Enhanced biological nutrients removal using the combined fixed-film reactor with bypass flow

The possibility of effective internal carbon source usage for removing nitrogen and phosphorus simultaneously in a fixed-film reactor was studied using the operation strategy with bypass flow. Tests were made to confirm whether nitrogen and phosphorus from municipal wastewater were eliminated effectively in the fixed-film reactor with bypass flow by increasing the bypass flow ratio from 0 to 0.4. The fixed-film reactor used in this experiment was a combined A2/O process and biofilm process. The bypass flow was applied in this experiment unit and the part of the influent was directly fed to an anoxic reactor for effective denitrification. The bypass flow ratio applied in this unit was 0, 0.3 and 0.4 based on the influent flow rate. The removal efficiencies of COD, NH+4–N and T–P were observed to be higher than 87.2%, 75.2% and 52.8% in all runs, respectively. Further, the optimal operational conditions for phosphorus removal were estimated when the bypass flow ratio was 0.4 with the internal recycle ratio of 0.5 and the external recycle ratio of 0.5 on the basis of the influent flow rate. The removal efficiencies in the bypass flow ratio of 0.4 were 88.0% for NH+4-N and 68.0% for T–P. Large differences in the removal of phosphorus resulted from varying the bypass flow ratio. With the bypass flow ratio of 0, 0.3 and 0.4, the removal efficiencies for T–P were 52.8%, 61.6% and 68.0%, respectively. It is suggested that the bypass flow in the fixed-film reactor can achieve complete denitrification and can be helpful for improving phosphorus removal.