Our paper presents the possibility of phenol removal from post-processing wastewater originating from an experimental simulation of the underground coal gasification process carried out in the Barbara Experimental Mine in Mikołów, Poland. The application of initial coagulation-flocculation processes with iron(II), iron(III), aluminium, titanium(IV), and zirconium(IV) and anionic flocculant has allowed phenols removal from 7.9% (pH 8, Zr⁴⁺ 400 mg/L) to 22.2% (for pH 8, Fe²⁺ 800 and 1,000 mg/L and for pH 8, Fe³⁺ 1,000 mg/L). The application of the coagulation-flocculation process and advanced oxidation in the H₂O₂/UV system resulted in a total reduction of phenolic concentrations in the treated wastewater by about 93% due to their reaction with OH• radicals, resulting from the UV rays impact on H₂O₂.