The effect of AQUI-S on nitrogen compound removal efficiency in recirculating aquaculture system (RAS)

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AQUI-S (AQUI-S® New Zealand Ltd., Lower Hutt, New Zealand) is a water dispersible liquid anaesthetic that is currently approved in Norway for sedation and anesthesia of Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss) prior and during handling events and during live fish transport. Its active ingredient is isoeugenol; 540 mg/ml. It is used both in flow-through and recirculating aquaculture systems (RAS). However, the potential effects of AQUI-S on the biological component of the water treatment and the possible accumulation within RAS water have not been closely investigated. This project aims to look into the bioreactor efficiency of nitrogen compound removal and AQUI-S stability and potential accumulation within RAS water. The initial concentration of AQUI-S in the RAS water was 5 ml/m3 total system volume, corresponding to 2,7 mg isoeugenol L-1. This represents the maximum load of Aqui-S that theoretically will be added to a RAS; usually only a few tanks in the circuit will be medicated at once, not the entire system. Six hours after addition of AQUI-S, 86-87% of the isoeugenol was degraded or converted although the total water exchange within RAS was very low (6.6 %). It is therefore anticipated that in commercial RAS where the level of total daily water exchange might be higher, the depletion of AQUI-S from water would occur even faster. The water quality and the removal of the TAN and NO2-N were moderately affected, particularly during first 12h of the experiment. One of the reasons for the disturbance of the bioreactor function could be foam formation that was visible prior and in all three chambers of the MBBR within 30 minutes after addition of the AQUI-S. Consequently the difficulty in the mixing and the drop in the oxygen saturation within MBBR could have affected proper operation of the bioreactor. In addition, the low and decreasing concentration of nitrogenous compounds within the system (lack of feeding and fish) is not favourable for the nitrification process within bioreactor. However, it is important to stress that certain amount of nitrogenous compounds was removed at all times. The increase in concentration of TAN and NO2-N was significant; however concentrations were within recommended values issued by Norwegian Food Safety Authorities at all times during the experiment. This conclusion will, however, depend on the initial load of the system. Hence, it is recommended that before AQUI-S use, the TAN levels should not be above 1 mg/l (at pH 7.4 or less), to allow some reduction in removal efficiency and still be within legislation regarding TAN concentration. The removal efficiency of nitrogenous compounds oscillated during first 12 h of experiment but was not significantly affected by addition of AQUI-S. After 12 h, a steady increase in removal efficiency was recorded for TAN and NO2-N, respectively. This conclusion will, however, depend on the initial load of the system. Hence, it is recommended that before AQUI-S use, the TAN levels should not be above 1 mg/l (at pH 7.4 or less), to allow some reduction in removal efficiency and still be within legislation regarding TAN concentration. The removal efficiency of nitrogenous compounds oscillated during first 12 h of experiment but was not significantly affected by addition of AQUI-S. After 12 h, a steady increase in removal efficiency was recorded for TAN and NO2-N, respectively. • The study indicates that Aqui-S is readily degraded by RAS bioreactors, minimizing the risk of isoeugenol accumulating following sequential medication of fish tanks in the RAS • AQUI-S used at doses recommended for sedation is not likely to have serious effects on bioreactor within 48 hours following its use in RAS • Increased foaming in the moving bed bioreactor during the first hours following Aqui-S administration might cause or contribute to a transient reduction in TAN and NO2 –N removal efficiency.