Innovations for nitrate removal in recirculating aquaculture systems

Due to the efficient reuse of water in modern intensive recirculating aquaculture systems (RAS) metabolic end products of fish and bacteria can enrich in the system water. It is important to clarify whether and at which concentration these substances have a negative effect on the respective cultivated fish species. For nitrate, a metabolic end product of microbial nitrification, negative effects have already been documented for many fish species. However, there are still significant gaps in knowledge for various aquaculture-relevant fish species. In this thesis, the effects of nitrate on production and health parameters of European sea bass (Dicentrarchus labrax) were investigated. It was shown that the European sea bass is less sensitive to increased nitrate concentrations. In order to reduce the nitrate concentration in RAS, the self-cleaning inherent gas denitrification reactor (SID-Reactor) was used. The main focus of the thesis was the optimization in order to remove the potentially harmful substances from the system water. In different experiments, effects of central operating parameters of the SID-Reactor (hydraulic retention time, backflushing interval & methanol dosage) on water quality and denitrification were investigated. In addition, the potential use of biodegradable plastic from polyhydroxyalkanoate (PHA) in the SID-Reactor was investigated. The results showed that PHA provided a lower denitrification efficiency (%) than methanol. However, the use of PHA outweighed the benefits in terms of safety, dosage effort and maintenance.