Nowadays, the use of xenobiotics is becoming more and more common, and thus increasing amounts are being released into the environment in an uncontrolled manner. Pesticides, antiparasitics and other xenobiotics and their metabolites, degradation and transformation products reach the soil and water causing damage to the plant and animal life. Since xenobiotics are compounds with a complex chemical structure, it is very difficult to completely remove them from water, and it is often necessary to use complex and expensive wastewater treatment methods. Activated sludge treatment methods are the most common methods in wastewater treatment. The aim of this work is to examine the possibilities of removing xenobiotics from wastewater by biodegradation using activated sludge. The possibility of biodegradation of the antiparasitics febantel, albendazole and mebendazole and pesticides from the group of insecticides thiacloprid, acetamiprid and clothianidin was determined within 48 hours by high-performance liquid chromatography with monitoring of pH-value, temperature, density, total carbon and impact on microorganisms. The biodegradation process took place in five reactors that were protected by aluminum foil to prevent the influence of sunlight on the degradation of xenobiotics. The removal of xenobiotics was determined by high-performance liquid chromatography with a diode array detector, and samples were taken at precisely determined time intervals. In addition, the samples were analyzed by measuring the total amount of carbon, and we monitored the density of activated sludge before the biodegradation process and the density after biodegradation for the reactor with sludge, microplastics and xenobiotics and for the reactor with sludge and xenobiotics. A microbiological analysis was also carried out for each xenobiotic in order to determine the influence of each xenobiotic on the microorganisms that participate in the biodegradation process. One hundred percent removal was achieved for all antiparasitics, while the removal was slightly worse for pesticides. In addition to the removal process itself, the adsorption and desorption of components on the surface of microplastics and activated sludge flakes were also taken into account, and the resulting chromatograms were used to detect the degradation/transformation products that arise during the biodegradation process.