Airlifts are used to reduce the energy cost associated with RAS circulation, aeration, and carbon dioxide stripping. They are generally powered by air from centralized blowers and facilitate the isolation of blocks of tanks providing barrier to the spread of disease.
The benefits are only realized when the RAS system is correctly designed to facilitate low lift circulation typically without the use of a centrifugal water pumps. The Lift to Submergence ratio (S/L ) is key to the design and airlift operational efficiencies. The lift is the height the water must be lifted above the hydrostatic head where the air is injected. The hydrostatic head is easily observed by a pitot tube placed just before air injection. L/S ratios in the range of 20-25 percent are considered ideal for RAS applications. The Gas to Liquid ratio (G/L) defines how much air must be injected (in liters per minute) to move a given flow (in liters per minute). This is the prime operational parameter defining air lift performance once the L/S ratio has been set. With L/S ratios in the 20-25% range, operational G/Ls tend to range close to 1.3.
Interim airlift design guidelines were generated in 2005 and have been refined by practice over the last 20 years. These design rules facilitate the design of airlifted RAS in a rational manner and are based on empirical design constants. Safety factors of about two, are inherent in the criteria . Oxygen and carbon dioxide stripping rates have been estimated. Airlifts a re compatible with a number of treatment devices. The criteria have proven robust in commercial practice.