Effect of Inoculum Type on Anaerobic Co-Digestion of Palm Oil Mill Effluent and Brewery Spent Grain for Biogas Production

The effect of different inocula on the anaerobic co-digestion of palm oil mill effluent (POME) and brewery spent grain (BSG) was evaluated for biogas production using laboratory scale bioreactors. The reactors (A: POME + BSG + cow dung, B: POME + BSG + swine dung, C: POME + BSG + swine dung + cow dung) were incubated at 35 ± 2°C for 30 days. The average biogas volume measured periodically by water displacement technique were 4.73, 38.64, and 36.45 mL gVS⁻¹ in reactors A, B and C respectively. The results indicated that reactor B produced 8.17 times more biogas than A and 1.06 times than C, whereas reactor C produced 7.71 times more gas than A, with significant differences at p = 0.05. The acidity of the digestates increased with a decrease in pH from 6.91 to 4.52, 7.33 to 5.20, and 6.73 to 5.46 in digesters A, B and C respectively. Whilst the total solid contents decreased from 18.22 ± 0.22 to 9.27 ± 0.01%, 19.05 ± 0.13 to 9.26 ± 0.01%, and 18.88 ± 0.03 to 8.62 ± 0.04%, volatile solids reduced from 10.9 ± 0.16 to 0.04 ± 0.01, 10.7 ± 0.07 to 0.28 ± 0.34, and 10.8 ± 0.09 to 0.09 ± 0.04% in reactors A, B and C respectively. The total anaerobic bacterial loads of reactors A, B and C respectively were 5.57± 0.46, 5.61 ± 0.39, 5.38 ± 0.48 Log₁₀ CFU g⁻¹. Methanogens associated with the biogas production were identified as members of the genera Methanothrix, Methanosarcina, Methanobrevibacter and Methanocorpusculum. Contrary to using cow dung only as inoculum, the combined use of swine and cow dung for anaerobic co-digestion enhanced biogas production and/or yield.