Process for the treatment of sewage by using conventional precipitants and flocculants, like silicates, iron- or aluminum salts or organic polymers, and in addition about 0.1-10 (0.03-3) g/l sewage of X-ray amorphous or crystalline, finely-divided alkali- and/or alkaline earth-alumosilicates of the formula:x cat.sub.2/n.Al.sub.2 O.sub.3.y SiO.sub.2where x=0.7-1.5; y=0.8-6; cat=Na, K, Mg or Ca; n=1-2. The compounds contain preferably bound water. The alumosilicate is added preferably in the form of an aqueous suspension (pH 3.5-5.0) to the sewage to be treated, adjusted if necessary to a weakly alkaline to weakly acid pH-value (5-10). The pH-value is preferably adjusted with strong acids, preferably sulfuric acid.In sewage from laundries, which can already contain x g alumosilicates, the amount is reduced by x g.Advantage: Compared to known processes, the dirt elimination rate is greater and/or the consumption of precipitants is reduced. The separated impurities are deposited in large floccules, and the separated sludges contain less water.