Abstract: The invention relates to a method for the treatment of (raw) sewage in a package plant consisting of three bioreactors in series. The treatment is being carried out using three types of biomass.In a first step phosphate is removed by biological means and, at the same time, the chemical and biological oxygen demand is lowered in a highly loaded active sludge system, in a second step a nitrification is carried out, ammonium being converted to nitrate, and in a third step a denitrification is carried out using a carbon source. In the denitrification step acidified surplus sludge from the P-removing step can also be used instead of an external carbon source. The nitrifying and denitrifying bioreactors are both fixed film processes. The thickness of the biofilm on the support material in the nitrifying bioreactor can be influenced by adjusting the aeration system or by adjusting the hydraulic loading.