Effect of hydraulic retention time on advanced nitrogen removal performance in denitrification and partial-denitrification anammox processes: NH4+-N and organic matter from secondary influent as electron donors

The hydraulic retention time (HRT) of a denitrifying filter (DF) is crucial in influencing total nitrogen (TN) removal. Therefore, the effect of HRT on the performance of advanced synergistic nitrogen removal (ASNR) processes, including denitrification and partial-denitrification anammox (PDA), was investigated using a labscale DF. NH4+-N and organic matter from the secondary influent were utilized as electron donors. As the HRT decreased from 8 to 2 h, the filtered effluent TN increased from 0.51 to 2.45 mg/L, with the corresponding removal efficiency decreasing from 97.7 % to 88.9 %. Besides, the effluent COD ranged from 17.26 to 18.98 mg/ L, with a removal efficiency between 58.5 % and 61.5 %, indicating the minimal impact from HRT variation. Furthermore, 60.9 %-63.2 % of the removed TN was eliminated via the PDA pathway, with 1.29 to 1.36 mg of COD consumed per mg of TN removal. The presence of denitrifying, anammox and hydrolytic bacteria confirmed the effectiveness of ASNR. These results demonstrated that the technique holds significant potential in practice due to its cost-effective and high TN and COD removal under low HRT.