The Effect of Temperature on Denitrification in Recirculating Aquaculture systems
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The primary objective of this master's thesis is to examine how temperature affects denitrification in recirculating aquaculture systems (RAS) effluent treatment to achieve efficient nitrate removal in aquaculture operations. To the best of our knowledge, existing research lacks focus on temperature's impact on denitrification in aquaculture systems. Similarly, only a few studies have focused on denitrification processes occurring at low temperatures. This thesis aims to address this knowledge gap. In this study, one reactor was utilized to adapt the sludge denitrifying bacteria to acetate and fresh sludge obtained from Tytlandsvik Aqua's RAS plant as carbon sources. The reactor was operated anaerobically through N2 flushing of the influent feeding water and the reactor content, while maintaining a temperature of 12°C over a period of 4 months. During this time, the pH and effluent concentrations of nitrate compounds were regularly measured to track the progress of denitrification activity. Once a high denitrification rate was observed at an optimal pH range between 6.5-8.2, the main scope of the project started with dividing the reactor into three separate 1-liter reactors. Each reactor was incubated at different temperatures: 8°C, 12°C, and 16°C, and fed with acetate and actual fish sludge as carbon sources in separate batch cycles. No significant difference in denitrification rates (absolute and specific) was observed between temperatures 8°C, 12°C, and 16°C during the endogenous phase. Temperature had an insignificant effect on denitrifying bacteria growth rate with acetate as the carbon source but had a more pronounced impact on hydrolysis with fish waste as the substrate. pH fluctuated during acetate denitrification, initially decreasing, and then gradually increasing due to intermittent nitrite accumulation. Denitrification rate was higher with acetate as the substrate compared to fish sludge. The findings derived from this study will contribute to the advancement of sustainable RAS practices, optimize the design of denitrification RAS effluent processing, and add to current knowledge on nitrogen removal in aquaculture systems.