2.2 Experimental design and operation
A continuous stirred tank reactor (CSTR) was setup with a whole volume
of 2.5 L (2 L of working volume) under thermophilic conditions (55±1°C).
The reactor was stirred automatically every 2 h for 10 min at a speed of
50-90 rpm and a thermostatic circulating water tank (HH-60, China) was
employed to keep the set-up temperature. The reactor was fed with a mix
of straw and food waste (1:1 on dry matter basis) for 270d in two
stages. During the first stage (1-218 days), the reactor was operated by
a stepwise OLR-increase program, i.e. from 2.6 to 5.4, 7.9, 10.2 and
17.1 g VS L-1 d-1. A peristaltic
pump (BT300N-YZ1515x, China) was applied for 4 times feeding per day and
controlled by a timer and a relay. The process upset in the first stage
was temporary and was resolved by stop feeding until the accumulated
VFAs from the previous period were depredated (from 219-230 day). During
the recovery period, the accumulated VFAs reduced from 3725 down to 295
mg L-1. Once process stability was restored by day 230
of operation, the second stage (231-270 days) was designed with a
stepwise HRT decrease from 5 to 3 and finally to 1.5 with the best OLR
obtained in first stage (Table 2). The feedstock was fed into into the
reactor 6 times per day by the timer and a peristaltic pump. The daily
effluent of the reactor was withdrawn before the influent addition by a
peristaltic pump.