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Modes of biogas digestion

The two modes of running a biogas plant are batch and continuous.

A batch plant is a container which is fed with a mixture of feed material and starter. The gas produced is collected. It takes time for the process to start (the lag time); the gas production rises quickly to a peak; it then drops steadily over several weeks. If a batch system is used for gas production, several plants are needed. These are started at different times, so that sufficient gas is available all the time.

A continuous plant (usually semi-continuous) is a container partly filled with a slurry of feed material and microbes. The plant is fed regularly (usually daily) with exhausted slurry replaced with new. Gas production is fairly steady, as long as the same amount of feed is used each day.   

Biogas models

There are a range of models to describe the amount of biogas generated from various feed materials. These range from very simple models to much more complex ones.

Biogas Potential

The simplest model defines the total amount of biogas that can be generated from a feed material. This is usually measured in small-scale laboratory batch plants.

The theoretical amount of biogas that can be produced from a feed material can be calculated from the relative amounts of carbon, hydrogen and oxygen in the material (Buswell’s formula). Starch, with a formula of (C6H12O5)100 when hydrolised with water (H2O), gives equal amounts of carbon dioxide and methane. Not all of the biomass material can be processed.

First order rate model

The weakness of the very simple model is that it does not explain how fast biogas is produced. A second constant can be  defined, the “rate” constant. Details are provided.

Complex models

There is a growing number of models developed by different researchers, such as Paul Harris at Adelaide University.