Abstract: This work reports the degradation of nitrogen compounds in a commercial marine Recirculating Aquaculture System (RAS) aimed at the culture of sea bream (Sparus aurata) and sea bass (Dicentrarchus labrax). The annual production of fingerlings is around 18 million and the process includes a drum filter and a biological treatment in order to enhance the water quality. Ammonia measurements at the inlet of the biological system showed that the concentration of this compound followed a diurnal pattern closely related to the feeding of the fingerlings; thus every day after feeding around 8 am, the concentration of ammonia started increasing, it reached a maximum about 8 h after feeding and then continued decreasing until the following morning. With regard to nitrite concentration, no significant differences were observed between the values measured at the inlet and the outlet of the biological system during the day, with an average concentration of this compound ranging between 0.08 and 3.66 mg NO-N l-1. A drawback of ammonia removal by means of nitrification is the subsequent increase of nitrate as the final product of ammonia oxidation in the culture system. The nitrate concentration in the biofilters inlet was found to fluctuate between 22.33 and 55.44 mg NO3-N l-1 during the characterization period. Partial water exchange was needed during the day in order to minimize the water losses during fish handling and to keep the concentration of nitrate below the maximum allowable level of 46 mg NO3-N l-1 due to production requirements in the hatchery under study. The ammonia degradation within the biological system, obtained by the ammonia measurements and comparison of the values at the inlet and outlet of the trickling filters has been fitted satisfactorily to ½-order/0-order kinetic expressions in good agreement with the results found in literature for laboratory and pilot plant studies. Rate constants k(1/2-order) = 0.49 g1/2 m-1/2 day-1 and k(0-order) = 0.64 g m-2 day-1, have been obtained in this study for commercial trickling biofilters. Thus, this work reports for the first time the kinetics of ammonia oxidation in trickling biofilters installed in a commercial recirculating aquaculture marine water system. These results will provide useful information for the design of an appropriately sized biofilter in order to optimize the water quality and reduce the need to exchange water in this activity.

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