Urban sewer systems, mostly situated in river plains are carrying high proportions of groundwater intruded through leaky joints and connected house drains. This unwanted flow component, also called parasite water, has a high seasonal variation corresponding to groundwater storage and long, slow recessions unlike those of urban storm drainage but similar to baseflow in rivers which is also fed by shallow groundwater exfiltrating into the surface waters. Applying the same methods as used for baseflow separation from time series of total flows in rivers, groundwater flow was separated from daily measured effluents to treatment plants in Germany. Found annual variation and recession characteristics allow to quantify and predict intruded groundwater flow in sewer systems.