Wastewater contains various harmful substances that pollute water and as such must not be discharged into nature, that is, such water must be subjected to an adequate purification process. The most common method of wastewater treatment is carried out through the Wastewater Treatment plant, which uses a combination of mechanical, physical-chemical and biological methods. The mechanical process of wastewater treatment removes coarse impurities and suspended substances, while further physical-chemical treatment removes colloidal and inorganic substances, and biological methods are designed for removing organic substances of natural origin. The target of this final work was to conduct a two-week test of the efficiency of wastewater treatment received at the Zaprešić City Wastewater Treatment Plant. The obtained results indicate a successful treatment, where in the first stage of wastewater treatment, the amount of harmful substances was reduced by up to 60%, and with the secondary treatment, it is possible to remove from 30 to 50% of the total nitrogen with the help of active sludge.