Recirculating aquaculture systems (RAS) are closed-containment systems where fish are farmed in reused water. RAS has become increasingly important as the available water is better utilized, achieving production continuity. However, such systems that are characterized by prolonged hydraulic retention time, high production intensity and feed loading, the water is heavily loaded by organic and inorganic compounds, where proteins, ammonia, urea, heavy metals and dissolved organic matter (DOM) are the most pronounced. In presence of high DOM, the overall water quality is deteriorating, creating a favourable environment for microorganisms’ abundances to thrive resulting in disease outbreaks or being responsible for a muddy taste of fish flesh making them not suitable for consumption. Disinfection and vaccination have been applied as part of the management of RAS to control or eliminate pathogens. To address the need for environmentally friendly disinfectants, drifting away from chemotherapeutants, ozone has been introduced as an alternative in RAS.
Ozone is a strong oxidizing agent, reacting rapidly with the non-biodegradable DOM. When ozone is added into water, protein degradation is enhanced, while the water clarity, UV transparency and the processes of filtration and nitrification are improved. Ozone as disinfectant is able to kill bacteria, parasites and viruses. In a non-meticulously designed system, ozone can reach the tanks affecting adversely the fish. Depending on the water matrix, additional issues emerge, e.g. formation of brominated by-products in salt water. The risk of losing fish from overdose, the failure to monitor the ozone concentration and the high investment and running costs are limiting parameters and lead to a reluctance by the aquaculture industry to use ozone.
Over the past years we have participated in several projects aiming to resolve a number of issues in RAS water but also issues related to ozonation itself. Several examples, based on our industrial experience, will be given, where ozone technology has been integrated in different fish farms to improve the overall water quality, enhance fish welfare, remove off-flavour in fish meat, optimize different water treatment technologies to strengthen green and blue effects and to regulate the ozone dose avoiding over or under dosing and without forming any brominated by-products in salt water facilities.