In recent years, interest in the presence of chemical and biological pollutants (drugs, pesticides, heavy metals etc.) in sewage water has been permanently increasing and, at the same time, significant effort to eliminate them has been shown. The aim of this work was to investigate the influence of ferrates activity on the decomposition of the drug diclofenac (DCF), which is commonly present in sewage water. The main task is to follow the effect of potassium ferrate dosage on DCF degradation and to evaluate the most effective ferrates concentration in solution. Subsequently, real samples of waste water containing a significant amount of various micro‑pollutants were treated by the same amount of ferrates as laboratory samples. High-performance liquid chromatography (HPLC) was used for the sample analysis. Ferrates removal effectiveness was determined by comparing the chromatographic peak areas of residual DCF in the treated samples. Removal efficiency of diclofenac is proportional to the concentration of potassium ferrate in the treated solutions.