The invention relates to a method for biological treatment of water, wherein macromolecular carbohydrates are added to the water to be treated, optionally together with vitamins and other additives, whereby a great reduction in residues is already achieved after adding small amounts of macromolecular carbohydrates, for example 0.4 to 600 mg/kg BSB.sub.5,zw or 0.014 to 14 mg/kg dry activated sludge daily.