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

Table 4.2 Surplus Energy of ITDAR During Various Runs
Run
Energy
production
(MJ/kg
VS)
Energy Consumption (MJ/kg VS)
Feeding
Heating and
Shredding and Recirculation maintaining
withdrawal
thermophilic
conditions
Surplus
energy
(%)
Feedstock 1 (avg. C/N ratio 27)
1 14.42 1.33 0.29 0.26 22.34 -68.01
2 28.61 1.27 0.28 0.23 10.15 58.28
3 30.64 1.03 0.23 0.27 6.79 72.86
Feedstock 2 (avg. C/N ratio 32)
4 22.33 0.87 0.19 0.27 6.17 66.38
5 14.71 0.93 0.21 0.69 3.70 62.43
6 15.90 1.49 0.33 1.99 8.89 20.12
7 11.53 1.12 0.25 1.36 6.10 23.42
8 17.97 1.24 0.27 1.45 6.55 47.07
In run 1, however, the net energy production was negative. The reason could be that the
very high retention time of 153 days that increased the energy consumption for
maintaining thermophilic conditions. Similarly, runs 6 and 7 had only a small surplus
energy (i.e. 20% and 23% for run 6 and 7, respectively). The reason could be that the runs
6 and 7 had relatively high retention time compared to run 5 and 8 and thus energy
consumption was higher. Moreover, with the increasing Digrr during runs 4 to 8 of ITDAR
operations resulted with the increasing energy consumption rate compared to that of runs
1–3. But, overall, it can be concluded that the decentralized system (ITDAR) can produce
surplus energy in the range of 50–73% and considered to be economically viable than the
centralized systems.
4.3 Optimization of a Pilot-Scale Thermophillic Dry Anaerobic Digester (Results of
Phase II Pilot Experiment)
In phase II pilot experiment, optimization of ITDAR treating OFMSW was performed by
testing different organic loading rates (OLRs). The C/N ratio of OFMSW, which
performed well in the earlier experiment, i.e., 32, was used in this study. The study was
started with a start-up phase (batch mode of operation) followed by continuous operation.
In continuous operation, effect of various organic loading rates on the stability and
performance of ITDAR was evaluated at a constant recirculation rate. The results have
been discussed in the following sections.
4.3.1 Start-up of ITDAR in phase II pilot experiment
For start-up phase, 40% of the reactor’s working volume was filled with inoculum, which
consisted of a mixture of digestate from thermophilic anaerobic reactor, anaerobic sludge
and cow dung. The remaining 60% of working volume was filled with Feedstock 3 (please
see detail of Feedstock 3 in section 3.1.2). The operating temperature in the start-up phase
was in thermophilic range (55°C), which was reached in 3 days by gradual increase. In the
first 50 days (start -up phase), the reactor was not fed and only mixing of the reactor
content was done at the rate of 2.4 Ldig/Lreactor vol.d. Under these conditions, the pH was
initially 7, which started to decrease and reached 6.36. Therefore, small quantities of
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