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

(i.e. 5.2 and 3.04 respectively, please refer to section 3.3.2 and 3.3.3). The detailed results
have been discussed in the following sections.
4.2.1 Performance of ITDAR during start-up and continuous operations
The performance of ITDAR for treating the OFMSW with two different C/N ratios (27 and
32) was evaluated under various organic loading rates using the mesophilic inoculum
sources. The overall recital during initial start-up phase, in association with the OLR and
Digrr (digestate recirculation rate) were measured using parameters like pH, alkalinity,
VFA and methane yield as depicted in Figure 4.3(a-e). The results have been discussed in
the following subsections.
a) Monitoring of ITDAR during start-up phase
Initial pH of the substrate contents within the ITDAR was acidic (5.5–5.8) due to
accumulation of very high VFA concentrations of 37,857 mg/L, which was mainly
released from the rapid degradation of readily available biodegradable components in the
simulated feedstock 1 (Table 4.1; Appendix C, Table C-1). The biogas yield was higher
with the higher CO2 and lesser CH4 contents during this period. Therefore, NaOH was
added between 3 rd and 6 th day to attain the pH of near natural range to support with the
methanogenic activity in ITDAR. With the continuous pH maintenance in ITDAR, it
showed the percentage increase in methane content of biogas samples i.e., increased from
3% to 33%, on 10 th day with the alkalinity value of 23,000–27,000 mg/L as CaCO3. Also,
the ammonia-N contents were measured in the range of 1800–2100 mg/L in ITDAR. The
ORP of liquid digestate, which was considered as a good indicator of anaerobic condition,
was measured as -260 mV and -450 mV in ITDAR before and after pH adjustment. Less
than -300 mV of ORP found to be favoring the methanogenesis process in anaerobic
system was reported by Alkaya and Demirer (2011).
Therefore, it was very clear that the ITDAR attained its methanogenesis stage within 14
days of batch operations with the continuous pH adjustment, which can be visualized from
the stable pH, ORP and methane contents.
b) Monitoring of ITDAR during continuous operation phase (Run 1 to 8)
The pH variation in ITDAR during different runs (1–8) has been depicted in Figure 4.3c. It
was observed that, the near neutral pH of 7 and above was maintained throughout the study
period (of 280 days) even with the increasing OLRs (Figure 4.3a) in ITDAR.
The results showed that the system was able to get naturally buffered, which means that the
acid producers and acid consumers were equally dynamic, and produced specific methane
yield of 219 L/kg VSadded on an average from different trials (Figure 4.3d and Appendix C,
Table C-2). In other sense, the ITDAR was continuously acting like an active
methanogenic reactor and the waste fed into the system was comparatively lesser than the
reactor contents, hence the system pH and associated methane yield was not getting much
affected with the increasing OLRs. Generally, the methanogenic system pH was reported
to be in between 6.8 and 7.8 (Lahav and Morgan, 2004).
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