Medium C/N with Low Aeration for Enhanced Aerobic Denitrifying MBBRs Start-Up Treating Mariculture Wastewater

The Moving Bed Biofilm Reactor (MBBR) is commonly used for NH4+-N removal from seawater recirculating aquaculture systems (RASs), while total nitrogen (TN) reduction is increasingly required. Aerobic denitrification offers a potential solution for simultaneous TN and NH4+-N removal, but rapid reactor start-up remains challenging. This study investigated the start-up of aerobic denitrifying MBBRs treating mariculture wastewater. Eight MBBRs were operated at 25 °C with four C/N ratios (5.5, 4.5, 3.5, and 0) and two aeration rates (1.0 and 0.4 L/min). Only the CN4.5-0.4 MBBR met the predefined start-up criteria, achieving start-up in 26 days with an NH4+-N removal efficiency of 93.8%. Effluent NH4+-N and NO2−-N remained below 0.5 mg/L without NO3−-N accumulation. External carbon addition was essential, but excessive carbon did not accelerate start-up; CN3.5 MBBRs showed slower and less stable NH4+-N removal, whereas CN5.5 MBBRs showed higher NO3−-N removal but still accumulated NO2−-N. Low aeration increased the average NO3−-N removal efficiency at C/N = 5.5 from 39.0% to 66.2%. Microbial community analysis identified Pseudomonas, Vitellibacter, and Paracoccus as the dominant nitrogen-removing genera, with a combined relative abundance of 41.0% in the CN4.5-0.4 MBBR. Metagenomic sequencing identified genes associated with multiple nitrogen-transformation pathways and supported the potential for aerobic denitrification.