AbstractTests to determine the mixing characteristics of the anaerobic downflow stationary fixed film (DSFF) reactor during startup showed that mixing characteristics affected performance. Different mixing profiles were obtained by keeping the same flow distribution system and by varying the number of clay channels (1, 4, and 25) in the DSFF reactors (2–32 L). Results with a clean bed reactor indicated a plug flow pattern with a relatively large extent of dispersion. Recirculation dramatically improved the mixing and the residence time distribution (RTD) changed to that of the completely mixed type. Multiple‐channel reactors exhibited a dead space of ca. 12% of the total volume, likely a result of a less than optimally designed flow distributor. A startup period of 90 days was necessary to achieve a maximum loading rate of between 10 and 15 kg COD/m3day, a volumetric methane production rate of up to 3 m3(STP)/m3day and a COD reduction efficiency of up to 90%. For the first 50 days of operation, the difference in achievable volumetric loading rate and volumetric methane production rate was only related to the surface‐to‐volume ratio of the reactors and was not affected by the number of channels present. After 90 days, the bacterial growth on the support material was sufficient to dramatically increase the amount of dead space in the reactors, especially in multiple‐channel reactors (up to 55% of their volume). As a result, the performance of these reactors deteriorated and overloading characteristics were observed. Other results showed that biogas production alone was not sufficient to improve reactor mixing and that little or no shortcircuiting or channelling occurred. Furthermore, the nonmethanogenic bacterial activity in the liquid phase was not affected by the degree of mixing but acetoclastic methanogenic and hydrogenophilic methanogenic activity in the liquid phase were reduced as the fluid flow pattern in the reactor