Design and engineering of biogas plants - The Biogas: Science, Production and Applications - page 193

. Degradation rate. The higher the temperature, the faster the degradation

of the organic matter. Thermophilic digesters require shorter retention
times and therefore smaller reactor volumes.

. Hygienisation effect. The higher the temperature, the better the

pathogen inactivation during digestion. In accordance with national
legislation, thermophilic digestion can replace feedstock pre-treatment
by hygienisation.

. Process stability. The higher the temperature, the higher the sensitivity

of the process to changes in temperature, pH and feeding rate.
Furthermore, high temperatures enhance the transformation of
ammonium (NH 4 ) to ammonia (NH 3 ), which increase the risk of
microbial inhibition.

. Energy consumption. The higher the temperature, the higher the energy

demand.

Psychrophilic temperatures occur in biogas plants without heating systems,
such as family-size biogas plants, mainly in developing countries.
Degradation is too slow for reactors with high efficiency requirements.
Mesophilic temperatures, which allow satisfactory retention times and
moderate energy demand, are the most commonly used. In particular,
CSTRs with a high water content should not have excessive energy
consumption for heating in order to maintain a reasonable global energy
efficiency. Thermophilic temperature ranges are mainly used for substrates
with a hygiene risk, typically food wastes. In plants with more than one
digestion tank, thermophilic and mesophilic reactors can be combined and
substrates can go through one or both of them, depending on their pathogen
risk and degradation rate.

8.2.5 Reactor volume

The reactor volume needs to be adapted to the amount of feedstock and the
degradation rate of the key substrates. On the one hand, micro-organisms
must have sufficient time for the degradation process and, on the other, the
concentration of organic matter must not be of a level that leads to over
feeding of the microbes and thus process inhibition. To ensure the right
balance is achieved, two parameters are used to calculate the digester
volume - the organic loading rate (OLR) and the hydraulic retention time
(HRT).
The OLR describes the amount of volatile dry matter (VDM) introduced
into the digester, expressed in kilogrammes VDM per day and per cubic
metre of digester (kg VDM/m 3 day). For CSTR digesters, the OLR is
typically between 2 and 3 kg VDM/m 3 day. It can go up to 4 or even
5 kg VDM/m 3 day, but the higher the organic load, the more sensitive the

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