night has not been taken into account.
Air and Oxygen Diffusers: Submerged diffuser AES Numbers assume diffuser placement at a depth of 3' and assumes that air or oxygen flows to the diffuser at the average recommended rate. Oxygen transfer is proportional to bubble size and contact time. Small bubbles have a greater air-to-water contact surface area than the same volume of gas in fewer large bubbles (see Bubble Size Tech Note). Although fine pore diffusers may produce finer bubbles, they will require more pressure and they must be cleaned much more frequently.
Regenerative blowers work best with medium pore diffusers. Medium-sized bubbles (1–3 mm diameter) rise at about one foot per second in water, which means that a bubble released at a depth of 3' will exchange gas with the water for 3 seconds. So, if you are operating your diffusers at 1.5' instead of 3', divide the AES number by 2—your diffusion system can only support half as many fish. If your diffusers are 4' deep, you can support 33 percent more than the AES Number shown. Be careful using air at depths greater than 4' because dissolved nitrogen can cause gas bubble disease (see Tech Note). When using air with a diffuser that is rated for pure oxygen, divide the pure oxygen AES Number by twelve.
Pure Oxygen: Pure oxygen AES Numbers are rated at 100 percent saturation (7.9 mg/l DO) rather than 5.2 mg/l (67 percent saturation). Oxygen purity near 100 percent, as you would get from a liquid or gas oxygen cylinder, is also assumed. For oxygen generators, multiply the AES Number by the oxygen content of your generator gas output. For example, if your oxygen generator delivers 90 percent pure oxygen, multiply the AES number by .90. If you are using pure oxygen with diffusers that are rated for air, multiply the AES Number by twelve.
Splash Type Aerators: Surface aerator AES Numbers assume that the water is being circulated efficiently from the point of aeration to where the fish are located and back to the aeration device—that is, the device is not re-aerating the same water.
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