Environmental Engineering Reference
In-Depth Information
Following this reasoning, the overall contribution of AD to different
chains can be expressed as
AD overall contribution ¼
E int output AD þ
E ext output AD
E input AD
X
þ
þ
D
E input biom i þ D
E input ind i þ D
E loss i
E env
½
7
:
4
In equation 7.4,
E loss i represent the change in
direct and indirect energy requirements in each of the chains (i) where AD
products are replacing other inputs or where by-products were previously
being used; E env represents the energy that AD is saving in terms of cleaning
polluting loads that are now receiving an added value. Basic to the
calculations is the energy characterization of the AD unit itself. In fact,
despite the simplicity of the AD process itself, the configuration of a full-
scale installation usually requires several units, such as storage, pre-
treatment, gas and digestate post-treatment, and the overall energy balance
of an AD facility is determined by its specific configuration.
The energy balance of an AD facility, E balanceAD , can be defined as the
difference between the energy outputs and the energy inputs of the system,
both direct and indirect. The direct energy output of an AD facility
corresponds to the gross energy produced in terms of methane, E methane ,
whereas the indirect energy outputs correspond mainly to the energy
embedded in the nutrients, E nutr , the indirect energy represented by the
water content in the digestate which when given a use is in fact replacing
fresh water, E water , and the energy content in the organic matter still present
in the solid digestate, which could be recovered for example as a soil
conditioner or by means of incineration, E OM . As an indication, 90% dry
digested sewage sludge has an energy value of about 10-14MJ/kg dry
matter, i.e. comparable to lignite. Direct energy is used in logistics, E log , pre-
treatments, E pret , digester operation, E dig.op , biogas post-treatment, E biog.
post , and digestate post-treatment, E dig.post , whereas indirect energy inputs
are mainly found in the energy used for inputs different from the energy
needed in the process, such as the energy embedded in chemical additives,
E add , and that used for the building and maintenance the different
operational units, E inf (equation 7.5).
E balanceAD MJ
yr
E input biomi ,
E input ind i , and
Δ
Δ
Δ
￿ ￿ ￿ ￿ ￿ ￿
¼
ð
E methane þ
E nutr þ
E water þ
E OM
Þ
E log þ
E pret
þ
E dig : op þ
E biog : post þ
E dig : post þ
E add þ
E inf Þ½
7
5
:
Depending on how the outputs provided by AD are effectively brought back
to supply the demands of the chain for energy, water and nutrients, equation
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