Design, construction and start-up of the biological wastewater treatment were performed in four equal phases. Works on construction of the biological waste water treatment plant commenced on 1 December 2004 and were completed thirty-three months later, according to the time schedule agreed with the City of Zagreb. The plant capacity is 1.2 million PE. In technical-technological terms, the process of biological wastewater treatment with associated sludge treatment represents the second stage of wastewater treatment, because it is connected to the first treatment stage i.e. mechanical treatment. The plant capacity may be increased to 1.5 million PE if required.