Abstract: A method for treating a gas liquor to remove polluting substances with a stable treating efficiency and at a high rate which comprises the steps of(A) pre-treating a gas liquor discharged from a quenching of coke oven gas, the gas liquor containing ammonia, phenols, thiocyanate compounds, cyanide compounds, suspended solids and oils, to reduce the ammonia content of the gas liquor to about 1,000 ppm or less;(B) subjecting the gas liquor from step (A) to a first biological treatment comprising treating the gas liquor treated in step (A) with microorganisms,(C) subjecting the gas liquor from step (B) to a second biological treatment comprising treating the gas liquor treated in step (B) in an aerating tank including a mixture of suspended solids comprising powdered activated carbon and activated sludge mixed with the gas liquor, and(D) adding an iron salt to the gas liquor from step (C) to cause coagulation and sedimentation.