Abstract: The biological reactor contains in a vessel (11) a bundle (13) of tubes (15). The bundle (13) is formed by a plurality of plastic tubes (15) being of preferably square cross-section. The walls of the plastic tubes serve as growth surfaces for microorganisms. A distributing device (17) feeds the different ducts (18) evenly with liquid organic refuse. The ducts (18) are not completely filled. The liquid level in the reactor is at a certain distance from the top of the tubes (15). Accordingly, if one of the ducts (18) tends to clog, it is filled more than the remaining ducts so that in the clogging duct a hydrostatic pressure is created having the tendency to free the duct. This freeing of the duct is assisted by the control unit 58 which from time to time lowers the liquid level in the reactor and then increases it again.