Two-stage thermophilic fermentation and mesophilic methanogenic process for biohythane production from palm oil mill effluent with methanogenic effluent recirculation for pH control

Two-stage thermophilic fermentation and mesophilic methanogenic process with methanogenic effluent recirculation to hydrogen reactor for biohythane production from palm oil mill effluent (POME) was investigated. Maximum hydrogen (4.1 L H2/L-POME) and methane production (16.6 L CH4/L-POME) from POME in batch tests were achieved at 30% recirculation rate of methanogenic effluent. Continuous hydrogen production of without and with recirculation phase was 2.2 and 3.8 L H2/L-POME, respectively. Continuous methane production of without and with recirculation phase was 12.2 and 14 L CH4/L-POME, respectively. Both reactor effluents were effective to keep at optimal pH with 2 times increasing in hydrogen production. The hydrogen and methane yield were 135 mL H2/gVS and 414 mL CH4/gVS, respectively. Biohythane gas composition was composed 13.3% of hydrogen, 54.4% of methane and 32.2% carbon dioxide. Two-stage process with methanogenic effluent recirculation flavored Thermoanaerobacterium sp. in the hydrogen reactor and efficiently for energy recovery from POME.