Effect of internal recycle ratio on the denitrification process and nirS-containing bacteria of an anaerobic/anoxic/oxic (A²/O) wastewater treatment process

Internal recycle (IR) ratio is an important operation parameter for the anaerobic/anoxic/oxic (A2O) wastewater treatment process. Three laboratory-scale A2O wastewater treatment processes with IR ratios of 100%, 200%, and 300% were set up to study its influence on the denitrification process and nirS gene-containing bacteria. Results showed the removal rate of chemical oxygen demand (COD), ammonia nitrogen (NH4-N), total nitrogen (TN) and total phosphorus (TP) increased at different levels as the IR rate augmented from 100% to 300%. NirS gene numbers were increased from 1.8×108 to 3.2×108 copies/g MLSS, which was positively correlated with the denitrification rate in anoxic areas. Moreover, similarities were observed in the community structures of denitrifying bacteria that contained the nirS gene under different operation modes. These results indicated that increasing the IR rate in the A2O treatment process could benefit nirS gene-containing bacteria and improve denitrification ability observably while maintaining the stability of the community structure of the system.