Design and engineering of biogas plants - The Biogas: Science, Production and Applications - page 189

Table 8.1 Processing options

Technology Key parameter Options

Feeding
system

Digester type
and matter
content of
feedstock

. Discontinuous feeding for batch digesters
. Continuous or semi-continuous feeding for
plug-flow or CSTR digesters

. Solid or liquid feeding system depending on

dry matter content of the substrate

Reactor
type

Dry matter
content of
feedstock

. CSTR for liquid substrates

. Plug-flow or batch digester for solid substrates

Reactor
temperature

Risk for
pathogens

. Mesophilic temperature when no risk for

pathogens

. Thermophilic temperatures when risk for

pathogens (organic household waste)

Number
of phases

Composition of
substrates,
acidification risk

. One phase systems when no acidification risk

. Two-phase system for substrates with a high

content of sugar, starch or proteins

Agitation
system

Dry matter
content of
feedstock

. Mechanical agitators for high solids

concentration in the digester

. Mechanical, hydraulic or pneumatic agitation

systems for low solids concentration in the
digester

weighing and feed-in control with full automation. The degree of
technological advancement is mainly dependent on budget.
The feeding system needs to be adapted to both the feedstock and the
reactor type. Batch digesters require discontinuous feeding, which is done by
wheel loaders, as only solid substrates are employed. Plug-flow and CSTR
(continuously stirred tank reactor) digesters are fed continuously or semi-
continuously, with different options for liquid and solid feeding. Feedstock
pre-treatment, altering the physical characteristics, may change the required
feeding system.
Liquid substrates are pumped from a holding tank into the digester (Fig.
8.2). Before pumping, the content must be fully homogenised by mixing.
Round and small tanks simplify this process. Solid substrates can be mixed
into the holding tank, but the material stream must stay fluid and the pumps
must be adapted to higher solid matter contents. More often, solid
substrates are fed in separately, through the sidewall or the ceiling of the
digester. The advantages of this independent feeding from liquid substrates
are the avoidance of clogging risk to the pumps and the possibility of
influencing the total solids concentration inside the digester. In the past,
chute or flushing systems were used; these are simple and cheap
technologies, but involve sudden feeding in high loads, and temperature

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