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

ii) Calculation of N2O emission from land applied digestate
The land applied fertilizer, compost or biosolids emits N2O equal to 0.01 of applied N as
stated by IPCC (Brown et al., 2010). However, Moller et al.,(2009) used N 2O emission
factor equal to 0.013-0.017 of applied N to soil. Here we have used N2O emission factor
0.013 of applied N in digestate.
The total amount of N provided by 1 kg of digestate was calculated from the N content of
each type of digestate (Table 4.5). The amount of N2O to be emitted from 1 kg of digestate
was then calculated using the factor 0.013 of applied N. The N2O to be emitted was then
converted to its equivalent amount of CO2 by multiplying with a factor of 298 (IPCC,
2011).
iii) Calculation of methane emission during storage
It is equal to the difference of methane emission potential of digestate before and after
storage. The methane emission potential was calculated by the same method as used by
IGES, (2011).
iv) Calculation of avoided GHG by land application of digestate
Land application of raw digestate, stored digestate and stored-cured digestate can somehow
replace the use of synthetic fertilizer and hence can reduce the emission of fossil carbon by
energy use for production of chemical fertilizer. The total amount of N and P provided by
1 kg of digestate was calculated from the N and P content of each type of digestate. The N
and P content of each type of digestate were measured by lab analysis and are given in
Table 4.5. The avoided or saved GHG by land application of digestate was then calculated
by emission factors 8.9 kg CO2-eq/ kg N and 1.8 kg CO2-eq/ kg P (Moller et al., 2009).
3.5 Analytical Methods
Simulated feedstocks, inoculum sources and digested residues (as solid samples) were
characterized for their physico-chemical characteristics such as TS, VS, Carbon (as TOC)
and Nitrogen (as TKN) by standard methods given in Table 3.5. These measurements were
performed once to twice a week to interpret the process performance.
Liquid portion of the digested residues (liquid digestate) was collected by simple
centrifugation (5000 rpm for 20 min). These extracted samples were used for analysis of
pH, oxidation reduction potential (ORP), alkalinity, VFA and total ammonia nitrogen i.e.
TAN (NH3 + NH4 + ). The pH and ORP of digestate were analyzed on daily basis, whereas
all other parameters of liquid digestate were analyzed once to twice a week. Biogas
production was measured using the wet gas flow meter (TG 05, Ritter) connected with
reactor gas outlet port. The biogas composition was analyzed once to twice a week
depending on sensitivity of the reactor conditions.
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