The aim of this study was to determine the extent of the effect of infiltration into a sewerage system during wet weather on the concentration of nitrogen and phosphorus compounds in raw sewage flowing into the treatment plant and in treated sewage discharged into the receiver. The study was conducted in four se-lected sewerage systems located in the Lesser Poland Voivodeship. The results showed that in each of the analyzed cases the concentration of to-tal nitrogen and total phosphorus in raw sewage was reduced by the infiltration into the sewerage system during wet weather. There was no adverse effect of infiltration on the concentration of total ni-trogen in treated sewage discharged into the receiver during wet weather. On the other hand, the research showed that when the share of infiltration in the daily flow increased by 10%, the concentration of total nitrogen was reduced by from 2.7 to 4.8%. The research demonstrated the adverse effect of infiltration on the concen-tration of total phosphorus in treated sewage in the case of objects with low hy-draulic capacity. Sewage flows, increased by infiltration water, caused the removal of activated sludge, containing phosphorus compounds, from bioreactor chambers and funnels of secondary settling tanks. However, ...
