Proceedings Venice 2010, Third International Symposium on Energy from Biomass and Waste Venice, Italy; 8-11 November 2010  2010 by CISA, Environmental Sanitary Engineering Centre, Italy MONITORING VOLATILE FATTY ACID PRODUCTION DURING MESOPHILIC ANAEROBIC DIGESTION EXPOSED TO INCREASING FEEDING RATES F. MAYER, C. ADAM, A. NOO, C. GUIGNARD, L. HOFFMANN AND P. DELFOSSE Centre de Recherche Public – Gabriel Lippmann, Department Environment and Agro-Biotechnologies, rue du Brill 41, L-4422 Belvaux, Luxembourg SUMMARY: Batch anaerobic digesters were fed with increasing amounts of maize silage in order to reach a failure in the biomethanation process. A methodology for quantitative and qualitative analysis was developed to follow the progress of volatile fatty acids (VFA) related to the biogas production. In overfed digesters, process failure was characterized by a decreased biogas production and a higher CO 2 /CH 4 ratio compared to the reasonably fed digesters. No fixed value in terms of total or individual VFA concentration was found to be a good indicator of process imbalance but the results indicated that the microbial consortium was able to degrade up to 837, 674 and 697 mg.L -1 of acetic, propionic and butyric acids, respectively. This study also highlighted the importance of quantifying the substrate to inoculum ratio in batch anaerobic digestion assays to get significant and valid estimation of biomethane potential and production kinetics. 1. INTRODUCTION 1.1 Anaerobic digestion Degradation of organic sources (wastes, agricultural products and residues, industrial by- products) through anaerobic digestions (AD) can be considered as an environmentally friendly process to generate energy and recover nutrients for agriculture. AD is a complex biological process where organic polymers are converted into biogas, mainly composed of methane (CH 4 ) and carbon dioxide (CO 2 ). A diverse bacterial consortium is involved in this conversion process, which is generally described in four phases:  Hydrolitic bacteria degrade polymers (carbohydrates, proteins, lipids) into smaller polymers and monomers (monosaccharides, amino acids, long-chain fatty acids).  Acidogenic microorganisms metabolize previous components and mainly produce acetic acid and larger volatile fatty acids (VFA) such as propionic, butyric and valeric acids, as well as hydrogen.  Acetogenesis step allows the conversion of VFA into acetic acid.  Methanogenic subpopulations finally produce CH 4 from acetic acid or from the reduction of CO 2 with hydrogen.