In the theoretical part of the Master's thesis we first summarized the legislation about the disposal and treatment of wastewaters. After this, there is a description of biological wastewater treatments and a presentation of the various systems for biological wastewater treatment. We presented the possible ways to deal with biological excess sludge, which appears as an inevitable side product of biological wastewater treatment. The theoretical part ends with a description of the system of disposal and treatment of wastewaters in the Municipality Zagorje ob Savi, where we focused on the process of wastewater treatment at the Central Wastewater Treatment Plant Zagorje ob Savi. In the experimental part we performed a simulation of the activity of the SBR reactor of the Central Wastewater Treatment Plant Zagorje ob Savi on the laboratory's SBR reactors. With the laboratory's SBR reactors we first established how the ventilation time of the SBR reactors influence the effects of the wastewater treatment and then we compared the effects of the wastewater treatment of the laboratory's SBR reactors with the effects of the wastewater treatment of the real SBR reactor of the Central Wastewater Treatment Plant Zagorje ob Savi. With the Automatic Methane Potential Test System II (AMPTS II) device we established the biomethane potential of the biological excess sludge of the Central Wastewater Treatment Plant Zagorje ob Savi for two different organic loads, in a mesophilic temperature zone at 38 °C and in a thermophilic temperature zone at 55 °C. On the basis of the obtained results of the biomethane potential test we analysed the expediency of setting up an anaerobic digester. In the conclusion of the thesis we introduced our findings and possible suggestions.