SIZING AND DESIGN OF LOW LIFT AIRLIFTS FOR RAS APPLICATIONS  

Ron Malone, Chief Technology Officer
 
Aquaculture Systems Technologies
108 Industrial Avenue
New Orleans, LA
Ron.Malone@astfilters.com

Use of airlifts in an RAS application reduces overall energy demands as a result of their concurrent contribution to the circulation, aeration, and degassing needs.  Airlifts have been conservatively designed to recirculate water at a flow rate of 1.5 ft/sec with submergence to lift ratio of 0.25.  Air pumps are sized with a conservative gas to liquid ratio of 2 with a typical operational ratio of 1.3.  These airlift criteria have proven robust across a wide variety pipe sizes in both marine and freshwater systems.   Math relationships have been developed defining the oxygen transfer and carbon dioxide stripping rates for airlifts rate.  These relationships are driven by the airflow and oxygen deficit/carbon dioxide surplus entering the lift tube.  However, most sizing is operations are based on simplified feed to air volume relationships.  In warm water systems utilizing exclusively blown air for gas transfer 3-4 cfm of air are required per pound of fed daily to maintain oxygen above 5 ppm and carbon dioxide below 10 ppm.  

The injection depth (lift) is typically limited to 5 feet to avoid problems with supersaturated gases.  This in turn limits the total lift to 15 inches with a favorable gas to liquid ratio below 1.5.   These constraints limit the RAS applications to low head systems.  However, the airlift technology has proven compatible with a variety of filtration components, including moving bed reactors, microscreens, and floating bead filters.  Airlifts are also frequently used to remove sludge from sludge chambers associated in unpressured solids capture devices.   Airlift applications in the commercial sector have be mostly successful prompting further adoption.