Method of decomposing phenolic by-products

A method of decomposing phenolic by-products, the method comprising:

• supplying a phenolic by-product stream having an amount of process water of 2 wt % or less to a decomposition device to perform thermal decomposition;
• separating an upper discharge stream including effective components and a lower discharge stream including materials having a high boiling point in the decomposition device;
• supplying the lower discharge stream from the decomposition device, a side discharge stream from the decomposition device, and a process water stream to a mixing device and mixing these streams;
• and
• supplying a discharge stream from the mixing device to a layer separation device to separate the discharge stream from the mixing device into an oil phase and an aqueous phase,
• wherein a portion of an aqueous stream discharged from the layer separation device is supplied to the mixing device, and a residual stream is discharged as waste water including salts.