Co-digestion of palm oil mill effluent and Ceratophyllum demersum in a two-stage anaerobic bioreactor to recovering gaseous biofuel

Abstract

Palm oil mill effluent (POME) and Cerathophyllum demersum (CD) were aimed to valorize for efficient bio-hythane production via the two-stage anaerobic co-digestion. Batch cultivation at the initial organic concentration of 10 g-VS/L and various POME:CD mixing ratios of 100:0, 90:10, 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, 20:80, 10:90, and 0:100 (VS basis) was firstly investigated. Using a POME:CD mixing ratio of 90:10, the first-stage batch fermentation provided the highest hydrogen yield of 65.4 mL-H2/g-VS, along with the highest synergistic index (SI) of 1.39, and the second-stage batch anaerobic digestion could generate a satisfactory methane yield of 360.7 mL-CH4/g-VS. The two-stage anaerobic process for continuous hydrogen and methane production was subsequently investigated in the Continuous Stirred Tank Reactor (CSTR) and the Plug Flow Reactor (PFR), respectively, by feeding POME and CD at the optimal mixing ratio of 90:10. Hydrogen yield of 80.6 mL/g-VS and methane yield of 415.8 mL/g-VS, accounting for bioenergy recovery of 17.55 kJ/g-VS, along with 80% COD removal efficiency, were achieved by operating the CSTR at 2-day hydraulic retention time (HRT) and the PFR at 30-day HRT, respectively. Dominant hydrogen-producing bacteria in CSTR were Clostridium, Enterobacter, Weissella, Leuconostoc, and Lactobacillus. Meanwhile, Methanocorpusculum, Methanothrix, Methanosarcina, and Methanoregula were the main populations detected in the PFR. This research work thus demonstrates a feasible approach for co-digesting POME with CD is utilized to efficiently produce valuable bio-hythane and mixed hydrogen-methane gas.