Optimizing the production of hydrogen and 1,3-propanediol in anaerobic fermentation of biodiesel glycerol

The conversion of glycerol in biodiesel waste streams to valuable products (e.g. hydrogen and 1,3-propanediol (1,3-PD)) was studied through batch-mode anaerobic fermentation with organic soil as inoculum. The production of hydrogen in headspace and 1,3-PD in liquid phase was examined at different hydrogen retention times (HyRTs), which were controlled by gas-collection intervals (GCIs) and initial gas-collection time points (IGCTs). Two purification stages of biodiesel glycerol (P2 and P3) were tested at three concentrations (3, 5 and 7 g/L). Longer HyRT (longer GCI and longer IGCT) led to lower hydrogen yield but higher 1,3-PD yield. The P3 glycerol at the concentration of 7 g/L had the highest 1,3-PD yield (0.65 mol/mol glycerol_consumed) at the GCI/IGCT of 20 h/65 h and the highest hydrogen yield (0.75 mol/mol glycerol_consumed) at the GCI/IGCT of 2.5 h/20 h), respectively. A mixed-order kinetic model was developed to simulate the effects of GCI/IGCT on the production of hydrogen and 1,3-PD. The results showed that the production of hydrogen and 1,3-PD can be optimized by adjusting HyRT in anaerobic fermentation of glycerol.     

Bioconversion of residual glycerol from biodiesel synthesis into 1,3-propanediol and ethanol by isolated bacteria from environmental consortia

Residual (raw) glycerol originated from biodiesel synthesis is becoming of great environmental and economical concern due to its ever-growing surplus. In the present study, several bacterial strains were isolated and characterized for their ability to convert this raw glycerol into 1,3-propanediol (1,3-PD) and ethanol. The best producers of both 1,3-PD and ethanol were identified by 16S rDNA sequences to be Klebsiella pneumoniae and Pantoea agglomerans strains. Batch bioreactor cultivations under anaerobic and aerobic conditions were carried out in order to access the kinetics of glycerol consumption and product formation. Results showed that one isolated, K. pneumoniae BLh-1, was able to simultaneously produce up to 9.4 g/L of 1,3-PD with yields of 0.41 mol product mol⁻¹ glycerol, and 6.1 g/L of ethanol with yields of 0.14 mol product mol⁻¹ glycerol under anaerobic conditions, showing great potential for bioprocesses.     

Electricity generation from rice straw using a microbial fuel cell

This study demonstrated electricity generation from rice straw without pretreatment in a two-chambered microbial fuel cell (MFC) inoculated with a mixed culture of cellulose-degrading bacteria (CDB). The power density reached 145 mW/m² with an initial rice straw concentration of 1 g/L; while the coulombic efficiencies (CEs) ranged from 54.3 to 45.3%, corresponding to initial rice straw concentrations of 0.5–1 g/L. Stackable MFCs in series and parallel produced an open circuit voltage of 2.17 and 0.723 V, respectively, using hexacyanoferrate as the catholyte. The maximum power for serial connection of three stacked MFCs was 490 mW/m² (0.5 mA). In parallelly stacked MFCs, the current levels were approximately 3-fold (1.5 mA) higher than those produced from the serial connection. These results demonstrated that electricity can be produced from rice straw by exploiting CDB as the biocatalyst. Thus, this method provides a promising way to utilize rice straw for bioenergy production.     

Enhanced sludge stabilization coupled with microbial fuel cells (MFCs)

This study presents research results on electricity production from waste activated sludge using MFCs during stabilization process. Different MFC configurations equipped with various electrodes were used. Voltage measurements were continuously done during 35 days of MFC operation. Experimental results showed that bioelectricity generation was linked to volatile solids (VS) and protein reductions as a fraction of extracellular polymeric substances (EPS). Double chamber MFC reactor equipped with graphite electrodes had better power and current densities as 312.98 mW/m² and 39.07 μA/cm² while single chamber MFC equipped with titanium electrodes revealed better power and current densities as 97.60 mW/m² and 17.63 μA/cm², respectively. Molecular results indicated that power outputs of MFCs effected by diverse microbial communities in anode biofilms. Although organic matter degradation is reported as 35%–55% VS reduction for digesters, this research provided a promising approach for sludge stabilization with enhanced degrading of organic matters up to 75% by using MFCs.     

Power recovery with multi-anode/cathode microbial fuel cells suitable for future large-scale applications

Multi-anode/cathode microbial fuel cells (MFCs) incorporate multiple MFCs into a single unit, which maintain high power generation at a low cost and small space occupation for the scale-up MFC systems. The power production of multi-anode/cathode MFCs was similar to the total power production of multiple single-anode/cathode MFCs. The power density of a 4-anode/cathode MFC was 1184 mW/m³, which was 3.2 times as that of a single-anode/cathode MFC (350 mW/m³). The effect of chemical oxygen demand (COD) was studied as the preliminary factor affecting the MFC performance. The power density of MFCs increased with COD concentrations. Multi-anode/cathode MFCs exhibited higher power generation efficiencies than single-anode/cathode MFCs at high CODs. The power output of the 4-anode/cathode MFCs kept increasing from 200 mW/m³ to 1200 mW/m³ as COD increased from 500 mg/L to 3000 mg/L, while the single-anode/cathode MFC showed no increase in the power output at CODs above 1000 mg/L. In addition, the internal resistance (R_in) exhibited strong dependence on COD and electrode distance. The R_in decreased at high CODs and short electrode distances. The tests indicated that the multi-anode/cathode configuration efficiently enhanced the power generation.     

Harvest of electrical energy from fermented microalgal residue using a microbial fuel cell

The application of microalgal biomass for fermentation has been highlighted as a means of producing a range of value-added biofuels and chemicals. On the other hand, the microalgal residue from the fermentation process still contains as much as 50% organic contaminants, which can be a valuable substrate for further bioenergy recovery. In this study, a microbial fuel cell and automatic external load control by maximum power point tracking (MPPT) were implemented to harvest the electrical energy from waste fermented microalgal residue (FMR). The MFC with MPPT produced the highest amount of energy (1.82 kJ/L) compared to the other MFCs with fixed resistances: 0.98 (1000 Ω), 1.16 (500 Ω), and 1.17 kJ/L (300 Ω). The MFC with MPPT also showed the highest maximum power density (88.6 mW/m²) and COD removal efficiency (620.0 mg COD/L removal with 85% removal efficiency). The implementation of MPPT gained an approximate 12.9% energy yield compared to the previous fermentation stage. These results suggest that FMR can be an appropriate feedstock for electrical energy recovery using MFCs, and the combined fermentation and MFC system improves significantly the energy recovery and treatment efficiency from FMR.     

Near stoichiometric reforming of biodiesel derived crude glycerol to hydrogen by photofermentation

Biodiesel manufacture produces crude glycerol as a major byproduct. At the scale estimated for future biodiesel production, extensive quantities of crude glycerol fraction will be generated, creating a large waste stream with potentially significant environmental impacts. The magnitude of projected future crude glycerol supplies suggests that its conversion to a biofuel is the only viable route to producing a product that does not cause market saturation. Previously it was shown that crude glycerol could be converted to hydrogen, a possible future clean energy carrier, by photofermentation using Rhodopseudomonas palustris through photofermentation. Here, the effects of nitrogen source and different concentrations of crude glycerol on this process were assessed. At 20 mM glycerol, 4 mM glutamate, 6.1 mol hydrogen/mole of crude glycerol were obtained under optimal conditions, a yield of 87% of the theoretical, and significantly higher than what was achieved previously.     

A pilot-scale study on utilizing multi-anode/cathode microbial fuel cells (MAC MFCs) to enhance the power production in wastewater treatment

A new type of microbial fuel cell (MFC), multi-anode/cathode MFC (termed as MAC MFC) containing 12 anodes/cathodes were developed to harvest electric power treating domestic wastewater. The power density of MAC MFCs increased from 300 to 380 mW/m² at the range of the organic loading rates (0.19-0.66 kg/m³/day). MAC MFCs achieved 80% of contaminant removal at the hydraulic retention time (HRT) of 20 h but the contaminant removal deceased to 66% at the HRT of 5 h. In addition, metal-doped manganese dioxide (MnO₂) cathodes were developed to replace the costly platinum cathodes, and exhibited high power density. Cu-MnO₂ cathodes produced 465 mW/m² and Co-MnO₂ cathodes produced 500 mW/m². Due to the cathode fouling of the precipitation of calcium and sodium, a decrease in the power density (from 400 to 150 mW/m²) and an increase in internal resistance (R_in) (from 175 to 225 Ω) were observed in MAC MFCs.     

Waste glycerol gasification to syngas in pure DC water vapor arc plasma

In this experimental study, the conversion of waste glycerol, derived from industrial biodiesel production process, to syngas in water vapor DC thermal arc plasma was carried out. Pure water vapor was used as a gasifying agent, a heat carrier and a plasma-forming gas. This enabled to avoid undesirable ballast products, such as nitrogen oxide, and obtain higher hydrogen and carbon monoxide concentration. It was found that a higher process efficiency (the concentration of H₂ 57.9 vol% and CO 21 vol%, the H₂/CO ratio 2.76, the yield of H₂ 70 mol.% and CO 37 mol.%, the tar content 0.822 g/m³, the carbon conversion efficiency 68%, the energy conversion efficiency 39.5%) with a lower specific energy requirement (227.8 kJ/mol) was obtained at the highest H₂O/C₃H₈O₃ ratio of 1.9 (or water vapor flow rate of 14.76 kg/h, waste glycerol content of 7.88 kg/h, and the plasma torch power of 57 kW). This study is expected to provide an effective and advanced waste-to-energy solution.     

Statistical optimization of biohydrogen and ethanol production from crude glycerol by microbial mixed culture

Design of Experiments (DoE) was applied to improve the ability of enriched activity sludge to efficiently convert crude glycerol from biodiesel industry into hydrogen and ethanol, using a very simple synthetic medium. Based on Plackett–Burman screening design, glycerol concentration, temperature and initial pH were identified as significant variables. Box–Behnken design and Response Surface Method (RSM) were then used for optimization. The maximum hydrogen yield of 0.96 mol H₂/mol glycerol was estimated at the temperature of 37.0 °C, initial pH of 7.9 and glycerol concentration of 15.0 g/L. Maximum hydrogen production rate of 2191 mL/L/d was estimated at the temperature of 37.3 °C, initial pH of 8.0 and glycerol concentration of 15.2 g/L. Finally maximum ethanol production of 7.92 g/L was estimated at an initial pH of 8.0 and glycerol concentration of 15.0 g/L (temperature had no significant effect). These results show that it is possible to obtain both, high yield and production of hydrogen and ethanol together, using a very simple synthetic medium, without trace element- and vitamin solution, tryptone or yeast extract.     

Media optimization for biohydrogen production from waste glycerol by anaerobic thermophilic mixed cultures

Media compositions affecting thermophilic biohydrogen production from waste glycerol were optimized using response surface methodology (RSM) with central composite design (CCD). Investigated parameters used were waste glycerol concentration, urea concentration, the amount of Endo-nutrient addition and disodium hydrogen phosphate (Na₂HPO₄) concentration. Waste glycerol concentration and the amount of Endo-nutrient addition had a significant individual effect on the cumulative hydrogen production (HP) (p ≤ 0.05). The interactive effect on HP was found between waste glycerol and urea concentration as well as waste glycerol concentration and the amount of Endo-nutrient addition (p ≤ 0.05). The optimal media compositions were 20.33 g/L of waste glycerol, 0.16 g/L of urea, 3.97 g/L of Na₂HPO₄ and 0.20 mL/L of the amount of Endo-nutrient addition which gave the maximum HP of 1470.19 mL H₂/L. The difference between observed HP (1502.84 mL H₂/L) and predicted HP was 2.22%. The metabolic products from the fermentation process were 1,3-propanediol (1,3-PD), ethanol, acetic, formic, lactic, butyric, and propionic acids. Results from polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis indicated that the hydrogen producers present in the fermentation broth was Thermoanaerobacterium sp.     

Effects of gas diffusion layer (GDL) and micro porous layer (MPL) on cathode performance in microbial fuel cells (MFCs)

This study focused on novel cathode structures to increase power generation and organic substrate removal in microbial fuel cells (MFCs). Three types of cathode structures, including two-layer (gas diffusion layer (GDL) and catalyst layer (CL)), three-layer (GDL, micro porous layer (MPL) and CL), and multi-layer (GDL, CL, carbon based layer (CBL) and hydrophobic layers) structures were examined and compared in single-chamber MFCs (SCMFCs). The results showed that the three-layer (3L) cathode structures had lower water loss than other cathodes and had a high power density (501 mW/m²). The MPL in the 3L cathode structure prevented biofilm penetration into the cathode structure, which facilitated the oxygen reduction reaction (ORR) at the cathode. The SCMFCs with the 3L cathodes had a low ohmic resistance (R_ohmic: 26-34 Ω) and a high cathode open circuit potential (OCP: 191 mV). The organic substrate removal efficiency (71-78%) in the SCMFCs with 3L cathodes was higher than the SCMFCs with two-layer and multi-layer cathodes (49-68%). This study demonstrated that inserting the MPL between CL and GDL substantially enhanced the overall electrical conduction, power generation and organic substrate removal in MFCs by reducing water loss and preventing biofilm infiltration into the cathode structure.     

Energy valorization of crude glycerol and sanitary sewage in hydrogen generation by biological processes

Biohydrogen production was studied with increased concentrations of crude glycerol (CG) co-digested with sanitary sewage by anaerobic consortium bacteria in anaerobic batch reactors, at 30 °C and initial pH 7.0. The CG was obtained during the biodiesel production from waste cooking oils (WCO). The anaerobic consortium was from a granular sludge of UASB reactor used in the treatment of poultry wastes heat treated to inhibit methanogenic activity previously. The higher H₂ generation was observed with 240.0 g COD L⁻¹ (35.82 mmol L⁻¹), being consumed 63.9% of CG. The co-digestion associated with sanitary sewage facilitated the CG consumption with the increase of organic load, favoring the acetic acid and 1,3-propanediol production in a liquid phase of the reactors. These results are promising, contributing effectively to the treatment of both wastes with concomitant generation of bioenergy.     

Biohydrogen production from waste glycerol and sludge by anaerobic mixed cultures

Key factors affecting biohydrogen production from waste glycerol and sludge by anaerobic mixed cultures were optimized using response surface methodology (RSM) with central composite design (CCD). Investigated parameters were waste glycerol concentration, sludge concentration, and the amount of Endo–nutrient addition. Concentrations of waste glycerol and sludge had a significant individual effect on hydrogen production rate (HPR) (p ≤ 0.05). The interactive effect on HPR (p ≤ 0.05) was found between waste glycerol concentration and sludge concentration. The optimal conditions for the maximum HPR were: waste glycerol concentration 22.19 g/L, sludge concentration 7.16 g-total solid (TS/L), and the amount of Endo–nutrient addition 2.89 mL/L in which the maximum HPR of 1.37 mmol H₂/L h was achieved. Using the optimal conditions, HPR from a co-digestion of waste glycerol and sludge (1.37 mmol H₂/L h) was two times greater than the control (waste glycerol without addition of sludge) (0.76 mmol H₂/L h), indicating a significant enhancement of HPR by sludge. Major metabolites of the fermentation process were ethanol, 1,3-propanediol (1,3-PD), lactate, and formate.     

Bioconversion of crude glycerol from waste cooking oils into hydrogen by sub-tropical mixed and pure cultures

This study compared the biohydrogen generation by sub-tropical mixed and pure cultures from the crude glycerol from the biodiesel production using waste cooking oils (WCO). The crude glycerol was pretreated by pH adjustment. The mixed culture was obtained from a subtropical granular sludge of the UASB (Upflow Anaerobic Sludge Blanket) reactor used in the treatment of vinasse from sugarcane of ethanol and sugar industry. It was heat treated in order to inactivate hydrogen-consuming bacteria, which was identified by Illumina MiSeq Sequencing with a relative abundance of 97.96% Firmicutes Philum, 91.81% Clostridia Class and 91.81% Clostridiales Order. The pure culture was isolated from a sub-tropical granular sludge from UASB reactor of treating brewery wastewater and identified as Enterobacter sp. (KP893397). Two assays were carried in anaerobic batch reactors in order to verify the hydrogen production from crude glycerol bioconversion with: (I) mixed culture and (II) pure culture. The experiments were conducted at 37 °C, initial pH of 5.5 for assay I and 7.0 for assay II, with 20 g COD L⁻¹ of crude glycerol. The crude glycerol consumption was 56.2% and 88.0% for the assay I and II, respectively. The hydrogen yields were 0.80 moL H₂ mol⁻¹ glycerol for the assay I and 0.13 moL H₂ mol⁻¹ glycerol for the assay II. Enterobacter sp. preferred the reductive metabolic route, generating 1460.0 mg L⁻¹ of 1,3-propanediol, and it showed to be more sensitive in the presence of methanol from crude glycerol than mixed culture that preferred the oxidative metabolic route with biohydrogen generation. The mixed culture was more able to generate H₂ than pure culture from the crude glycerol coming from the biodiesel production using WCO.     

Conversion of residual glycerol from biodiesel synthesis into 1,3-propanediol by a new strain of Klebsiella pneumoniae

A stepwise strategy was devised to optimize the bioconversion of residual glycerol from biodiesel synthesis into 1,3-propanediol (1,3-PD) by a new strain Klebsiella pneumoniae. This strain is unique in its ability to convert glycerol simultaneously into 1,3-PD. The influence of glycerol concentration, pH, temperature, nitrogen and mineral sources were first investigated using the Plackett–Burman (P–B) statistical design to screen the variables that significantly affect the production of 1,3-PD. Seven variables were statistically significant at 95% significance and a 2^7-3 fractional factorial design (FFD) was applied to further refine the optimization of culture conditions. Results showed that the ideal medium composition and culture conditions for the syntheses of 1,3-PD are: glycerol 65 g/L, yeast extract 5 g/L, peptone 5 g/L, (NH₄)₂SO₄ 7 g/L, K₂HPO₄ 7 g/L, and temperature of 37 °C. Experiments in batch bioreactors under controlled pH produced up to 23.80 g/L of 1,3-PD and 12.30 g/L of ethanol, while in fed-batch cultivations a three-fold increase of 1,3-PD production (36.86 g/L) was obtained compared with the results of the FFD design.     

Miniature microbial fuel cells and stacks for urine utilisation

MFCs are becoming a stronger contender in the area of alternative energy sources and show great promise in utilising a wide variety of organic sources. This paper describes the utilisation of neat undiluted urine as the main feedstock for different types of individual MFCs and stacks of small-scale MFCs, for direct electricity production, with conversion efficiencies of >50%. The smallest MFC (1.4 mL total volume) produced equal amounts of power to that produced by larger MFCs (6.25 mL), resulting in increased power densities. Power densities of 4.93 mW/m² (absolute power of 1.5 mW) were recorded when 48 small-scale MFCs were connected as a stack and fed with urine. This study demonstrates the feasibility of using urine as an untreated fuel and that improved power outputs can be achieved through MFC miniaturisation and multiplication into stacks.     

Comparison of fermentative hydrogen production from glycerol using immobilized and suspended mixed cultures

This study explored the fermentative hydrogen production by immobilized microorganisms from glycerol, which is the byproduct of biodiesel production, and compared it with suspended fermentation. The effect of immobilization on hydrogen production process was examined. Results showed that both cumulative hydrogen production (CHP) and hydrogen yield (HY) were enhanced by microbial immobilization. The highest CHP and HY of 64 mL/100 mL and 0.52 mol H₂/mol glycerol were obtained by immobilized microorganisms, compared to 9 mL/100 mL and 0.29 mol H₂/mol glycerol in suspended microorganisms. Immobilization enhanced CHP and HY by 611.1% and 79.3%. In addition, immobilized microorganisms showed stronger tolerance to high substrate concentration and higher capability in glycerol utilization, which is of great significance for hydrogen production from glycerol. The enhanced hydrogen production may be due to the favorable micro-environment for different microorganisms in immobilized beads.     

Optimizing energy harvest in wastewater treatment by combining anaerobic hydrogen producing biofermentor (HPB) and microbial fuel cell (MFC)

This study focused on the optimization of energy harvest from wastewater treatment by integrating two novel biotechnologies: anaerobic hydrogen production and microbial fuel cell (MFC). The simultaneous production of hydrogen and electricity from wastewater was examined at continuous flow at different organic loading rates (OLR) by changing chemical oxygen demand (COD) and hydraulic retention time (HRT). The experimental results showed that the specific hydrogen yield (SHY, mole H₂/mole glucose) increased with the decrease in OLR, and reached at the maximum value of 2.72 mol H₂/mole glucose at the lowest OLR of 4 g/L.d. The effluent from hydrogen producing biofermentor (HPB) was fed to a single chamber MFC (SCMFC), obtaining the highest power density and coulombic efficiency (CE) of 4200 mW/m³ and 5.3%, respectively. The energy conversion efficiency (ECE) increased with OLR and reached the peak value of 4.24% at the OLR of 2.35 g/L.d, but decreased with higher OLR. It was demonstrated that the combination of HPB and MFC improved the ECE and COD removal with the maximum total ECE of 29% and COD removal of 71%. The kinetic analysis was conducted for the HPB-MFC hybrid system. The maximum hydrogen production was projected to be 2.85 mol H₂/mole glucose. The maximum energy recovery and COD removal efficiency from MFC were projected to be 559 J/L and 97%, respectively.