Effect of sudden substrate-composition change on anaerobic digestion of organic waste


Prof. Christof Holliger, Dr. Pierre Rossi, Marc Deront, Elmar Büehler, Noam Shani

Funding agency

Internal project, no specific funding

Project period

August 2002 – December 2003



Production of biogas from organic waste is a promising technology for renewable energy. Anaerobic digestion is the responsible microbial process of biogas production, a process that is catalyzed by different interdependent microbial guilds such as fermentative bacteria that hydrolyze the polymeric substrates and ferment the monomers to organic acids, alcohols, carbon dioxide, and hydrogen, also referred to as acidogens, acetogenic bacteria that transform organic acids to acetate and hydrogen, also referred to as syntrophs, and finally the methanogens producing methane from acetate or hydrogen plus carbon dioxide. Full scale installations have sometimes encountered problematic situations when abruptly changing the composition of the organic waste fed to the fermenter. The biogas production collapsed and the fermenter had to be emptied. Neither low pH nor high ammonium concentrations could explain this phenomenon. The objective of this study was to simulate such an abrupt substrate composition change and to investigate biogas production as well as the microbial guilds involved.


Two lab-scale reactors have been operated in parallel with a mixture of cellulose and peptone with a ratio of 9:1 (w/w). Once steady-state conditions have been reached, the ratio in the cellulose-peptone mixture has been changed to 4:6 from one day to the other. Biogas production decreased from the day of the substrate composition change on. Acetate and later also propionate and butyrate started to accumulate. The pH started to decrease only at a later stage when the buffer capacity of the reactor has been used up. Ammonium concentration increase due to fermentation of the nitrogen-rich substrate peptone fed to the bioreactor but did not exceed 2 g N-NH4/L which is normally not inhibiting concentration for methanogenesis. The composition of the eubacterial community changed quite rapidly after the substrate change and one operational taxonomic unit with a terminal fragment in the T-RFLP analysis of 233 bp became predominant accounting for 30-60% of all eubacteria present. The sequence of the clones containing the 16S rRNA gene of the metagenomic DNA isolated from a fermenter biomass sample taken at the end of the reactor operation that produced the terminal fragment 233 bp affiliated with Clostridia-like clones isolated from other methanogenic systems. The archaeal community was dominated by Methanosaeta-like clones before the substrate composition change and remained on DNA-level dominated by this acetoclastic genus up to three weeks after the substrate change. The population then decreased from 50-60% to levels of about 10% of the whole community based on the fluorescence signal of the T-RFLP analysis. The substrate change apparently selected for a specific Clostridia-like population that had an inhibitory effect on either the syntrophs or the acetoclastic Methanosaeta-like methanogens.