Influence of recirculation flow in a pilot scale downflow stationary fixed film anaerobic reactor treating piggery slurry

Some aspects of the operational studies of a pilot scale downflow stationary fix film reactor (DSFF), treating sieved piggery slurries (1 mm mesh), are presented. Conditions affecting anaerobic digestion of the slurries were examined. The effect of the variation in the recirculation factor, which was defined as the relation between the recirculation and the feeding flows (Qr/Qo), was analyzed in this phase. The organic loading rate to the reactor and the temperature were kept almost uniform along the process. The experiments were conducted in the mesophilic range (around 35°C). Hydraulic retention time was between 2.1 and 1.6 days and the system did not go sour. The recirculation factor was varied from four up to 20, with limitations for the higher rates due to the equipment. Average organic concentration in the influents were 24×103 ppm Total chemical oxygen demand (COD) and 13×103 ppm volatile solids (VS). A medium load reduction was achieved, about 62% COD and 56% VS, with factors greater than 15 r, descending to 48 and 42%, respectively, when shortening the factors to under six. Biogas productivity increased from 2.8 to 3.3 m3 m−3 d−1 when the factor was enlarged from four to 18. The gas composition displayed a certain uniformity in methane, between 80 and 70%, while carbon dioxide showed no significant changes for those factors. Nitrogen varied in a complementary way regarding the aforementioned gases, whereas only traces of hydrogen and sulphur were found.