Abstract: A prefabricated biological purification system for treatment of residential wastewater having a primary purification unit and a secondary purification unit containing a filling material having a grain size distribution of about 2 cm to about 4 cm in diameter. The primary purification unit having modular element forming a container, and internal elements including an inflow tube, an internal separation panel with a fluid passage duct defining a first internal sector and a second internal sector, a trapezoidal diaphragm, and a device for controlling liquid outflow. A second diaphragm in a form of an octagonal modular element is positioned within the container and a horizontal fluid passage duct is formed as a horizontal opening between portion of the internal separation panel and the second diaphragm. A flow control element forms an intake space between a bottom end portion of an outlet pipe and the trapezoidal diaphragm.