We analyse the interactions between the biofilm growth and the liquid flow distribution in a trickling filter. Experiments are carried out on 0.5 m3 pilot scale reactor. We determine both the liquid flowrate distribution by local measurements and the filter performance by measuring the conversion. The liquid superficial flowrate ranges between 0 and 0.9 m/h. The synthetic effluent consists of a water solution of beet molasses and potassium dihydrogenophosphate. The hydrodynamic modelling allows us to relate the characteristics of the liquid flow distribution and the reactor performance to a single set of packing parameters. The packing irrigation rate plays a key role. Our results confirm the definite effect that the biofilm activity may have on the quality of the liquid flow distribution.