In several slow-sand-filter experiments the behaviour of phages during the subsurface passage was measured and modelled. Here the focus is on the effect of the velocity. The observed data show a strong effect of decreasing filter efficiency with increasing velocity. Using a modelling approach, which is based on the transport differential equation, the theoretical influence of velocity on filter efficiency is examined. Finally an extrapolation of the results to large scale filtration units or bank filtration processes is attempted.