The invention relates to the complex processing of a nuclear power plant's NPP's liquid, boron-containing waste with a complex composition, being generated during the operation of NPPs, including of ones VVER-type, and can be used to isolate boric and nitric acids and hydroxides of sodium and potassium for their reuse in the NPP process cycle. The invention allows to obtain crystalline boric acid and highly concentrated solutions of nitric acid and hydroxides of sodium and potassium, suitable for reuse in the NPP process cycle and for general industrial use. Conducting electrodialysis at low values of current and voltage provides a reduction of the method's energy intensity. The involvement of all major components of waste mother liquors into the processing reduces the amount of stored and disposed hazardous waste.