Acclimated chitosan for inorganic nitrogen removal in recirculating aquaculture system for <i>Penaeus vannamei</i> cultivation

Abstract This research study aimed to apply acclimated chitosan for inorganic nitrogen removal in the recirculating aquaculture system (RAS). The chitosan flakes were acclimated under conditions with the presence of ammonia and nitrite at a salinity of 15-20 ppt to activate the nitrification activity. Ammonia oxidation was rapidly initiated after one week of acclimation, while nitrite oxidation required ten weeks to complete. The indices of total ammonia nitrogen (TAN) and nitrite removal rates were 0.09±0.03 and 0.05±0.0 mg-N/g-chitosan/day, respectively. The abundance of ammonia-oxidizing bacteria (AOB) amoA gene was relatively constant (8.41 to 15.14 copies/g-chitosan), while nitrite-oxidizing bacteria (NOB) nrxB gene increased to 2.30 × 10 4 copies/g-chitosan. Acclimated chitosan was subsequently installed as a biofilter unit in the RAS for Penaeus vannamei cultivation, operated for two months under a zero-discharge scheme. The control tanks with non-acclimated chitosan showed elevated concentrations of ammonia (0.9 ± 0.3 mg-N/L) and nitrite (4.4 ± 0.8 mg-N/L) during the first month of shrimp cultivation. In contrast, the acclimated chitosan effectively controlled ammonia (0.2 ± 0.1 mg-N/L) and nitrite (0.2 ± 0.2 mg-N/L) concentrations within acceptable limits throughout the cultivation period, resulting in significantly higher shrimp productivity in terms of weight and specific growth rate (SGR).