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 The daily effluent of the reactor was withdrawn before the influent addition by a peristaltic pump.