Ozone in recirculating aquaculture systems
Introduction
Recirculating Aquaculture Systems (RAS) provide potential advantages over pond or cage-based forms of aquaculture. These include flexibility in site selection, reduced water usage, lower effluent volumes, better environmental control, and higher intensity of production. However, as stock densities and levels of water re-use increase, wastes accumulate rapidly and environmental control becomes more difficult. Sophisticated systems capable of removing both particulate and dissolved organic wastes become necessary.
Conventional means of solids removal, such as microscreen filters and sedimentation tanks address the removal of coarse settleable and filterable solids, but not the removal of fine colloidal solids. Similarly, bacterial nitrification in biofilters removes dissolved ammonia and nitrite, but not other dissolved wastes. As the organic loading increases with intensity of production, the bacteria that convert nitrite to nitrate operate less efficiently, resulting in increased nitrite levels. The accumulation of fine colloidal solids, dissolved organics and nitrite in RAS can impair biofilter function, and increase biochemical oxygen demand and stress levels in the cultured stock. The net effect of this residual organic waste is a less stable, less productive system.