Producing fish in conventional RAS, in which a large volume of water is refreshed and a limited number of water treatments units are used (essentially mechanical waste removal and biofiltration) has a smaller environmental impact (sensu eutrophication) than flow-through systems. Recent innovations such as denitrification reactors, sludge thickening technologies and ozone treatments led to a further decrease in water use, waste discharge and energy use
in RAS. In addition, the discharged waste is more concentrated, facilitating waste (re-)use options as fertilizer or in integrated, eventually completely closed, systems (reviewed in Section3.3). Combined, these developments certainly improve the environmental sustainability of RAS.