Evaluation of an anaerobic baffled reactor (ABR)-downflow hanging sponge (DHS) system in treatment of black wastewater from a closed recirculating aquaculture system

This study focused on evaluating the nitrogen and solids removal performance of a laboratory-scale anaerobic baffled reactor (ABR) - downflow hanging sponge (DHS) system for treating black wastewater from a closed recirculating aquaculture system (RAS) under a salinity of 15 parts per thousand. The system was investigated under a controlled temperature (35 degrees C) with total organic carbon/total nitrogen (TOC/TN) ratios of 1.8, 2.5, 0.3, and 3.0, which corresponded to Phases 1, 2, 3, and 4, respectively, for 500 days. The ABR achieved a nitrate removal efficiency of 95-99 % at TOC/TN ratios higher than 1.8. The nitrate removal efficiency significantly decreased from 98 % to 12 % when the TOC/TN ratio decreased from 3.0 to 0.3. The highest TN removal efficiency (94 +/- 4 %) was achieved with a TOC/TN ratio of 3.0. The ABR could remove 97 +/- 6 % of the suspended solids during the entire experimental period. The DHS reactor achieved a nitrification efficiency of 70 % with short hydraulic retention times of 0.11-0.28 min. Total ammonia nitrogen in the DHS effluent was less than 1.0 mg N/L in the stable condition. The denitrifying Thauera genera predominant in the ABR reactor led to a high degree of denitrifi-cation. Methanogenic organisms such as Methanobacterium and Methanosaeta contributed to methane production of 0.8 +/- 0.4 L/d in the ABR at a TOC/TN ratio of 3.0. Microbial community structure analysis showed that nitrifying microorganisms, such as Nitrosomonas, Nitrospira, and Candidatus Nitrosopumilus, were detected in the DHS-retained sludge. These results indicate that the ABR-DHS system evaluated could be applied for the removal of nitrogen and solids from RAS derived black wastewater to meet the water quality level needed for production -scale RAS.

Automated summary

This study evaluated the performance of a laboratory-scale anaerobic baffled reactor (ABR) - downflow hanging sponge (DHS) system for treating black wastewater from a closed recirculating aquaculture system (RAS) with a salinity of 15 parts per thousand. The results showed that the ABR achieved a nitrate removal efficiency of 95-99 % at TOC/TN ratios higher than 1.8, but significantly decreased to 12 % when the ratio decreased to 0.3. The highest removal efficiency for total nitrogen and suspended solids was achieved with a TOC/TN ratio of 3.0.