Efficient treatment of high-density bullfrog farming tailwater through shortcut nitrification and denitrification

Aquaculture production has surged over the last two decades and is now in a stage of stable growth globally. Bullfrog farming has become a flourishing aquaculture industry in China since 2010s and is being considered as the pillar industry in many southern provinces of China. However, its sustainability is being restricted due to the increasingly stringent discharge standards for tailwater. In this study, shortcut nitrification and denitrification was established in two configurations, i.e., one-stage sequencing batch reactor (SBR) and two-stage continuous stirred tank reactors (CSTR), to treat real bullfrog farming tailwater. SBR and CSTR achieved stable removal of nitrogen (i.e., 89.8 ± 1.0% and 93.3 ± 0.7%, respectively) and chemical oxygen demand (COD) (i.e., 93.3 ± 0.9% and 93.3 ± 0.9%, respectively), producing high-quality effluent (i.e., 2.2 ± 0.2 mgTN/L and 2.5 ± 0.3 mgCOD/L, and 1.6 ± 0.3 mgTN/L and 2.5 ± 0.4 mgCOD/L, respectively), which not only complies with the current discharge standards for COD and nitrogen, but also the more stringent standards (effective from May 1st, 2026). 16S rRNA gene amplicon sequencing analysis revealed that Nitrosomonas dominated in SBR (24.7%), and Nitrosomonas (1.9%) and Nitrosospira (1.7%) coexisted in CSTR nitrification reactor, performing the ammonia oxidation. Nitrite oxidizing bacterium was mostly negligible. Diverse aerobic denitrifying populations, e.g., Aridibacter, Comamonas, and Thauera, sustained the dominance in CSTR denitrification reactor (64.8%) and SBR (27.1%). These results hinted that shortcut nitrification and aerobic denitrification likely accomplished the concurrent removal of nitrogen and COD from the tailwater with reduced aeration. This study offers high-efficiency and energy-saving solutions for the treatment of bullfrog farming tailwater, which underpins the sustainable development of bullfrog farming and probably other aquaculture industries in China and elsewhere.