Effect of hydraulic retention time on biohydrogen production from palm oil mill effluent in anaerobic sequencing batch reactor

The feasibility of hydrogen generation from palm oil mill effluent (POME), a high strength wastewater with high solid content, was evaluated in an anaerobic sequencing batch reactor (ASBR) using enriched mixed microflora, under mesophilic digestion process at 37 °C. Four different hydraulic retention times (HRT), ranging from 96 h to 36 h at constant cycle length of 24 h and various organic loading rate (OLR) concentrations were tested to evaluate hydrogen productivity and operational stability of ASBR. The results showed higher system efficiency was achieved at HRT of 72 h with maximum hydrogen production rate of 6.7 LH₂/L/d and hydrogen yield of 0.34 LH₂/g COD_feeding, while in longer and shorter HRTs, hydrogen productivity decreased. Organic matter removal efficiency was affected by HRT; accordingly, total and soluble COD removal reached more than 37% and 50%, respectively. Solid retention time (SRT) of 4-19 days was achieved at these wide ranges of HRTs. Butyrate was found to be the dominant metabolite in all HRTs. Low concentration of volatile fatty acid (VFA) confirmed the state of stability and efficiency of sequential batch mode operation was achieved in ASBR. Results also suggest that ASBR has the potential to offer high digestion rate and good stability of operation for POME treatment.     

Pilot-scale of biohythane production from palm oil mill effluent by two-stage thermophilic anaerobic fermentation

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.     

UASFF start-up for biohydrogen and biomethane production from treatment of Palm Oil Mill Effluent

A start-up study was conducted to produce biohydrogen and biomethane from Palm Oil Mill Effluent (POME) using a two-stage up-flow anaerobic sludge fixed-film (UASFF) bioreactor. 100% molasses was used to start the system, and POME was added at 10% increments until it reached 100% after 59 days. During this period of continuous operation, the HRT and temperature were adjusted in order to optimize the condition for biogas production. Hydrogen and methane gas production fluctuated between 53–70% and 90–95%, respectively, in the last four days of operation (days 56–59), with POME percentage being increased from 70% to 100% (30%–0% molasses). Using 100% raw POME led to a total COD removal of 83.70%, average gas production rates of 5.29 L H₂ d⁻¹ (57.11% H₂) and 9.60 L CH₄ d⁻¹ (94.08% CH₄), in their respective units. This output is comparable to, if not better than using 100% molasses as substrate. This work concludes that based on the relative consistency of biogas production on days 56–59, the two-stage UASFF bioreactor operating at a final HRT of 4 h and temperature of 43 °C has taken a period of two months for start-up.     

An approach of auxiliary carbohydrate source on stabilized biohythane production and energy recovery by two-stage anaerobic process from swine manure

In this study, a two-stage biohythane production system was used to treat swine manure to solve the high Chemical Oxygen Demand (COD) concentration and verify the total energy recovery between the two-stage and a traditional single-stage system. Experiments were carried out in single-stage methane production, two-stage biohythane production in long Hydraulic Retention Time (HRT), and short HRT. The COD removal efficiency and energy recovery were finally compared between single-stage (CH₄ fermenter) and two-stage (H₂+ CH₄ fermenter) systems. The results showed that the methane production rate of 53.2 ± 2.7 mL/d.L, the COD removal efficiency of 29.6 ± 5.8%, and total energy recovery of 2.9 ± 0.1 kJ/L.d was obtained in the single-stage of methane production system with HRT 11.08 d, pH 7, and temperature 55 °C, respectively. In the two-stage of hydrogen and methane productions system, the hydrogen production rate of 1.8 ± 0.7 mL/d.L, the methane production rate of 65.7 ± 2.5 mL/d.L, the COD removal rate was 97.8 ± 1.7%, and the total energy recovery of 3.6 ± 0.1 kJ/L.d was obtained and stabilized when the sugary wastewater content gradually reduced to 0%. This study shows that the methane production rate increases 20%, COD removal efficiency increases to 97.8 ± 1.7%, and total energy recovery increases 30%. At the same time, the single-stage (CH₄ fermenter) switched to a two-stage (H₂+ CH₄ fermenter) system. The two-stage anaerobic biohythane production system successfully treated the high organic swine manure and obtained a higher energy recovery against the traditional single-stage of the biomethane production system.     

Production of methane from ozonated palm oil mill effluent

Methane (CH₄) production from palm oil mill effluent (POME) pre-treated by ozonation was conducted under mesophilic (37 °C) condition. The results demonstrated that methane can be produced from both non-ozonated and ozonated POME at a concentration range of 3,000 to 15,000 mg COD L⁻¹. Methane yield rised 54% when POME was pre-treated by ozonation at POME concentration of 15,000 mg COD L⁻¹. The methane yield increased the POME concentration was increased. At POME above 15,000 mg COD L⁻¹, the methane yield was dropped dramatically. The methane production rates (R_max) and yields exerted similar trend regarding the POME concentration. Accumulation of volatile fatty acids in the reactor posed the drop of methane production. Ozonation pretreatment process of POME can improve the biodegradability of the complex organic matter in POME and enhanced methane yield and rate at POME concentration range of 3,000–15,000 mg COD L⁻¹.     

Hydrogen production from alcohol wastewater with added fermentation residue by an anaerobic sequencing batch reactor (ASBR) under thermophilic operation

The objective of this study was to investigate the enhancement of hydrogen production from alcohol wastewater by adding fermentation residue using an anaerobic sequencing batch reactor (ASBR) under thermophillic operation (55 °C) and at a constant pH of 5.5. The digestibility of the added fermentation residue was also evaluated. For a first set of previous experiments, the ASBR system was operated to obtain an optimum COD loading rate of 50.6 kg/m³ d of alcohol wastewater without added fermentation residue and the produced gas contained 31% H₂ and 69% CO₂. In this experiment, the effect of added fermentation residue (100–1200 mg/l) on hydrogen production performance was investigated under a COD loading rate of 50.6 kg/m³ d of the alcohol wastewater. At a fermentation residue concentration of 1000 mg/l, the produced gas contained 40% H₂ and 60% CO₂ without methane and the system gave the highest hydrogen yield and specific hydrogen production rate of 128 ml/g COD removed and 2880 ml/l d, respectively. Under thermophilic operation with a high total COD loading rate (51.8 kg/m³ d) and a short HRT (21 h) at pH 5.5, the ASBR system could only break down cellulose (41.6%) and hemicellulose (21.8%), not decompose lignin.     

Biohydrogen production from palm oil mill effluent (POME) by two stage anaerobic sequencing batch reactor (ASBR) system for better utilization of carbon sources in POME

The production of biohydrogen through dark fermentation of palm oil mill effluent (POME) was evaluated in two-stages of biohydrogen in an anaerobic sequencing batch reactor (ASBR) system using enriched mixed culture for the first time. This study attempts to examine the effect of HRT and its interaction behavior with the solid retention time (SRT), and the sugar consumption. The effluent after discharged from the thermophilic reactor contained 7.61 g/L TC and 22.87 g/L TSS was fed to the secondary mesophilic reactor system. Results indicated that the overall sugar consumption reached 88.62% at the optimum HRT of 12 h with the SRT set to 20 h. The optimum hydrogen yield and HPR in the thermophilic stage were 2.99 mol H₂/mol-sugar and 8.54 mmol H₂/L·h respectively, while for the mesophilic stage were 1.19 mol H₂/mol-sugar and 1.47 mmolH₂/L·h respectively. The overall HPR showed an improvement and increase from 8.54 mmol H₂/L·h to 10.34 mmol H₂/L.h. Microbial community analysis of mixed culture in the two-stage thermophilic (55.0 °C) and mesophilic (37.0 °C) ASBR reactor was dominated by Thermoanaerobacterium sp. based on the PCR-DGGE technique.     

Hydrogen and methane production in extreme thermophilic conditions in two-stage (upflow anaerobic sludge bed) UASB reactor system

Two-stage hydrogen and methane production in extreme thermophilic (70 °C) conditions was demonstrated for the first time in UASB-reactor system. Inoculum used in hydrogen and methane reactors was granular sludge from mesophilic internal circulation reactor and was first acclimated for extreme thermophilic conditions. In hydrogen reactor, operated with hydraulic retention time (HRT) of 5 h and organic loading rate (OLR) of 25.1 kg COD/m³/d, hydrogen yield was 0.73 mol/mol glucose_added. Methane was produced in second stage from hydrogen reactor effluent. In methane reactor operated with HRT of 13 h and OLR of 7.8 kg COD/m³/d, methane yield was 117.5 ml/g COD_added. These results prove that hydrogen and methane can be produced in extreme thermophilic temperatures, but as batch experiments confirmed, for methane production lower temperature would be more efficient.     

Hydrogen production from alcohol wastewater by an anaerobic sequencing batch reactor under thermophilic operation: Nitrogen and phosphorous uptakes and transformation

The objective of this study was to investigate hydrogen production from alcohol wastewater using an anaerobic sequencing batch reactor (ASBR) under thermophilic operation and at a constant pH of 5.5. Under the optimum COD loading rate of 68 kg/m³d, the produced gas contained 43% H₂ without methane and the system provided a hydrogen yield and specific hydrogen production rate of 130 ml H₂/g COD removed and 2100 ml H₂/l d, respectively, which were much higher than those obtained under the mesophilic operation. Under thermophilic operation, both nitrogen and phosphate uptakes were minimal at the optimum COD loading rate for hydrogen production and most nitrogen uptake was derived from organic nitrogen. Under the thermophilic operation for hydrogen production, the nutrient requirement in terms of COD:N:P was found to be 100:6:0.5, which was much higher than that for the methenogenic step for methane production under both thermophilic and mesophilic operations and for the acidogenic step for hydrogen production under mesophilic operation.     

Optimization and microbial community analysis for production of biohydrogen from palm oil mill effluent by thermophilic fermentative process

The optimum values of hydraulic retention time (HRT) and organic loading rate (OLR) of an anaerobic sequencing batch reactor (ASBR) for biohydrogen production from palm oil mill effluent (POME) under thermophilic conditions (60 °C) were investigated in order to achieve the maximum process stability. Microbial community structure dynamics in the ASBR was studied by denaturing gradient gel electrophoresis (DGGE) aiming at improved insight into the hydrogen fermentation microorganisms. The optimum values of 2-d HRT with an OLR of 60 gCOD l⁻¹ d⁻¹ gave a maximum hydrogen yield of 0.27 l H₂ g COD⁻¹ with a volumetric hydrogen production rate of 9.1 l H₂ l⁻¹ d⁻¹ (16.9 mmol l⁻¹ h⁻¹). The hydrogen content, total carbohydrate consumption, COD (chemical oxygen demand) removal and suspended solids removal were 55 ± 3.5%, 92 ± 3%, 57 ± 2.5% and 78 ± 2%, respectively. Acetic acid and butyric acid were the major soluble end-products. The microbial community structure was strongly dependent on the HRT and OLR. DGGE profiling illustrated that Thermoanaerobacterium spp., such as Thermoanaerobacterium thermosaccharolyticum and Thermoanaerobacterium bryantii, were dominant and probably played an important role in hydrogen production under the optimum conditions. The shift in the microbial community from a dominance of T. thermosaccharolyticum to a community where also Caloramator proteoclasticus constituted a major component occurred at suboptimal HRT (1 d) and OLR (80 gCOD l⁻¹ d⁻¹) conditions. The results showed that the hydrogen production performance was closely correlated with the bacterial community structure. This is the first report of a successful ASBR operation achieving a high hydrogen production rate from real wastewater (POME).     

Thermophilic hydrogen and methane production from sugarcane stillage in two-stage anaerobic fluidized bed reactors

This study evaluated the feasibility of H₂ and CH₄ production in two-stage thermophilic (55 °C) anaerobic digestion of sugarcane stillage (5,000 to 10,000 mg COD.L⁻¹) using an acidogenic anaerobic fluidized bed reactor (AFBR-A) with a hydraulic retention time (HRT) of 4 h and a methanogenic AFBR (AFBR-S) with HRTs of 24 h–10 h. To compare two-stage digestion with single-stage digestion, a third methanogenic reactor (AFBR-M) with a HRT of 24 h was fed with increasing stillage concentrations (5,000 to 10,000 mg COD.L⁻¹). The AFBR-M produced a methane content of 68.4 ± 7.2%, a maximum yield of 0.30 ± 0.04 L CH₄.g COD⁻¹, a production rate of 3.78 ± 0.40 L CH₄.day⁻¹.L⁻¹ and a COD removal of 73.2 ± 5.0% at an organic loading rate (OLR) of 7.5 kg COD.m⁻³.day⁻¹. In contrast, the two-stage AFBR-A system produced a hydrogen content of 23.9 ± 5.6%, a production rate of 1.30 ± 0.16 L H₂.day⁻¹.L⁻¹ and a yield of 0.34 ± 0.08 mmol H₂.g CODap⁻¹. Additionally, the decrease in the HRT from 18 h to 10 h in the AFBR-S favored a higher methane production, improving the maximum methane content (74.5 ± 6.0%), production rate (5.57 ± 0.38 L CH₄.day⁻¹.L⁻¹) and yield (0.26 ± 0.06 L CH₄.g COD⁻¹) at an OLR of 21.6 kg COD.m⁻³.day⁻¹ (HRT of 10 h) with a total COD removal of 70.1 ± 7.1%. Under the applied COD of 10,000 mg L⁻¹, the two-stage system showed a 52.8% higher energy yield than the single-stage anaerobic digestion system. These results show that, relative to a single-stage system, two-stage anaerobic digestion systems produce more hydrogen and methane while achieving similar treatment efficiencies.     

Continuous fermentative hydrogen and methane production from Laminaria japonica using a two-stage fermentation system with recycling of methane fermented effluent

In this study, a two-stage fermentation system to produce H₂ and CH₄ from Laminaria japonica was developed. In the first stage (dark fermentative H₂ production, DFHP), response surface methodology (RSM) with a Box-Behnken design (BBD) was applied for optimization of operational parameters, including cycle-frequency, HRT, and substrate concentration, using an intermittent-continuously stirred tank reactor (i-CSTR). Overall performance revealed that the degree of importance of the three variables in terms of H₂ yield is as follows: cycle-frequency > substrate concentration > HRT. In the confirmation test, H₂ yield of 113.1 mL H₂/g dry cell weight (dcw) was recorded, corresponding with 96.3% of the predicted response value under desirable operational conditions (cycle-frequency of 17 hr, HRT of 2.7 days, and substrate concentration of 31.1 g COD/L). In the second stage, an anaerobic sequencing batch reactor (ASBR) and an up-flow anaerobic sludge blanket reactor (UASBr) were employed for CH₄ production from H₂ fermented solid state (HFSS) and H₂ fermented liquid state (HFLS), respectively. The CH₄ producing ASBR and UASBr showed a stable CH₄ yield and COD removal until a HRT of 12 days and OLR of 3.5 g COD/L/d, respectively. Subsequently, for recycling of CH₄ fermented effluent from the UASBr (MFE_UASBr) as diluting water in DFHP, the tap water and MFE_UASBr mixing ratio (T/M ratio) was optimized (a T/M ratio of 5:5) in a batch test using heat pretreated MFE_UASBr at 90 °C for 20 min, resulting in the best performance. Although slight decreases of H₂ yield (7.6%) and H₂ production rate (3.5%) were recorded, 100% reduction of alkali addition was possible, indicating potential to maximize economic benefits. However, a drastic decrease of H₂ productivity and a change of liquid-state metabolites were observed with the use of non-heat pretreated MFE_UASBr. These results coincided with those of the microbial analysis, where non-H₂ producing bacteria, such as Selenomonas sp., were detected. The results indicate that pretreatment of MFE_UASBr may be required in order to recycle it in DFHP.     

Direct fermentation of Laminaria japonica for biohydrogen production by anaerobic mixed cultures

A few studies have been made on fermentative hydrogen production from marine algae, despite of their advantages compared with other biomass substrates. In this study, fermentative hydrogen production from Laminaria japonica (one brown algae species) was investigated under mesophilic condition (35 ± 1 °C) without any pretreatment method. A feasibility test was first conducted through a series of batch cultivations, and 0.92 mol H₂/mol hexose_added, or 71.4 ml H₂/g TS of hydrogen yield was achieved at a substrate concentration of 20 g COD/L (based on carbohydrate), initial pH of 7.5, and cultivation pH of 5.5. Continuous operation for a period of 80 days was then carried out using anaerobic sequencing batch reactor (ASBR) with a hydraulic retention time (HRT) of 6 days. After operation for approximately 30 days, a stable hydrogen yield of 0.79 ± 0.03 mol H₂/mol hexose_added was obtained. To optimize bioenergy recovery from L. japonica, an up-flow anaerobic sludge blanket reactor (UASBr) was applied to treat hydrogen fermentation effluent (HFE) for methane production. A maximum methane yield of 309 ± 12 ml CH₄/g COD was achieved during the 90 days operation period, where the organic loading rate (OLR) was 3.5 g COD/L/d.     

Biological saccharification by Clostridium thermocellum and two-stage hydrogen and methane production from hydrogen peroxide-acetic acid pretreated sugarcane bagasse

The present work aimed at establishing an efficient degradation and energy recovery system form sugarcane bagasse (SCB) through hydrogen peroxide-acetic acid (HPAC) pretreatment, thermophilic hydrogen production and mesophilic methane production. The degradation ratio of HPAC pretreated SCB (HPAC-SCB, 2%, w/v) exceeded 90% under the biological hydrolysis of C. thermocellum without enzyme addition. The hydrogen yield in the co-culture fermentation of T. thermosaccharolyticum and C. thermocellum from HPAC-SCB (2%, w/v) reached 226 mL/g substrate. The long-term hydrogen fermentation was successfully established with 1.59 L/(L·d), 0.159 L/g substrate for average hydrogen productivity and yield, respectively. Methane production of 0.341 L/g COD (chemical oxygen demand)_added was recovered by semi-continuous methane fermentation from hydrogen-producing effluent at 12 days of hydraulic retention time (HRT). Average energy recovery of 8.79 MJ/kg SCB was obtained under the optimal conditions. The present work indicated the promising application of the established process in valorization of lignocellulosic bio-waste.     

Two-stage UASB reactor converting coffee drink manufacturing wastewater to hydrogen and methane

In this study, the feasibility of a continuous two-stage up-flow anaerobic sludge blanket (UASB) reactor system, consisted of thermophilic (55 °C) dark fermentative H₂ production and mesophilic (35 °C) CH₄ production from coffee drink manufacturing wastewater (CDMW), was tested. A recently proposed operational strategy was used to overcome a major drawback of the long start-up period of the UASB reactor. Firstly, a completely stirred tank reactor (CSTR) was operated for 8 days to prepare seeding. The seed was then directly transferred to the UASB reactor. Microbial aggregation took place in the initial period, and the floc size was gradually increased over time. In UASB reactor, the maximum H₂ yield of 2.57 mol H₂/mol hexose_added and a stable H₂ production rate of 4.24 L H₂/L/h were observed at a hydraulic retention time (HRT) of 6 h and substrate concentration of 20 g Carbo. COD/L. In this novel method using CDMW, thermophilic H₂-producing granules with an average particle size of 1.3 mm was successfully developed after 100 days. The more bioenergy recovery was attempted in a post-treatment process using a mesophilic UASB reactor for CH₄ production from the H₂ fermented effluent. The maximum CH₄ yield of 325 mL of CH₄/g COD was achieved with removing 93% of the COD at an organic loading rate of 3.5 g COD/L/d. The developed two-stage UASB reactor system achieved biogas conversion by 88.2% (H₂ 15.2% and CH₄ 73%) and COD removal by 98%.     

Application of immobilized upflow anaerobic sludge blanket reactor using Clostridium LS2 for enhanced biohydrogen production and treatment efficiency of palm oil mill effluent

Polyethylene glycol (PEG) gel was used to immobilize hydrogen producing Clostridium LS2 bacteria for hydrogen production in an upflow anaerobic sludge blanket (UASB) reactor. The UASB reactor with a PEG-immobilized cell packing ratio of 10% weight to volume ratio (w/v) was optimal for dark hydrogen production. The performance of the UASB reactor fed with palm oil mill effluent (POME) as a carbon source was examined under various hydraulic retention time (HRT) and POME concentration. The best volumetric hydrogen production rate of 365 mL H₂/L/h (or 16.2 mmol/L/h) with a hydrogen yield of 0.38 L H₂/g COD_added was obtained at POME concentration of 30 g COD/L and HRT of 16 h. The average hydrogen content of biogas and COD reduction were 68% and 65%, respectively. The primary soluble metabolites were butyric acid and acetic acid with smaller quantities of other volatile fatty acid and alcohols formed during hydrogen fermentation. More importantly, the feasibility of PEG-immobilized cell UASB reactor for the enhancement of the dark-hydrogen production and treatment of wastewater is demonstrated.     

Hydrogen and methane production via a two-stage processes (H2-SBR + CH4-UASB) using tequila vinasses

The feasibility of producing hydrogen and methane via a two-stage fermentation of tequila vinasses was evaluated in sequencing batch (SBR) and up-flow anaerobic sludge blanket (UASB) reactors. Different vinasses concentrations ranging from 500 mg COD/L to 16 g COD/L were studied in SBR by using thermally pre-treated anaerobic sludge as inoculum for hydrogen production. Peak volumetric hydrogen production rate and specific hydrogen production were attained as 57.4 ± 4.0 mL H₂/L-h and 918 ± 63 mL H₂/gVSS-d, at the substrate concentration of 16 g COD/L and 6 h of hydraulic retention time (HRT). Increasing substrate concentration has no effect on the specific hydrogen production rate. The fermentation effluent was used for methane production in an UASB reactor. The higher methane composition in the biogas was achieved as 68% at an influent concentration of 1636 mg COD/L. Peak methane volumetric, specific production rates and yield were attained as 11.7 ± 0.7 mL CH₄/L-h, 7.2 ± 0.4 mL CH₄/g COD-h and 257.9 ± 13.8 mL CH₄/g COD at 24 h-HRT and a substrate concentration of 1636 mg COD/L. An overall organic matter removal (SBR + UASB) in this two-stage process of 73–75% was achieved.     

Hydrogen production from alcohol distillery wastewater containing high potassium and sulfate using an anaerobic sequencing batch reactor

In this study, the feasibility of hydrogen production from alcohol distillery wastewater containing high potassium and sulfate was investigated using an anaerobic sequencing batch reactor (ASBR). The seed sludge taken from an anaerobic tank treating the distillery wastewater was boiled for 15 min before being fed to the ASBR. The ASBR system was operated under different feed chemical oxygen demand (COD) values and different COD loading rates at a mesophilic temperature of 37 °C, a controlled pH at 5.5, and a cycle time of 6 cycles per day. When the studied ASBR was operated under the best conditions (providing a maximum hydrogen production efficiency) of a feed COD of 40,000 mg/l, a COD loading rate of 60 kg/m³ d, and a hydraulic retention time of 16 h, the produced gas was found to contain 34.7% H₂ and 65.3% CO_2, without any methane being detected. Under these best conditions, the specific hydrogen production rate (SHPR) of 270 ml H₂/g MLVSS d (or 3310 ml H₂/l d), and hydrogen yield of 172 ml H₂/g COD removed, were obtained. When the feed COD exceeded 40,000 mg/l, the process performance in terms of hydrogen production decreased because of the potassium and sulfate toxicity.     

Biological hydrogen production from sweet sorghum syrup by mixed cultures using an anaerobic sequencing batch reactor (ASBR)

Continuous biological hydrogen production from sweet sorghum syrup by mixed cultures was investigated by using anaerobic sequencing batch reactor (ASBR). The ASBR was conducted based on the optimum condition obtained from batch experiment i.e. 25 g/L of total sugar concentration, 1.45 g/L of FeSO₄ and pH of 5.0. Feasibility of continuous hydrogen fermentation in ASBR operation at room temperature (30 ± 3 °C) with different hydraulic retention time (HRT) of 96, 48, 24 and 12 hr and cycle periods consisting of filling (20 min), settling (20 min), and decanting (20 min) phases was analyzed. Results showed that hydrogen content decreased with a reduction in HRT i.e. from 42.93% (96 hr HRT) to 21.06% (12 hr HRT). Decrease in HRT resulted in a decrease of solvents produced which was from 10.77 to 2.67 mg/L for acetone and 78.25 mg/L to zero for butanol at HRT of 96 hr-12 hr, respectively. HRT of 24 hr was the optimum condition for ASBR operation indicated by the maximum hydrogen yield of 0.68 mol H₂/mol hexose. The microbial determination in DGGE analysis indicated that the well-known hydrogen producers Clostridia species were dominant in the reacting step. The presence of Sporolactobacillus sp. which could excrete the bacteriocins causing the adverse effect on hydrogen-producing bacteria might responsible for the low hydrogen content obtained.     

Optimization of biohydrogen production of palm oil mill effluent by ozone pretreatment

The biohydrogen (H₂) production in batch experiments under varying concentrations of raw and ozonated palm oil mill effluent (POME) of 5000–30,000 mg COD.L⁻¹, at initial pH 6, under mesophilic (37 °C), thermophilic (55 °C) and extreme-thermophilic (70 °C) conditions. Effects of ozone pretreatment, substrate concentration and fermentation temperature on H₂ production using mesophilic seed sludge was undertaken. The results demonstrated that H₂ can be produced from both raw and ozonated POME, and the amounts of H₂ production were directly increased as the POME concentrations were increased. H₂ was successfully produced under the mesophilic fermentation of ozonated POME, with maximum H₂ yield, and specific H₂ production rate of 182 mL.g⁻¹ COD_removed (30,000 mg COD.L⁻¹) and 6.2 mL.h⁻¹.g⁻¹ TVS (25,000 mg COD.L⁻¹), respectively. Thus, indicating that the ozone pretreatment could elevate on the biodegradability of major constituents of the POME, which significantly enhanced yields and rates of the H₂ production. H₂ production was not achieved under the thermophilic and extreme-thermophilic fermentation. In both fermentation temperatures with ozonated POME, the maximum H₂ yield was 62 mL.g⁻¹ COD_removed (30,000 mg COD.L⁻¹) and 63 mL.g⁻¹ COD_removed (30,000 mg COD.L⁻¹), respectively. The highest efficiency of total and soluble COD removal was obtained at 44 and 37%, respectively following the mesophilic fermentation, of 24 and 25%, respectively under the thermophilic fermentation, of 32 and 20%, respectively under the extreme-thermophilic fermentation. The production of volatile fatty acids increased with an increased fermentation time and temperature in both raw and ozonated POME under all three fermentation temperatures. The accumulation of volatile fatty acids in the reactor content were mostly acetic and butyric acids. H₂ fermentation under the mesophilic condition of 37 °C was the better selection than that of the thermophilic and extreme-thermophilic fermentation.