dry anaerobic digestion of municipal solid waste and digestate ...

The GHG emission potentials of various types of digestate shown in Figure 4.18
correspond to the GHG emissions, only if they are dumped to the shallow dumpsite.
However, the emissions can be minimized if they are managed well in some better way.
For this purpose, various digestate management options in the form of 5 different scenarios
have been considered and compared with regard to GHG emissions and have been
presented in the following section.
ii) Comparison of GHG emissions from different digestate management scenarios
As digestate has certain residual GHG emission potential, it tends to emit methane to the
atmosphere, if not stored properly and hence can contribute to the climate change.
Therefore, it is necessary to carefully analyze various digestate management options based
on their net GHG emission reductions under various scenarios, so that the best scenario can
be selected for digestate management. In this part of the study, various digestate
management options were considered to assess the reduction in GHG emissions. Results of
GHG emissions from each scenario of digestate management have been presented in Table
4.6.
Scenario 1: The baseline scenario shows that the net GHG emission is 190 g CO2-eq/kg
digestate. All the emission is from dumpsite or landfill. The emission comes directly from
biodegradation of digestate in CH4 form. Since no flaring, collection and recovery of
methane is considered in scenario 1, thus all the produced methane is released into
atmosphere and contributes to GHG emissions. There is no GHG saving here in this
scenario as well.
Scenario 2: In this scenario, GHG emission is mainly in N2O form, which is equal to 7.85 g
CO2-eq/kg digestate from the land applied digestate. But here the N and P as nutrients
provided by the land applied digestate is the most important factor, which replaces the use
of chemical fertilizer and hence the GHG from fertilizer manufacturing is avoided. Larger
avoidance of GHG emissions from fertilizer production ( -19 g CO2-eq/kg digestate) than
N2O emission (8 g CO 2-eq/kg digestate) from the land applied digestate results in the
negative net GHG emission as shown in Table 4.6.
Scenario 3: In this scenario, GHG emission is in both CH4 and N2O forms. Storage of
digestate for 2 months is the major contributor of CH4 from the stored digestate which is
equal to 20 g CO2-eq/kg digestate. Moreover, N2O from land applied digestate further
contributes to GHG emission. However, the GHG saving from the fertilizer substitution by
application of digestate is lesser than scenario 2. It is because of loss of nutrients during
storage. For instance, GHG saving from fertilizer substitution is mainly through N content
of digestate, but during storage, ammonia is lost through its volatilization from the stored
digestate due to favorable condition of pH. Since the GHG emissions from storage and
land applied digestate (25 CO2-eq/kg digestate) is larger than GHG savings (-13 CO2-eq/kg
digestate), the net GHG emission is positive in this scenario. It can be concluded from the
results of scenario 3 that if land application is the fate of digestate, its storage time should
be minimized as much as possible to reduce the CH4 emission and maximize the GHG
savings from fertilizer substitution by digestate.
Scenario 4: In this scenario, although, curing of digestate has reduced the GHG emission
potential of stored digestate as shown earlier in Figure 4.18. But still the net GHG
emissions here are just like scenario 3, which are during storage (before curing) and land
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