2.2. 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 A given rate of liquid was withdrawn from the reactor top and injected at the reactor bottom. Gas produced during the process was continuously withdrawn from the reactor top, then piped to the dehumidification system and then to the thermal gas flow meter (±0.1 Full Scale).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 The reactor was fed using a volumetric pump, whereas the digestate was withdrawn manually using the valve at the reactor bottom (Fig. 1).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Tests were started by introducing 100 L of digestate, withdrawn from the digester of the full-scale WWTP facility, into the pilot apparatus.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.