Pilot apparatus and procedure of runs
The pilot apparatus used in the analysis consisted of a 100-liter
gas-tight anaerobic reactor (Di Maria et al., 2012b) with a removable
top (Fig. 1). The digester temperature was controlled by a
thermal heating jacket wrapping the digester and by a 2 cm thick
insulating layer. The temperature was continuously monitored
with a resistance temperature detector inserted inside the processed
substrate. Stirring was maintained by a temporized circulating
pump. A given rate of liquid was withdrawn from the reactor
top and injected at the reactor bottom. Gas produced during the
process was continuously withdrawn from the reactor top, then piped to the dehumidification system and then to the thermal gas
flow meter (±0.1 Full Scale).
Gas composition was measured with infrared sensors for CH4
and CO2 (% v/v) (±1%) and with electrochemical cells for O2 and
H2S (% v/v) (±2%). The reactor was fed using a volumetric pump,
whereas the digestate was withdrawn manually using the valve
at the reactor bottom (Fig. 1). Operating conditions similar to the
full-scale digester of the existing 90,000 PE WWTP were reproduced
in the pilot apparatus. Process temperature was maintained
at 35 C ± 2 C and TS concentration at about 4%. About 7 L/day of
WMS were introduced in the pilot reactor and the same amount
of digestate was extracted. Feeding lasted for about 1 h/day. If necessary
TS concentration was adjusted by diluting with deionized
water. In accordance with the mean operating conditions of the
full-scale digester, the initial HRT was 14 days, which corresponded
to the sludge retention time, and the OLR was about
1.46 kgVS/m3 day. Three different OLR of the digester
(1.46 kgVS/m3 day, 2.1 kgVS/m3 day and 2.8 kgVS/m3 day) were
investigated by adding to the daily feed, respectively, 0 L, 1.4 L
and 2.8 L of FVW with TS = 4% w/w.
Tests were started by introducing 100 L of digestate, withdrawn
from the digester of the full-scale WWTP facility, into the pilot
apparatus. For each new OLR an adaptation period >3 HRT was
used. Then, the constant loading rate was held for at least the following
14 days before a further increase in OLR. Both digestate
samples for HTS and process parameters were determined during
this operating period.