Some aspects of the operational studies of a Pilot Scale Downflow Stationary Fix Film Anaerobic Reactor treating sieved piggery slurries (1 mm mesh) are presented. Conditions affecting anaerobic digestion were analyzed. The organic loading rate to the reactor was modified, controlling variation in the recirculation rate and the temperature. The experiments were accomplished in the mesophilic range (35 degrees C). Hydraulic retention time was operated between 6 and 0.9 days with no souring of the system. A medium organic load reduction was achieved, about 60% Total COD and 50% VS, both related to HRT longer than 3 days. Biogas productivity resulted in 4.5 m(3)/m(3) d(-1) for that HRT. The gas composition displayed a certain uniformity in methane and carbon dioxide content with little variation in nitrogen, traces of hydrogen were found. A decrease in the gas production was observed when reducing the HRT below 2 days, leading to a stabilization around 4.0 m(3)/m(3) d(-1). The results were adjusted to the Chen and Hashimoto kinetic model, attaining the wash-out HRT, and the value of the kinetic constants. The study at a pilot scale suggests the convenience of the DSFF system for the design of full-scale plants for slurries from piggery farms.