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Production of Gaseous Biofuels and Electricity from Cheese Whey Katerina Stamatelatou, Georgia Antonopoulou, Asimina Tremouli, and Gerasimos Lyberatos* Department of Chemical Engineering, UniVersity of Patras, Karatheodori 1 Street, 26500 Patras, Greece and Institute of Chemical Engineering and High-Temperature Chemical Processes, 26504 Patras, Greece
In this paper, the potential of energy recovery from cheese whey in the form of gas biofuels (hydrogen, methane) as well as electricity generation through application of the microbial fuel cell technology are studied. Hydrogen and methane production from cheese whey in a two-stage process has already been studied at a lab scale in a continuous stirred tank reactor (CSTR) of 3 L and a periodic anaerobic baffled reactor of 15 L, respectively. In this work, to scale up the hydrogen production step, the cheese whey fermentation process was studied at a larger scale using a 13.8 L bioreactor of CSTR type. The chemical oxygen demand concentration of the cheese whey fed to the CSTR was 53.3 ( 3.8 g/L. The pH in the bioreactor was maintained at 5.14 ( 0.15 with NaOH addition. The hydrogen produced reached up to 4.8 ( 0.5 L/L/d at an hydraulic retention time of 24 h (corresponding to a yield of 1.3 mol H2/mol consumed carbohydrates), approximately twice as much obtained in previous lab-scale experiments in a 3 L bioreactor. In addition, diluted cheese whey at different initial concentrations, was used as feedstock for electricity generation, using a two-chamber microbial fuel cell (MFC). The experiments showed that the MFC performance was not limited by the wastewater strength since the substrate removal efficiency and maximum power output were not affected by the increase of the initial concentration. The time needed for complete substrate degradation increased linearly with the wastewater strength. Introduction The energy captured in the organic wastewaters is exploitable if the electrons of the chemical bonds can be transferred to form high energy carriers (such as hydrogen, methane, etc.) or used directly to electrical circuits. Electron transfer can be realized through (bio)chemical processes. Predominant biochemical processes for this purpose include biohydrogen and biomethane production as well as the relatively novel technology of microbial fuel cell. The potential of energy exploitation of a high organic-loaded wastewater such as cheese whey through the aforementioned technologies is addressed in this paper. Biohydrogen is a clean, CO2-neutral energy source, which can be used in fuel cells to produce electricity efficiently. Contrary to other fossil fuels, the oxidation of which is accompanied by CO2, NOx, particulate and other emissions, water is the only byproduct. Biohydrogen can be produced by both light-driven and dark fermentation bioprocesses, but dark fermentation seems to be far more promising in terms of feasibility and viability.1,2 It is common knowledge that the main barriers to fermentative hydrogen production are the relatively low yield of hydrogen, relatively low production rates, and the cost of the feedstock. To make the biohydrogen economy viable and given that only 15% of the energy captured in the chemical bonds of the organic compounds can be retrieved as hydrogen through fermentation,3 there is a major challenge to increase the yield and production rates through integrated strategies consisting of two steps with the first one being fermentative hydrogen production or modified microbial fuel cell-based hydrogen production, and the second one being either photobiological hydrogen production or methane production. In this way, more energy or energy carriers in the form of hydrogen or methane can be harvested per mole of substrate utilized. The feedstock choice is another important factor that affects the biohydrogen economy. It is widely considered that the use * To whom correspondence should be addressed. E-mail:
[email protected].
of feedstocks such as corn and sugar for energy production is unacceptable.4 On the other hand, agricultural wastes (crop residues or wastes from agroindustrial processes or food industries) have negligible cost and appear as promising feedstocks not only for hydrogen but also more other biofuels (bioethanol, biobutanol, biodiesel) production. Some researchers have studied anaerobic H2 fermentation from wastes, such as municipal solid waste and wastewaters,5-8 sugar manufacturer wastewater,9 starch manufacturer waste10 and rice slurry.11 Cheese whey is another type of wastewater rich in soluble carbohydrates (4.5-5% w/v) that can be used for hydrogen production. It is the main byproduct of cheese manufacturing. Apart from lactose, it contains soluble proteins (0.6-0.8% w/v), lipids (0.4-0.5% w/v), and mineral salts (8-10% of dried extract).12 Most studies focusing on hydrogen production from whey are based on batch experiments.13,14 However there are recent studies exploring the potential of biohydrogen production in continuous bioreactors such as continuous stirred tank reactor (CSTR) under mesophilic15-17 and thermophilic conditions18 or in an upflow anaerobic sludge blanket (UASB) reactor.19 These studies have been performed at a lab scale (2-4.6 L), but not at a larger scale. In this paper, the efficiency of fermentative hydrogen production from cheese whey at a larger scale is compared with the results from previous lab-scale experiments. Hydrogen produced during fermentation of sugars is the result of the abundant electrons released and transferred through the reduction of the nicotinamide adenine dinucleotide (NAD+) electron carrier. Methane formation is also the result of electron transfer resulting in the partial reduction and oxidation of the parent organic compound. Both hydrogen (directly) and methane (after reforming or even directly) can be used in fuel cells for the production of electricity. Another concept studied in this work, is to direct the electrons transferred during the anaerobic biological processes to a closed loop circuit to generate electricity directly. This can be carried out in microbial fuel cells (MFCs), which are considered to be a new alternative to
10.1021/ie1002262 2011 American Chemical Society Published on Web 08/13/2010
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hydrogen and methane production for recovering energy from a wastewater.20 An MFC consisting of two electrodes (anode and cathode) in two separate chambers was used in this work. The bacteria grow on dissolved organic matter (cheese whey) in the anode chamber under anaerobic conditions. These bacteria transfer electrons to the anode electrode. The electrons then pass through an external circuit to the cathode electrode producing electrical current. Protons migrate through the solution and a proton exchange membrane to the cathode, where they combine with oxygen and the electrons transferred through the circuit to form water.21 In a previous work22 cheese whey was used without any pretreatment and it was seen that electricity production was possible but at a reduced efficiency, attributed to significant substrate bioconversion in the bulk of the anode, rather than on the electrode. In this work, the cheese whey was filteredsterilized before using in the MFC. The maximum wastewater concentration that could be used in the MFC was assessed. In these experiments, the substrate removal efficiency was determined and the duration of electricity supply was correlated with the initial substrate concentration. Materials and Methods Analytical Methods. Samples were taken from the feeding tube and the interior of the bioreactor daily. The parameters determined included the total suspended solids (TSS), volatile suspended solids (VSS), and total and dissolved (after centrifugation and filtration of the supernatant liquid) chemical oxygen demand (COD).23 The total carbohydrates were measured according to the analysis of Josefsson24 and L-lactic acid was determined using the enzymatic reagent kit (Megazyme D-/Llactic acid kit, K-DLATE 10/05). For volatile fatty acids (VFA) and alcohol (ethanol and butanol) quantification, acidified samples with 20% H2SO4 were analyzed on a gas chromatograph (Varian CP-30), equipped with a flame ionization detector and a capillary column (Agilent Technologies, Inc., 30 m × 0.53 mm). The oven was programmed from 105 to 160 °C at a rate of 15 °C/min and subsequently to 235 °C (held for 3 min) at a rate of 20 °C/min for VFA analysis and from 60 °C (held for 1 min) to 230 °C (held for 0.5 min) at a rate of 45 °C/min for alcohols analysis. Helium was used as the carrier gas at 15 mL/min, and the injector temperature was set at 175 °C and the detector at 225 °C. The hydrogen content of biogas were determined in a gas chromatograph (Varian STAR 3600) equipped with a thermal conductivity detector and a packed column (Poropak Q, 80-100 mesh) with nitrogen as the carrier gas. The injector, column, and detector temperatures were set at 70, 80, and 180 °C, respectively. The biogas production rate was measured using a water displacement technique. Finally, pH was measured with a Hanna pH-meter. Feedstock. The cheese whey used in this study was obtained from a cheese factory, producing mainly the white cheese “feta”, located in the Achaia prefecture nearby Patras, Western Greece. The wastewater was maintained in a 550 L refrigerator at 4 °C, and every 10 d the refrigerator was emptied, cleaned, and filled up with fresh wastewater. Lactate fermentation took place while cheese whey was maintained in the refrigerator, resulting in lactic acid production (from
