Environmental Engineering Reference
In-Depth Information
7.4
Biomass chain configuration after introduction of the anaerobic
digestion step.
inputs
required
in
the
second
system,
that
is
E
input biom2
þ
E
input ind2
þ
E
input AD
,
as
compared with the first
case,
E
input biom1
þ
E
input ind1
and
the
difference
in
other
energy
losses,
E
loss1
E
loss2
provided no changes other than the introduction of the AD
unit in the system exist:
AD
chain contribution
¼
E
input biom1
þ
E
input ind1
E
input biom2
E
input AD
þ
E
input ind2
þ
þ
ð
E
loss1
E
loss2
Þ
½
7
3
:
It is clear that if expanding beyond the system borders (i.e. getting into the
multifunctional perspective), the contribution of AD can be much higher
than that specific to the chain to which it belongs. This is because AD
external outputs are also replacing energy, water, fertilizers and soil
amendments in other systems. In addition, and also looking beyond system
borders, it is possible that the by-products being used by the AD unit
already had a use in other chains in the first case, meaning that when using
those substrates as input for AD, energy would be needed to be spent
replacing them. Therefore, the overall contribution of AD should also
account, for example, for the energy used to produce animal feed which
before was coming from the by-products now being transformed by AD.
Conversely, in the case where by-products are polluting the environment, a
new term should be introduced in the equation that accounts for the energy
that would be needed to clean the environment from such polluting load.
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