Continuous biogas production from fodder beet silage as sole substrate

Abstract

Since April 2000 a two-step anaerobic plant with two subsequent 500 m3 reactors has been producing biogas from fodder beet silage (pH 3.4-4.1) as the sole substrate. The plant is located at Kirchlengern near Bielefeld, Germany. Initially the reactors were inoculated with swine manure at 37 degrees C. After a start-up phase the process was sustained at pH 7.5-8.0 by feeding the silage as sole substrate with an HRT of about 55 d twice a day. Parallel to the biogas plant at Kirchlengern four one-step laboratory reactors were continuously driven at temperatures of 37 degrees C, 45 degrees C, 60 degrees C and 65 degrees 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 degrees C reactor ceased and a high amount of approximately 65 mM propionate could be determined. By decreasing the temperature down to 60 degrees C a stable reactor performance was recovered for a period of at least 600 further days. Interestingly microscopic analyses revealed that the morphology of methanogenic bacteria in the 60 degrees C was quite different from the 37 and 45 degrees C reactor exhibiting only rodlike methanogens whereas at 37 degrees C coccoid morphotypes besides rodlike methanogens were dominant. In a 55 degrees C reactor (separate experiment) a mixture of coccoid and rodlike methanogens established. During impulse feeding with 3.9 g (VS)/(l*d) it was observed that the quickest recovery of gas production, the pH, CH4 and CO2 content as well as the redox value could be observed at 37 degrees C or at 45 degrees 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 degrees C reactor needed 16 h. Slight significant differences were seen in the spectrum of volatile fatty acids (VFA) reaching at 37 degrees or 45 degrees 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 degrees C reactor). Therefore the 37 degrees C reactor was favoured. A double experiment with a second 37 degrees C reactor was started by a somewhat different inoculation procedure from the remaining 3 reactors, but revealed similar results. An impulse feeding experiment with a very high OLR of 16.5 g (VS)/(l*d) lasting 1 week offered a stable reactor performance with a peak GPR of up to 24 l/(l reactor *d) and an HRT of 5.45d. Therefore a long term operation with an HRT of only 7.5 days and an OLR of 12 g (VS)/(l*d) should be possible. By increasing the temperature no significantly different specific gas production rates and methane yields could be observed, e.g. it gave 600-7001 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.