Single-tank sequencing batch bioreactor with a cellulose substrate for simultaneous nitrification and denitrification of an aquarium

Processing and detoxifying nitrogenous waste in an aquarium is difficult with one tank because nitrifying and denitrifying bacteria need contradictory conditions to grow and survive. Herein, a single-tank sequencing batch bioreactor was used to simultaneously perform aerobic nitrification and denitrification for aquarium filtration. To evaluate its performance at the laboratory-scale, the 8 L bioreactor was connected to a 20 L aquarium with a carbon water filter and containing Oranda goldfish. The bioreactor comprised a bottom layer filled with 2.5 L of polyester sponge, covered with 1 L of carbon pellets and then 2.5 L of cellulose beads or polyester sponge. A submersible pump in the aquarium continuously pumped water into the bioreactor, which created an oxygenated environment with a water filtration rate of 4,608 L/day. Over 117 days, the bioreactor significantly lowered the total ammonia nitrogen, nitrite, and nitrate concentrations with minimal water filling to compensate for evaporation. Therefore, the bioreactor can simultaneously perform aerobic nitrification and denitrification in a single-tank and maintain good water quality for long-term care of ornamental fish. However, aerobic denitrification successfully occurred only in the bioreactor containing cellulose beads. Therefore, metagenomic analysis was performed to examine the microbial communities on the filters, and Proteobacteria were identified as the dominant phylum in all samples. The nitrifying and denitrifying bacteria in the filters mainly belonged to Nitrospirota (Nitrospira spp.) and Proteobacteria (Comamonadaceae and Rhodobacteraceae families), respectively. Beta diversity analysis indicated that the differences in bacterial communities could be attributed to changes in the substrate and incubation time. Proteobacteria were the most significant contributors to the physiological activity in the simultaneous nitrification and aerobic denitrification bioreactor.