With the progress and improvement of the pharmaceutical industry, the number of pharmaceuticals used daily is increasing. Since there is a large number of different medicines, the interest of science is growing for the investigation of their influence on humans and other target organisms as well as their impact on the environment. Pharmaceuticals reach into the environment in many ways, from which one of the most significant ones is entry by conventional wastewater treatment plants (WWTPs) caused by inadequate removal. During the last 15 years, a great effort is invested into finding new highly-effective technologies for waste removal from wastewaters, and for the removal of pharmaceuticals or micro-pollutants, advanced oxidation processes have been shown as a possible solution. In this research, the oxidative effect of peracetic acid on the removal of diclofenac was investigated by statistical design of experiments. Diclofenac is an active pharmaceutical ingredient available on the market as an oral, transdermal, parenteral and ophtalmic dosage forms, hence it is omnipresent in wastewater. This research showed that the degradation of diclofenac with peracetic acid is a kinetically slow process and to increase the rate of reaction, an iron-oxide heterogenous catalyst was chosen, in order to achieve a modified-Fenton process. The PAA/γ-Fe2O3 has achieved a kinetically fast and complete removal during the investigated period of time and achieved an overall high degree of organic content removal.