Design and engineering of biogas plants - The Biogas: Science, Production and Applications - page 194

system becomes and more monitoring is required (Eder and Schulz, 2006).
Plug-flow digesters function with a higher OLR, up to 10 kg VDM/m 3 day.
The formula for calculating the organic load is given by equation 8.1. The
digester volume includes the volume of the post-digesters.

Substrate input (kg/day) 6 DM (%)

¼

ð Þ
Digester volume m 3 ½

6 VDM % of DM

m 3 day OLR kg VDM = 8 : 1

The HRT describes the theoretical time period that the substrates stay in the
digester. It describes the mean retention time that, in reality, deviates from
this value, especially in CSTR systems where shortcuts occur. The HRT
must be chosen in order to allow adequate substrate degradation without
increasing the digester volume too much. Washout of the microbes must
absolutely be avoided, therefore the HRT must not be below 10 days (Eder,
2006). The HRT is calculated from

Net digester volume m 3
Substrate input m 3

HRT days ð Þ¼ ½ 8 : 2

ð = day Þ

Both the OLR and the HRT make reference to the effective digester volume,
which is the volume actually available to the substrates. To obtain the total
digester volume, the headspace above the liquid level (eventual gas storage)
needs to be taken into account. Box 8.1 shows a worked example.

8.2.6 Reactor material and protection

Digestion tanks of reinforced concrete and steel are most widely used.
Reinforced concrete tanks benefit from the high tensile strength of steel and
high compression strength of concrete. Appropriate concrete quality (blast-
furnace cement and low lime content) and professional construction are
important to prevent corrosion and leaks in the tank wall. Concrete
digesters can be built partially or completely in the ground. Steel digesters
are built on concrete foundations above ground. Steel plates are welded or
bolted together and seams are tightened. For parts in contact with corrosive
fluids highest quality stainless steel is recommended (Eder and Schulz,
2006). Glass-coated or galvanised steel is used when there is no risk of
corrosion.
Vulnerable parts of the reactor should be protected by coatings or liners
in order to avoid corrosion. Substrates, biogas and condensate can contain
aggressive substances (e.g. hydrogen sulphide, ammonia, organic acids and
even microbes) that disintegrate concrete and plastic. In steel and concrete
reactors built from high-quality materials, the zone in contact with the
substrate does not normally need protection, but the zone in contact with

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