Anaerobic co-digestion of hydrolysate from alkali pre-treated oil palm empty fruit bunches with biodiesel waste glycerol

With an attempt to utilize bio-waste, oil palm empty fruit bunches (OPEFB) were investigated as feedstock for biogas production. Bench scale experiments were carried out employing a two-stage process for the digestion of pre-treated OPEFB with and without the addition of biodiesel waste glycerol (WG). Two continuous stirred tank reactors (CSTR) were operated for acid and methane production. Alkali pre-treated OPEFB were initially acidified to produce an acid hydrolysate solution. Different proportions of WG, between 0.4 and 0.8% (by volume of feed), were increased stepwise. The results indicated that the acidification of these pre-treated brunches generated approximately 4.25 g/L of volatile fatty acid at an optimum pre-treatment condition with soaking in 2% of NaOH for 48 h. This was an increase of 47% compared to the acidification of raw OPEFB. The maximum methane production was achieved with the addition of 0.4% WG, which yielded a gas production of 0.542 and 0.369 L g CODrem biogas and methane, respectively. These conditions can improve the gas yield of biogas and methane by a factor of 1.67 and 2.84, respectively. However, a higher supplementation of WG seemed to promote more instability, which was reflected by a lower methane concentration and organic removal efficiency.