NOVELTY - The procedure for the anaerobic treatment of sewage sludge in a putrefying tank or biogas plant for wastewater purification, comprises adding an additive (16), which contains a stabilized dihydrofolic acid or tetrahydrofolic acid and/or its derivatives, to the sewage sludge. In the treatment, the sewage sludge is partially taken off from the putrefying tank (11) or biogas plant and is supplied again into putrefying tank or biogas plant by a return line (21). The additive in an area of an outlet of the sewage sludge is supplied to the return line.    USE - For the anaerobic treatment of sewage sludge in a putrefying tank or biogas plant for wastewater purification (claimed).    ADVANTAGE - Reduces the operating cost of the biogas plant and ensures good and homogeneous mixing of the additive with the sewage sludge for effective anaerobic treatment of sewage sludge.    DETAILED DESCRIPTION - The procedure for the anaerobic treatment of sewage sludge in a putrefying tank or biogas plant for wastewater purification, comprises adding an additive (16), which contains a stabilized dihydrofolic acid or tetrahydrofolic acid and/or its derivatives, to the sewage sludge. In the treatment, the sewage sludge is partially taken off from the putrefying tank (11) or biogas plant and is supplied again into putrefying tank or biogas plant by a return line (21). The additive in an area of an outlet of the sewage sludge is supplied to the return line. The additive in an area of an inlet (12) or in a supply pipe (14) and in the return stroke, is added to the sewage sludge. Concentration of the folic acid and/or its derivatives is 1.0-10 ppm with regard to the mass of the sewage sludge. The folic acid and/or its derivatives are mixed before the supply with demineralized water (17). The additive contains further enzymes and/or co-enzyme as biologically stimulating substances or the enzymes and/or the co-enzyme are supplied to the additive before the addition into the sewage sludge.    An INDEPENDENT CLAIM is included for an additive for use in the anaerobic treatment of sewage sludge.    DESCRIPTION OF DRAWING(S) - The figure shows an installation schema of an anaerobic treatment of sewage sludge in a putrefying tank of a clarifier.    Putrefying tank; (11)    Inlet; (12)    Supply pipe; (14)    Additive; (16)    Deminaralized water; (17)    Return line. (21)