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

Chapter 4
Results and Discussion
The results and discussion chapter has been divided into four sections. In the first section,
biogas and methane formation potential of waste simulations have been discussed. In the
next section, effect of C/N ratio and ammonia-N accumulation on dry anaerobic digestion
is discussed. In this part of study, effect of two feedstocks with different C/N ratio (27 and
32) and its associated accumulation of ammonia-N have been investigated on the operation
and performance of a pilot-scale thermophilic dry anaerobic digester. In third section, the
effect of different organic loading rates on the stability and performance of the same pilotscale
thermophilic dry digester has been discussed. For this purpose, a third feedstock with
C/N ratio 32 (for detail, please refer to section 3.1.2) was fed to the digester, because feed
with C/N ratio 32 performed well in the earlier experiment. In the final section,
characteristics of digestate have been described and various digestate management options
from perspective of GHG emissions have been discussed.
4.1 Gas Formation Potential of Waste
In this initial part of the thesis, the potential of prepared waste simulations to produce
biogas was measured experimentally. For this purpose, GP21 test set-up was used. Sludge
from anaerobic process was used as a standard inoculum in this study. Similarly, to make
sure that inoculum is active, cellulose was used as a standard substrate, because its biogas
production potential is known in literature. Thus to standardize the test conditions,
experimental runs of cellulose (cellulose + inoculum) were conducted in parallel with the
waste simulations (waste + inoculum). Moreover, blanks (inoculum alone) were also run
for correction. Two separate sets of experiments were conducted for Feedstock 1 and 2.
The results have been discussed below.
The inoculum (alone) used for Feedstock 1 produced lesser biogas (107 NmL) as
compared to that used for Feedstock 2 (125 NmL). The two inoculums used for Feedstock
1 and Feedstock 2 are hereby called as inoculum 1 and inoculum 2 respectively. The same
trend of biogas production was observed from 1 g cellulose by inoculum 1 and 2 (176 and
402 NmL) as shown in Figure 4.1 and 4.2. The inoculum used in both the cases, had TS
and VS content of around 5.5% and 45%TS respectively. The specific biogas production of
inoculum 1 and 2 was 91 and 102 NmL/g VS respectively. These results show that
inoculum 2 was more active as compared to inoculum 1. The results similar to those of
inoculum 2 were obtained from the GP21 test study conducted by Heerenklage and
Stegmann, (2005), where the active inoculum produced around 100 and 400 NmL biogas
from inoculum (alone) and 1 g cellulose respectively.
The cumulative biogas production of Feedstock 1 and 2 (having different TS and VS
characteristics) was almost similar, i.e., 2365 NmL (Figure 4.1 and 4.2). Therefore, the
specific biogas production of Feedstock 1 and 2 was different (i.e. 348 and 225 NmL/g
VSadded respectively) as found in this experiment. The percentage of methane varied
between 65-70 % in biogas samples collected at different time intervals from reaction
bottles.
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