The disclosed denitrification method with least cost compared with prior art comprises: an in-out water system, a filter contained filter medium with size reducing gradually along the inlet direction, a backwash system, and a PLC control system. Wherein, the inlet pump (1) connects with the filter (11) with lower end linked both with the outlet valve (6) and the backwash pump (7) and upper end linked with the backwash outlet valve (9); it arranges the sensor (10) in the filter (11) to contact with water. When the SCOD achieves stable value but less than 100mg/L, putting the medium into filter; using PLC control program to adjust water residence time; backwashing for 2-3h with flux adjusted by the PLC control box; and repeating to filter and backwash. This invention simplifies operation and reduces cost without secondary pollution. |