Nitrate removal efficiency and bacterial community of polycaprolactone-packed bioreactors treating water from a recirculating aquaculture system

A denitrification reactor packed with polycaprolactone (PCL) as a carbon source and biofilm carrier was developed to remove nitrate nitrogen (NO3--N) from the water of a recirculating aquaculture system for 115 days. The hydraulic retention time was set to 6 h, and the water flow rate was 0.95 L h(-1). The removal rates of NO3--N ranged from 34.67 to 155.7 g NO3--N m(-3) day(-1) when the effluent concentrations ranged between 1.62 and 72.25 mg L-1. No obvious changes in the Fourier transform infrared spectra of the PCL before and after use were observed. Bacterial community structure in the biofilm of the PCL granules was analyzed using Illumina MiSeq sequencing technology. Proteobacteria and Bacteroidetes were predominant in the biofilm, with relative abundances of 63.9 and 27.4%, respectively. Three genera, Acidovorax, Azospira, and Diaphorobacter, were capable of both denitrification and PCL degradation. This study indicates that PCL-packed reactors may be used and optimized for removing nitrate from aquaculture effluents.