Continuous biogas production from fodder beet silage as sole substrate

Since April 2000 a two-step anaerobic plant with two subsequent 500 m³ reactors has been producing biogas from fodder beet silage (pH 4.1) as the sole substrate. The plant is located at Kirchlengern near Bielefeld, Germany. Initially the reactors were inoculated with swine manure at 37°C. After a start-up phase the process was sustained at pH 7.5-8.0 by feeding with the silage as sole substrate twice a day. Parallel to the biogas plant at Kirchlengern four one-step laboratory reactors were continuously driven at temperatures of 37°C, 45°C, 60°C and 65°C. They were fed with the same silage, but only once per day (one impulse). The organic loading rate (OLR) was adjusted to 3.9 g volatile solids (VS)/(l*d) with a concomitant hydraulic retention time (HRT) of 27 d. There was no problem with starting the reactors, but after 86 days the volumetric gas production of the 65°C reactor ceased and a high amount of approximately 130 mM propionate could be determined. By decreasing the temperature down to 60°C a stable reactor performance was recovered for a period of at least 250 further days. During impulse feeding it was observed that the quickest recovery of gas production could be observed at 37°C or at 45°C. Recovery of 75% gas volume (related to the value before or after impulse feeding) was obtained after 5.5 and 7.5 h of feeding time point whereas the 60°C reactor needed 16 h. Slight significant differences were seen in the spectrum of volatile fatty acids (VFA) reaching at 37° or 45°C its maximum with 10-30 mM total VFA at 2-3 h after feeding. After this the VFA level declined to nearly zero (except for the 60°C reactor). Therefore the 37°C reactor was favoured. A double experiment with a second 37°C reactor was started by a somewhat different inoculation procedure from the remaining 3 reactors, but revealed similar results. By increasing the temperature no significantly different specific gas production rates and methane yields could be observed, e.g. it gave 600-700 l biogas from 1 kg VS. The corresponding methane content ranged between 62-64%. With a methane content of 63 ± 1% a yield of 40.1 ± 2 m³ methane/ton fresh fodder beet silage was obtained.