A biological reactor for wastewater treatment from minor sources of pollution using activation process with activated sludge in upflow and/or with growth culture on a bio-carrier, where all processes of biological treatment with activation process take place in single-tank biological reactor, where functional areas of mechanical pre-treatment, aeration and activated sludge separation are spatially separated, containing a tank with an inflow and an outflow, in which a separation chamber with a bottom is situated, an area between a casing of the separation chamber and a casing of the tank is divided by vertical partition walls and near the bottom of the aeration area aeration elements are situated in front of the vertical partition walls characterized in that the area between the casing of the separation chamber and the casing of the tank is divided into two parts by two vertical partition walls A and B spanning from a tank bottom up to over a bottom of a discharge piping, namely to an aerated chamber and a mechanical pre-treatment chamber having an anaerobic sludge zone in the lower portion, an anoxic non-aerated zone in the upper portion, the mechanical pre-treatment chamber containing a raking basket with a perforated grate and with a funnel-shaped bottom arranged under the perforated grate, and a vertical intake piping having a mouth and ending with an outlet opening, the mouth being watertight connected with the funnel shaped bottom and the outlet opening being located above the bottom of the mechanical pre-treatment chamber; the biological reactor also comprises an air-lift pump, which has a suctioning mouth located near the bottom of the separation chamber, for suction of the settled sludge from the bottom of the separation chamber, and a pipe introduced into the intake piping, the pipe of the air-lift pump ending by a pump opening under the perforated grate of the raking basket, so that the incoming wastewater, which is, after the coarse pre-treatment, directed by the funnel shaped bottom into the mouth of the intake piping, is mixed with the activated sludge pumped by the air-lift pump, and the mixture of incoming wastewater and activated sludge is directed by the intake piping towards the bottom of the mechanical pre-treatment chamber; the mechanical pre-treatment chamber and the aerated chamber are connected by an opening (19, 22) in the upper parts of both vertical partition walls A and B; and wherein recirculation of activated mixture from the oxic aerated chamber into mechanical pre-treatment chamber is provided in such manner that the aeration elements A and B located near the bottom of the aerated chamber in front of the partition walls A and B are situated so that a height of the water column over the aeration element A is lower than a height of the water column over the aeration element B and/or an air flow regulator (42, 43) for the aeration elements A and B provides air flow regulation in such manner that the amount of the air conveyed into the aeration element A is higher than the amount of the air conveyed into the aeration element B, which allows to create rotational, horizontal circulation of water surface layer of activated mixture around the upper part of the casing of the separation chamber, namely from the opening in the partition wall A through the mechanical pre-treatment chamber to the opening in the partition wall B and through the opening in the partition wall B to the aerated chamber and further to the opening in the partition wall A; and wherein an opening is provided in the lower conical portion of the casing of the separation chamber through which the activated sludge can flow from the aerated chamber to the separation chamber; and wherein a discharging piping is provided so that the clear treated water separated by sedimentation from the activated sludge in the separation chamber and having flown up in the cylindrical part of the separation chamber up to the discharging pipe can be discharged.