Environmental Engineering Reference
In-Depth Information
can be degraded to a higher extent and have a much higher biogas potentials of 100
−
200 m
3
per tonne (wet) substrate.
Codigestion concerns the anaerobic digestion of mixtures of feedstocks. Examples
of full-scale trials described in the literature include the digestion of sewage sludge
with OFMSW (Italy and Slovenia), pig manure and energy crops (Switzerland),
and algae biomass and wastepaper. Codigestion provides a number of potential advan-
tages over digestion of a single substrate:
Mixing of substrates may improve the properties of the feedstock through opti-
mization of the nutrient concentration (e.g., using manure as nutrient supply),
water content, or alkalinity concentration.
When more readily degradable substrates are added to, e.g., manure digesters,
the biogas production is increased, minimizing the need for mechanical mixing
and associated electricity consumption.
Codigestion of a mixture of locally produced substrates minimizes transport dis-
tances and allows for larger centralized anaerobic digesters, which can be con-
structed and operated more efficiently (economies of scale).
Codigestion also imposes limitations on the process. Whereas anaerobic digestion of a
specific feedstock enables reuse of the digestate as fertilizer in agriculture, the use of
digestate of codigestion cannot in all cases be used in agriculture due to the presence
of specific compounds in the digestate of specific substrates (e.g., metals in manure).
To which extent digestate can be reused in agriculture is strongly dependent on the
local legislation and the nutrient requirements of local agriculture. Some degree of
hygienization will be required as typically is established by heating the feedstock
to 70
C. In terms of process operation, it has been demonstrated that the performance
of codigestion basically is comparable to the sum of the individual substrates,
provided that specific limitations (e.g., N) are overcome.
14.4.3 Process Integration
Successful implementation of the anaerobic digestion process for bioenergy produc-
tion largely depends on the development of an efficient processing scheme. In prac-
tice, the anaerobic digestion process is the central process in a treatment scheme
involving a large range of technologies. Additional processing steps required for
implementation of the anaerobic digestion process include:
Upgrading of the biogas, through removal of hydrogen sulfide (H
2
S) for effec-
tive combustion of biogas in a CHP plant and removal of carbon dioxide (CO
2
)to
facilitate distribution in the natural gas network.
Feedstocks like municipal solid waste (MSW) require several pretreatment steps
for separation of specific fractions (like sand, plastics, metals) from the waste
prior to anaerobic digestion. A typical mass flow chart including mass balances
for MSW treatment is shown in Figure 14.11.
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