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

the reactor was increased from 35°C to 55°C with a gradual increment of 2°C per day to
avoid reactor upset and was maintained at 55°C throughout the study. The system was
operated in a batch mode, without loading any additional feedstock, for first 14 days and
denoted as start-up phase. During the initial start-up phase, the system pH was neutralized
using commercial caustic soda (NaOH) for quick onset of methanogenesis in ITDAR. The
amount of NaOH required for the pH adjustment was calculated based on the simple
laboratory tests using 100 mL of digestate from the ITDAR and pH meter. The reactor
contents were mixed at the rate of about 1 Ldig/Lreactor vol.d (one liter digestate recirculated
per liter reactor volume per day. So Digrr = 1 means that whole contents in reactor is
recirculated for one time in a day) to mix up for homogenous distribution of reactor
contents in these periods.
From day 15 onwards, ITDAR was operated in a continuous mode by loading with the
designed feedstocks under different organic loading rate (OLR) as detailed in below
sections. Table 3.3 provides the details of OLR, solid retention time (SRT) and digestate
recirculation rate (Dig rr) during continuous operation of ITDAR. As the reactor volume
was fixed, the increase in waste loading rate i.e., the OLR, in subsequent runs thus
decreased the SRT. During these periods, the system was continuously monitored for the
fluctuations in process parameters such as biogas, methane, ammonia-N, volatile fatty
acids (VFA), alkalinity and pH as well as other digestate parameters (TS, VS, C and N).
Table 3.3 Operating Conditions of ITDAR for Phase I Pilot Experiment
Run Duration
(d)
OLR
(kg VS/m 3 .d)
Solid retention
time (d)
Digrr
(Ldig/Lreactor vol.d)
Feedstock 1 (avg. C/N ratio 27)
Start-up 1-14 - 14 1.00
1 15-38 0.65 a
153 0.05
2 39-67 1.60 89 0.10
3 68-99 2.60 54 0.19
Feedstock 2 (avg. C/N ratio 32)
4 100-148 4.00 45 0.34
5 149-170 10.65 13 2.12
6 171-215 4.35 29 2.40
7 216-245 7.70 21 2.90
8 246-280 7.30 19 2.96
a
Table gives average values of OLR, retention time and recirculation rate for each run.
b) Continuous operation with feedstock 1 (C/N ratio of 27) - Run 1 to 3
As detailed in Table 3.3, the feedstock 1 with the C/N ratio of 27 was loaded into ITDAR
under different OLR of 0.65 to 2.60 kg VS/m 3 .d in three consecutive runs (1 to 3). Each
run was preceded until the biogas yield attained to its steady state, with no further
increment, in ITDAR.
The SRT of 153 days was given during run 1 to avoid the reactor upset with lower Digrr. In
subsequent runs the step-up increase in OLR was studied with decreasing SRT and
increasing Digrr. Every time, one part of the fresh feedstock was mixed up with the two
parts (wt/wt basis) of digestate collected from the ITDAR before loading. Based on mass
balance calculation, specified amount of digestate was removed from the ITDAR. Feeding
and digestate withdrawal was done once a day. To balance the moisture loss due to evapo-
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