Effect of dissolved oxygen on nitrate removal using polycaprolactone as an organic carbon source and biofilm carrier in fixed-film denitrifying reactors

Nitrate-nitrogen (NO₃⁻-N) always accumulates in commercial recirculating aquaculture systems (RASs) with aerobic nitrification units. The ability to reduce NO₃⁻-N consistently and confidently could help RASs to become more sustainable. The rich dissolved oxygen (DO) content and sensitive organisms stocked in RASs increase the difficulty of denitrifying technology. A denitrifying process using biologically degradable polymers as an organic carbon source and biofilm carrier was proposed because of its space-efficient nature and strong ability to remove NO₃⁻-N from RASs. The effect of dissolved oxygen (DO) levels on heterotrophic denitrification in fixed-film reactors filled with polycaprolactone (PCL) was explored in the current experiment. DO conditions in the influent of the denitrifying reactors were set up as follows: the anoxic treatment group (Group A, average DO concentration of 0.28 ± 0.05 mg/L), the low-oxygen treatment DO group (Group B, average DO concentration of 2.50 ± 0.24 mg/L) and the aerated treatment group (Group C, average DO concentration of 5.63 ± 0.57 mg/L). Feeding with 200 mg/L of NO₃⁻-N, the NO₃⁻-N removal rates were 1.53, 1.60 and 1.42 kg/m³ PCL/day in Groups A, B and C, respectively. No significant difference in NO₃⁻-N removal rates was observed among the three treatments. It was concluded that the inhibitory effects of DO concentrations lower than 6 mg/L on heterotrophic denitrification in the fixed-film reactors filled with PCL can be mitigated.