Recirculation Aquaculture Systems (RAS) reduce water consumption but require costly filtering methods needed in order to maintain adequate levels of accumulating compounds, such as nitrogenous compounds and sludge. Sludge (faeces and waste diet) is mechanically removed by drum filters and settling systems and disposed of, while nitrogen compounds are reduced via nitrification-denitrification filters, the latter of which requires of supplementary commercial carbon sources. Sludge is a source of particulate carbon, potentially for use in denitrification, equivalent to ca 20% of feed input (Lekang 2013). The reutilization of sludge implies the correct accumulation and storage to avoid uncontrolled decomposition by partial exposure to aerobic and anaerobic conditions and the disintegration of complex molecules into easily biodegradable nutrients. Ozone exposure is a potential method to promote mineralization and allows formation of biodegradable intermediates. The present study investigated the potential of sludge as an alternative carbon source for denitrification and asks: i) How much time should the sludge be exposed to ozone in order to achieve the highest levels of carbon intermediates in solution? ii) What are the remaining constituents of the solid phase of the sludge post- ozonisation prone be used as nutrients by denitrifying bacteria? iii) Is there an influence of different fish diet composition on the fatty acid content of sludge? iv) Is denitrification possible when using ozone-treated sludge as a carbon source? As particulate matter represents a great amount of the sludge discharge, this study aimed to estimate the dissociative effect of ozonisation on the solid phase and the physicochemical changes that this process could bring to the water matrix in terms of carbon and nitrogen content present in solution and in particulate state. The preliminary results of a batch pilot study using ozone-activated sludge vs. a common commercial carbon source for denitrification are also provided.