2.2. Anaerobic digestion experiment
The experimental set-up is shown in Fig. 1. It consisted of 1 L bottle which functioned as the anaerobic digester, burette as the biogas collector, and container as the receiver for the water discharged from the collector. The biogas generated in the digester was transported by a rubber pipe into the inverted burette filled with acidified water. The anaerobic digestion was carried out at room temperature (30 ± 1 °C). Each digester was manually mixed once a day to avoid stratification. The daily biogas production for each anaerobic digester was recorded using the water displacement method [21], and the corresponding cumulative biogas volume was calculated.