To provide a method and a device for treating a waste liquid capable of efficiently treating a waste liquid while suppressing the generation of nitric acid, particularly, paying attention to characteristics of the waste liquid per se that is to be a treated liquid, in order to solve the problem that there is a fear of device corrosion as nitric acid is concurrently produced by the reaction of nitrogen compounds, although a high temperature of about 630°C is required to decompose a hardly degradable hazardous substance when the waste liquid containing the hardly degradable hazardous substance and the nitrogen compounds are supercritically water-oxidated.

The method and device for treating a waste liquid including the step of supercritically water-oxidating the waste liquid containing the hardly degradable substance and nitrogen compounds and with calorific value of a certain value or more or/and with mole ratio TOC/TN of total organic carbon (TOC) and total nitrogen (TN) of a certain value or more at a temperature required to decompose a hardly degradable hazardous substance, and the step of adding a nitric acid production suppressant consisting of KOH and NaOH to the waste liquid before heating the waste liquid to the temperature required to decompose a hardly degradable hazardous substance.

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