Acclimatization and application of biofilters for nitrogen removal in marine recirculating shrimp culture system

Abstract

This research involved in the complete nitrogen removal in marine recirculating aquaculture system (RAS) through sequential nitrification and denitrification processes using internal biofilter within a single tank. The study was divided into two experimental parts. The first study was to estimate the effects of salinity (5, 15 and 25 PSU) and stocking density (50 and 100 shrimp m-2) on nitrification and denitrification efficiencies, as well as on microbial diversity in the biofilm. Also, the nitrogen removal efficiencies of fibrous BiocordTM biofilter and Japanese filter mat were compared. Results showed that the nitrification was stimulated in low-salinity (5 PSU) system with intensive (100 shrimp m-2) shrimp cultivation at the maximum ammonia removal rates of 100.42±5.97 mg-N m-2 day-1 for fibrous biofilter and 145.43±1.17 mg-N m-2 day-1 for filter mat, respectively. While the highest denitrification efficiencies were also found in the intensive system as 81.86±4.40 mg-N m-2 day-1 at 25 PSU for fibrous biofilter and 165.80±50.17 mg-N m-2 day-1 at 5 PSU for filter mat, respectively. Results from the next-generation sequencing (NGS) on Illumina MiSeq demonstrated that Proteobacteria and Bacteroidetes were the dominant bacterial groups in all experimental systems. For microorganisms in nitrogen cycle, however, the predominant nitrifiers and denitrifiers observed in low-salinity system was different from that under medium- (15 PSU) and high-salinity (25 PSU) conditions. The second study was to evaluate the performance of fibrous biofilter and to monitor the microbial community dynamics during long-term (210 days) operation of marine (25 PSU) RAS at the initial shrimp density of 1 kg-shrimp m-3. Results showed that the complete nitrification was achieved after approximately 2 months of biofilter acclimation in parallel with shrimp cultivation. Throughout the two rounds replication of aerobic nitrification followed by anoxic denitrification, ammonia and nitrite were controlled within the acceptable condition while nitrate was then remove after shrimp harvest under anoxic condition with methanol supplement at COD:Nitrate-N of 5:1. Microbial results demonstrated that the uncultured bacterium clone PI1AB88 and the uncultured bacterium clone SF_NOB_Cd08 were the main players in ammonia and nitrite oxidation, respectively, while Methylophaga and Methylotenera were the predominant denitrifying bacteria in anoxic denitrification.