The present work studies the destruction of a silica-free solution of alkaline hydrolysate of rice husk (pH 7.25, COD = 22.8O g·L−1) using the photo-Fenton process in the UV layer at a wavelength of 365 nm. Oxidation of lignin was carried out at a constant mass ratio of COD:H2O2 = 1:2 in the solutions with different dilution times (1:10, 1:50) and with varying iron(II) concentrations in the range from 10 to 100 mg·L−1. It was established that the oxidative degradation of alkaline lignin effectively proceeds in the UV layer when the solution is diluted 50 times, at an iron(II) concentration from 50 to 70 mg·L−1. Color, the content of polyphenols and chemical oxygen demand in the silica-free solution with a dilution ratio of 1:50, after oxidation, is reduced by several tens of times. The products of lignin degradation were identified using gas chromatography.