• Energy Research

Abstract Self-fermentation of cellulosic substrates to produce biohydrogen without inoculum addition nor pretreatments was investigated. Dark fermentation of two different substrates made of leaf-shaped vegetable refuses (V) and leaf-shaped vegetable refuses plus potato peels (VP), was taken in consideration. Batch experiments were carried out, under two mesophilic anaerobic conditions (28 and 37 °C), in order to isolate and to identify potential H2-producing bacterial strains contained in the vegetable extracts. The effect of initial glucose concentration (at 1, 5 and 10 g/L) on fermentative H2 production by the isolates was also evaluated. H2 production from self-fermentation of both biomasses was found to be feasible, without methane evolution, showing the highest yield for V biomass at 28 °C (24 L/kg VS). The pH control of the culture medium proved to be a critical parameter. The isolates had sequence similarities ≥98% with already known strains, belonging to the family Enterobacteriaceae (γ-proteobacteria) and Streptococcaceae (Firmicutes). Four genera found in the samples, namely Pectobacterium, Raoultella, Rahnella and Lactococcus have not been previously described for H2 production from glucose. The isolates showed higher yield (1.6–2.2 mol H2/mol glucoseadded) at low glucose concentration (1 g/L), while the maximum H2 production ranged from 410 to 1016 mL/L and was obtained at a substrate concentration of 10 g/L. The results suggested that vegetable waste can be effectively used as both, substrate and source of suitable microflora for bio-hydrogen production.

Microbial fuel cells (MFCs) fed with wastewater are currently considered a feasible strategy for production of renewable electricity. A membrane-less MFC with biological cathode was built from a compact wastewater treatment reactor and fed with synthetic wastewater. When operated with an external resistance of 250 Omega, the MFC produced a long-term power of about 70 mW/m(2) for 10 months. Denaturing Gradient Gel Electrophoresis (DGGE) analysis of the cathode biomass when the MFC was closed on a 2100 Omega external resistance showed that the sequenced bands were affiliated with Firmicutes, alpha-Proteobacteria,beta-Proteobacteria, gamma-Proteobacteria, and Bacteroidetes groups. When the external resistance was varied between 250 and 2100 Omega, minimum sustainable resistance decreased from 900 to 750 Omega, while maximum sustainable power output decreased from 32 to 28 mW/m(2). It is likely that these effects were caused by changes in the microbial ecology of anodic and cathodic biomass attached to the electrodes. Results suggest that cathodic biomass enrichment in "electroactive" bacteria may improve MFCs power output in a similar fashion to what has been already observed for anodic biomass.

In this study, we evaluated the effectiveness of lake sediment as inoculum for hydrogen production through dark fermentation in a repeated batch process. In addition, we investigated the effect of heat treatment, applied to enrich hydrogen-producing bacteria, on the bacterial composition and metabolism. Denaturing gradient gel electrophoresis and molecular cloning, both performed using the 16S rDNA gene as target gene, were used to monitor the structure of the bacterial community. Hydrogen production and bacterial metabolism were analysed via gas chromatography and high-performance liquid chromatography. Both treated and non-treated inocula were able to produce high amounts of hydrogen. However, statistical analysis showed a clear difference in their bacterial composition and metabolism. The heat treatment favoured the growth of different Clostridia sp., in particular of Clostridium bifermentans, allowing the production of a constant amount of hydrogen over prolonged time. These cultures showed both butyrate and ethanol fermentation types. Absence of heat treatment allowed species belonging to the genera Bacillus, Sporolactobacillus and Massilia to outgrow Clostridia sp. with a reduction in hydrogen production and a significant metabolic change. Our data indicate that lake sediment harbours bacteria that can efficiently produce hydrogen over prolonged fermentation time. Moreover, we could show that the heat treatment stabilizes the bacterial community composition and the hydrogen production.