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Methane production from solid-state anaerobic digestion (SS-AD) of empty fruit bunches (EFB) using recycling solid-anaerobic digested (S-AD) sludge and liquid-anaerobic digested (L-AD) sludge as inoculum was investigated under thermophilic (55 °C) and mesophilic conditions (40 °C). The methane production rate of L-AD sludge (2.98–3.27 L-CH4 L−1 reactor d −1) was 1–2 times higher than recycling S-AD sludge (1.36 L-CH4 L−1 reactor d −1) as inoculum. Methane production of L-AD and S-AD sludge inoculum was 53.5–54.1 and 31.2–39.0 m3 CH4 tonne-1 EFB, respectively. The SS-AD of EFB with L-AD and S-AD sludge under thermophilic condition had specific methane activity of 34–96 times higher than the mesophilic condition. Clostridium sp., Methanosaeta sp., and Methanoculleus sp. were responsible for the SS-AD of EFB with L-AD sludge inoculum. The L-AD sludge was suitable inoculum for SS-AD of EFB in terms of methane production rate, cellulose degradation efficiency, biogas production, and microbial activity.

Improvement of biohythane production from oil palm industry solid waste residues by co-digestion with palm oil mill effluent (POME) in two-stage thermophilic fermentation was investigated. A two-stage co-digestion of solid waste with POME has biohythane production of 26.5–34 m3/ton waste. The co-digestion of solid waste with POME increased biohythane production of 67–114% compared to digestion POME alone. Co-digestion of solid waste with POME enhanced hydrolysis constant (kh) from 0.07 to 0.113 to 0.120–0.223 d−1. The hydrolysis constant (kh) of co-digestion was 10 times higher than the single digestion of solid waste. Clostridium sp. was predominated in the hydrogen stage, while Methanosphaera sp. was predominant in methane stage. The co-digestion of solid waste with readily biodegradable organic matter (POME) could significantly increase biohythane production with achieving the significant cost reduction for pretreatment of solid wastes.

The pilot-scale of two-stage thermophilic (55 °C) for biohythane production from palm oil mill effluent (POME) was operated at hydraulic retention time (HRT) of 2 days and organic loading rate (OLR) of 27.5 gCOD/L⋅d) for first stage and HRT of 10 days and OLR of 5.5 gCOD/L⋅d for second stage. Biohythane production rate was 1.93 L-gas/L⋅d with biogas containing 11% H2, 37% CO2, and 52% CH4. Recirculation of methane effluent mixed with POME at a ratio of 1:1 can control pH in the first stage at an optimal range of 5.0–6.5. Microbial community in hydrogen stage dominated by Thermoanaerobacterium sp., while methane stage dominated by Methanosarcina sp. The H2/CH4 ratio of biohythane was 0.13–0.18 which suitable for vehicle fuel. Biohythane production from POME could be promising cleaner biofuel with flexible and controllable H2/CH4 ratio.

A sudden pH drops always inhibits the anaerobic digestion (AD) reactor for biogas production from palm oil mill effluent (POME). The pH adjustment of POME by oil palm ash addition and the biogas effluent recycling effect on the preventing of pH drop and change of the archaea community was investigated. The pH adjustment of POME to 7.5 increased the methane yield two times more than raw POME (pH 4.3). The optimal dose for pH adjustment by oil palm ash addition was 5% w/v with a methane yield of 440 mL-CH4/gVS. The optimal dose for pH adjustment by biogas effluent recycling was 20% v/v with a methane yield of 351 mL-CH4/gVS. Methane production from POME in a continuous reactor with pH adjustment by 5% w/v oil palm ash and 20% v/v biogas effluent recycling was 19.1 ± 0.25 and 13.8 ± 0.3 m3 CH4/m3-POME, respectively. The pH adjustment by oil palm ash enhanced methane production for the long-term operation with the stability of pH, alkalinity, and archaea community. Oil palm ash increased the number of Methanosarcina mazei and Methanothermobacter defluvii. Oil palm ash is a cost-effective alkali material as a source of buffer and trace metals for preventing the pH drop and the increased methanogen population in the AD process.

Third generation biomass, i.e. microalgae, has emerged as a promising alternative to first and second generation biomass for biohydrogen production. However, its utilization is still low at present, due to several reasons including the strong and rigidity of the microalgal cell wall that limit the hydrolysis efficiency during dark fermentation (DF) and photofermentation (PF) processes. To improve the utilization efficiency of microalgal biomass, it is crucial that important aspects related to the production of the biomass and the following processes are elaborated. Thus, this article provides detailed overview of algal strains, cultivation, and harvesting. It also presents recent research and detailed information on microalgal biomass pretreatment, and biohydrogen production through DF, PF, and co-digestion of microalgal biomass with organic materials. Furthermore, factors affecting fermentation processes performance and the use of molecular techniques in biohydrogen production are presented. This review also discusses challenges and future prospects towards biohydrogen production from microalgal biomass.

Background Anaerobic digestion (AD) is a suitable process for treating high moisture MSW with biogas and biofertilizer production. However, the low stability of AD performance and low methane production results from high moisture MSW due to the fast acidify of carbohydrate fermentation. The effects of organic loading and incineration fly ash addition as a pH adjustment on methane production from high moisture MSW in the single-stage AD and two-stage AD processes were investigated. Results Suitable initial organic loading of the single-stage AD process was 17 gVS L−1 at incineration fly ash (IFA) addition of 0.5% with methane yield of 287 mL CH4 g−1 VS. Suitable initial organic loading of the two-stage AD process was 43 gVS L−1 at IFA addition of 1% with hydrogen and methane yield of 47.4 ml H2 g−1 VS and 363 mL CH4 g−1 VS, respectively. The highest hydrogen and methane production of 8.7 m3 H2 ton−1 of high moisture MSW and 66.6 m3 CH4 ton−1 of high moisture MSW was achieved at organic loading of 43 gVS L−1 at IFA addition of 1% by two-stage AD process. Biogas production by the two-stage AD process enabled 18.5% higher energy recovery than single-stage AD. The 1% addition of IFA into high moisture MSW was useful for controlling pH of the two-stage AD process with enhanced biogas production between 87–92% when compared to without IFA addition. Electricity production and energy recovery from MSW using the coupled incineration with biogas production by two-stage AD process were 9,874 MJ ton−1 MSW and 89%, respectively. Conclusions The two-stage AD process with IFA addition for pH adjustment could improve biogas production from high moisture MSW, as well as reduce lag phase and enhance biodegradability efficiency. The coupled incineration process with biogas production using the two-stage AD process was suitable for the management of MSW with low area requirement, low greenhouse gas emissions, and high energy recovery.

Empty fruit bunches (EFB) have low biodegradability and restrict their commercial utilization in biogas plants. Integration of straw mushroom (Volvariella volvacea) cultivation as a function of bio-pretreatment on EFB to improve biodegradability and methane production by solid-state anaerobic digestion (SS-AD) was investigated. The mushroom yield was 47.3 kg·tonne−1 EFB with remaining weight in spent mushroom-EFB (S-mEFB) of 82%. The cellulose, hemicellulose, and lignin of EFB were degraded by 3.3%, 21.3%, and 17.6%, respectively, with an increased surface area of S-mEFB. The biodegradability of S-mEFB (62.7%) was 2 times higher than raw EFB (33.5%) with the highest methane yield and production of 281 mL CH4·g−1 VS and 50.6 m3·tonne−1 S-mEFB, respectively. The co-digestion of S-mEFB with 5% v/w POME had highest methane yield of 405 mL CH4·g−1 VS with biodegradability of 90.8%. Integrating straw mushroom cultivation with SS-AD is a promising strategy for achieving an environmentally friendly and economically feasible process.

Homoacetogens consortium was enriched from peatland soil, digested sludge, and rumen fluid for simultaneous carbon dioxide (CO2) consumption and acetic acid production in the biogas upgrading process. The homoacetogens consortium enriched from peatland soil (PL3) has a maximum CO2 consumption and acetic acid production of 95% and 120 mg/L, respectively. The methane content in upgraded biogas and acetic acid production of 98% and 543 mg/L was obtained at optimum hydrogen (H2) to CO2 at a ratio of 2:1, pH 8, and temperature 30 °C. The upgraded biogas (CH4 > 95%) can be used as vehicle fuel or injected into the natural gas grid. The homoacetogens consortium PL3 was dominated by Clostridium sp., Proteiniclasticum sp., and Petrimonas sp. Clostridium species are the main homoacetogens responsible for CO2 reduction using H2 as an energy source. The homoacetogens consortium PL3 could provide an opportunity for simultaneous biogas upgrading to biomethane and acetic acid production.

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.

Co-fermentation of palm oil mill effluent (POME) for hydrogen production by Thermoanaerobacterium thermosaccharolyticum PSU-2 under thermophilic condition was investigated. Hydrogen production from co-fermentation POME with 5 g/L decanter cake and without was 2.6 and 1.7 L-H 2 /L-POME. Co-fermentation of POME with decanter cake enhanced hydrogen production. Tbm. thermosaccharolyticum PSU-2 augmentation at 20% v/v was found to be an efficient start-up strategy in continuous hydrogen production from co-fermentation of POME with decanter cake. Tbm. thermosaccharolyticum PSU-2 augmentation could suppress normal flora bacteria in POME and decanter cake. Hydrogen production of 7.2 L-H 2 /L-POME and hydrogen production rate of 18.1 mmol-H 2 /L/h was achieved from continuous hydrogen in long term operation. The soluble metabolites were dominated by acetic acid and butyric acid. Tbm. thermosaccharolyticum PSU-2 augmentation could use for control undesired bacteria in the hydrogen production system with flavored Thermoanaerobacterium sp. as dominant species.

Moderate thermophilic fermentation appears to be superior on hydrogen production rate (98–152 mmol/L/h) with satisfactory hydrogen yield of 2–2.4 mol/mol C6, while extreme thermophilic fermentation has a lower hydrogen production rate (15–30 mmol/L/h) with rather high hydrogen yield of 2.8–3.5 mol/mol C6 when using pure sugar as substrate. Both moderate thermophilic and extreme thermophilic fermentation has similar hydrogen yield for the real-world substrate (2.4–2.8 mol H2/mol C6), such as wastewater and organic wastes. Extreme thermophilic fermentation has less variety of side-products than moderate thermophilic fermentation. Significant improvement in H2 production yield could be achieved to near maximum of 4 mol H2/mol C6 by in situ gas separation, microbial population optimization, and metabolic engineering in the microorganisms. Bioreactor design by integrating dark fermentation with volatile fatty acids oxidation by microbial electrolysis could have the potential to increase the hydrogen yield gone beyond the current maximum of 4 mol H2/mol C6 and address technical barriers that currently limit the technoeconomic feasibility of fermentative hydrogen production.

A two-stage thermophilic fermentation and mesophilic methanogenic process for biohythane production from palm oil mill effluent (POME) was investigated. The hydrogen and methane potential from POME were 170–200 L H 2 kgCOD −1 and 210–292 L CH 4 kgCOD −1 , respectively. A continuous two-stage process with hydraulic retention time (HRT) of 2 d for hydrogen reactor and 15 d for methane reactor, obtained 34% higher energy yield than single stage methane production. The hydrogen and methane yields from two-stage were 210 L H 2 kgCOD −1 and 315 L CH 4 kgCOD −1 , respectively with total energy yield of 15.34 MJ kgCOD −1 . Mixed hydrogen and methane (biohythane) production rate was 4.4 L biogas L −1 d −1 with containing of 51% CH 4 , 14% H 2 and 35% CO 2 . Hydrogen reactor was dominated with hydrogen producing bacteria of Thermoanaerobacterium thermosaccharolyticum , while acetoclastic Methanoculleus sp. was the dominant methanogen in methane reactor. Two-stage process for biohythane production could efficiently for energy recovery from POME.

Thermoanaerobacterium-rich sludge was used for hydrogen production and phenol removal from palm oil mill effluent (POME) in the presence of phenol concentration of 100–1000 mg/L. Thermoanaerobacterium-rich sludge yielded the most hydrogen of 4.2 L H2/L-POME with 65% phenol removal efficiency at 400 mg/L phenol. Butyric acid and acetic acid were the main metabolites. The effects of oil palm ash, NH4NO3 and iron concentration (Fe2+) on hydrogen production and phenol removal efficiency from POME by Thermoanaerobacterium-rich sludge was investigated using response surface methodology (RSM). The RSM results indicated that the presence of 0.2 g Fe2+/L, 0.3 g/L NH4NO3 and 20 g/L oil palm ash in POME could improved phenol removal efficiency, with predicted hydrogen production and phenol removal efficiency of 3.45 L H2/L-POME and 93%, respectively. In a confirmation experiment under optimized conditions highly reproducible results were obtained, with hydrogen production and phenol removal efficiency of 3.43 ± 0.12 L H2/L-POME and 92 ± 1.5%, respectively. Simultaneous hydrogen production and phenol removal efficiency in continuous stirred tank reactor at hydraulic retention time (HRT) of 1 and 2 days were 4.0 L H2/L-POME with 85% and 4.2 L H2/L-POME with 92%, respectively. Phenol degrading Thermoanaerobacterium-rich sludge comprised of Thermoanaerobacterium thermosaccharolyticum, Thermoanaerobacterium aciditolerans, Desulfotomaculum sp., Bacillus coagulans and Clostridium uzonii. Phenol degrading Thermoanaerobacterium-rich sludge has great potential to harvest hydrogen from phenol-containing wastewater.

Effect of organic loading rate (OLR), hydraulic retention time (HRT) and temperature variation on process stability of fermentative hydrogen production from palm oil mill effluent (POME) under thermophilic condition was investigated. High OLR (>70 gCOD/l/d) and low HRT (4 d) has significant affected on hydrogen productivity, process stability and microbial community. Hydrogen production at OLR of 55, 60, 65 and 70 gCOD/l/d was 2.4, 3.4, 4.9 and 3.7 L H 2 /L-POME, respectively. The decreasing in pH and alkalinity and increasing in total VFA could cause the instability of process. The microbial community was changed at OLR >70 gCOD/l/d, HRT 4 d and stop feeding. The dominant microbial community changed from Thermoanaerobacterim sp. to Clostridium sp., Thermoanaerobacterim sp. and Bacillus sp. Hydrogen production at HRT of 2, 4, stop feeding and 2 d was 2.7, 0.8, 0, and 2.6 L H 2 /L-POME, respectively. Hydrogen production, process stability and microbial community was not affected by variation of temperature. Hydrogen production before and after temperature variation was 2.5 and 2.45 L H 2 /L-POME.