For bacterial control and water clarity, ozone (O3) or ultraviolet (UV) lamps or a combination of both can be applied. The task in RAS is usually not to completely disinfect the water but to keep the bacterial counts at a reasonable level.
UV light inactivates microorganisms through photochemical damage. The irradiance of UV lights at 254 nm damages the cellular RNA and DNA making the bacteria unable to replicate or infect. An advantage of UV is the lack of residues, because it is a physical process. The disadvantage is that the target organisms are only affected if the light can directly hit them.
Ozone is generated on-site and can be used to oxidize organic molecules. O3 is formed by forcing an O2 molecule to bind with a third O atom. This super-oxide is highly unstable resulting in an excellent oxidizing agent. Therefore, ozone is ideal to remove dissolved organic compounds (DOC). DOC can give the water a tea-coloured stain. For salmon culture this tea colour can severely limit the salmon’s feed intake, as they are visual feeders. Next to the oxidizing effect, ozone can also improve micro flocculation, by binding small particles (<30µm). This enhances the efficiency of the foam fractionator. Ozone is not fully without risk, as it is highly toxic to fish, even in low concentrations. In salt water ozone can react with chloride and bromide ions in toxic unwanted by-products, a high level of knowledge about water quality is desired considering the use of ozone.