The primary alternative to granular media filters in recirculation aquaculture is some type of microscreen filter. The screens must be in constant motion and continually back flushed because, unlike granular filters, screens have only one layer of porosity and it immediately clogs. An advantage of continual back flushing is that screens have the advantage of not needing periodic shut down and back flushing. The disadvantage is that this continual back flushing requires that more water be diverted down the drain with the waste. Also, they typically do not remove as small a particle as granular media. Moving screen filters have been created in a variety of configurations, but drums and belts are the most common. Whatever their configuration, dirty water moves through the screen which traps solids and then the screen moves to a back flush area where clean water is sprayed from behind the screen to clean it.
Foam fractionators are for specialized removal of very fine solids (<30 microns) and colloids. They work on compressed air. In a typical design, an air lift in the smaller section of a "U" shaped pipe generates a downward flow of water in the larger leg of the "U". Fine bubbles are released in the larger leg and as the bubbles rise through the water and break on the surface the very fine waste clings to the surface film making foam. The continual pressure of the air carries the foam upward through a tube and out of the tank.
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