LAUSR

Abstract Removal of total ammonium nitrogen (TAN) is essential during transportation of live seafood due to its high toxicity to aquatic cultures. Recirculating TAN removal system has been proposed and installed for the transportation of live seafood, while TAN removal performance often deteriorates at low temperature range (10–20 °C) due to the decrease of nitrifying activity. The authors have previously developed down-hanging sponge (DHS) reactors as a recirculating TAN removal system, which is a trickling filter system that uses polyurethane-sponge media to retain nitrifying biomass. The present study aimed to examine TAN removal performance of DHS reactor operated at 10 and 20 °C, and the following 4 DHS reactors were operated for the purpose; the DHS reactor operated 1) at 20 °C, 2) at 20 °C with addition of powdered activated carbon (AC) as an additional biomass carrier, 3) at 10 °C, and 4) at 10 °C with AC (designated as 20 °C-DHS, 20 °C-DHS-AC, 10 °C-DHS, and 10 °C-DHS-AC reactors, respectively). The DHS reactors were operated for 472 d with feeding of artificial seawater media, and TAN loading rates increased stepwisely up to 200 mg-N L-sponge media−1 d−1. TAN removal performance was examined by determining the concentrations of TAN, nitrite and nitrate in influent and effluent. Additionally, amplicon sequencing of prokaryotic 16S rRNA gene sequence was performed at Day 34, 212 and 374 to identify the nitrifying bacteria involved in the TAN removal. Both the 20 °C-DHS and 20 °C-DHS-AC reactors showed 200 mg-N L-sponge media−1 d−1 of TAN removal rate, whereas the 10 °C-DHS reactor showed only 15 mg-N L-sponge media−1 d−1. Notably, the addition of AC enhanced TAN removal performance of the DHS reactor operated at 10 °C. The 10 °C-DHS-AC reactor showed 60 mg-N L-sponge media−1 d−1 of TAN removal rate, which was 4-folds higher than those found in the 10 °C-DHS reactor. The amplicon sequencing analysis revealed that ammonia-oxidizing (i.e., Nitrosomonas) and nitrite-oxidizing (Nitrospira and Nitrospina) bacteria (AOB and NOB, respectively) proliferated in the operated DHS reactors, and the relative abundance of those AOB and NOB were greater in the DHS reactors with AC addition (i.e., the 20 °C-DHS-AC and 10 °C-DHS-AC reactors). Intriguingly, temperature difference (20 °C and 10 °C) resulted in occurrence of different NOB communities, and Nitrospira and Nitrospina were prominent in the DHS reactors operated at 20 °C and 10 °C, respectively. DHS reactor is capable of removing TAN even at 10 °C, which is applicable as a recirculating TAN removal system for transportation of live seafood.