Biogas production
The substrate used for reactor R1 was initially pre-treated with oyster mushroom and co-digested with CM and inoculated with POME. The operation was kept at 5 days batch mode at the beginning and semi-continuous started with daily feeding in the reactor. Fig. 3 showed that the biogas production started immediately on day 1 and gradually increased until day 6. The initial rapid increase is due to readily available biodegradable organic matter in the substrate of the biological pre-treatment that was conducted on the OPMF using oyster mushroom and the influence CM used as the co-substrate. The pH increased from 6.2 to 7.8 within the first 5 days of operation concurring with the less acid formation via acidogenesis process (Omar et al., 2008). R1 reached its peak of daily biogas volume of 1.95 L/day on the day 10.