Mathieu Brulé

Exponential model describing methane production kinetics in batch anaerobic digestion: a tool for evaluation of biochemical methane potential assays

Biochemical methane potential assays, usually run in batch mode, are performed by numerous laboratories to characterize the anaerobic degradability of biogas substrates such as energy crops, agricultural residues, and organic wastes. Unfortunately, the data obtained from these assays lacks common, universal bases for comparison, because standard protocols did not diffuse to the entire scientific community. Results are usually provided as final values of the methane yields of substrates. However, methane production curves generated in these assays also provide useful information about substrate degradation kinetics, which is rarely exploited. A basic understanding of the kinetics of the biogas process may be a first step towards a convergence of the assay methodologies on an international level. Following this assumption, a modeling toolbox containing an exponential model adjusted with a simple data-fitting method has been developed. This model should allow (a) quality control of the assays according to the goodness of fit of the model onto data series generated from the digestion of standard substrates, (b) interpretation of substrate degradation kinetics, and (c) estimate of the ultimate methane yield at infinite time. The exponential model is based on two assumptions: (a) the biogas process is a two-step reaction yielding VFA as intermediate products, and methane as the final product, and (b) the digestible substrate can be divided into a rapidly degradable and a slowly degradable fraction.

Einfluss einer Enzymzugabe auf die Methanerträge von Gärsubstraten einer Praxis-Biogasanlage

Biogas plants fed with a high share of fi bre-rich energy crops (e.g. ensiled grass) often resort to enzyme additives in order to increase substrate degradation as well as the resulting methane yield. In order to evaluate the effect of enzyme additives on the digestion process, effl uents were collected from the fi rst and second reactor of a full-scale on-farm biogas plant. The sampled effl uents were digested again in batch anaerobic digestion assays at the biogas laboratory of the University of Hohenheim. Enzyme addition even at high dosage could not yield any signifi cant increase of the methane yields of effl uents.

Einfluss der Temperatur und des pH-Wertes auf die Aktivität von Enzymen im Biogasprozess

In landwirtschaftlichen Biogasanlagen werden oft Enzymzusatze eingesetzt, um den Abbau der pfl anzlichen Fasern (z.B. Zellulose und Hemi-Zellulose) zu beschleunigen und dadurch den Biogasertrag zu erhohen. Die Wirkung dieser Enzymzusatze wurde jedoch unter Laborbedingugen nur unausreichend getestet. Um die Faktoren, die die Enzymaktivitat beeinfl ussen, systematisch zu untersuchen wurden an der Universitat Hohenheim Versuche zur enzymatischen Hydrolyse von Maisstroh durchgefuhrt. Dadurch sollte die Wirkung kommerzieller Enzymzusatze unter kontrollierten Bedingungen auserhalb des Biogasverfahrens uberpruft werden.