Nowadays, hydrogen produced globally has been synthesized from fossil fuel with limited source. Therefore, research has been developed in order to explore biological H2 production by dark fermentation. The purpose of this work was to evaluate the effect of initial pH and ferrous sulfate and ammonium sulfate concentrations on the production of biohydrogen by dark fermentation. The process was carried out in batch mode under anaerobic conditions, in the absence of light, and at standard room temperature and pressure. A microbial consortium provided by the effluent treatment plant of a local dairy company was inoculated into a synthetic medium supplemented with cheese whey permeate (20 g/L of lactose) as a carbon source. The influence of three variables was analyzed by a central composite design 2((3)), and the optimum results of hydrogen yield (4.13 mol H2/mol lactose) and productivity (86.31 mmol H2/L/day) were achieved at initial pH 7.0 and FeSO4 and (NH4)2SO4 concentrations of 0.6 and 1.5 g/L, respectively. Under these conditions, the kinetic parameters of fermentation were investigated by analyzing the profile of H2 yield and productivity, metabolite concentrations, pH, and concentration of dissolved iron. In the kinetic analysis, the modified Gompertz equation described adequately the fermentative hydrogen production from cheese whey permeate (R (2) = 0.98). The profile of ethanol and volatile organic acids showed that lactic acid and butyric acid were the main metabolites produced, and the sum of both by-products corresponded to about 58 % of the total metabolites.
The aim of this work was to produce ethanol using the acid hydrolysis of soybean molasses followed by alcoholic fermentation via submerged Saccharomyces cerevisiae. The influence of the acid type, pH, and absolute pressure of the hydrolysis on the ethanol yield and the total residual sugar concentration was evaluated using a factorial design (FD). The absolute pressure ranged from 1 to 2 atm; the pH ranged from 3 to 5; and three different acids were studied in the hydrolysis process: sulfuric, hydrochloric, and nitric acids. The experiments were conducted in an Applicon batch reactor with a useful volume of 1.5 L at a stirring speed of 230 rpm and with an inoculum concentration of 30 g/L. The inoculum volume used was 30% of the total volume. The best results, as determined by FD, were obtained at pH 4 and an absolute pressure of 1.5 atm for all of the acids studied. The highest ethanol yield was 46% for sulfuric acid, 48% for hydrochloric acid, and 54% for nitric acid. After the concentration of inoculum and the fermentation kinetics profiles were investigated, a 62% yield relative to the initial sugar content was obtained under optimum conditions after 14 h of fermentation and an inoculum concentration of 35 g/L.
Highlights 1) Hydrogen produced by dark fermentation based on repeated-batch cycles 2) Extension superior to 900 h was succeeded with alternated cycles of sugars 3) Maximum H2 yield (3.4 mol H2/mol hexose) resulted from alternated addition of sugars 4) Maximum hydrogen productivity was 168.27 mmol H2/L/day in 24 h of process 5) Microorganisms followed the butyric-type fermentation AbstractHydrogen is considered a very clean energy source, since its combustion releases mainly water as a reaction product. Besides, it has the advantage of having the highest energy density when compared to any other fuel. This work studied the hydrogen production applying dark fermentation by a heat shock pre-treated microbial 2 consortium. A repeated batch cycle operation was evaluated by adding glucose or lactose in an isolated, alternated or simultaneous ways, in order to keep the production of hydrogen for a longer time. Fermentations with simultaneous addition of glucose and lactose promoted maximum productivity of 168.27 mmol H2·L -1 ·day -1 . Nevertheless, the alternation of two carbon source (glucose and lactose) allowed keeping the culture active with potential to hydrogen production for a period of time higher than 900 h. At the end of fermentation, the main products were lactic acid and butyric acid, followed by acetic acid, ethanol and propionic acid.
The production of biofuels as an alternative to the fossil fuels has been mandatory for a cleaner and sustainable process. Hydrogen is seen as the fuel of the future because it has a very high energy density and its use produces only water instead of greenhouse gases and other exhaust pollutants. The biological synthesis of hydrogen by dark fermentation complies with these criteria. In the current work, the use of cheese whey permeate was evaluated aiming hydrogen production by dark fermentation using a microbial consortium in the semi-continuous process, with a reaction volume of 700 mL. The volume of the medium renewal and the frequency of replacements of fresh medium were evaluated to extend the production of H 2. It is important to note decreases in the hydrogen production after 84 h. The target-product content became higher particularly when 466 mL of medium were withdrawn, in every 24 h in the first two replacements and, subsequently, in every 12 h. Besides, it was observed lower lactic acid concentration under this condition, suggesting that the shorter removal time of the medium could inhibit lactic acid bacteria, which may secrete bacteriocins that inhibit the hydrogen-producing microorganisms.
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